localbrain qm01 brain01 stanford Everett's Relative-State Formulation of Quantum Mechanics; the standard collapse formulation of quantum mechanics, like the Copenhagen interpretation, required observers always to be treated as external to the system described by the theory. One consequence of this was that neither the standard collapse theory nor the Copenhagen interpretation can be used to describe the physical universe as a whole.“波函数坍塌”的假设完全是多余的。纯粹的量子理论实际上并不产生任何矛盾。它预示着这样一种情形：一个现实状态会逐渐分裂成许多重叠的现实状态，观测者在分裂过程中的主观体验仅仅是经历完成了一个可能性恰好等于以前“波函数坍塌假设结果”的轻微的随机事件。

http://plato.stanford.edu/entries/qm-everett/

http://www.astronomy.com.cn/bbs/forum.php
电磁势作为可观测量

http://www.weibo.com/234525857?from=hissimilar_home


小青虫0408

+关注

初九之初

微能见著，博可达通。

+关注

货币Diors

专注货币及相关。仅代表个人浅陋之见。

由于投资者不可能获得完整的资讯，且投资者会因个别问题影响其对市场的认知，令投资者对市场预期产生不同的意见，索罗斯把这种“不同的意见”解释为“投资偏见”，并认为“投资偏见”是金融市场的根本动力。当“投资偏见”零散的时候，其对金融市场的影响力是很小的，当“投资偏见”在互动中不断强化并产生群体影响时就会产生“蝴蝶效应”，从而推动市场朝单一方向发展，最终必然反转。                                                                                

feng1734 | 只看Ta

2013-01-07 15:32

个人观点
“可观测量”的定义为“可以由时空坐标的测量和计算得到的东西”即“可观测量就是时空坐标的函数”
以电场E为例，在一个惯性系中，给定时刻给定位置无初速的释放一个点电荷，在电荷的运动轨迹上测量一列以初始位置为极限的点列（事件列），则“物理量a（点电荷在初始时刻的加速度）是可观测量”的意思就是“a是这列事件列（时空坐标构成的数列）的函数”；“物理量E（初始时刻电荷所在位置的电场）是可观测量”的意思就是“E是这列事件列的函数”，具体函数关系就是“E等于初始时刻的加速度乘以点电荷的质量再除以点电荷的电量”
进一步的，“xx规范电磁势（满足xx规范的电磁势）是可观测量”的含义就是“xx规范电磁势是某些事件集合的函数”，这里的函数形式包含了“xx规范”的限制
同样的，“oo规范电磁势是可观测量”的含义就是“oo规范电磁势是某些事件集合的函数”，这里的函数形式包含了“oo规范”的限制
因为“xx规范电磁势”与“oo规范电磁势”作为事件集合的函数有着一定的相似性，所以“xx规范电磁势”与“oo规范电磁势”被认为是同一种物理量，统称“电磁势”。“电磁势是可观测量”的含义就是“xx规范电磁势和oo规范电磁势（等等各种规范电磁势）是可观测量”
类似的，“电磁场张量是可观测量”的含义就是“电场和磁场是可观测量”

分享到：

收藏 |

+ 加入我的果篮

发表评论 0

来点赞吧，让这个帖子成为小组热帖。

评论 (3) 只看楼主

Everett's Relative-State Formulation of Quantum Mechanics

First published Wed Jun 3, 1998; substantive revision Mon Jul 7, 2014

Hugh Everett III's relative-state formulation of quantum mechanics is an attempt to solve the quantum measurement problem by dropping the collapse dynamics from the standard von Neumann-Dirac formulation of quantum mechanics. Everett then wanted to recapture the predictions of the standard collapse theory by explaining why observers nevertheless get determinate measurement records that satisfy the standard quantum statistics. There has been considerable disagreement over the precise content of his theory and how it was suppose to work. Here we will consider how Everett himself presented the theory, then briefly compare his presentation to the popular many-worlds interpretation.

• 1. Introduction
• 2. The Measurement Problem
• 3. Everett's Proposal
• 4. Empirical Faithfulness
• 5. Four Arguments
• 6. Faithfulness and the Problem of Empirical Adequacy
• 7. Many Worlds
• 8. Summary
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

---

1. Introduction

Everett developed his relative-state formulation of quantum mechanics while a graduate student in physics at Princeton University. His doctoral thesis (1957a) was accepted in March 1957 and a paper (1957b) covering essentially the same material was published in July of the same year. DeWitt and Graham (1973) later published Everett's longer, more detailed description of the theory (1956) in a collection of papers on the topic. The published version was revised from a longer draft thesis that Everett had given John Wheeler, his Ph.D. adviser, in January 1956 under the title “Wave Mechanics Without Probability”. While Everett always favored the description of the theory as presented in the longer thesis, Wheeler, in part because of Bohr's disapproval of Everett's critical approach, insisted on the revisions that led to the much shorter thesis that Everett ultimately defended.
Everett took a job outside academics as a defense analyst in the spring of 1956. While subsequent notes and letters indicate that he continued to be interested in the conceptual problems of quantum mechanics and, in particular, in the reception and interpretation of his formulation of the theory, he did not take an active role in the debates surrounding either. Consequently, the long version of his thesis (1956) is the most complete description of his theory. Everett died in 1982. See Byrne 2010 and Barrett and Byrne 2012 for further biographical details.
Everett's no-collapse formulation of quantum mechanics was a direct reaction to the measurement problem that arises in the standard von Neumann-Dirac collapse formulation of the theory. Everett understood this problem in the context of a version of the Wigner's Friend story. Everett's solution to the problem was to drop the collapse postulate from the standard formulation of quantum mechanics then deduce the empirical predictions of the standard collapse theory as the subjective experiences of observers who were themselves modeled as physical systems in the theory.
There have been a number of mutually incompatible presentations of Everett's theory. Indeed, most no-collapse interpretations of quantum mechanics have at one time or another either been directly attributed to Everett or suggested as charitable reconstructions. The most popular of these, the many worlds interpretation, is often simply attributed to Everett directly and without comment even when Everett himself never described his theory in terms of many worlds.
In order to understand Everett's proposal for solving the quantum measurement problem, one must first understand what he took the measurement problem to be. We will start with this, then consider Everett's presentation of his relative-state formulation of pure wave mechanics quantum mechanics and contrast it briefly with two versions of the many worlds interpretation.

2. The Measurement Problem

Everett presented his relative-state formulation of pure wave mechanics as a way of avoiding conceptual problems encountered by the standard von Neumann-Dirac collapse formulation of quantum mechanics. The main problem, according to Everett, was that the standard collapse formulation of quantum mechanics, like the Copenhagen interpretation, required observers always to be treated as external to the system described by the theory. One consequence of this was that neither the standard collapse theory nor the Copenhagen interpretation can be used to describe the physical universe as a whole. He took the von Neumann-Dirac collapse theory to be inconsistent and the Copenhagen interpretation to be essentially incomplete. We will follow the main argument of Everett's thesis and focus here on the measurement problem as encountered by the standard collapse theory.
In order to understand what Everett was worried about, one must first understand how the standard collapse formulation of quantum mechanics works. The theory involves the following principles (von Neumann, 1955):

1. Representation of States: The state of a physical system S is represented by an element of unit length in a Hilbert space (a vector space with an inner product).
2. Representation of Observables: Every physical observable O is represented by a Hermitian operator O on the Hilbert space representing states, and every Hermitian operator on the Hilbert space corresponds to some observable.
3. Eigenvalue-Eigenstate Link: A system S has a determinate value for observable O if and only if the state of S is an eigenstate of O. If it is, then one would with certainty get the corresponding eigenvalue as the result of measuring O of S.
4. Dynamics: (a) If no measurement is made, then a system S evolves continuously according to the linear, deterministic dynamics, which depends only on the energy properties of the system. (b) If a measurement is made, then the system S instantaneously and randomly jumps to a state where it either determinately has or determinately does not have the property being measured. The probability of each possible post-measurement state is determined by the system's initial state. More specifically, the probability of ending up in a particular final state is equal to the norm squared of the projection of the initial state on the final state.

Everett referred to the standard von Neumann-Dirac theory the “external observation formulation of quantum mechanics” and discussed it beginning (1956, 73) and (1957, 175) in the long and short versions of his thesis respectively. While he took the standard collapse theory to encounter a serious conceptual problem, he also used it as the starting point for his presentation of pure wave mechanics, which he described as the standard collapse theory but without the collapse dynamics (rule 4b). We will briefly describe the problem with the standard theory, then turn to Everett's discussion of the Wigner's Friend story and his proposal for replacing the standard theory with pure wave mechanics.
According to the eigenvalue-eigenstate link (rule 3) a system would typically neither determinately have nor determinately not have a particular given property. In order to determinately have a particular property the vector representing the state of a system must be in the ray (or subspace) in state space representing the property, and in order to determinately not have the property the state of a system must be in the subspace orthogonal to it, and most state vectors will be neither parallel nor orthogonal to a given ray.
The deterministic dynamics (rule 4a) typically does nothing to guarantee that a system will either determinately have or determinately not have a particular property when one observes the system to see whether the system has that property. This is why the collapse dynamics (rule 4b) is needed in the standard formulation of quantum mechanics. It is the collapse dynamics that guarantees that a system will either determinately have or determinately not have a particular property (by the lights of rule 3) whenever one observes the system to see whether or not it has the property. But the linear dynamics (rule 4a) is also needed to account for quantum mechanical interference effects. So the standard theory has two dynamical laws: the deterministic, continuous, linear rule 4a describes how a system evolves when it is not being measured, and the random, discontinuous, nonlinear rule 4b describes how a system evolves when it is measured.
But the standard formulation of quantum mechanics does not say what it takes for an interaction to count as a measurement. Without specifying this, the theory is at best incomplete since it does not indicate when each dynamical law obtains. Moreover, if one supposes that observers and their measuring devices are constructed from simpler systems that each obey the deterministic dynamics, as Everett did, then the composite systems, the observers and their measuring devices, must evolve in a continuous deterministic way, and nothing like the random, discontinuous evolution described by rule 4b can ever occur. That is, if observers and their measuring devices are understood as being constructed of simpler systems each behaving as quantum mechanics requires, each obeying rule 4a, then the standard formulation of quantum mechanics is logically inconsistent since it says that the two systems together must obey rule 4b. This is the quantum measurement problem in the context of the standard collapse formulation of quantum mechanics. See the entry on measurement in quantum theory.
The problem with the theory, Everett argued, was that it was logically inconsistent and hence untenable. In particular, one could not provide a consistent account of nested measurement in the theory. Everett illustrated the problem of the consistency of the standard collapse theory in the context of an “amusing, but extremely hypothetical drama” (1956, 74–8), a story that was a few years later famously retold by Eugene Wigner.
Everett's version of the Wigner's Friend story involved an observer A who knows the state function of some system S, and knows that it is not an eigenstate of the measurement he is about to perform on it, and an observer B who is in possession of the state function of the composite system A+S. Observer A believes that the outcome of his measurement on S will be randomly determined by the collapse rule 4b, hence A attributes to A+S a state describing A as having a determinate measurement result and S as having collapsed to the corresponding state. Observer B, however, attributes the state function of the room after A's measurement according to the deterministic rule 4a, hence B attributes to A+S an entangled state where, according to rule 3, neither A nor S even has a determinate quantum-mechanical state of its own. Everett argued that since A and B make incompatible state attributions to A+S, the standard collapse theory yields a straightforward contradiction.
It would be extraordinarily difficult in practice for B to make a Wigner's Friend interference measurement that would determine the state of a composite system like A+S, hence the “extremely hypothetical” nature of the drama. Everett was careful, however, to explain why this was entirely irrelevant to the conceptual problem at hand. Indeed, he explicitly rejected that one might simply “deny the possibility that B could ever be in possession of the state function of A+S.” Rather, he argued, that “no matter what the state of A+S is, there is in principle a complete set of commuting operators for which it is an eigenstate, so that, at least, the determination of these quantities will not affect the state nor in any way disrupt the operation of A,” nor, he added, are there “fundamental restrictions in the usual theory about the knowability of any state functions.” And he concluded that “it is not particularly relevant whether or not B actually knows the precise state function of A+S. If he merely believes that the system is described by a state function, which he does not presume to know, then the difficulty still exists. He must then believe that this state function changed deterministically, and hence that there was nothing probabilistic in A's determination” (1956, 76). And, Everett argued, B is right in so believing.
That Everett took the Wigner's Friend story, which involves an experiment that, on the basis of decoherence considerations, would be virtually impossible to perform, to pose the central conceptual problem for quantum mechanics is essential to understanding how he thought of the measurement problem and what it would take to solve it. In particular, Everett held that one only has a satisfactory solution to the quantum measurement problem if one can provide a consistent account of nested measurement. And concretely, this meant that one must be able to tell the Wigner's Friend story consistently.
Being able to consistently tell the Wigner's Friend story then was, for Everett, a necessary condition for any satisfactory resolution of the quantum measurement problem.

3. Everett's Proposal

In order to solve the measurement problem Everett proposed dropping the collapse dynamics (rule 4b) from the standard collapse theory and taking the resulting physical theory to provide a complete and accurate description of all physical systems in the context of all possible physical interactions. Everett called the theory pure wave mechanics. He believed that he could deduce the standard statistical predictions of quantum mechanics (the predictions that depend on rule 4b in the standard collapse formulation of quantum mechanics) in terms of the subjective experiences of observers who are themselves treated as ordinary physical systems within pure wave mechanics.
Everett described the proposed deduction in the long thesis as follows:

We shall be able to introduce into [pure wave mechanics] systems which represent observers. Such systems can be conceived as automatically functioning machines (servomechanisms) possessing recording devices (memory) and which are capable of responding to their environment. The behavior of these observers shall always be treated within the framework of wave mechanics. Furthermore, we shall deduce the probabilistic assertions of Process 1 [rule 4b] as subjective appearances to such observers, thus placing the theory in correspondence with experience. We are then led to the novel situation in which the formal theory is objectively continuous and causal, while subjectively discontinuous and probabilistic. While this point of view thus shall ultimately justify our use of the statistical assertions of the orthodox view, it enables us to do so in a logically consistent manner, allowing for the existence of other observers (1956, 77–8).

Everett's goal, then, was to show that the memory records of an observer as described by quantum mechanics without the collapse dynamics would agree with those predicted by the standard formulation with the collapse dynamics. More specifically, he wanted to show that observers, modeled as servomechanisms within pure wave mechanics, would have fully determinate relative measurement records and the probabilistic assertions of the standard theory will correspond to statistical properties of typical sequences of such relative records.
Note that, in the context of his version of the Wigner's Friend story, Everett was insisting on three things simultaneously: (1) there are no collapses of the quantum-mechanical state, hence B is correct in attributing to A+S a state where A is in an entangled superposition of having recorded mutually incompatible results, (2) there is a sense in which A nevertheless got a fully determinate measurement result, and (3) such determinate results satisfy the standard quantum statistics.
The main problem in understanding what Everett had in mind is in figuring out precisely how the correspondence between the predictions of the standard collapse theory and the pure wave mechanics was supposed to work. Part of the problem is that the former theory is stochastic with fundamentally chance events and the latter deterministic with no mention of probabilities whatsoever, but there is also a problem even accounting for determinate measurement records in pure wave mechanics. In order to see why, we will consider how Everett's no-collapse proposal plays out in a simple interaction like A's measurement in the Wigner's Friend story.
Consider measuring the x-spin of a spin-1/2 system. Such a system will be found to be either “x-spin up” or “x-spin down”. Suppose that J is a good observer. For Everett, being a good x-spin observer meant that J has the following two dispositions (the arrows below represent the time-evolution of the composite system as described by the deterministic dynamics of rule 4a):

If J measures a system that is determinately x-spin up, then J will determinately record “x-spin up”; and if J measures a system that is determinately x-spin down, then J will determinately record “x-spin down” (and we assume, for simplicity, that the spin of the object system S is undisturbed by the interaction).
Now consider what happens when J observes the x-spin of a system that begins in a superposition of x-spin eigenstates:

The initial state of the composite system then is:

Here J is determinately ready to make an x-spin measurement, but the object system S, according to rule 3, has no determinate x-spin. Given J's two dispositions and the fact that the deterministic dynamics is linear, the state of the composite system after J's x-spin measurement will be:

On the standard collapse formulation of quantum mechanics, somehow during the measurement interaction the state would collapse to either the first term of this expression (with probability equal to a squared) or to the second term of this expression (with probability equal to b squared). In the former case, J ends up with the determinate measurement record “spin up”, and in the later case J ends up with the determinate measurement record “spin down”. But on Everett's proposal no collapse occurs. Rather, the post-measurement state is simply this entangled superposition of J recording the result “spin up” and S being x-spin up and J recording “spin down” and S being x-spin down. Call this state E.
On the standard eigenvalue-eigenstate link (rule 3) E is not a state where J determinately records “spin up”, neither is it a state where J determinately records “spin down”. So Everett's interpretational problem is to explain the sense in which J's entangled superposition of mutually incompatible records represents a determinate measurement outcome that agrees with the empirical prediction made by the standard collapse formulation of quantum mechanics when the standard theory predicts that J either ends up with the fully determinate measurement record “spin up” or the fully determinate record “spin down”, with probabilities equal to a-squared and b-squared respectively. More specifically, here the standard collapse theory predicts that on measurement the quantum-mechanical state of the composite system will collapse to precisely one of the following two states:

and that there is thus a single, simple matter of fact about which measurement result J recorded.
Everett, then, faced two closely related problems. The determinate-record problem requires him to explain how a measurement interaction like that just described might yield a determinate record in the context of pure wave mechanics. And the probability problem requires him to somehow recover the standard quantum statistics for such determinate records.
Everett took the key to the solution of both problems to be the principle of the fundamental relativity of states:

There does not, in general, exist anything like a single state for one subsystem of a composite system. Subsystems do not possess states that are independent of the states of the remainder of the system, so that the subsystem states are generally correlated with one another. One can arbitrarily choose a state for one subsystem, and be led to the relative state for the remainder. Thus we are faced with a fundamental relativity of states, which is implied by the formalism of composite systems. It is meaningless to ask the absolute state of a subsystem--one can only ask the state relative to a given state of the remainder of the subsystem. (1956, 103; 1957, 180)

One might understand Everett as adding the fundamental principle of relativity of states to pure wave mechanics to allow for a richer interpretation of states than that provided by just the eigenvalue-eigenstate link (rule 3). The resulting theory is the relative-state formulation of pure wave mechanics. Central to this theory is the distinction between absolute and relative states. This distinction played an essential explanatory role for Everett.
While the absolute state E is one where J has no determinate measurement record and S has no determinate x-spin, each of these systems also has relative states by dint of the correlation between J recording variable and S's x-spin. In particular, in state E, J recorded “x-spin up” relative to S being in the x-spin up state and that J recorded “x-spin down” relative to S being in the x-spin down state.
So while J has no absolute determinate record in state E, in each of these relative states, J has a determinate relative record. It is these relative records that Everett's takes to solve the determinate record problem:

Let one regard an observer as a subsystem of the composite system: observer + object-system. It is then an inescapable consequence that after the interaction has taken place there will not, generally, exist a single observer state. There will, however, be a superposition of the composite system states, each element of which contains a definite observer state and a definite relative object-system state. Furthermore, as we shall see, each of these relative object system states will be, approximately, the eigenstates of the observation corresponding to the value obtained by the observer which is described by the same element of the superposition. Thus, each element of the resulting superposition describes an observer who perceived a definite and generally different result, and to whom it appears that the object-system state has been transformed into the corresponding eigenstate. (1956, 78).

Absolute states, then, provide absolute properties for complete composite systems by way of the standard eigenvalue-eigenstate link, and relative states provide relative properties for subsystems of a composite system. And on Everett's account of the empirical faithfulness of pure wave mechanics, he identifies an observer's determinate measurement records with the modeled observer's relative memory states.
In particular, it is that each relative memory state describes a relative observer with a determinate measurement result that explains determinate measurement records on Everett's view. Why this was enough to fully explain our experience of determinate measurement records ultimately rests on his understanding what it means for a physical theory to be empirically faithful.

4. Empirical Faithfulness

While the physicist Bryce DeWitt would later argue for his own particular reconstruction of Everett's theory (see below), when DeWitt first read Everett's description of pure wave mechanics, he objected because its surplus structure made the theory too rich to represent the world we experience. In his 7 May 1957 letter to Everett's adviser John Wheeler, DeWitt wrote

I do agree that the scheme which Everett sets up is beautifully consistent; that any single one of the [relative memory states of an observer] ... gives an excellent representation of a typical memory configuration, with no causal or logical contradictions, and with “built-in” statistical features. The whole state vector ... , however, is simply too rich in content, by vast orders of magnitude, to serve as a representation of the physical world. It contains all possible branches in it at the same time. In the real physical world we must be content with just one branch. Everett's world and the real physical world are therefore not isomorphic. Barrett and Byrne (eds) (2012, 246–7)

The thought was that the richness of pure wave mechanics indicated an empirical flaw in the theory because we do not notice other branches. As DeWitt put it:

The trajectory of the memory configuration of a real observer ... does not branch. I can testify to this from personal introspection, as can you. I simply do not branch. Barrett and Byrne (eds) (2012, 246)

Wheeler showed Everett the letter and told him to reply. In his 31 May 1957 letter to DeWitt, Everett began by summarizing his understanding of the proper cognitive status of physical theories.

First, I must say a few words to clarify my conception of the nature and purpose of physical theories in general. To me, any physical theory is a logical construct (model), consisting of symbols and rules for their manipulation, some of whose elements are associated with elements of the perceived world. If this association is an isomorphism (or at least a homomorphism) we can speak of the theory as correct, or as faithful. The fundamental requirements of any theory are logical consistency and correctness in this sense. Barrett and Byrne (eds) (2012, 253)

In the final long version of his thesis, Everett further explained in a footnote that “[t]he word homomorphism would be technically more correct, since there may not be a one-one correspondence between the model and the external world” (1956, 169). The map is a homomorphism because (1) there may be elements of the theory that do not directly correspond to experience and because (2) a particular theory may not seek to explain all of experience. It is case (1) that is particularly important here: Everett considered the surplus experiential structure represented in the various branches of the absolute state to be explanatorily harmless.
In his letter to DeWitt, Everett described how he understood the aim of physical inquiry: “There can be no question of which theory is ‘true’ or ‘real’ — the best that one can do is reject those theories which are not isomorphic to sense experience” Barrett and Byrne (eds) (2012, 253). The task then was to find our experience in an appropriate way in the relative-state model of pure wave mechanics.
So, for Everett, a theory was empirically faithful and hence empirically acceptable if there was a homomorphism between its model and the world as experienced. What this amounted to here was that pure wave mechanics is empirically faithful if one can find observers' experiences appropriately associated with modeled observers in the model of the theory. In short, Everett took pure wave mechanics to be empirically faithful because one could find quantum mechanical experience in the model as relative memory records associated with relative modeled observers.
While he left significant room in precisely how one might interpret the theory, the core of Everett's interpretation involved four closely related arguments.

5. Four Arguments

Together the following four arguments indicate the sense in which Everett took pure wave mechanics to be empirically faithful and to recapture the empirical predictions of the standard collapse theory.

5.1 Experience is found in the relative memory records of observers

As suggested earlier, Everett held that one can find our actual experience in the model of pure wave mechanics as relative measurement records associated with modeled observers. In the state E, for example, since J has a different relative measurement record in each term of the superposition written in the determinate record basis and since these relative records span the space of quantum-mechanically possible outcomes of this measurement, regardless of what result the actual observer gets, we will be able to find his experience represented as a relative record of the modeled observer in the the interaction as described by pure wave mechanics.
More generally, if one performs a sequence of measurements, it follows from the linearity of the dynamics and Everett's model of an ideal observer that every quantum-mechanically possible sequence of determinate measurement results will be represented in the entangled post-measurement state as relative sequence of determinate measurement records. This is also true in the theory if one only relatively, rather than absolutely, makes the sequence of observations. In this precise sense, then, it is possible to find our experience as sequences of relative records in the model of pure wave mechanics.
Everett took such relative records to be sufficient to explain the subjective appearances of observers because in an ideal measurement, every relative state will be one where the observer in fact has, and, as we will see in the next section, would report that she has, a fully determinate, repeatable measurement record that agrees with the records of other ideal observers. As Everett put it, the system states observed by a relative observer are eigenstates of the observable being measured (1957, 188). For further details of Everett's discussion of this point see (1956, 129–30), (1955, 67), (1956 121–3 and 130–3), and (1957, 186–8 and 194–5).
Note that Everett did not require a physically preferred basis to solve the determinate record problem to show that pure wave mechanics was empirically faithful. The principle of the fundamental relatively of states explicitly allows for arbitrarily specified decompositions of the absolute universal state into relative states. Given his understanding of empirical faithfulness, all Everett needed to explain a particular actual record was to show that is that there is some decomposition of the state that represents the modeled observer with the corresponding relative record. And he clearly has that in pure wave mechanics under relatively weak assumptions regarding the nature of the actual absolute quantum mechanical state.

5.2 Pure wave mechanics predicts that one would not ordinarily notice that there are alternative relative records

It was important to Everett to explain why one would not ordinarily notice the surplus structure of pure wave mechanics. In his reply to DeWitt, Everett claimed that pure wave mechanics “is in full accord with our experience (at least insofar as ordinary quantum mechanics is) ... just because it is possible to show that no observer would ever be aware of any ‘branching,’ which is alien to our experience as you point out” Barrett and Byrne (eds) (2012, 254).
There are two distinct arguments that Everett seems to have had in mind.
First, one would only notice macroscopic splitting if one had access to records of macroscopic splitting events, but records of such events will be rare precisely insofar as measurements that would show that there are branches where macroscopic measurement apparata have different macroscopic measurement records for the same measurement would require one to perform something akin to a Wigner's Friend measurement on a macroscopic system, which, as Everett indicated in his characterization of his version of the Wigner Friend story as “extremely hypothetical,” would be extraordinarily difficult to do. The upshot is that, while not impossible, one should not typically expect to find reliable relative measurement records indicating that there are branches corresponding to alternative macroscopic measurement records.
Second, Everett repeatedly noted in his various deductions of subjective appearances that it follows directly from the dynamical laws of pure wave mechanics that it would seem to an ideal agent that he had fully determinate measurement results. Albert and Loewer presented a dispositional version of this line of argument in their presentation of the bare theory (a version of pure wave mechanics) as a way of understanding Everett's formulation of quantum mechanics (Albert and Loewer, 1988, and Albert, 1992; see also the bare theory chapter of Barrett 1999).
The idea is that if there are no collapse of the quantum mechanical state an ideal modeled observer like J would have the sure-fire disposition falsely to report and hence to believe that he had a perfectly ordinary, fully sharp and determinate measurement record. The trick is to ask the observer not what result he got, but rather whether he got some specific determinate result. If the post-measurement state was:

then J would report “I got a determinate result, either spin up or spin down”. And he would make precisely the same report if he ended up in the post-measurement state:

So, by the linearity of the dynamics, J would falsely report “I got a determinate result, either spin up or spin down” when in the state E:

Thus, insofar as his beliefs agree with his sure-fire dispositions to report that he got a fully determinate result, it would seem to J that he got a perfectly determinate ordinary measurement result even when he did not (that is, he did not determinately get “spin up” and did not determinately get “spin down”).
So, while which result J got in state E is a relative fact, that it would seem to J that he got some determinate result is an absolute fact. (See Albert (1992) and Barrett (1999) for further discussion of such dispositional properties of modeled observers on the “bare theory.” See Everett (1956, 129–30), (1955, 67), (1956 121–3 and 130–3), and (1957, 186–8 and 194–5) for parallel discussions in pure wave mechanics.)

5.3 The surplus structure of pure wave mechanics is in principle detectable and hence isn't surplus structure at all

While sometimes extremely difficult to detect, Everett insisted that alternative relative states, even alternative relative macroscopic measurement records, were always in principle detectable. Hence they do not, as DeWitt worried, represent surplus structure at all. Indeed, since all branches, in any basis, are in principle detectable, all branches in any decomposition of the state of a composite system were operationally real on Everett's view. As he put it in the long thesis:

It is ... improper to attribute any less validity or “reality” to any element of a superposition than any other element, due to [the] ever present possibility of obtaining interference effects between the elements, all elements of a superposition must be regarded as simultaneously existing (1956, 150).

While Everett understood decoherence considerations, he did not believe that they rendered the detection alternative measurement records impossible. Indeed, as indicated in his discussion of the Wigner's Friend story above, Everett held that it was always in principle possible to measure an observable that would detect an alternative post-measurement branch, and it was this that he used to argue the other direction. It is precisely because the linear dynamics requires that all branches of the global wave function are at least in principle detectable that pure wave mechanics requires that all branches are equal real.
And, again, note that this does not mean that only branches in one physically preferred basis are real. Rather, it means that every branch in every decomposition of a composite system is real in Everett's operational sense of ‘real’ since any such state might in principle entail observational consequences.
So while one should not typically expect to find relative records of alternative, macroscopically distinct branches, such alternative branches do not represent surplus structure on Everett's view insofar as they are required by the linear dynamics and in principle possible to detect. In this sense, pure wave mechanics provides the simplest possible theory compatible with the operational consequences of the linear dynamics.
One upshot of this is that pure wave mechanics allows one to have a specific sort of inductive empirical evidence in favor of the theory. In particular, since real for Everett means has observable consequences, any experiment that illustrates quantum interference provides empirical evidence for the operational existence of alternative Everett branches on some decomposition of the state. Again, Everett was an operational realist concerning all branches in every basis insofar as they might be detected. Specifically, one provides increasingly compelling evidence in favor of pure wave mechanics correctly describing macroscopic measurement interactions the closer one gets to being able to perform something like a Wigner's Friend interference experiment.

5.4 One should expect to find the standard quantum statistics in a typical relative sequence of measurement records

Everett did not solve the probability problem by finding probabilities in pure wave mechanics. Indeed, as suggested by his original thesis title, he repeatedly insisted that there were no probabilities and took this to be an essential feature of the theory. Rather, what it meant for pure wave mechanics to be empirically faithful with respect to the statistical predictions of quantum mechanics is that one can find the standard quantum statistics we experience in the distribution of a typical relative relative sequence of a modeled observer's measurement records. In explaining this, Everett appealed to a measure of typicality given by the norm squared of the amplitude associated with each relative state in an orthogonal decomposition of the absolute state.
The thought then is that if an observer supposes that his relative measurement records will be faithfully represented by a typical relative sequence of measurements records, in Everett's norm-squared measure of typicality, he will expect to observe the standard statistical predictions of quantum mechanics.
Everett got to the result in two steps. First, he found a well-behaved measure of typicality over relative states whose value is fully determined by the model of pure wave mechanics. Then he showed that, in the limit as the number of measurement interactions gets large, almost all relative sequences of measurement records, in the sense of almost all given by the specified measure, will exhibit the standard quantum statistics. Note that it is typically false that most relative sequences by count will exhibit the standard quantum statistics, and Everett knew this. This is why his explicit choice of how to understand typicality is essential to his account of the standard quantum statistics. See Everett (1956, 120–30) and (1957, 186–94) for discussions of typicality and the quantum statistics.
It is then left to the reader to notice that if one assumes that one's relative records are typical, in the precise sense that Everett specified, then they should be expected to exhibit the standard quantum statistics. Were such an assumption added to the theory, then one should expect to see the standard quantum statistics as determinate relative records.
Everett took this deduction to establish that the relative state formulation of pure wave mechanics is empirically faithful over the standard quantum statistics.

6. Faithfulness and the Problem of Empirical Adequacy

Pure wave mechanics, then, is empirical faithful since (1) one can find an observer's determinate measurement records as the relative records of an idealized modeled observer in the theory and (2) the model of pure wave mechanics provides a typicality measure over relative states corresponding such that a typical relative sequence of measurement records in that measure will exhibit the standard quantum statistics. The first result is Everett's resolution of the determinate record problem, and the second his resolution of the probability problem.
The upshot is that if one associates one's experience with relative records and if one expects one's relative sequence of records to be typical in the norm-squared-amplitude sense, then one should expect one's experience to agrees with the standard statistical predictions of quantum mechanics, wherever it makes coherent predictions. And where the standard collapse theory and the Copenhagen interpretation do not make coherent predictions, as in the Wigner's Friend story, one should expect to see evidence that the linear dynamics always correctly describes the evolution of every physical system whatsoever. So while pure wave mechanics explains why one would not typically observe other branches, it also predicts that other branches are in principle observable, and hence do not represent surplus structure.
One might, of course, want more than empirical faithfulness from a satisfactory formulation of quantum mechanics. In keeping with his view that pure wave mechanics is quantum mechanics without probabilities, Everett simply conceded that every relative state under every decomposition of the absolute state in fact obtains. The resulting problem, one might feel, is that empirical faithfulness, in Everett's sense at least, is a relatively weak form of empirical adequacy. This can be seen by considering how one should understand the very notion of having a differential expectations when every physically possible measurement result is in fact fully realized in the model of the theory.
For Everett to call his norm-squared-amplitude measure a measure of typicality might suggest that a sample relative state is somehow selected with respect to the measure. If that were the case, then it would be natural to expect, by stipulation as suggested earlier, one's relative sequence of measurement records to be typical. But then it would also be natural to suppose that it would be probable that a relative sequence of measurements records exhibit the standard quantum statistics, and, for Everett, there were no probabilities in the theory. And indeed, there are no probabilities whatsoever in the statement of the theory, and hence no way to derive them without adding something to the theory.
But the problem here is more fundamental that that might suggest. Insofar as a probability is a measure over possibilities where precisely one is in fact realized and insofar as all possibilities are realized in pure wave mechanics, there simply can be no probabilities associated with alternative relative sequences of measurement records. Similarly, any understanding of typicality that somehow involves the selection of a typical relative sequence of records rather than an atypical sequence of records is incompatible with pure wave mechanics since the theory describes no such selection. Neither can the typicality measure represent an expectation of the standard quantum statistics obtaining for one's actual relative sequence of measurement records at the exclusion of the rest since all such sequences are equally actual in Everett's operationalist sense of actual. Insofar as the theory describes any possible result as occurring, it describes every possible result as occurring, so there is no particular sequence of measurement records that is realized satisfying, or failing to satisfy, one's prior expectations.
That Everett's notion of empirical faithfulness is a relatively weak version of empirical adequacy, then, is exhibited in what pure wave mechanics, being empirically faithful, does not explain. In particular, it does not explain what it is about the physical world that makes it appropriate to expect one's relative sequence of records to be typical in the norm-squared-amplitude sense, or any other sense. In short, while one can get subjective expectations for future experience by stipulation, the theory itself does not describe a physical world where such expectations might be understood as expectations concerning what will in fact occur. One might take Everett's typicality measure to determine the subjective degree to which I should expect a particular relative sequence of records being (relative) mine, but to get even this would require careful explanatory amendments to Everett's presentation of the theory. One can get a concrete sense of what such a strategy would involve by contrasting pure wave mechanics with something like Bohmian mechanics the many-thread or many-maps formulations of quantum mechanics where one has a clear notion of subjective quantum probabilities (see Barrett 1999 and 2005 for discussions of this approach).

7. Many Worlds

While he was initially skeptical of Everett's views, DeWitt became an ardent proponent of the many-worlds interpretation, a theory that DeWitt presented as the EWG interpretation of quantum mechanics after Everett, Wheeler, and DeWitt's graduate student R. Neill Graham. In his description of the many-worlds interpretation DeWitt (1970) emphasized that its central feature was the metaphysical commitment to physically splitting worlds. DeWitt's description subsequently became the most popular understanding of Everett's theory (see Barrett (2011b) for further discussion of Everett's attitude toward DeWitt and the many worlds interpretation).
DeWitt described the theory in the context of the Schroedinger's cat thought experiment.

The animal [is] trapped in a room together with a Geiger counter and a hammer, which, upon discharge of the counter, smashes a flask of prussic acid. The counter contains a trace of radioactive material—just enough that in one hour there is a 50% chance one of the nuclei will decay and therefore an equal chance the cat will be poisoned. At the end of the hour the total wave function for the system will have a form in which the living cat and the dead cat are mixed in equal portions. Schrodinger felt that the wave mechanics that led to this paradox presented an unacceptable description of reality. However, Everett, Wheeler and Graham's interpretation of quantum mechanics pictures the cats as inhabiting two simultaneous, noninteracting, but equally real worlds. (1970, 31)

DeWitt took this view to follow from “the mathematical formalism of quantum mechanics as it stands without adding anything to it.” More specifically, he claimed that EWG had proven a metatheorem that the mathematical formalism of pure wave mechanics interprets itself:

Without drawing on any external metaphysics or mathematics other than the standard rules of logic, EWG are able, from these postulates, to prove the following metatheorem: The mathematical formalism of the quantum theory is capable of yielding its own interpretation. (1970, 33)

He gave Everett credit for the metatheorem, Wheeler credit for encouraging Everett, and Graham credit for clarifying the metatheorem. DeWitt and Graham later described Everett's formulation of quantum mechanics as follows:

[It] denies the existence of a separate classical realm and asserts that it makes sense to talk about a state vector for the whole universe. This state vector never collapses and hence reality as a whole is rigorously deterministic. This reality, which is described jointly by the dynamical variables and the state vector, is not the reality we customarily think of, but is a reality composed of many worlds. By virtue of the temporal development of the dynamical variables the state vector decomposes naturally into orthogonal vectors, reflecting a continual splitting of the universe into a multitude of mutually unobservable but equally real worlds, in each of which every good measurement has yielded a definite result and in most of which the familiar statistical quantum laws hold (1973, v).

For his part, DeWitt conceded that this constant splitting of worlds whenever the states of systems become correlated was counterintuitive:

I still recall vividly the shock I experienced on first encountering this multiworld concept. The idea of 10¹⁰⁰ slightly imperfect copies of oneself all constantly splitting into further copies, which ultimately become unrecognizable, is not easy to reconcile with common sense. Here is schizophrenia with a vengeance (1973, 161).

That said, he strongly promoted the theory at every turn, and Everett's views quickly came to be identified with DeWitt and Graham's many-worlds interpretation.
While Everett's presentation of his theory was unclear at several points, DeWitt's exegesis did little to help clarify pure wave mechanics. Since a number of these confusions persist in discussions of Everett, we will briefly consider DeWitt and Graham's interpretation and compare it against Everett's description of the relative-state formulation of pure wave mechanics.
To begin, since purely mathematical postulates entail only purely mathematical theorems, one cannot deduce any metaphysical commitments whatsoever regarding the physical world from the mathematical formalism of pure wave mechanics alone. The formalism of pure wave mechanics might entail the sort of metaphysical commitments that DeWitt and others have envisioned only if supplemented with sufficiently strong metaphysical assumptions, strong enough to determine a metaphysical interpretation for the theory. Concerning the claim that pure wave mechanics interprets itself by way of a metatheorem that Everett proved, on even a broad understanding of what might count as such a metatheorem, there is nothing answering to DeWitt's description in either the long or short versions of Everett's thesis.
Second, contrary to what DeWitt, Graham, and others have supposed, Everett was not committed to causally isolated worlds. In contrast, as we have seen, Everett held that it is always in principle possible for branches to interact. More specifically, he argued that “no matter what the state of [Wigner's Friend] is, there is in principle a complete set of commuting operators for which it is an eigenstate, so that, at least, the determination of these quantities will not affect the state nor in any way,” he denied that there are fundamental restrictions about the “knowability of any state functions,” and he believed that the sense in which all branches of the global state are equally actual is given by the ever-present possibility of interaction between branches. So while one clearly can describe situations where there is no post-measurement interference between the branches representing incompatible measurement records, one can also describe interactions where there is, and for Everett there was no special physical distinction to be made between the two cases.
Third, there was no consensus between Everett, Wheeler, DeWitt, and Graham concerning what Everett's theory was. In particular, we know what Everett thought of Graham's formulation of the theory. In his personal copy of DeWitt's description of the many worlds interpretation, Everett wrote the word “bullshit” next to the passage where DeWitt presented Graham's clarification of Everett's views (See Barrett and Byrne (2012, 364–6) for scans of Everett's handwritten marginal notes).
Finally, as indicated in the discussion of empirical faithfulness above, Everett's understanding of pure wave mechanics was decidedly non-metaphysical. In particular, He carefully avoided talk of multiple, splitting worlds, his understanding of the reality of branches was purely operational, and he explicitly denied that the aim of physics was to produce true theories. That the proper aim, rather, was to produce empirically faithful theories in the sense that he described, was an essential part of Everett's argument for why his theory was not only acceptable but ought to be preferred to the other formulations of quantum mechanics that he knew (which explicitly included the standard collapse theory, the Copenhagen interpretation, and Bohmian mechanics; see Barrett and Byrne (eds.) 2012, 152–5).
For Everett, the relative states of its subsystems provided a way to characterize branches of the absolute state of a composite system. Insofar as the principle of the fundamental relativity of states allows one to consider the quantum-mechanical state in any specified basis, there is no canonical way to individuate branches. This makes it natural perhaps to think of the existence of branches operationally, as Everett did. Rather than take the branches determined by a physically preferred basis or those determined by, or roughly determined by, some decoherence condition to determine which physically possible worlds were real, he took every branch in any basis to have observational consequences and hence to be real in his operational sense. Given how he understood branches and their role in determining the empirical faithfulness of the theory, Everett never had to say anything concerning how a particular physically preferred basis is selected because none was required.
While Everett himself did not do so, one might nevertheless designate a special set of branches of the global absolute state, say, those that satisfy exhibit an appropriate sort of stable diachronic identity, to represent worlds, or emergent worlds, or approximate emergent worlds. But how one understands such physical entities cannot be determined solely by the mathematical formalism of pure wave mechanics.
This has led recent many-worlds proponents like David Wallace (2010 and 2012) to add explicit interpretative assumptions to the formalism of pure wave mechanics. In contrast with DeWitt, who seems to have taken worlds to be basic entities described by the global absolute state, Wallace takes the quantum state as basic, then seeks to characterize worlds as emergent entities represented in its structure. The analogy he gives is that pure wave mechanics describes the quantum state just as classical field theory describes physical fields (2010, 69). Worlds then are understood as physically real but contingently emergent entities that are identified with approximate substructures of the quantum state, or as Wallace puts it, “mutually dynamically isolated structures instantiated within the quantum state, which are structurally and dynamically ‘quasiclassical’” (2010, 70). Just a bit more carefully, one would expect such emergent worlds to be more or less isolated depending on the physical situation and properties one seeks to describe and the degree of decoherence in fact exhibited by the systems as characterized.
On this account, there is no simple matter of fact concerning what or even how many emergent worlds there are because such questions depend on one's level of description and on how well-isolated one requires the worlds to be for the explanatory considerations at hand. But, however one individuates them, the emergent worlds correspond to approximately determinate decoherent substructures of the quantum state. Hence, only some relative states describe physically real worlds.
In contrast, as we have seen, when Everett claimed that all branches were equally real, he had something less metaphysical and more empirical in mind, which, in turn, suggests a quite different understanding of branches. In particular, since every branch in every decomposition of the state has potential empirical consequences for the results of one's future observations, every branch, not just those represented in a favored decohering basis, is operationally real. In short, every relative state describes something that the linear dynamics requires one to take as real in the only sense that Everett understood.
There is certainly a place for a decoherence account of quasiclassicality akin to the sort that Wallace and others favor as an extension of Everett’s project insofar as it yields a yet richer sense in which one might find our experience in the model of pure wave mechanics. But, given how he understood his theory and what was required for it to be empirically acceptable, Everett's explanatory goals were arguably more modest than those of many Everettians and hence more readily attained.
Consider probability again. If one were to take pure wave mechanics to be directly descriptive of the real physical world, one might feel that one should explain what it is about the world that makes it appropriate to expect one's relative sequence of records to be typical in the norm-squared-amplitude sense when every physically possible outcome is in fact realized as a relative state. For his part, however, Everett believed that all that was required to explain the standard quantum statistics was that one be able to find them somehow associated in a precise and unambiguous way with the relative records of an ideal modeled observer. And he arguably did just this. That such an account does not, without additional assumptions, explain why one should expect one's measurement records to exhibit the standard quantum statistics in a world directly described by pure wave mechanics is a weakness of the account, but, arguably, one that need not have worried Everett given the relatively modest explanatory aim of empirical faithfulness.

8. Summary

Everett took his version of the Wigner's Friend story to reveal the inconsistency of the standard collapse formulation of quantum mechanics and the incompleteness of the Copenhagen interpretation. The problem was that neither could make sense of nested measurement. And since pure wave mechanics allowed one to provide a consistent account of nested measurement, he took it to immediately resolve the measurement problem. The task then was to explain the sense in which pure wave mechanics might be taken to be empirically faithful over determinate measurement records exhibiting the standard quantum-mechanical statistics.
Everett's relative-state formulation of pure wave mechanics has a number of salient virtues. It eliminates the collapse dynamics and hence immediately resolves the potential conflict between the two dynamical laws. It is consistent, applicable to all physical systems, and perhaps as simple as a formulation of quantum mechanics can be. And it is empirically faithful in that one can find an observer's quantum experience as relative records in the model of pure wave mechanics and one can find a measure over relative sequences of records such that most such sequences exhibit the standard quantum statistics.
Insofar as Everett's standard of empirical faithfulness just involved finding measurement records associated with a modeled observer in the theory that agree with one's experience, it is a relatively weak variety of empirical adequacy. The relative weakness of this condition is illustrated by the fact that the way that one's experience is found in the model of pure wave mechanics does not explain why one should expect to have that particular experience in a world described by the theory. Judging a theory to be empirically adequate when it tells us that there is a sense in which everything physically possible in fact happens clearly puts pressure on the very idea of empirical adequacy. But one might nevertheless argue that the empirical faithfulness of the relative-state formulation of pure wave mechanics represent a nontrivial empirical virtue.
There remain a number of alternative reconstructions of Everett's relative-state formulation of pure wave mechanics. Insofar as one takes pure wave mechanics to provide a clear starting point for addressing the quantum measurement problem, one might find such alternatives naturally compelling



平行宇宙理论 [复制链接]

天文摄星人 天文摄星人 当前离线 积分 10782 IP卡 狗仔卡	电梯直达 楼主 发表于 2007-4-21 10:50 | 只看该作者 |倒序浏览 |阅读模式 是否有另一个你正在阅读和本文完全一样的一篇文章？那个家伙并非你自己，却生活在一个有着云雾缭绕的高山、一望无际的原野、喧嚣嘈杂的城市，和其它8颗行星一同围绕一颗恒星旋转，并且也叫做“地球”的行星上？他（她）一生的经历和你每秒钟都相同。然而也许她此刻正准备放下这篇文章而你却打算看下去。  这种“分身”的想法听起来奇怪而又难以置信，但似乎我们不得不接受它，因为它已为各种天文观测的结果所支持。如今最流行同时也最简单的宇宙模型指出，离我们大约10^(10^28)米外之处存在一个和我们的银河一摸一样的星系，而那其中正有个一摸一样的你。虽然这距离大得超乎人们的想象，却毫不影响你的“分身”存在的真实性。该想法最初起源于很简单的“自然可能性”而非现代物理所假设：宇宙在尺寸上无限大（或者至少足够大），并且象天文观测指出的那样－－均匀的分布着物质。既然如此，按照统计学规律便可以断定，所有的事件（无论多么相似或者相同）都会发生无数次：会有无数个孕育人类的星球，它们之中会有和你一摸一样的人－－一摸一样的长相、名字、记忆甚至和你一摸一样的动作、选择－－这样的人还不止一个，确切的说，是无穷多个。      最新的宇宙学观测表明，平行宇宙的概念并非一种比喻。空间似乎是无限的。如果真是这样,一切可能会发生的事情必然会发生，不管这些事有多荒唐。在比我们天文观测能企及范围远得多的地方，有和我们一摸一样的宇宙。天文学家甚至计算出它们距地球的平均距离      你很可能永远见不到你的“影子”们。你能观测到的最远距离也就是自大爆炸以来光所行进的最远距离：大约140亿光年，即4X10^26米－－该距离为半径的球体正好定义了我们可观测视界的大小，或者简单地说，宇宙的大小，又叫做哈勃体积。同样的，另一个你所在的宇宙也是个同样大小的球体。以上便是对“平行宇宙”最直观的解释。每个宇宙都是更大的“多重宇宙”的一小部分。      对“宇宙”的如此定义，人们也许会认为这只是种形而上学的方式罢了。然则物理学和形而上学的区别在于该理论是否能通过实验来测试，而不是它看起来是否怪异或者包含难以察觉的东西。多年来，物理学前沿不断扩张，吸收融合了许多抽象的（甚至一度是形而上学的）概念，比如球形的地球、看不见的电磁场、时间在高速下流动减慢、量子重叠、空间弯曲、黑洞等等。近几年来“多重宇宙”的概念也加入了上面的名单，与先前一些经过检验的理论，如相对论和量子力学配合起来，并且至少达到了一个经验主义科学理论的基本标准：作出预言。当然作出的论断也可能是错误的。科学家们迄今讨论过多达4种类型独立的平行宇宙。现在关键的已不是多重宇宙是否存在的问题了，而是它们到底有多少个层次。  第一层次：视界之外      所有的平行宇宙组成第一层多重宇宙。－－这是争论最少的一层。所有人都接受这样一个事实：虽然我们此时此刻看不见另一个自己，但换一个地方或者简单地在原地等上足够长的时间以后就能观察到了。就像观察海平面以外驶来的船只－－观察视界之外物体的情形与此类似。随着光的飞行，可观察的宇宙半径每年都扩大一光年，因此只需要坐在那里等着瞧。当然，你多半等不到另一个宇宙的另一个你发出的光线传到这里那天，但从理论上讲，如果宇宙扩张的理论站得住脚的话，你的后代就有可能用超级望远镜看到它们。      怎么样，第一层多重宇宙的概念听起来平平无奇？空间不都是无限的么？谁能想象某处插着块牌子，上书“空间到此结束，当心下面的沟”？如果是这样，每个人都会本能的置疑：尽头的“外面”是什么？实际上，爱因斯坦的重力场理论偏偏把我们的直觉变成了问题。空间有可能不是无限，只要它具有某种程度的弯曲或者并非我们直觉中的拓扑结构（即具有相互联络的结构）。    一个球形、炸面圈形或者圆号形的宇宙都可能大小有限，却无边界。对宇宙微波背景辐射的观测可以用来测定这些假设。【见另一篇文章《宇宙是有限的吗？》by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks; Scientific American, April 1999】然而，迄今为止的观察结果似乎背逆了它们。无尽宇宙的模型才和观测数据符合，外带强烈的限制条件。     另一种可能是：空间本身无限，但所有物质被限制在我们周围一个有限区域内－－曾经流行的“岛状宇宙”模型。该模型不同之处在于，在大尺度下物质分布会呈现分形图案，而且会不断耗散怠尽。这种情形下，第一层多重宇宙里的几乎每个宇宙最终都将变得空空如也，陷入死寂。但是近期关于三维银河分布与微波背景的观测指出物质的组织方式在大尺度上呈现出某种模糊的均匀，在大于10^24米的尺度上便观测不到清晰的细节了。假定这种模式延伸下去，我们可观测宇宙以外的空间也将充满行星、恒星和星系。     有资料支持空间延伸于可观测宇宙之外的理论。WMAP卫星最近测量了微波背景辐射的波动(左图)。最强烈的振幅超过了0.5开，暗示着空间非常之大，甚至可能无穷(中图)。另外，WMAP和2dF星系红移探测器发现在非常大的尺度下，空间均匀分布着物质     生活在第一层多重宇宙不同平行宇宙中的观察者们将察觉到与我们相同的物理定律，但初始条件有所不同。根据当前理论，大爆炸早期的一瞬间物质按一定的随机度被抛出，此过程包含了物质分布的一切可能性，每种可能性都不为0。宇宙学家们假定我们所在的当初有着近似均匀物质分布和初始波动状态（100，000可能性中的一种）的宇宙，是一个相当典型的（至少在所有产生了观察者的平行宇宙中很典型）个体。那么距你最近的和你一模一样那个人将远在10^(10^28)米之外；而在10^(10^92)米外才会有一个半径100光年的区域，它里面的一切与我们居住的空间丝毫不差，也就是说未来100年内我们世界所发生的每件事都会在该区域完全再现；而至少10^(10^118)米之外该区域才会增大到哈勃体积那么大，换句话说才会有一个和我们一模一样的宇宙。     上面的估计还算极端保守的，它仅仅穷举了一个温度在10^8开以下、大小为一个哈勃体积的空间的所有量子状态。其中一个计算步骤是这样：在那温度下一个哈勃体积的空间最多能容纳多少质子？答案是10^118个。每个质子可能存在，也可能不存在，也就是总共2^(10^118)个可能的状态。现在只需要一个能装下2^(10^118)个哈勃空间的盒子便用光所有可能性。如果盒子更大些－－比如边长10^(10^118)米的盒子－－根据抽屉原理，质子的排列方式必然会重复。当然，宇宙不只有质子，也不止两种量子状态，但可用与此类似的方法估算出宇宙所能容纳的信息总量 。     与我们宇宙一摸一样的另一个宇宙的平均距离距你最近那个“分身”没准并不象理论计算的那么远，也许要近得多。因为物质的组织方式还要受其他物理规律制约。给定一些诸如行星的形成过程、化学方程式等规律，天文学家们怀疑仅在我们的哈勃体积内就存在至少10^20个有人类居住的行星；其中一些可能和地球十分相像。     第一层多重宇宙的框架通常被用来评估现代宇宙学的理论，虽然该过程很少被清晰地表达。举例来说，考察我们的宇宙学家如何通过微波背景来试图得出“球形空间”的宇宙几何图。随着空间曲率半径的不同，那些“热区域”和“冷区域”在宇宙微波背景图上的大小会呈现某种特征；而观测到的区域表明曲率太小不足以形成球形的封闭空间。然而，保持统计学上的严格是非常重要的事。每个哈勃空间的这些区域的平均大小完全是随机的。因此有可能是宇宙在愚弄我们－－并非空间曲率不足以形成封闭球形使得观测到的区域偏小，而恰巧因为我们宇宙的平均区域天生就比别的来的小。所以当宇宙学家们信誓旦旦保证他们的球状空间模型有99.9%可信度的时候，他们的真正意思是我们那个宇宙是如此地不合群，以至1000个哈勃体积之中才会出一个象那样的。     这堂课的重点是：即使我们没法观测其他宇宙，多重宇宙理论依然可以被实践验证。关键在于预言第一层多重宇宙中各个平行宇宙的共性并指出其概率分布－－也就是数学家所谓的“度量”。我们的宇宙应当是那些“出现可能性最大的宇宙”中的一个。否则－－我们很不幸地生活在一个不大可能的宇宙中－－那么先前假设的理论就有大麻烦了。如我们接下来要讨论的那样，如何解决这度量上的问题将会变得相当有挑战性。 第二层多重宇宙示意图。     第二层次：膨胀后留下的气泡 如果第一层多重宇宙的概念不太好消化，那么试着想象下一个拥有无穷组第一层多重宇宙的结构：组与组之间相互独立，甚至有着互不相同的时空维度和物理常量。这些组构成了第二层多重宇宙－－被称为“无序的持续膨胀”的现代理论预言了它们。   “膨胀”作为大爆炸理论的必然延伸，与该理论的许多其他推论联系紧密。比如我们的宇宙为何如此之大而又如此的规整，光滑和平坦？答案是“空间经历了一个快速的拉伸过程”，它不仅能解释上面的问题，还能阐释宇宙的许多其他属性。【见《膨胀的宇宙》 by Alan H. Guth and Paul J. Steinhard; Scientific American, May 1984; 《自我繁殖的膨胀宇宙》 by Andrei Linde, November 1994 】“膨胀”理论不仅为基本粒子的许多理论所语言，而且被许多观测证实。“无序的持续”指的是在最大尺度上的行为。作为一个整体的空间正在被拉伸并将永远持续下去。然而某些特定区域却停止拉神，由此产生了独立的“气泡”，好像膨胀的烤面包内部的气泡一样。这种气泡有无数个。它们每个都是第一层多重宇宙：在尺寸上无限而且充满因能量场涨落而析出的物质。     对地球来说，另一个气泡在无限遥远之外，远到即使你以光速前进也永远无法到达。因为地球和“另一个气泡”之间的那片空间拉伸的速度远比你行进的速度快。如果另一个气泡中存在另一个你，即便你的后代也永远别想观察到他。基于同样的原因，即空间在加速扩张，观察结果令人沮丧的指出：即便是第一层多重空间中的另一个自己也将看不到了。     第二层多重宇宙与第一层的区别非常之大。各个气泡之间不仅初始条件不同，在表观面貌上也有天壤之别。当今物理学主流观点认为诸如时空的维度、基本粒子的特性还有许许多多所谓的物理常量并非基本物理规律的一部分，而仅是一种被称作“对称性破坏”过程的结果而已。举例言之，理论物理学家认为我们的宇宙曾一度由9个相互平等的维度组成。在宇宙早期历史中，只有其中3个维度参与空间拉神，形成我们现在观察到的三维宇宙。其余6个维度现在观察不到了，因为它们被卷曲在非常微小的尺度中，而且所有的物质都分布在这三个充分拉伸过的维度“表面”上（对9维来说，三维就是一个面而已，或者叫一层“膜”）。     我们生活在3+1维时空之中，对此我们并不特别意外。当描述自然的偏微分方程是椭圆或者超双曲线方程时，也就是空间或者时间其中之一是0维或同时多维，对观测者来说，宇宙不可能预测(紫色和绿色部分).其余情况下(双曲线方程)，若n>3，原子无法稳定存在，n<3，复杂度太低以至于无法产生自我意识的观测者(没有引力，拓扑结构也成问题).由此，我们称空间的对称性被破坏了。量子波的不确定性会导致不同的气泡在膨胀过程中以不同的方式破坏平衡。而结果将会千奇百怪。其中一些可能伸展成4维空间；另一些可能只形成两代夸克而不是我们熟知的三代；还有些它们的宇宙基本物理常数可能比我们的宇宙大.产生第二层多重宇宙的另一条路是经历宇宙从创生到毁灭的完整周期。科学史上，该理论由一位叫Richard C的物理学家于二十世纪30年代提出，最近普林斯顿大学的Paul J. Steinhardt和剑桥大学的Neil Turok两位科学家对此作了详尽阐述。Steinhardt和Turok 提出了一个“次级三维膜”的模型，它与我们的空间相当接近，只是在更高维度上有一些平移。【see "Been There, Done That," by George Musser; News Scan, Scientific American, March 2002】该平行宇宙并非真正意义上的独立宇宙，但宇宙作为一个整体－－过去、现在和未来－－却形成了多重宇宙，并且可以证明它包含的多样性恰似无序膨胀宇宙所包含的。此外，沃特卢的物理学家Lee Smolin还提出了另一种与第二层多重宇宙有着相似多样性的理论，该理论中宇宙通过黑洞创生和变异而非通过膜物理学。     尽管我们没法与其他第二层多重宇宙之中的事物相互作用，宇宙学家仍能间接地指出它们的存在。因为他们的存在可以用来很好地解释我们宇宙的偶然性。做一个类比：设想你走进一座旅馆，发现了一个房间门牌号码是1967，正是你出生那年。多么巧合呀，在那瞬间你惊叹到。不过你随即反应过来，这完全不算什么巧合。整个旅馆有成百上千的房间，其中有一个和你生日相同很正常。然而你若看见的是另一个与你毫无干系的数字，便不会引发上面的思考。这说明什么问题呢？即便对旅馆一无所知，你也可以用上面的方法来解释很多偶然现象。     让我们举个更切题的例子：考察太阳的质量。太阳的质量决定它的光度（即辐射的总量）。通过基本物理运算我们可知只有当太阳的质量在1.6X10^30～2.4X10^30千克这么个狭窄范围内，地球才可能适合生命居住。否则地球将比金星还热，或者比火星还冷。而太阳的质量正好是2.0X10^30千克。乍看之下，太阳质量是种惊人的幸运与巧合。绝大多数恒星的质量随机分布于10^29～10^32千克的巨大范围内，因此若太阳出生时也随机决定质量的话，落在合适范围的机会将微乎其微。然而有了旅馆的经验，我们便明白这种表面的偶然实为大系统中（在这个例子里是许多太阳系）的必然选择结果（因为我们在这里，所以太阳的质量不得不如此）。这种与观测者密切相关的选择称为“人择原理”。虽然可想而知它引发过多么大的争论，物理学家们还是广泛接收了这一事实：验证基础理论的时候无法忽略这种选择效应.适用于旅馆房间的原理同样适用于平行宇宙。有趣的是：我们的宇宙在对称性被打破的时候，所有的（至少绝大部分）属性都被“调整”得恰到好处，如果对这些属性作哪怕极其微小的改变，整个宇宙就会面目全非－－没有任何生物可以存在于其中。如果质子的质量增加0.2%，它们立即衰变成中子，原子也就无法稳定的存在。如果电磁力减小4%，便不会有氢，也就不会有恒星。如果弱相互作用再弱一些，氢同样无法形成；相反如果它们更强些，那些超新星将无法向星际散播重元素离子。如果宇宙的常数更大一些，它将在形成星系之前就把自己炸得四分五裂.虽然“宇宙到底被调节得多好”尚无定论，但上面举的每一个例子都暗示着存在许许多多包含每一种可能的调节状态的平行宇宙。【see "Exploring Our Universe and Others," by Martin Rees; Scientific American, December 1999】第二层多重宇宙预示着物理学家们不可能测定那些常数的理论值。他们只能计算出期望值的概率分布，在选择效应纳入考虑之后。 第三层次：量子平行世界     第一层和第二层多重宇宙预示的平行世界相隔如此之遥远，超出了天文学家企及的范围。但下一层多重宇宙却就在你我身边。它直接源于著名的、备受争议的量子力学解释－－任何随机量子过程都导致宇宙分裂成多个，每种可能性一个。     量子平行宇宙。当你掷骰子，它看起会随机得到一个特定的结果。然而量子力学指出，那一瞬间你实际上掷出了每一个状态，骰子在不同的宇宙中停在不同的点数。其中一个宇宙里，你掷出了1，另一个宇宙里你掷出了2……。然而我们仅能看到全部真实的一小部分－－其中一个宇宙。     20世纪早些年，量子力学理论在解释原子层面现象方面的成功掀起了物理学革命。在原子领域下，物质运动不再遵守经典的牛顿力学规律。在量子理论解释它们取得瞩目成功的同时却引发了爆炸性激烈的争论。它到底意味着什么？量子理论指出宇宙并不像经典理论描述的那样，决定宇宙状态的是所有粒子的位置和速度，而是一种叫作波函数的数学对象。根据薛定鄂方程，该状态按照数学家称之为“统一性”的方式随时间演化，意味着波函数在一个被称为“希尔伯特空间”的无穷维度空间中演化。尽管多数时候量子力学被描述成随机和不确定，波函数本身的演化方式却是完全确定，没有丝毫随机性可言的。     关键问题是如何将波函数与我们观测到的东西联系起来。许多合理的波函数都导致看似荒谬不合逻辑的状态，比如那只在所谓的量子叠加下同时处于死和活两种状态的猫。为了解释这种怪异情形，在20实际20年代，物理学家们做了一种假设：当有人试图观察时，波函数立即“坍塌”成经典理论中的某种确定状态。这个附加假设能够解决观测发现的问题，然而却把原本优雅和谐统一的理论变得七拼八凑，失去统一性。随机性的本质通常归咎于量子力学本身就是这些不顺眼假设的结果。     许多年过去了，物理学家们逐渐抛弃了这种假设，转而开始接受普林斯顿大学毕业生Hugh Everett在1957年提出的一种观点。他指出“波函数坍塌”的假设完全是多余的。纯粹的量子理论实际上并不产生任何矛盾。它预示着这样一种情形：一个现实状态会逐渐分裂成许多重叠的现实状态，观测者在分裂过程中的主观体验仅仅是经历完成了一个可能性恰好等于以前“波函数坍塌假设结果”的轻微的随机事件。这种重叠的传统世界就是第三层多重宇宙。     四十多年来，物理界为是否接受Everett的平行世界犹豫不决，数度反复。但如果我们将之区分成不同视点分别来看待，就会更容易理解。研究它数学方程的物理学家们站在外部的视点，好像飞在空中的鸟审视地面；而生活在方程所描述世界里的观测者则站在内部的视点，就好比被鸟俯瞰的一只青蛙。在鸟看来，整个第三层多重宇宙非常简单。只用一个平滑演化的、确定的波函数就能就能描绘它而不引发任何分裂或平行。被这个演化的波函数描绘的抽象量子世界内部却包含了大量平行的经典世界。它们一刻不停的分裂、合并，如同经典理论无法描述的一堆量子现象。在青蛙看来，观察者感知的只有全部真相的一小部分。它们能观测到自己所在那个第一层宇宙，但是一种模仿波函数坍塌效果而又保留统一性、被称为“去相干”的作用却阻碍他们观测到与之平行的其他宇宙。每当观测者被问及一个问题、做一个决定或是回答一个问题，他大脑里的量子作用就导致复合的结果，诸如“继续读这篇文章”和“放弃阅读本文”。在鸟看来，“作出决定”这个行为导致该人分裂成两个，一个继续读文章而另一个做别的去了。而在青蛙看来，该人的两个分身都没有意识到彼此的存在，它们对刚才分裂的感知仅仅是经历了个轻微的随机事件。他们只知道“自己”做了什么决定，而不知道同时还有一个“他”做了不同的决定。尽管听起来很奇怪，这种事情同样发生在前面讲过的第一层多重宇宙中。显然，你刚作出了“继续阅读本文”的决定，然而在很远很远的另一个银河系中的另一个你在读过第一段之后就放下了杂志。第一层宇宙和第三层宇宙唯一的区别就是“另一个你”身处何处。第一层宇宙中，他位于距你很远之处－－通常维度空间概念上的“远”。第三层宇宙中，你的分身住在另一个量子分支中，被一个维度无限的希尔伯特空间分隔开来。     第三层多重宇宙的存在基于一个至关重要的假设：波函数随时间演化的统一。所幸迄今为止的实验都不曾与统一性假设背离。在过去几十年里我们在各种更大的系统中证实了统一性的存在：包括碳-60布基球和长达数公里的光纤中。理论反面，统一性也被“去相干”作用的发现所支持。【see "100 Years of Quantum Mysteries," by Max Tegmark and John Archibald Wheeler; Scientific American, February 2001】只有一些量子引力方面的理论物理学家对统一性提出置疑，其中一个观点是蒸发中的黑洞有可能破坏统一性，应该是个非统一性过程。但最近一项被叫做“AdS/CFT一致”的弦理论方面的研究成果暗示：量子引力领域也具有统一性，黑洞并不抹消信息，而是把它们传送到了别处。如果物理学是统一的，那么大爆炸早期量子波动是如何运作的那幅标准图画将不得不改写。它们并非随机产生某个初始条件，而是产生重叠在一起的所有可能的初始条件，同时存在。然后，“去相干”作用保证它们在各自的量子分支里像传统理论那样演化下去。这就是关键之处：一个哈勃体积内不同量子分支（即第三层多重宇宙）演化出的分布结果与不同哈勃体积内同一个量子分支（即第一层多重宇宙）演化出的分布结果是毫无区别的。量子波动的该性质在统计力学中被称为“遍历性”。     同样的原理也可以适用在第二层多重宇宙。破坏对称性的过程并不只产生一个独一无二的结果，而是所有可能结果的叠加。这些结果之后按自己的方向发展。因此如果在第三层多重宇宙的量子分支中物理常数、时空维度等各不相同的话，那些第二层平行宇宙同样也将各不相同。换句话说，第三层多重宇宙并没有在第一层和第二层上增加任何新东西，只是它们更加难以区分的复制品罢了－－同样的老故事在不同量子分支的平行宇宙间一遍遍上演。对Everett理论一度激烈的怀疑便在大家发现它和其他争议较少的理论实质相同之后销声匿迹了。    第三层和第一层区别的示意图    毫无疑问，这种联系是相当深层次的，物理学家们的研究也才处于刚刚起步阶段。例如，考察那个长久以来的问题：随着时间流逝，宇宙的数目会以指数方式暴涨吗？答案是令人惊讶的“不”。在鸟看来，全部世界就是由单个波函数描述的东西；在青蛙看来，宇宙个数不会超过特定时刻所有可区别状态的总数－－也即是包含不同状态的哈勃体积的总数。诸如行星运动到新位置、和某人结婚或是别的什么，这些都是新状态。在10^8开温度以下，这些量子状态的总数大约是10^(10^118)个，即最多这么多个平行宇宙。这是个庞大的数目，却很有限.从青蛙的视点看，波函数的演化相当于从这10^(10^118)个宇宙中的一个跳到另一个。现在你正处在宇宙A－－此时此刻你正在读这句话的宇宙里。现在你跳到宇宙B－－你正在阅读另一句话那个宇宙里。宇宙B存在一个与宇宙A一摸一样的观测者，仅多了几秒中额外记忆。全部可能状态存在于每一个瞬间。因此“时间流逝”很可能就是这些状态之间的转换过程－－最初在Greg Egan在1994所著的科幻小说[Permutation City]中提出的想法，而后被牛津大学的物理学家David Deutsch和自由物理学家Julian Barbour等人发展开来。 第四层次：其他数学界构     虽然在第一、第二和第三层多重宇宙中初始条件、物理常数可能各不相同，但支配自然的基础法则是相同的。为什么要到此为止？为何不让这些基础法则也多样化？来个只遵守经典物理定律，让量子效应见鬼去的宇宙如何？想象一个时间像计算机一样一段一段离散地流逝，而非现在那样连续地流逝的宇宙？再想象一个简单的空心十二面体宇宙？在第四层多重宇宙里，所有这些形态都存在。平行宇宙的终极分类，第四层。包含了所有可能的宇宙。宇宙之间的差异不仅在表现物理位置、属性或者量子状态，还可能是基本物理规律。它们在理论上几乎就是不能被观测的，我们能做的只有抽象思考。该模型解决了物理学中的很多基础问题。     为什么说上述的多重宇宙并非无稽之谈？理由之一就是抽象推理和实际观测结果间存在着密不可分的联系。数学方程式，或者更一般地，数字、矢量、几何图形等数学结构能以难以置信的逼真程度描述我们的宇宙。1959年的一次著名讲座上，物理学家Eugene P. Wigner阐述了“为何数学对自然科学的帮助大得神乎其神？”反言之，数学对它们（自然科学）有着可怕的真实感。数学结构能成为基于客观事实的主要标准：不管谁学到的都是完全一样的东西。如果一个数学定理成立的话，不管一个人，一台计算机还是一只高智力的海豚都同样认为它成立。即便外星文明也会发现和我们一摸一样的数学界构。从而，数学家们向来认为是他们“发现”了某种数学结构，而不是“发明”了它。关于如何理解数学与物理之间的关系，有两个长存已久并且完全对立的模型。两种分歧的形成要追溯到柏拉图和亚里斯多德。“亚里斯多德”模型认为，物理现实才是世界的本源，而数学工具仅仅是一种有用的、对物理现实的近似。“柏拉图”模型认为，纯粹的数学结构才是真正的“真实”，所有的观测者都只能对之作不完美的感知。换句话说，两种模型的根本分歧是：哪一个才是基础，物理还是数学？或者说站在青蛙视点的观测者，还是站在鸟视点的物理规律？“亚里斯多德”模型倾向于前者，“柏拉图”模型倾向于后者。在我们很小很小，甚至尚未听说过数学这个词以前，我们都先天接受“亚里斯多德”模型。而“柏拉图”模型则来自于后天体验。现代理论物理学家倾向于柏拉图派，他们怀疑为何数学能如此完美的描述宇宙乃是因为宇宙生来就是数学性的。这样，所有的物理都归结于一个根本的数学问题：一个拥有无穷知识与资源的数学家理论上能从鸟视点计算出青蛙的视点－－也就是说，为任何一个有自我意识的观测者计算出他所观测的宇宙有些什么东西、它将发明何种语言来向它的同类描述它看到的一切。        宇宙的数学结构是抽象、永恒的实体，独立于时空之外。如果把历史比作一段录像，数学结构不是其中一桢画面，而是整个录像带。试设想一个由四处运动的点状粒子构成的三维世界。在四维时空－－也就是鸟的视点－－看来，世界类似一锅缠绕纠结的意大利面条。如果青蛙观测到一个总是拥有恒定速率，方向的粒子，那么鸟就直接看到它的整个生命周期－－一根长长的、直直的面条。如果青蛙看到两个相互围绕旋转的粒子，鸟就看到两根以双螺旋结构缠在一起的面条。对青蛙来说，整个世界以牛顿运动定律和引力定律为规则运作；而对鸟来说，世界被描绘成“意大利面条几何学”－－一种数学结构。青蛙本人也仅是面条－－一大堆复杂到构成它们的粒子能存储和处理信息的面条。我们的宇宙要比上述例子复杂的多，科学家们还没有找到－－如果有的话－－那个能正确描述它的数学结构.“柏拉图”派模型带来了一个新的问题，为何我们的宇宙是现在这个样子。对“亚里斯多德”派来说，这个问题是没有意义的：因为宇宙的物理本源就是我们观测到的样子。但“柏拉图”派不仅无法回避它，反而会困惑为什么它不能是别的样子。如果宇宙天生是数学性的，为什么它仅仅基于“那一个”数学结构？要知道数学结构是多种多样的。似乎在真实的核心地带有某种最基本的不公平存在。作为解决该难题的一条路径，我认为数学结构有着完全的对称性：基于任何数学结构的宇宙都确实存在。每一个数学结构都有与之相关的平行宇宙。构成这个宇宙的基础并不在该宇宙内而是游离于时间和空间之外。大部分平行宇宙内很可能不存在观测者。这种假说可以看成是本质上的柏拉图主义，它断言柏拉图领域提及的数学结构或是圣荷西州立大学的数学家Rudy Rucker所谓的“精神领域(mindscape)”都存在对应的物理真实。它也类似于剑桥大学的宇宙学家John D. Barrow提到的“天空中的π”，或是哈佛大学的哲学家Robert Nozick提出的“多产性原理”，或是普林斯顿的哲学家David K. Lewis所谓的“形式现实主义”。第四层终于宣告了多重宇宙在层次上的终结，因为任何自相容的物理理论都能表达成某种数学结构。     第四层多重宇宙的假设作出了可验证的预言。在第二个层次上，它包含了全体可能（全体数学结构）和选择效应。数学家们还在继续为这些数学结构分门别类，而他们最终应该发现，用来描绘我们世界的那个数学结构将会是所有符合我们观测结果的结构中最简单那个。类似地，我们将来的观测结果将会是那些最简单的、与过去观测结相一致的东西；而过去的观测结果也应该是最简单的、与我们存在相符合的那些。 想要定量化这种“简单”是个严峻的考验，与之相关的研究才刚刚起步。但最具震撼性和令人鼓舞的是，对称和恒定的数学结构力图表现出的简明与整洁也正是我们宇宙所展现的。数学结构趋向于越简单越好，那些复杂的附加公理无疑破坏了简洁。 奥卡姆如是说:以上便是我们所讨论的平行宇宙理论，它分为由低到高四个层次，与我们熟知宇宙的差异也随层次不同越来越大。这些差异可以来自不同的初始条件（第一层）；不同的物理常数、粒子种类和时空维数（第二层）；不同的物理规律（第四层）。有意思的是，第三层才是最近几十年研究最火热的东西，因为它本质上没有增添任何新的宇宙类型。     未来十年内，发展迅猛的对宇宙微波背景和空间大尺度物质分布的测量会进一步确定空间的准确曲率和拓扑结构，其结果将直接支持或驳倒第一层多重宇宙的假说。这些测量结果也会验证“无序持续膨胀”理论，从而间接探测第二层多重宇宙。同时天体物理学与高能物理领域的巨大进展也将进一步阐明到底我们宇宙的哪些物理常数被“调节”过了，以此加强或削弱第二层多重宇宙的可信度。     如果当前研制量子计算机的大量努力成功的话，将为第三层平行宇宙提供更加深远的证据。不仅如此，量子计算机的工作是在本质上利用第三层多重宇宙的平行性。大量的试验同时也在寻找违反统一性－－最终决定量子平行宇宙存在于否－－的证据。现代物理学在其面对的最重大挑战－－将广义相对论与量子场论统一起来－－中成功与失败会改变对第四层多重宇宙的看法：最终会找到那个描述我们宇宙的数学结构，抑或是碍于数学的局限性而停止不前，最终放弃第四层次。 四层多重宇宙的关系 左上角那N圈蚊香就是无数个第一层平行宇宙黄色的连线显示着它们包容于一个气泡中 这些气泡构成了第二层多重宇宙（左下)右下角是所谓的量子平行宇宙（即第三层） 中间那只猫就是著名的猫佯谬猫佯谬是一个假想的用来连结微观量子现象和宏观世界的实验一个微观粒子在特定场合出现与否取决于波函数的概率这个箱子就被做成如果粒子出现了，就杀掉猫，否则不杀.现在问题来了，根据量子理论，粒子既会出现，又不会出现，是该波函数载空间的弥散,那么猫是死是活呢？物理学家没办法，只好承认猫同时处在死和活两种状态.现在第三层平行宇宙理论解决了这个问题,宇宙分裂成两个，猫在其中一个里面活着，在另一个里面死了. 左上角是第四层平行宇宙，亦即和我们的基本物理概念都不同的宇宙 . 图上画的从左到右，从上到下分别是形如曼德勃罗集的宇宙。曼德勃罗集是数学上最美丽的集合，产生规则简单得一句话就能说清楚，图形却比整个已知宇宙复杂得多. 第二个是正12面体宇宙 第三个有点象洛伦兹轨迹形状 下面那个方的叫谢尔宾斯基海绵，是一个体积为0 的立方体，也是分形里面的东东， 下面一排左边是一般的平滑空间；马鞍面空间；封闭的球状空间，最后一个是相互连通的怪异拓扑结构的空间 . 黄线表明量子平行宇宙和第二层多重宇宙是等价的 但可以看到量子平行宇宙只对应第四层的一小部分 是因为第四层的基本物理规律都不同了，绝大部分根本没有“量子”这种概念.你是否该相信平行宇宙？主要争论集中在：它们很浪费并且很奇怪。最首要的争论是，平行宇宙似乎不遵循“奥卡姆的剃刀”原则，因为它假设永远观测不到的其他宇宙存在。为何老天爷如此浪费并沉醉于这些多到无穷无尽的不同世界？争论充斥平行宇宙的每一个层次，为什么自然界偏偏要如此浪费？空间、物质或原子－－毫无疑问地，仅第一层多重宇宙就已经包含了无限的上述事物，谁在乎它多浪费点呢？关键是让理论显式地变得简单。怀议论者担心要描述所有不可见世界所需的信息量。     然而，一个整体集合往往要比集合中的单个元素简单得多。该原理在描述算法的时候很常用。我们知道，一个非常简短的计算机程序程序就能输出异常庞大的信息量。举例而言，考察整数集。哪个更简单些，整数集还是其中某个特定整数？也许你会天真的觉得单个整数简单些，但事实上整个整数集能用非常简单的规则表达出来，寥寥几行计算机程序就能产生它们；相反单个整数却可能难以置信的大。因此，真正简单的是整个集合。     同样，爱因斯坦的整套引力场方程要比其中某个特定的要简单。前者只需要很少几个方程就能描述，而后者要求在某些超平面指定大量的初始数据。由此我们学到，当我们把注意力局限在全体元素的一小部分上，复杂性就会大大增加，也就失去了整个系统原本应有的对称性和简洁性。     从这种意义上说，更高层次的多重宇宙意味着更简单。为了从我们居住的宇宙走向整个第一层多重宇宙，需要指定许多初始条件来消除彼此的差异；若是升级到第二层，需要指定一些物理常数；到了第四层则完全不用指定任何东西。多余的复杂性完全来自观测者的主观视点－－也就是青蛙的视点。从鸟的视点来讲，多重宇宙要简单的多。而抱怨该理论太奇怪的人出发点多半来自审美上而非科学上。然而这种看法只有在亚里斯多德派中才有意义。我们期待着什么？当我们提出“现实的本源是什么”如此意义深远的问题时，难道我们仅期待一个听起来不那么奇怪的答案？进化赋予我们对日常生活中物理现象的直觉，然而它仅对我们远古的祖先有用。现在，当我们遨游于远超日常物理的世界，我们应当预见到它们也许会很奇怪。     四层多重宇宙的共通特色是最简洁与最优雅的理论自然而然地包含着平行宇宙。要否认它们的存在，你必须复杂化你的理论，增加没有观测结果支持的过程和特殊的假定：无限的空间、波函数坍塌和天性不对称。那么，哪个才是真正的浪费和不雅，许多宇宙还是许多规则？也许我们将逐渐习惯宇宙的奇妙而终将发现这种不可思议的奇妙正是它魅力的一部分。      更多相关资料: http://post.baidu.com/f?kz=83733770 宇宙, 平行, 理论 200306047.jpg (36.13 KB, 下载次数: 44) 下载附件 2007-4-21 10:50 上传 000F1EDD-B48A-1E90-8EA5809EC5880000_p44.gif (30.76 KB, 下载次数: 47) 下载附件 2007-4-21 10:50 上传 000F1EDD-B48A-1E90-8EA5809EC5880000_p43.jpg (99.28 KB, 下载次数: 52) 下载附件 2007-4-21 10:50 上传 000A942B-FC34-1E95-8EA5809EC5880000_2.jpg (65.3 KB, 下载次数: 43) 下载附件 2007-4-21 10:50 上传 000F1EDD-B48A-1E90-8EA5809EC5880000_p50.jpg (97.17 KB, 下载次数: 48) 下载附件 2007-4-21 10:50 上传
	分享到:  !connect_viewthread_share_to_qq! QQ空间 腾讯微博 腾讯朋友 收藏0 顶~0 砸~0 相关帖子 • 大爆炸或产生2个宇宙 时间倒流可去另一个 • 了解宇宙如何运行三季全 • 宇宙中的美丽岛屿——星系 • 了解宇宙如何运行中所涉及的望远镜 • 宇宙真的是没有中心的吗？ • 了解宇宙如何运行中所涉及的望远镜 • 对宇宙中星系、黑洞等物体自转等的疑惑。 • 对宇宙中星系、黑洞等物体自转等的疑惑。(好像发错了版块 • 宇宙时空之旅 Cosmos: A SpaceTime Odyssey • 《了解宇宙如何运行》Ι 全集
	回复 使用道具 举报 置顶卡 变色卡 千斤顶 显身卡
	购买望远镜就在牧夫天文商城

天文摄星人 天文摄星人 当前离线 积分 10782 IP卡 狗仔卡	沙发  楼主| 发表于 2007-4-21 11:06 | 只看该作者 探讨真实的多重宇宙时空世界 霍金和维勒金一起发展了平行宇宙的想法。宇宙学家亚力山大维勒金说：“有非常多的平行世界，在那里，戈尔(美国前副总统，后来在与小布什的总统竞选中落败)是美国的总统，而埃尔维斯普雷斯利 (摇滚巨星猫王)还活着。” 广义相对论在大爆炸初始失效的原因是它没有和不确定原理相合并。爱因斯坦基于“上帝不玩骰子”的论断而反对量子理论中的随机因素。然而，所有证据表明上帝完全是一名“赌徒”。霍金认为，人们可以将宇宙看做一个庞大的赌场，在每一个场合下骰子都在滚动或者轮子都在旋转。宇宙的情景也是一样。当宇宙尺度很大，正如它今天这样时，骰子被投掷的次数极为巨大，其平均结果就会得出某种可预见的东西。这就是为什么经典定律对于大系统有效的原因。当然，宇宙尺度非常微小时，正如它在临近大爆炸的时刻，投掷骰子的次数很少，而不确定性原理则非常重要。 因为宇宙不停地投掷骰子，它就不像人们以为的那样仅仅存在一个历史。相反地，宇宙应该拥有所有可能的历史，每种历史各有其存在的概率。 霍金还设想自己到了另一并行宇宙。然后，所发生的一切就如他在科幻小说《星际旅行》中所经历的一样：霍金和牛顿、爱因斯坦玩扑克，玛丽莲?梦露坐在他的旁边。“任何一个想得到的故事都会在其中的一个宇宙中发生，”霍金说，“肯定有这样一个故事，在其中我和玛丽莲?梦露结婚了。也有另一个故事，在那里克娄巴特拉(埃及托勒密王朝末代女王，貌美，有强烈的权势欲望，一开始是恺撒情妇，然后与安东尼结婚，安东尼溃败后又欲勾引屋大维，未遂，以毒蛇自杀)成了我的妻子。 简单来说宇宙由不规则的量子组成，万物由不规则的分量子组成，还有一位宇宙学家杜治说的不规则属更大层面，量子力学认为世上不止一个宇宙，有些相似，有些不一样，有些空间和我们的空间只相差一颗光子或量子，差别较大的不会见到我们，有些甚至没有我或没有某个国家的存在。这是以不规则定律所作的大胆推论，杜治认为每颗量子代表一个空间，他说的不是梦话，他有真凭实据，而且是物理学生必会做的著名实验，这个实验历史悠久，源自1909年。在实验中要有光源（光束型），光源会被镜子反射到这些缝隙里去，隔片上有两缝隙，在摄影机里可以看见那初步形成的交错图案，所谓交错图案就是镜头中间一组昏暗的条纹，有两条缝时才会出现。因为有两条缝时，光源就会分走两边，然后就像涟漪交错，有时候涟漪会融合，有时会互相抵消产生条纹，如果用滤镜将光的强度减弱至只有一颗光子能穿越缝隙，在液体氮下就会清楚看见一束光，在氮气下光束四散，去到滤镜前便停止，而滤镜隔开了肉眼所见的一切，但摄影机会拍摄到流过去的光子。光子一颗一颗的穿过夹缝，因此在萤幕上该只见到两条明亮的线，而不是会交错的条纹，但事实上并非如似，从电脑中可看到提示中的条纹结果。做这个单光子实验时，多颗光子所造成的图案，在强光下完全一样，人们想不透个中原因，书本中所读到的理论是由于光子是粒子，也是波纹，但现在才发现那是谬论。光子其实既固定同时又四散，杜治认为单一的光子能造出条纹，是因为附近另有肉眼看不见的光子，我们看不见的光子是另一个空间的光子，这宇宙在我们附近撞击原有的光子，令它们改变方向，这实验结果是杜治所见过最古怪的。它充份证明了世上不止有一个宇宙空间而是有非常多的平行宇宙空间存在，否则不会有以上的实验结果。最新的超弦理论研究还指出，宇宙相当于全息成像的产物。例如马尔达塞纳的模型认为4维的场可以认为是5维场的全息投影，就象激光全息摄影中三维物体被投射到二维的平面上一样。在全息的宇宙里，某一体积内所有事物的信息会以某种方式显示在表面。全息成像与意识有关吗？有过这样一个现象。      全息图像是“编码”到一种特殊玻璃上的三维图像。这样制作的图像有一种特别的效果，即它虽然是做在平面上的，但是如果在激光照射下人眼从某个角度看去，图像却是三维立体的。 全息图像的奥秘在于它使用“单向”的激光，在激光中，光子都朝同一个方向运动，在拍全息照片时，拍摄物体被放在黑暗中，用一束激光向它照射。实际上是分开的两束激光，一束射向物体，一束射向一面镜子。一半的光子被物体的轮廓反射回来，另一半被镜子反射回来。所有的光子又都射到感光的全息玻璃片上。玻璃片中的分子随即改变了颜色深浅。这样所形成的图像在激光的照射下呈现出三维立体的效果。 这真是个奇妙的过程，好像在某种程度上从物体上反射回来的光子可以与从平面镜上反射回来的光子相互“沟通”似的。不过这样来解释这种现象是受人争议的，因为一般物理学原理也能解释这一过程。 真正令物理学家们震惊的还是全息照片玻璃摔碎时发生的现象。全息照片的碎片并没有变成拼图似的小片；可以拼在一起恢复原来的画面，而是每一个小片中都显示出整个画面。根据传统物理学的定律知识，这是不可能发生的。这一小片玻璃中的每个分子瞬间就改变了外观，却没有直接的物理作用能够解释这种变化。这一发现暗示着在一个更深的或者是非物质的层面上，甚至是简单的无生命的无机玻璃分子都知道它们自己的角色，它们有自我意识，知道自己的状态，并知道它们在整体中的地位。这也暗示着这些玻璃分子有集体“记忆”，并且它们之间还能交流沟通。要不是这样，单个的碎片又怎能知道该重组成什么样的图像呢？ 当然人的意识是否单独存在还有着很大的争议，但是某些人的意识是可以做时空旅行的！ 世上有其他不同空间存在，这对意识时空穿梭是可能的，如果做意识的时空旅行，意识回到过去是不一定会回到自己的空间，而是到了别的平行宇宙空间。 但是要身体的时空旅行就需要用相对论来阐释宇宙结构，又要用量子力学来阐释当中细节，但这理论互有矛盾，因此要有一套能结合两者的新理论。 回顾人类时空观的发展，可以分为三个阶段。牛顿由经典物理学的成果出发，把宇宙看成是机械的，看成是一个以固定规律运转的精密的机器，比如地球围着太阳旋转，星系如同一个巨大钟表的很多齿轮。这就是机械的时空观，是绝对时间、绝对空间的框架体系，把时间与空间完全割裂开来；第二阶段，以爱因斯坦的相对论为基础，摒弃了牛顿的绝对时空的概念，将空时结合起来建立了相对论时空观。对于任何一个惯性系，时间是用相对于这个惯性系静止的、同样构造的钟来度量的。广义相对论时空观还取消了惯性系的概念，以弯曲的空间概念把物质、运动、时空联系在一起，否认了时间、空间割裂的思想。但是爱因斯坦的相对论是描述静态的、孤立的、均匀分布的时空，没有建立高维动态空时流形的物理概念，没有分析时空结构的演变。而且新的观测事实，如水星进动、X射线爆发源对爱因斯坦的广义相对论发起了挑战；第三阶段，现代科学已经认识到我们存在的时空世界事实上是十分复杂的，决不是只存在着我们人类用肉眼可以感知到的空间。在这基础上，人们发展了现代时空理论。 一、现代时空论及量子物理时空观 现代时空论的观点是宇宙是由各种不同维度的空时结构组成。高维空时流形的实质是负能量流，即空间的实质是能量流Zi8RD` { 2 }比如超弦理论认为：真实的时空是高维的，可能是10维，也可能是26维等/ [3]。若以10维的为例，量子力学认为一切粒子均有波动性，其波长为λ＝h/p,其中p为动量，h为普朗克常数。若粒子的波长比空间尺度L大许多，则这一维必将被紧致。根据卡鲁查克莱因理论，要在紧致后的4维时空中获得正确的引力常数G，则其余6维的尺度L 都要在普朗克线度lp之内（lp=h/(mp, Ec)，其中分母表示动量）。由此可见，要探测余下的这6维时空，则发射的粒子应具有大于(mp, Ec)的动量，这样使得λ E4.$. [4]。 关于量子物理时空观，著名的量子物理学家、牛津大学教授德伊兹教授作了最好的表述，他说：“量子力学本质上是关于多个平行宇宙的解释，有些宇宙和我们这个宇宙很相似，而另一些则很不一样。在我们附近的宇宙可能只跟我们宇宙相差一个光子，而最远的那些宇宙则跟我们的宇宙完全不同。”，“现实世界并不是由一个宇宙构成，但是我们只能在它的一个层面上活动。 [5] 另外近十余年来，随着量子物理学和广义相对论的交汇，特别是规范场理论中对称破缺相变的最新进展，现代宇宙学提出了许多宇宙形成假说。宇宙大爆炸理论、暴胀宇宙论和宇宙弦理论就是这些理论中很重要的一部份。例如，根据1983年A.Linde提出的混沌暴胀宇宙模型，极早期宇宙中存在着若干个空间畴，每一个空间畴将作指数膨胀，形成大小超过可观测宇宙的微宇宙泡。每一个微宇宙泡可以演化为一个对应的宇宙，而我们生活的这个宇宙只是由其中一个微宇宙泡暴胀演化而来的. [6]。这些宇宙是连通的。根据1935年爱因斯坦提出的“虫洞”（Wormholes）理论。虫洞能扭曲空间，是宇宙的隧道，可以令原来相隔亿万公里的地方变成近在咫尺。也就是说不同的子宇宙空间可通过虫洞联结，但“虫洞”的引力极其大，以至于可以毁灭所有进入它的物质，只有人的意识可以从“虫洞”中去通往其它世界。 [7] 今年2003.7月的《科学美国人》权威杂志中，标题做 《平行宇宙（ParallelUniverses）真的存在》。请注意：这“存 在”是“真的”，不是“也许”。 文章的作者，是美国宾夕法尼亚大学天体物理学家马克斯· 泰格马克。他毕生研究的领域，是广袤的宇宙。他称“平行宇宙 是真的”，并不因为他造访过“宇宙外的宇宙”，而是（他说） 能获证于经验的证据。 那平行宇宙是什么意思呢？它是说：在我们的宇宙外，尚有 其他的宇宙存在，其数量，或许是无穷的。其中的某些和我们的 宇宙几乎相同，活像是双胞胎。在这无数的“外宇宙”中的某处， 也许还有你的“对等人”———你的“复制品”。也许还有一帮 这样的人，生活在一颗和我们的地球处处相同的行星上。 二、、多层面时空论 如前所述，现代科学已经认识到了多个时空的存在，并提出了以上各种理论，但是这些理论还存在很多问题。比如对于宇宙大爆炸理论，我们无法解释0~10^-43秒之间宇宙是什么状态？为什么极早期宇宙中粒子和反粒子数目不对称？为什么宇宙中光子数和粒子数之比为10^-9数量级[1]？1964年发现的所谓的“大爆炸火球”，在1992年后的观测中有温度的波动，即其密度是在密与疏之间波动，这与宇宙大爆炸理论的推论不相符。 又例如，1997年1月9日，权威科学杂志《Nature》上的一篇关于星系分布的文章指出，超级星云是按规则晶格排列的，每个长方形的格子边长三亿六千万光年。据Estonia的Tartu观测中心的J.Einasto博士报导，超级星云的整个分布就象一个三维的棋盘。1990年2月，英国Durham大学天文学家T.J.Broadhurst和一个多国科学家组成的小组，在范围不大的一块天空区域向纵深观察，观测范围至60亿光年。他们使用一种笔心式(Pencil Beam)扫描仪器确定，星云间是以3亿光年间隔的密度按周期分布的。天文学家已经知道星系会组成片状或丝状的星云，围绕着没有星系的空间运转，但周期性的结构却是非常出乎意料的。所有这些观测结果向我们原先对宇宙的认识提出了挑战。根据宇宙大爆炸理论，星云应该是随机地分布在整个宇宙中的。加州大学伯克利分校的Marc Davis博士说，如果星系的分布真的是周期性的，那么完全可以说，我们对宇宙的早期情况实际上一无所知。 超弦理论也存在困难，比如弦论提出的量子色动力学（QCD）的标准模型能把强作用力、弱作用力、电磁力统一起来，但很难将引力也统一进去[7]。再有，是不是宇宙间就只存在这四种基本作用力？宇宙间的γ射线超能爆发就不能简单归结为这四种力的作用。超弦理论对此也无法解释。另外，弦论中维数的概念本身没有解释宇宙演化的物理本质，它的结论是不可检验的（物理学家必须建造一个周长为1000光年的粒子加速器，而整个太阳系的周长才一光天）。超弦理论在物理学领域把数学工具用到了极致，被称为“数学之舞”，把宇宙的演化变成了纯“数学游戏”，甚至失去了物理意义，成了“美学”的作品。 总之，现在人类对于宇宙、时空的认识还是很不完善的。 事实上，现代时空论也已经认识到空间的实质是能量流[2]。另外量子力学告诉我们，微观粒子在不同条件下可以分别表现出波动性和粒子性，此即“波粒二象性”。然而在亚原子层面，波粒的分界消失，物质不能只被作为其中的一种来解释，它们既是波又是粒子。波是一种能量，它不表现为我们可见的粒子状态，但是不能说它不是物质。此时物质的概念发生了变化，也就是说能量也是物质。爱因斯坦相对论的质能关系式E=mc^2告诉我们，物质的质量是能量的一种表征形式，所以物质即能量。物质与能量是统一的，“波粒二象性”就是这种统一性的证明。既然能量是物质的本质，那么也就是宇宙的本质。宇宙从根本上来说，就是由能量构成的。 现代科学已经认识到，物质是由分子、原子、原子核、电子、质子、中子、各种介子、超子、共振态粒子、一层一层一直到中微子等组成的[8]。宇宙不同层次中各种物质状态维持依靠的是能量，粒子越微观，能量越大，宇宙的演化就是能量在同一层次间、不同层次间的相互作用、转移和转化。不同层次能量包括：庞大天体（宇宙岛、银河系、恒星系）的运动动能、我们周围物体的机械能、生物能、分子间的作用能（如热能、化学能）、原子间的作用能（如核能）、被夸克禁闭的空间的能量、中微子放射能（可轻松穿透1000光年的钢板），还有更微观的及更宏观的未被认识的能量状态等。       晶体和生物分子之间对应几电子伏特；一般无机和有机分子对应几千电子伏特；原子核对应几百万电子伏特；质子、中子对应几十亿电子伏特；夸克、中微子对应更强大的能量级别，是现有科技水平无法探知的。 现代科学只能研究微观粒子的点状存在，不能看到微观粒子存在的整个面，是因为研究越微观的粒子要求越高的能量去探测。目前在实验室中的最高能量可以探测到中微子，虽然中微子离物质的本源相差还很远很远，但再往微观下去就无能为力了。而在微观上，物质内部不同粒子的不同整体层面、不同能量级别就构成了相应的不同空间。 现代物理学已经认识到普朗克常数h标志着宏观物理规律和微观物理规律的界限，是体现空间层次性的一个例证。任何物体都是存在于许许多多个同时同地存在的空间中。任何空间都有各自的时空结构和生命的特定存在形式，我们人感觉、接触到的基本上是分子构成的宏观物体，我们所在的是位于分子和星球之间的空间。事实上，现代科学还认识到，电子到原子核之间也是一个极其广阔的空间。现代弦理论的T对偶性，把一根弦绕着一个紧致维形成环圈时所出现的两类粒子（即振动粒子和环绕粒子），联系了起来。T对偶性理论认为，半径为R的圆的环绕粒子与半径为1/R的振动粒子是相同的，反之亦然。这样，如果宇宙缩小到小于普朗克长度（10^-35米），宇宙将转变为一个对偶的宇宙，随着原先宇宙的缩小而不断增大。因此在这样极小的尺度上，宇宙仍然看起来象大尺度一样. 不同层次时空的能量层次存在差异，所以不同时空有不同的时空结构，宇宙特性在各层次时空的体现不同，就有不同的演化规律。海森堡的测不准原理（即不能同时地对微观粒子的坐标和动量进行绝对精确的测量）事实上就反映出宏观时空概念及规律已经不再适用于微观世界。试想，生物分子的能级只有几电子伏特，而到质子和中子则对应几十亿电子伏特，这样大的能级差异，必然暴露出宏观理论的局限性，必须要有适合高能量层次的新概念、新理论才行，否则就会遇到“螳臂当车”的尴尬。又如“宇宙遗失90%质量”这一问题，很可能就是把时间的概念不适当地用来描述不同层次的能量稳定状态，对能量的作用层次认识不足所造成的。显然宇宙中不只存在现有的能量形式。“γ射线超能爆发”之所以现有的物理规律难以适用，就是因为现有的规律是基于分子到星球之间这个空间层次得到的，而对于更高能量层次时空的现象就难以解释了。 不同层面的空间是客观存在的，不过肉眼看不到它，但是看不到并不意味着它们不存在。19世纪初意大利天文学家皮亚齐用望远镜发现了第一颗名为谷神星的小行星。但是，当这颗行星一接近太阳，就消失了。人们就说他的发现是虚假的。德国数学家卡尔?高斯听到这个消息后，根据皮亚齐的发现，用自己的计算结果，证实了谷神星所在位置。天文学家用望远镜朝卡尔?高斯所指方向看去，果然发现了这颗小行星。显然，如果当时没有卡尔?高斯的计算结果，人们就不会承认谷神星的存在。然而，不管你有没有计算结果，能不能看不到它，承认不承认它，它都是真实的客观存在，都是太阳系中的一个成员。 宇宙具有十分复杂的时空结构，另外层面的空间，无穷多的平行世界，无穷多的其它世界的您，我们现代科学水平还不能去深入研究，但却是实实在在客观存在的，有的现象从古至今都影响着我们这个世界，有待与人们的进一步探索 .
	回复 支持 反对 使用道具 举报 显身卡

天文摄星人 天文摄星人 当前离线 积分 10782 IP卡 狗仔卡

板凳  楼主| 发表于 2007-4-21 11:27 | 只看该作者 我们目前生活的世界仅仅是许多平行世界中的一片 澳大利亚科学家保罗·戴维斯的科学研究包括黑洞、量子场论、宇宙起源、意识的本质和生命起源等诸多涉及人类和宇宙本源的问题。他曾经带领一个研究小组，得出光速在变化的结论；他曾经写过不少关于时间的书；如果他设想的一切都成为现实，人可以回到过去见一见自己的老爷爷、老奶奶，也可以到未来看一看自己的重孙子、重孙女，关于时空的基本定义就要改写，物理学的根基就会动摇。　　 　　“有限时间旅行”肯定可行 　　人类乐于梦想，也盼望知晓过去和未来。若要将历史和命运活生生地展现在我们面前，时间旅行似乎是个最为简便的方法。然而，时间机器的制造仍然停留在幻想阶段，而即便是幻想，也多产生于小说家和电影特效师的手中，少有人知科学家对时间机器的具体构想。保罗·戴维斯是享誉世界的理论物理学家，他决定当第一个吃螃蟹的人，尝试制造时间机器，哪怕只是“理论上的制造”。 　　要制造时间机器首先需要弄清楚的问题是，你想回到过去还是飞到未来。在戴维斯看来，在时间中到未来旅行是很容易的，如果你接近于光速运动或者身处强大的引力场中，会感到时间流逝得比其他人更缓慢——你进入了他们的未来。 　　飞向未来的旅行又叫“有限时间旅行”。保罗·戴维斯在《怎样制造时间机器》的开头非常肯定的提出，“有限时间旅行”是可行的，但回到任何时代的“无限时间旅行”只是“有可能可行”。 　　虫洞：回到过去的关键 　　爱因斯坦的相对论允许这一旅行发生在特定的时空结构里：一个旋转的宇宙，一个旋转的柱体，以及非常著名的虫洞——一条贯穿空间和时间的隧道。也就是说，只要能够建造一个稳定的虫洞，就可以跨越时间和空间。那么，到底什么是虫洞？它和黑洞有什么联系呢？ 　　在斯蒂芬·霍金的《时间简史》里有这样的解释：虫洞是连接宇宙遥远区域间的时空细管，它可以把平行的宇宙或者婴儿宇宙连接起来，并提供时间旅行的可能性。而黑洞是时空的一个区域，那里引力是如此之强，以至于任何东西，甚至光都不能从该处逃逸出来。 　　在戴维斯的计划里，建造一个虫洞要分3步： 　　第一步，寻找或建立一个虫洞，开辟一个隧道用来连接太空中两个不同的区域。 　　第二步，使虫洞稳定下来。由量子产生的负能量，虫洞便允许信号和物体安全地穿越它。负能量会抵制虫洞变为密度无穷大或接近无穷大。换句话说，它阻止了虫洞演变成黑洞。 　　第三步是牵引虫洞。一艘具有高度先进技术的太空船将虫洞的入口互相分离开。如果两个端口都放置在空间中合适的地方，那么时间差将保持恒定状态。假设这一差值是10年，一名宇航员从一个方向穿越虫洞，他将跳到10年后的未来，反之，宇航员若是从另一方向穿越虫洞，他将跳到10年前的过去。 　　时间机器的悖论 　　假如技术上的诸多难题都被克服了，时间机器的生产将会打开充满悖论的潘多拉盒子。对于这些，戴维斯也表现出一丝忧虑。他说：“我本人不打算把我描述的时间旅行和其它控制自然的行为区分开来。所有的技术都在以某种方式干预自然。在一些科幻小说里，有人通过回到过去而改变了现状。这种事出现在小说里当然无伤大雅，但如果发生在现实中会带来严重的伦理问题。谁给你回到过去改变历史的权利？” 　　既然过去、现在和未来紧密联系，过去能影响现在，那么现在影响过去在逻辑上也说得通。如果不能想干什么就干什么，人好不容易有了在时间中穿梭的自由，却又失去了行动的自由，眼睁睁看着历史从身边滑过，却无力改变什么，岂不是一个巨大的损失？ 　　对此，戴维斯给出的解决方案是“多宇宙”理论——世界不是只有一个，而是有许多平行的世界。你回到过去，但那不是你自己的世界，而是和你的历史相似的世界。这样，即便你打死了自己的母亲，她在那个世界也的确死了，但当你回到未来时，她依然活得好好的。 　　这种想法近乎疯狂，但除了戴维斯外还有许多著名物理学家相信有平行世界的存在。在《时间简史》里，霍金这样说：解决时间旅行的其他可能的方法是选择历史假想。其思想是，当时间旅行者回到过去，他就进入和历史记载不同的另外的一个历史中去。这样，他们可以自由地行动，不受和原先的历史相一致的约束。如果这样，回到过去和做一场梦又有什么区别？ ---------------------------------------------------------------- 在梦中, 很多时候就是在平行宇宙里. 晚上睡着后，人的身体是休息了但人的7魂6魄会到处去游荡（不分时间，空间也可能去平行世界另一宇宙附体到另一个你身上），但他们都会在醒来时回到你的肉体里，所以他们在游荡时（也就是你做梦时）会看到以前发生或未来要发生的事情，不过都是很零散的，这也是为什么做梦总是梦见一会在这里一会在那里。里面出现的人物也是混乱的。。。。 当你醒来时，好象电脑内存上的信息并没有储存的电脑硬盘上！通常你醒后就不会记得了.在平时也会发现有些场景很熟悉，似乎自己已经经历过，估计是在其他时空的自己跟自己这个时空的自己在产生感应，人是在利用睡觉的时候犹豫人的大脑休息，会产生一种独特的脑电波，这种脑电波也许就是俗称的灵魂在与以往时空或未来时空的多个自己进行连接，跟多台电脑联局域网一样，爱因斯坦称为“遥远地点间幽灵般的相互作用”，所以相同电波平率会连接上，而每个人在那种状态下的脑电波是是不同的，所以就不会串到别人的脑电波上，如果串到别人的脑电波上的就会产生多重人格，只是由于大脑在接受这种电波信息的同时进行筛选过程，结果就是大部分的未来与过去的信息被排除在外，而且由于这种信号过于微弱导致给与大脑刺激太小，因此大脑很难在事后清楚记得，有时候在梦境时所反映出来的未来的预测是经过自己大脑再处理过的，所以会通过某些自己比较喜欢的类型将未来重新预演。 几乎每一个人在睡觉时都会离开身体，等他们醒来的时候，他们说他们做了梦，因为我们现在的科学知识是只相信在世的这个物质身体的存在，不相信睡眠时灵魂能到不同的平行世界去旅行，其实大脑在休息,灵魂在平行宇宙中旅行.所以你有时候会 突然感觉很恍惚 眼前的一切都好象发生... 这也是“似曾相识”经历普遍存在的一个原因. 不是那么容易, 但是可能的.  时间旅行自从1895年H. G. Wells 的著名小说时间机器问世以来一直是个科幻小说的常见主题. 但真的能实现吗? 真能建造机器把人送到过去或未来吗? 几十年来, 时间旅行一直被排斥在正统科学之外. 但是最近这个话题开始成为理论物理学家自留地里种的大白菜了. 动机部分是因为休闲旅游时想想乐乐. 但这种研究也有它的严肃的一面. 理解因果关系是试图建立统一物理理论的一个关键部分. 如果毫无限制的时间旅行可能的话, 那么, 就是原则上, 也会使这样的统一理论的基础被强烈的影响. 我们对时间最透彻的理解来自爱因斯坦的相对论. 在这之前, 时间被认为是绝对和普适的, 不论其物理环境如何对每个人都是一样的. 在狭义相对论中, 爱因斯坦提出事件的时间间隔是依赖观测者如何移动的. 特别是, 两个运动不同的观测者将测量到同样两个事件有不同的时间间隔. 这个效应经常用"孪生子佯谬"来描述. 假设莎丽和山姆是孪生姐弟. 莎丽乘火箭高速飞向一个临近恒星, 转过恒星回到地球, 而山姆则待在地球上. 不妨说莎丽的旅行持续了一年, 而她回来时发现地球已经过了十年. 她的兄弟现在已经比她大了九岁. 莎丽和山姆不再同岁, 尽管同日出生. 这个例子描述了一个时间旅行的个例. 效果上看, 莎丽跳入了地球九年后的未来. 时间变慢 这个效应, 叫时间膨胀, 发生在当两观测者相对运动时. 在日常生活中我们是不会注意到这样的时间异常的, 因为它只有在运动接近光速时才变得显著. 甚至以飞机的速度, 一个典型的旅行的时间膨胀也就几个纳秒而已. 不管怎样, 原子钟有足够的精度证明时间确实被运动伸长了. 所以去未来的旅行是被证实了的, 尽管目前旅行的"时间距离"实在没什么可激动的. 要观测到明显的时维扭曲, 你得观察日常环境以外的世界. 亚原子粒子在大型加速器里可以被加速到接近光速. 一些粒子, 如μ介子, 随身就揣着个"钟", 因为它们有固定的半衰期; 根据爱因斯坦理论, 加速器内快速移动的μ介子衰变得要慢些. 一些宇宙射线也经历了强烈的时间扭曲. 这些粒子是如此的接近光速, 以至于在它们自己看来, 它们穿越银河系只要几分钟. 而在地球坐标系内, 它们则要用几万年. 如果时间膨胀不存在的话, 这些粒子永远也不会实现这样的旅行. 速度是一种去未来的方式。重力是另一个。在爱氏的广义相对论里，他预言重力会使时间慢下来。阁楼里的时钟要比地下室的时钟快，因为地下室离地心更近，在更深的引力场中。同样，在空间的时钟要比地面的快。这个效应也很小，不过用精确的钟可以直接测量。实际上，全球定位系统（GPS）就考虑了这些时间扭曲效应。如果没有考虑这些效应的话，那水手，出租车司机，巡航导弹会发现自己可能偏离航线达几公里之多。 在中子星的表面，引力是如此之强，以至于时间要比地球上慢30%。从这样的星星上往外看，外面就象快进的录像。黑洞代表着终极的时间扭曲。在黑洞的表面，时间相对与地球来说不再变化。这意味着，如果你从附近掉入黑洞，你很短的时间内就会到黑洞的表面，但这期间广袤的宇宙中的一切都已经从生到死了。因此黑洞内部对外面的世界来说是属于时间终结后的世界。如果一个宇航员能够很接近黑洞后又全身而退，（这自然是假设，想象中的事），他就能跳入遥远的未来。 到目前为止我们讨论了到未来旅行,那么咋能回到过去呢?这个问题更难整了.1948年，普林斯顿高级研究学院的Kurt .G根据爱因斯坦重力场方程得出的一个解描述了一个转动的宇宙。在这个宇宙中，一个宇航员可以穿越空间到达他自己的过去。这是由于重力影响了光。宇宙的转动将拖拽周围的光线（并且），使得一个物体在空间的闭环路运动，同时也在时间的闭环中运动，在整个过程中微观尺度上没有超光速，kurt.G的解作为一个数学奇迹被怀疑，毕竟，观测未能显示任何宇宙总体上转动的迹象。但他的结果却说明相对论并不排斥回到过去。实际上，爱因斯坦公开承认他也曾为他的理论中在某些环境中准许回到过去的想法所困惑。 除此以外，还有很多关于到过去旅行的想法，例如，1974年 土伦大学的Frank J.Tipler计算出一个巨大的无限长的圆筒绕轴心已接近光速转动能够使宇航员观察到他自己的过去，同样会使光线拖拽从而形成环绕圆筒的闭环。1991年 普林斯顿大学的J.Richard Gott预言了宇宙弦－一种宇宙学家认为在大爆炸初期产生的结构－－能够产生类似的结果。但是在80年代中期，大部分关于时间机器的现实情节，都是基于虫洞概念。 在科幻小说的情节中，虫洞有时被称作“星门”（也就是游戏中的传送门^0^），他们为空间中距离遥远的两点提供了捷径。通过假定的虫洞跳跃，只需片刻便从星系另一侧出现了。虫洞符合广义相对论，由此重力不仅弯曲时间而且弯曲空间。这个理论允许空间两点间对等的通路.数学家指出这样的空间类似于乘法连接(乘法有交换律).这就像通过山底的隧道远比通过山体表面的道路近,一个虫洞可能比通常空间中的通路短得多. 转自《科学美国人》杂志：如何建造时光机器

.