The WHY of speed of light vs. the FACT thereof

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  • #26
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In the symmetry-based axiomatizations, this is not a postulate but a theorem.

The bulk of your #24 seems to be based on a misunderstanding of quantum-mechanical entanglement. Entanglement does not propagate signals at >c.
My understanding is that Bell's theorem, which is considered to eliminate all deterministic causes for quantum events, assumes that no signal can travel faster than c. I further understand that at least one theory has been proposed which explains quantum events as effects caused by signals which propagate faster than c. I can look up the references upon which this understanding is based, if you feel that I have misunderstood what I have read.

My point is that if one starts with the notion that an event is the effect of a cause, then our observations of quantum events can be taken as evidence that the effect of at least one cause can propagate faster than c.

On the other hand, if one starts with the notion that no effect can propagate faster than c, then other observations can be taken as evidence that at least some events have no cause.

Is there any definitive empirical evidence which eliminates one of these alternatives?


We've been discussing SR. GR is locally equivalent to SR. GR does not allow for objects to have speeds greater than c on a local basis, i.e., relative to other nearby objects. When you get to the case of distant objects, GR does not even provide an unambiguous definition of the notion of relative velocity. E.g., you can say that distant galaxies are receding from us at >c, but someone who picks a different coordinate system can get a different answer.
Wald, in his discussion of the Hubble constant, notes that the relative velocity of two galaxies can exceed c if the distance between them is large enough. "This does not contradict the fundamental tenet of SR and GR that "nothing can travel faster than the speed of light", since this tenet refers to the locally measured relative velocity of two objects at the same spacetime event, not a globally defined velocity between distant objects." Wald is not suggesting that quantum events can be explained by a signal which travels faster than the speed of light. But he is saying that on the global scale--invisible to our measurements--objects can indeed have a relative velocity greater than c. I am suggesting that if there are global conditions which we cannot measure, there may also be local conditions which we cannot measure--effects which propagate faster than c.

In short, GR allows for a reality which we cannot measure. That being the case, we ought not to be dogmatic about the existence (or non-existence) of causes for quantum events--knowing as we do that we are unable to measure these events dynamically. (That is, no one has measured the actual collapse of a photon, for example.)
 
  • #27
Dale
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I think the Duff paper constitutes the math that is required in order to back up the conjecture that you stated.
Thanks, I'll check it out.
 
  • #28
Dale
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It seems to me that here, too, we run up against the distinction between reality and our measurement of reality.
Have you a good definition of "reality" yet? If not, then this statement is literally nonsense.

What I find surprising in this discussion is that no mention is made (in the context of this discussion, in the books which I have read) that GR allows for speeds of real objects which are faster than c.
Not in a coordinate-independent sense. You can certainly make coordinate systems, even in SR, where the coordinate speed of some object is greater than c. But a timelike worldline will be timelike in all coordinate systems and therefore will always be slower than c in a coordinate-independent sense.
 
  • #29
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My understanding is that Bell's theorem, which is considered to eliminate all deterministic causes for quantum events, assumes that no signal can travel faster than c. I further understand that at least one theory has been proposed which explains quantum events as effects caused by signals which propagate faster than c. I can look up the references upon which this understanding is based, if you feel that I have misunderstood what I have read.
Yes, one explanation for the observed violation of Bell's inequalities is that entangled photons can communicate with each other at FTL speeds, the so called "non local" condition. There are other competing theories such as the Many Worlds Interpretation which does not require non local FTL communication. What is certain is that if Bob and Alice have a source of entangled photons between them, then however they manipulate their detectors, they are not able to transmit information to each other at superluminal velocities. With this in mind it might be better to restate "c is the maximum speed of propagation of cause and effect" as "c is the maximum speed that matter or information can be transmitted between inertial observers". So while it does seem, in one interpretation of quantum entanglement experiments, that quantum particles can communicate/interact with each other at superluminal velocities, this does not translate into superluminal communications between macroscopic sentient observers. There are many other scenarios where there are apparent superluminal relations (e.g. a wave hitting a shore obliquely or sweeping a laser pointer across a distant surface) but none of these can be used to transmit information or matter at superluminal speeds.
 
  • #30
bcrowell
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My understanding is that Bell's theorem, which is considered to eliminate all deterministic causes for quantum events, assumes that no signal can travel faster than c.
As in #24, I think you're confusing an assumption of a theorem with the result of a theorem. This may also be relevant: http://en.wikipedia.org/wiki/No-communication_theorem

I further understand that at least one theory has been proposed which explains quantum events as effects caused by signals which propagate faster than c.
This would seem to violate the no-communication theorem.

I can look up the references upon which this understanding is based, if you feel that I have misunderstood what I have read.
Sure, please do.

My point is that if one starts with the notion that an event is the effect of a cause, then our observations of quantum events can be taken as evidence that the effect of at least one cause can propagate faster than c.
No, this is incorrect.

On the other hand, if one starts with the notion that no effect can propagate faster than c, then other observations can be taken as evidence that at least some events have no cause.
The notion of causality that we're discussing here really doesn't require any notion that A causes B. The laws of physics, e.g., Newton's laws or the Einstein field equations, are just differential equations. Differential equations don't make statements like "A causes B." The notion of causality that is relevant here only really requires that if A comes earlier in time than B according to one observer, then this is also true for all other observers.

Is there any definitive empirical evidence which eliminates one of these alternatives?
There is a sticky at the top of this forum, titled "FAQ: Experimental Basis of Special Relativity." Every experiment in that sticky constitutes empirical evidence that causality is satisfied, as predicted by SR.

Wald, in his discussion of the Hubble constant, notes that the relative velocity of two galaxies can exceed c if the distance between them is large enough. "This does not contradict the fundamental tenet of SR and GR that "nothing can travel faster than the speed of light", since this tenet refers to the locally measured relative velocity of two objects at the same spacetime event, not a globally defined velocity between distant objects." Wald is not suggesting that quantum events can be explained by a signal which travels faster than the speed of light.
Wald is not discussing quantum mechanics at all.

But he is saying that on the global scale--invisible to our measurements--objects can indeed have a relative velocity greater than c.
Your interpretation of Wald assumes that such a relative velocity is uniquely defined, which it isn't.

I am suggesting that if there are global conditions which we cannot measure, there may also be local conditions which we cannot measure--effects which propagate faster than c.
But there is no logical connection between the given information you start with and the conclusion you claim, and there is a century's worth of experimental evidence against your conclusion.

In short, GR allows for a reality which we cannot measure.
As DaleSpam has pointed out, you haven't provided a definition of "reality."
 
  • #31
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I believe that a local rescaling of c would also leave the results of experiments unchanged provided the dimensionless constants were unchanged, but I have to admit that I have not worked the math on that. Have you done so?

I'm not sure about any of this anymore. You might also like to look at reference [2] of the Duff paper.
 
  • #32
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Is it possible that the invariant speed of light could be a result of the limit at which a spatial expansion can occur? In other words, could it be possible that as an object moves through space it creates a wave front due to the displacement and expansion of space created by the object? So could it be that if the fastest space can expand (or be displaced by mass) as an object moves through it is C then the limit at which the object can move through it would also be C? Am I making any sense here?
 
  • #33
bcrowell
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Is it possible that the invariant speed of light could be a result of the limit at which a spatial expansion can occur?
No, because the invariant speed of light occurs in SR, where there is no such thing as spatial expansion.
 
  • #34
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As in #24, I think you're confusing an assumption of a theorem with the result of a theorem. This may also be relevant: http://en.wikipedia.org/wiki/No-communication_theorem
I read the linked article and several other related articles.


The notion of causality that we're discussing here really doesn't require any notion that A causes B. The laws of physics, e.g., Newton's laws or the Einstein field equations, are just differential equations. Differential equations don't make statements like "A causes B." The notion of causality that is relevant here only really requires that if A comes earlier in time than B according to one observer, then this is also true for all other observers.
The causality I have in mind is the notion that the act of measuring one particle determines the state of that particle and the state of a distant particle. The measurement is thus the cause; the effect is the combination of states [spin up and spin down, e.g.] taken on by the two particles.

There is a sticky at the top of this forum, titled "FAQ: Experimental Basis of Special Relativity." Every experiment in that sticky constitutes empirical evidence that causality is satisfied, as predicted by SR.
I read through the full page. I don't think there were any experiments in the list which dealt with entangled particles. If one accepts the premise that the measurement of a particle is a cause and the state of a distant (non-local) particle is an effect of that cause, then it seems true on the face of it that the effect of the cause has propagated at a speed faster than c.

[edit]
That said, I read this statement in an article about the De Broglie-Bohm theory:

The de Broglie–Bohm theory describes the physics in the Bell test experiments as follows: to understand the evolution of the particles, we need to set up a wave equation for both particles; the orientation of the apparatus affects the wavefunction. The particles in the experiment follow the guidance of the wavefunction. It is the wavefunction that carries the faster-than-light effect of changing the orientation of the apparatus. An analysis of exactly what kind of nonlocality is present and how it is compatible with relativity can be found in Maudlin.[18] Note that in Bell's work, and in more detail in Maudlin's work, it is shown that the nonlocality does not allow for signaling at speeds faster than light.

I don't doubt the truth of the two fragments in bold--but neither can I make sense of them with my current knowledge of quantum mechanics [which is even more limited than my knowledge of relativity--draw your own conclusions :)]


Your interpretation of Wald assumes that such a relative velocity is uniquely defined, which it isn't.
I'm not sure what you are getting at here. If you mean that I cannot identify two Lorentz inertial frames which have a relative velocity greater than c, I agree. But as I understand Wald, that failure is a consequence of the fact that GR is a manifold onto a Lorentz space. (I'm sure I'm not speaking the technical language with precise correctness, but I believe I have sufficient gist of the math for the purposes of this discussion.) That is, inertial frames are inherently local ['nearby'] in GR, while the two objects which have a relative velocity greater than c are necessarily not local ['nearby']. Born asserts that "if gravitational fields are present the velocity either of material bodies or of light can assume any numerical value". ("Einstein's Theory of Relativity", VII-11)

But there is no logical connection between the given information you start with and the conclusion you claim
I believe the logical connection is clarified in the above.

and there is a century's worth of experimental evidence against your conclusion.
As noted, the listed experiments do not deal with entangled particles.

As DaleSpam has pointed out, you haven't provided a definition of "reality."
I don't believe that a comprehensive definition of reality is a precondition for this discussion. Einstein, Born, Taylor-Wheeler, Schumm ("Deep Down Things"), Ford ("The Quantum World") all address the issue of reality as distinct from measurement; none of them presents a comprehensive definition of reality (in the books I have read).

I would argue that the assertion of fundamental probability in quantum theory is itself a statement about the nature of reality, as distinct from the measurements we make in the lab. Yet so far as I know neither the standard model nor the Copenhagen model presents a formal definition of reality.

Here's Treiman on the subject of quantum mechanics and classical reality (in "The Odd Quantum"): "Within quantum mechanics itself, there seems to be an unbridgeable divide between the future and the present instant. The future is intrinsically statistical, with probabilities governed by the equations of quantum mechanics. The trouble is that this way of looking at the situation seems something of a cop-out. In effect, it abandons the idea of explaining how facts come about, taking as the main function of science merely to correlate them. When a fact in fact happens, the quantum mechanical wave function is simply declared to have collapsed; after all, it's only a correlational tool! And that's that."

Treiman is no doubter of quantum mechanics--for that matter neither am I insofar as the measurements are concerned--yet he expresses dissatisfaction with its approach to reality. He makes these comments without attempting a formal definition of reality.

Which brings us back to the question in the original post: The WHY vs. the FACT of c.

I agree with Treiman that science is intimately concerned with the WHY, because scientists, as human beings, are motivated to understand what they observe in nature, not merely to organize the facts.
 
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  • #35
Dale
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I don't believe that a comprehensive definition of reality is a precondition for this discussion.
Having a definition of a word certainly is a precondition if you want to make meaningful statements using the word. If I were to try to discuss my opinion about "the distinction between farglmoger and our measurement of farglmoger" wouldn't you consider it necessary for me to define "farglmoger"?

If you can't define "real" then stop using the word. Otherwise you are literally writing nonsense.
 
  • #36
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The causality I have in mind is the notion that the act of measuring one particle determines the state of that particle and the state of a distant particle. The measurement is thus the cause; the effect is the combination of states [spin up and spin down, e.g.] taken on by the two particles.
OK. Your use of the word "causality" is then different from mine, and probably different from that of every physicist in the world. In particular, your idea of causality implies that the order of cause and effect depends on the frame of the observer. This is an undesirable feature for a definition of causality, which is why nobody else uses the word to include examples like this one.

I read through the full page.
Reading through a page of links is not the same as understanding the physics in the links.

bcrowell said:
There is a sticky at the top of this forum, titled "FAQ: Experimental Basis of Special Relativity." Every experiment in that sticky constitutes empirical evidence that causality is satisfied, as predicted by SR.
I don't think there were any experiments in the list which dealt with entangled particles.
But this would only be relevant if others accepted your nonstandard definition of causality.

If one accepts the premise that the measurement of a particle is a cause and the state of a distant (non-local) particle is an effect of that cause, then it seems true on the face of it that the effect of the cause has propagated at a speed faster than c.
But nobody else accepts this premise as falling within the definition of "causality," which has a technical definition in physics. For example, I know you're currently studying Spacetime Physics, by Taylor and Wheeler. Have you gotten to section 6.3 yet? It has a clear discussion of causality, and it makes it clear that causality is frame-independent ("Cause and effect preserved by light cone"), which would be inconsistent with your notion of causality.
 
  • #37
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If one accepts the premise that the measurement of a particle is a cause and the state of a distant (non-local) particle is an effect of that cause, then it seems true on the face of it that the effect of the cause has propagated at a speed faster than c.

In the case of entangled particles, it is hard to distinguish cause from effect as opposed to the causal interactions of macroscopic particles. For example if someone loads a bullet into a gun and fires it at some else so that the bullet ends up in victims's head, then we can reasonably say the that firing the gun is the cause and the victims head exploding and subsequent death is the effect. We never observe the opposite, i.e. dead body coming back to life and ejecting a bullet from its brain that ends up in the breach of a gun. The firing of the gun is indisputably the cause and the demise of the victim is indisputably the effect. Now things are not so clear in quantum mechanics. Alice observing up spin in an entangled particle might be described as the cause of Bob observing down spin in the other entangled particle. However it is equally likely that Bob observing down spin in his particle is the cause of Alice observing up spin so it not at all clear which is the cause and which is the effect as both observations have equal statistical likelihood. Even in a single instance of an entanglement experiment, different observers with relative velocity will disagree on which observer observation was the cause and which observers observation was the effect. There is no violation of entropy change or reversal of the "arrow of time." In the case of quantum observations, cause and effect are complete mirror images of each other and it does not matter which order they happen in, so it hard to claim that entanglement experiments contradict the normal light speed relationship between cause and effect. Also as I said before, EPR type experiments with entangled photons can not be used to pass any information from one observer to another at greater than light speed.
 
  • #38
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Having a definition of a word certainly is a precondition if you want to make meaningful statements using the word. If I were to try to discuss my opinion about "the distinction between farglmoger and our measurement of farglmoger" wouldn't you consider it necessary for me to define "farglmoger"?

If you can't define "real" then stop using the word. Otherwise you are literally writing nonsense.
I'm afraid we will have to disagree on this point. I readily understood all of the authors who were brash enough to discuss the subject of reality without first defining 'real'. The reason I was able to understand their meaning (to within a reasonably small margin of error) is that they were discussing reality, in which we all live (to one degree or another), and not 'farglmoger', with which no one has any experience.
 
  • #39
Dale
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I readily understood all of the authors who were brash enough to discuss the subject of reality without first defining 'real'. The reason I was able to understand their meaning (to within a reasonably small margin of error) is that they were discussing reality, in which we all live
If understanding the word is so easy for you then why is defining it so difficult? I believe it is because you do not understand it as well as you think you do and are simply deluding yourself. How can you even talk about a "reasonably small margin of error" for something that you cannot even define enough to measure?

Your posts are nonsense because you cannot define a key term.

and not 'farglmoger', with which no one has any experience.
And who do you think has experience with "reality" as distinct from our measurements of "reality"?
 
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  • #40
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OK. Your use of the word "causality" is then different from mine, and probably different from that of every physicist in the world. In particular, your idea of causality implies that the order of cause and effect depends on the frame of the observer. This is an undesirable feature for a definition of causality, which is why nobody else uses the word to include examples like this one.
I don't know enough to disagree with you, but I will observe that, as the experiments have been described to me, the two "stations" at which the entangled particles are measured are in the same frame. If so, then the cause and the effect are in the same frame.

While I understand from personal experience the need to properly understand and use technical terms, it is also true that most technical terms may be used to adequately communicate, without rigorous adherence to their formal definitions. In the books which I have read so far, it has been necessary for the authors to speak with less precision than they might otherwise, because those books were written for a general audience, not as college textbooks.

Of course, Einstein saw fit to use the less-than-precise, "spooky action at a distance", and still communicated very well.

I believe that I have correctly expressed the concept of "cause and effect", or "determinism", as it has been used by these authors in their explanations of the concept of fundamental probability.

This is not to say that I can continue being imprecise in my use of technical terms. As you know, I am working toward a technical grounding in relativity. If I live through that, I'll see what I can do in quantum mechanics.

It happens that my interest in these subjects, apart from the fact that I'm a nerd, is the relationship between scientific theories and reality. This explains why I keep coming back to the subject--and it is the driver for learning to handle the subjects with some degree of technical understanding.


But nobody else accepts this premise as falling within the definition of "causality," which has a technical definition in physics. For example, I know you're currently studying Spacetime Physics, by Taylor and Wheeler. Have you gotten to section 6.3 yet? It has a clear discussion of causality, and it makes it clear that causality is frame-independent ("Cause and effect preserved by light cone"), which would be inconsistent with your notion of causality.
No, I'm not to 6.3 yet. Again, without presuming to disagree with your assertion, I note that the association of cause and effect with the light cone assumes (as I understand it) that no information can be transmitted faster than the speed of light. But that is precisely the conundrum which quantum entanglement presents to us: the [seeming] transmission of information at speeds faster than light.

I am well aware, again from experience in other endeavors, that one must learn how to do things in the established way before one can seriously consider alternate approaches. (Which is why I was wrong to use the word 'contradict' in those three posts. But I was so sure.....)
 
  • #41
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If understanding the word is so easy for you then why is defining it so difficult? I believe it is because you do not understand it as well as you think you do and are simply deluding yourself. How can you even talk about a "reasonably small margin of error" for something that you cannot even define enough to measure?

Your posts are nonsense because you cannot define a key term.
I guess I'll just fall in line behind those other spouters of nonsense--all of whom are much smarter than I, and have much better credentials for writing the nonsense that they do.

And who do you think has experience with "reality" as distinct from our measurements of "reality"?
Everyone. There are many things which I know are real, but have never measured because they are not subject measurement. Indeed, they are similar to the photon in that the attempt to measure them destroys them. Some of these things I would not consider to be purely "physical", but the materialist would. So on the materialist's terms, I am certain of the reality of physical things which cannot be measured.

More along the lines of our discussion, we don't know what we don't know. So long as our knowledge of nature is incomplete, there is (in my opinion) an undeniable distinction between reality and our measurement of reality. As I mentioned in another post, that distinction takes on practical importance when we act at the limits of our knowledge. In those circumstances, reality has a way of biting us in the butt.
 
  • #42
JesseM
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I don't know enough to disagree with you, but I will observe that, as the experiments have been described to me, the two "stations" at which the entangled particles are measured are in the same frame. If so, then the cause and the effect are in the same frame.
Events aren't "in" any frame! A frame is just a coordinate system, a way of labeling events with position and time coordinates, nothing more. Any set of events can be described in any frame you like, and for a given set of initial conditions all frames make the same predictions about local events at later times, that's a basic principle of relativity.
GregAshmore said:
I believe that I have correctly expressed the concept of "cause and effect", or "determinism", as it has been used by these authors in their explanations of the concept of fundamental probability.
What specific quotes by these authors are you thinking of? Surely you realize that you may have misunderstood, so if you give a quote then others who are more familiar with the technical details of the theory may be able to give you an alternate interpretation more in line with what the theory actually says.
GregAshmore said:
No, I'm not to 6.3 yet. Again, without presuming to disagree with your assertion, I note that the association of cause and effect with the light cone assumes (as I understand it) that no information can be transmitted faster than the speed of light. But that is precisely the conundrum which quantum entanglement presents to us: the [seeming] transmission of information at speeds faster than light.
It's been proven that there's no way to exploit quantum entanglement to transmit messages FTL. If you believe in "hidden variables" (like the idea that particles have well-defined positions at all times, even when not measured), then quantum entanglement may suggest "hidden" FTL influences between these hidden variables, but there are other interpretations of QM which don't involve such hidden variables. Incidentally, if you want a simple explanation of why quantum statistics rule out any "local hidden variables" explanation, see my post #8 on this thread.
 
  • #43
Dale
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I guess I'll just fall in line behind those other spouters of nonsense--all of whom are much smarter than I, and have much better credentials for writing the nonsense that they do.
Appeal to authority is a fallacy, regardless of how smart and well credentialed the authorities. It is also an attempt to avoid an issue where you know you have taken an untenable position.

Everyone. There are many things which I know are real, but have never measured because they are not subject measurement.
I am going to call BS on this. There is nothing which anyone knows is real which has never been measured, certainly not if you are speaking scientifically. If you are not speaking scientifically then I would remind you about the forum rules that you agreed to. This is not a forum for philosophically or religiously motivated speculation.

More along the lines of our discussion, we don't know what we don't know.
First you assert some mysterious knowledge of reality independent of observation and then you talk about the limits of knowledge. That is a pretty rapid turn around, and I am much more supportive of this second position than your previous one.

there is (in my opinion) an undeniable distinction between reality and our measurement of reality.
If it is so undeniable then how come you can't even tell me clearly what you mean?
 
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  • #44
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Appeal to authority is a fallacy, regardless of how smart and well credentialed the authorities. It is also an attempt to avoid an issue where you know you have taken an untenable position.

I am going to call BS on this. There is nothing which anyone knows is real which has never been measured, certainly not if you are speaking scientifically. If you are not speaking scientifically then I would remind you about the forum rules that you agreed to. This is not a forum for philosophically or religiously motivated speculation.
Please define "science" so that I will know when I have crossed the boundary. (You will observe, I hope, that I have been careful not to actually cross the boundary as I understand it, in spite of the fact that you have not rigorously defined science.)

If any hint of philosophical speculation is out of order on this forum, then this thread is out of order. Simply define any discussion of reality as philosophical, and we can be done.

The problem with that approach is that an understanding of reality was a primary motivator for Einstein as he developed the theory. Not coincidentally, the theory challenges our intuitive conception of reality--this is no doubt why the theory is of interest to so many non-physicists. So I don't know how one would go about separating a discussion of reality from the discussion of relativity.

I have taken note of your earlier dismissal of the concept of reality--"whatever that is"--and thus have refrained from wasting my time trying to define it. I expected that no definition of reality would satisfy you.

Now I see that you do have at least a negative conception of reality--a thing cannot be known to be real if it has not been measured. On that much we can agree, at least in the lab.

First you assert some mysterious knowledge of reality independent of observation and then you talk about the limits of knowledge. That is a pretty rapid turn around, and I am much more supportive of this second position than your previous one.

If it is so undeniable then how come you can't even tell me clearly what you mean?
I contend that the universe exists apart from, and distinct from, our measurement of it. Thus, so long as our measurements give us an incomplete or distorted view of the universe, there exists some portion of reality which is unknown to us. In that context, the question as to whether the length contraction of SR is real or merely apparent ought to be clear enough.

You have chosen to dismiss such questions on the basis that no experiment can be devised which will answer them. This is a logical fallacy. Our inability to answer a question does not invalidate the question. Furthermore, your response assumes that our technology will never improve to the point where we can devise experiments which will answer such questions. It is precisely that over-confidence in our current understanding which motivates my questions.

In my view, our inability to answer questions as to how closely our conception of reality conforms to reality is a sign of weakness in our theories. I do not mean any disrespect for what has been accomplished; I do suggest that we should recognize that the residual ignorance leaves open the possibility that we are wrong to some degree.

Take the photon for example. What happens in the interval between the creation of a photon and its destruction? We have developed an equation which expresses the probability of the various modes of its destruction, but that equation is not the real photon. What is the reality of the photon's existence as it travels through spacetime? Or does it pass through spacetime in a sort of zombie state, becoming truly real only as it is destroyed? I can't answer these questions--but they are valid questions nonetheless.

What is to say that our ignorance of the nature of the photon's "being" will not manifest itself in some practical way as our technology advances? Is it wise to make sweeping claims about the nature of the universe on the basis that we cannot measure it? I don't think so.

How many particles can seethe on the head of a pin?
 
  • #45
Dale
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Please define "science" so that I will know when I have crossed the boundary. (You will observe, I hope, that I have been careful not to actually cross the boundary as I understand it, in spite of the fact that you have not rigorously defined science.)
Unlike you, when I use an important term I am always willing to provide a definition. Science is the body of knowledge obtained by using the scientific method. I can also explain the scientific method if you need, but the key elements are theories and experiments. If an idea is, in principle, not experimentally testable then it cannot be scientific and therefore does not belong on this forum.

The problem with that approach is that an understanding of reality was a primary motivator for Einstein as he developed the theory. Not coincidentally, the theory challenges our intuitive conception of reality--this is no doubt why the theory is of interest to so many non-physicists. So I don't know how one would go about separating a discussion of reality from the discussion of relativity.
Easy. Don't use terms you can't define.

Our inability to answer a question does not invalidate the question.
Maybe not, but your inability to even formulate the question certainly does invalidate it.
 
  • #46
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Unlike you, when I use an important term I am always willing to provide a definition. Science is the body of knowledge obtained by using the scientific method. I can also explain the scientific method if you need, but the key elements are theories and experiments. If an idea is, in principle, not experimentally testable then it cannot be scientific and therefore does not belong on this forum.

Easy. Don't use terms you can't define.

Maybe not, but your inability to even formulate the question certainly does invalidate it.
I've taken a couple of days to think about this. Two thoughts:

1. The fact that I have not provided a clear positive definition of reality--"Reality is....."--is a consequence of my conception of reality. I start with the notion that our knowledge is limited, that there is more to the physical universe than we have been able to measure. Given that assumption, a comprehensive definition of physical reality is probably out of reach, because one cannot define positively what one does not know. The best I can do (at this point) is a tautology: Reality is what is. I'm not happy with that; I doubt that you are.

On the other hand, it is undeniable (in my view) that our knowledge is limited. That being the case, there is some portion of reality which is beyond us, at least for the moment.

2. The scientific community has a tendency to blur--or at times ignore--the line between aspects of a theory which are backed up by direct measurement and those which are inferred from those measurements. In other words, scientists tend to treat their theories as completely "real", forgetting that our knowledge of reality is limited.

Example: Bruce Schumm, in "Deep Down Things", says, "The charge of the electron, theorists tell us, is infinite. But they also tell us that no experiment you ever mount will measure that infinity." Yet several pages later he concludes the discussion with, "It must be right." To which I must respond, "Really?"
 
  • #47
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I didn't read the whole topic and I'm not a theoretical physicist, but I think It's only an assumption. if I'm not making a mistake, Einstein based his special theory of relativity on two assumptions, the first one is the equivalence principle for inertial frames and the second one was the speed of light is invariant for all observers in inertial frames. the second assumption was added to escape from the idea of an existing ether. in fact equations of Einstein's special relativity had been developed by Lorentz many years before but because Lorentz believed in ether he gave wrong interpretations of the equations, Einstein said there is no need to insist on the existence of ether when it's not easy or maybe impossible to prove its existence therefore he added the 2nd assumption as an experimental FACT to develop his theory. I think asking why "the speed of light is constant for all inertial frames" is like asking why the equivalence principle is true which is not a question that you can ask from scientists. maybe philosophers would like to talk about such questions, but scientists don't.
 
  • #48
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The value for the speed of light in vacuum is a historical artefact.
It is the consequence of an unfortunate choice of units. In better chosen systems of units accounting for the equivalence of space and time, c is without dimension having the value 1.

http://en.wikipedia.org/wiki/Natural_units
 
  • #49
ghwellsjr
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I didn't read the whole topic and I'm not a theoretical physicist, but I think It's only an assumption. if I'm not making a mistake, Einstein based his special theory of relativity on two assumptions, the first one is the equivalence principle for inertial frames and the second one was the speed of light is invariant for all observers in inertial frames. the second assumption was added to escape from the idea of an existing ether. in fact equations of Einstein's special relativity had been developed by Lorentz many years before but because Lorentz believed in ether he gave wrong interpretations of the equations, Einstein said there is no need to insist on the existence of ether when it's not easy or maybe impossible to prove its existence therefore he added the 2nd assumption as an experimental FACT to develop his theory. I think asking why "the speed of light is constant for all inertial frames" is like asking why the equivalence principle is true which is not a question that you can ask from scientists. maybe philosophers would like to talk about such questions, but scientists don't.
Lorentz did not give wrong interpretations of the equations but you are right that Einstein said there is no need to insist on the existence of ether but you are not right when you say that his 2nd assumption is an experimental FACT. It is not and never can be. It is an assumption which doesn't matter if it is true or not. What is an experimental fact is the round trip speed of light is a constant. The 2nd postulate is that both halves of that round trip are the same which can not be measured or demonstrated to be either true or false.

It is no more possible to disprove the existence of the ether than it is to prove the existence of the ether. In fact, what Einstein's second postulate says is that you can assume that ANY inertial frame IS the one and only absolute ether frame.
 
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  • #50
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Lorentz did not give wrong interpretations of the equations but you are right that Einstein said there is no need to insist on the existence of ether but you are not right when you say that is 2nd assumption is an experimental FACT. It is not and never can be. It is an assumption which doesn't matter if it is true or not. What is an experimental fact is the round trip speed of light is a constant. The 2nd postulate is that both halves of that round trip are the same which can not be measured or demonstrated to be either true or false.

It is no more possible to disprove the existence of the ether than it is to prove the existence of the ether. In fact, what Einstein's second postulate says is that you can assume that ANY inertial frame IS the one and only absolute ether frame.

The quote below is from Craig and Smith, Einstein, Relativity and Absolte Simultaneity, page 14. An intereting read if you can overlook the religeous agenda. These are purported to be Lorentz's own words regarding SR and the undecideable choice between the Lorentz and Einstein views of the ether.

""One thus comes to the same results as when one in agreement with Einstein and Minkowski denies the existence of the aether and the true time and treats all coordinate systems as equivalent. Which of the two modes of thought one may agree with is best left to the individual.""

(Lorentz 1934)

Matheinste.
 

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