Models for making sense of relativity - physical space vs physical spacetime

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-- This is a spin-off of the thread "is space-time a physical entity [..]" --

A lot of people think that special relativity doesn't make sense and that it's hopeless to try to understand it; we are condemned to "shut up and calculate". However, I know of two physical models that can be used to explain the theoretical predictions, and possibly there is another model that I don't know of.

: To be perfectly clear, with "physical models" I here mean two competing hypothetical physical entities that have been proposed to make sense of the phenomena as described by relativity theory.

Right from the start it was perfectly possible to make sense of relativity by means of Lorentz's ether model. That model is quite different from the preceding material ether theories; one may just as well call it physical space. Einstein hoped to get rid of it for philosophical reasons, but others such as Langevin still made use of it[1] and in 1920 Einstein even expanded on the Lorentz ether for general relativity[2].

But in the meantime Minkowski had introduced a new interpretation of the Lorentz transformations, according to which "only a certain union of space and time shall retain substantiality". He even replaced the first postulate with a stronger one, the "Postulate of the absolute world"[3]. In the beginning his radical interpretation was mostly ignored (as in [1]), but in later years this physical spacetime interpretation became popular, and it is commonly called "block universe".

It will be useful to discuss these models and their implications here, and in particular the explanatory features of each model, as each answers questions such as "what is really happening" slightly differently.

Other things worth mentioning, are presentism vs eternalism and true symmetry vs. phenomenological symmetry.

I may start off with discussing the different explanations of mutual time dilation; but if there is a more urgent aspect of interest, we can go with the flow. I can imagine that this topic will generate several child topics.

Note: Once more, this is a spin-off of several current threads in the relativity forum as well as in this forum, such as http://www.scienceforums.net/topic/97105-is-space-time-a-physical-entity-or-a-mathematical-model/

[edit:] please don't comment on the general question "is space-time a physical entity [..]" in this thread but in that "mother"thread! This thread is for discussing two competing physical entities that can be used for explanations.

Also, there were short discussions of more limited scope in 2010 and 2012:

http://www.scienceforums.net/topic/44388-the-block-universe-concept-pros-and-cons/

http://www.scienceforums.net/topic/69780-the-block-universe/

Edited by Tim88

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A lot of people think that special relativity doesn't make sense and that it's hopeless to try to understand it...

And there are lots of people who do have a reasonable understanding of special relativity.

However, I know of two physical models that can be used to explain the theoretical predictions, and possibly there is another model that I don't know of.

What do you think a physical model is?

The standard formulation of special relativity - in terms of Minkowski space-time and its isometries - is a physical theory. That is we have a mathematical framework in which to make predictions that can be tested against nature.

The aether is now not thought to be needed - either using such a notion leads to predictions that do not agree with nature, or the aether is unobservable and so not needed.

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Bold face emphasis mine:

And there are lots of people who do have a reasonable understanding of special relativity.

What do you think a physical model is?

The standard formulation of special relativity - in terms of Minkowski space-time and its isometries - is a physical theory. That is we have a mathematical framework in which to make predictions that can be tested against nature.

The aether is now not thought to be needed - either using such a notion leads to predictions that do not agree with nature, or the aether is unobservable and so not needed.

Perhaps I should have summarized the thread from which this is a spin-off, in order to prevent such misunderstandings... I use here "physical" in contrast to your "mathematical". [edit:] I now added a short clarification to the first post - thanks!

If you deem that the phenomena can be reasonably explained without a physical space or spacetime, then your comments are welcome in the thread from which this is a continuation in part.

In contrast, this thread is meant for those who do believe that there must be more to spacetime than a mathematical framework, as well as for those who are looking for more than mathematical understanding.

Edited by Tim88
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So you really mean a kind of mechanical aether theory? (Which of course should still be a mathematical model)

... do believe that there must be more to spacetime than a mathematical framework, as well as for those who are looking for more than mathematical understanding.

This is deep philosophical question that I fear cannot have a decent answer. If the mathematical model works well, then it works well. I don't see that one can say much more.

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A lot of people think that special relativity doesn't make sense and that it's hopeless to try to understand it;

Perhaps this world SR example would help?

Imagine you have unstable particle, unstable isotope.

It has rest-mass m0 (we're in the same frame of reference as that particle).

But unstable isotope is decaying.

To daughter isotope,

and to

secondary particles, such as alpha (Helium-4 nucleus), beta (electron, positron), neutrinos/antineutrinos, or others.

If you sum the all relativistic-mass of products, you will get initial particle rest-mass prior decay.

If you sum the all energies of products, you will get initial particle energy prior decay.

Decay to two new particles:

$m_0c^2 = m_1c^2\gamma_1 + m_2c^2\gamma_2$

$m_0 = m_1\gamma_1 + m_2\gamma_2$

Decay to two new equal particles (f.e. Helium-2 ->2p+, Beryllium-8 -> 2 He-4):

$m_0c^2 = m_1c^2\gamma_1 + m_1c^2\gamma_1$

$m_0 = m_1\gamma_1 + m_1\gamma_1$

Decay to three new particles:

$m_0c^2 = m_1c^2\gamma_1 + m_2c^2\gamma_2 + m_3c^2\gamma_3$

$m_0 = m_1\gamma_1 + m_2\gamma_2 + m_3\gamma_3$

Edited by Sensei
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[..] Lorentz's ether model. That model is quite different from the preceding material ether theories; one may just as well call it physical space.

[..] Minkowski [..] physical spacetime interpretation became popular, and it is commonly called "block universe".

[..]

So you really mean a kind of mechanical aether theory? (Which of course should still be a mathematical model)

[..]

If the mathematical model works well, then it works well. I don't see that one can say much more.

On your question: I doubt that "mechanical" is a fitting term for either Lorentz ether or Minkowski block universe.

And on your comment: much more has been said already in the thread from which this is a continuation in part. Please discuss that there, and not here.

PS. I see that you participated there in the beginning, and I fully agree with your remark that "it becomes confusing very quickly, this is the nature of metaphysics".

Perhaps this world SR example would help?

Imagine you have unstable particle, unstable isotope.

It has rest-mass m0 (we're in the same frame of reference as that particle).

But unstable isotope is decaying.

To daughter isotope, and to secondary particles, such as alpha (Helium-4 nucleus), beta (electron, positron), neutrinos/antineutrinos, or others.

If you sum the all relativistic-mass of products, you will get initial particle rest-mass prior decay.

If you sum the all energies of products, you will get initial particle energy prior decay.

Decay to two new particles:

$m_0c^2 = m_1c^2\gamma_1 + m_2c^2\gamma_2$

[..]

You may have overlooked the last part of my sentence: "we are condemned to "shut up and calculate"." I'm not aware that such equations are paradoxical or mind-boggling in nature; and as I explained, this discussion is not about mathematical ability. But perhaps you can (qualitatively) explain why nature imposes a "relativistic" mass term $\gamma m$ instead of a "Newtonian" mass term m, and how nature does it, physically.

Edited by Tim88
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You may have overlooked the last part of my sentence: "we are condemned to "shut up and calculate"."

No, I didn't overlook it. I ignored it.

Science theory is verified or disproved using calculations.

Observation of the real world event leads to equations.

Equations are used again, after seeing the same or similar event.

If there is difference between what equation is predicting and observed event, investigation is started, why there is difference.

Say electron has shorter trace than expected. Why? After careful investigation scientist could find that decaying nucleus is emitting gamma photon after a while.

And energy of this photon is equal to what is missing in electrons kinetic energy. And end up as discoverer of nuclear isomers, and new decay mode.

In my radioactivity SR example, you put radioactive isotope to particle detector,

and see traces leaved by particles..

The larger kinetic energy of particle, the longer trace it leaves.

But perhaps you can qualitatively explain why nature imposes a "relativistic" mass term $\gamma m$ instead of a "Newtonian" mass term m, and how nature does it, physically.

Not without hijacking your thread, and running into what mods would call speculation.

Newtonian kinetic energy is:

$E.K.=\frac{1}{2}mv^2$

SR kinetic energy is:

$E.K.=m_0c^2\gamma-m_0c^2$

If you would open OpenOffice SpreadSheet, make 1st column velocity, 2nd column Newtonian EK, 3rd column SR EK.

And fill dozen rows with velocities going from 0...c

You would see that difference between Newtonian EK and SR EK for very small values of velocity, is negligible.

Newtonian EK equation has been established in times when there was no super precise instruments, either for measuring time and measuring distance, or measuring energy or force.

Now they're available, and used in experiments. Or you can simply see trace of particle accelerated to velocity near to speed of light passing through..

Edited by Sensei
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On your question: I doubt that "mechanical" is a fitting term for either Lorentz ether or Minkowski block universe.

Why not?

But perhaps you can (qualitatively) explain why nature imposes a "relativistic" mass term $\gamma m$ instead of a "Newtonian" mass term m, and how nature does it, physically.

No one knows why nature 'uses' Minkowski space-time (locally at least), we just know that our models fit well. I think you will not get a good answer to your question.

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No one knows why nature 'uses' Minkowski space-time (locally at least), we just know that our models fit well. I think you will not get a good answer to your question.

Allow me to pose the question: Is it reasonable to assume that Minkowski space-time is the best explanation for what nature "uses", or is the scientific jury still out?

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Bold face emphasis mine:

Perhaps I should have summarized the thread from which this is a spin-off, in order to prevent such misunderstandings... I use here "physical" in contrast to your "mathematical". [edit:] I now added a short clarification to the first post - thanks!

If you deem that the phenomena can be reasonably explained without a physical space or spacetime, then your comments are welcome in the thread from which this is a continuation in part.

In contrast, this thread is meant for those who do believe that there must be more to spacetime than a mathematical framework, as well as for those who are looking for more than mathematical understanding.

I don't really understand the question. Is there a "physical" model of anything? Ultimately all physics has is a set of mathematical models that seem to work. Why is space-time any different than, say, the electromagnetic field or quark color charge? They are all just mathematical abstractions that describe how things work.

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Allow me to pose the question: Is it reasonable to assume that Minkowski space-time is the best explanation for what nature "uses", or is the scientific jury still out?

I prefer ajb's way of putting it

ajb

we just know that our models fit well.

But Minkowski and relativity only address part of the story.

The do not address granularity v continuity , stochastic processes, scale invariance, nor do they form a complete set when assembling the constitutive relations and the conditions of compatibility.

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[..]

If you deem that the phenomena can be reasonably explained without a physical space or spacetime, then your comments are welcome in the thread from which this is a continuation in part.

In contrast, this thread is meant for those who do believe that there must be more to spacetime than a mathematical framework, as well as for those who are looking for more than mathematical understanding.

Why not ["mechancal"]?

No one knows why nature 'uses' Minkowski space-time (locally at least), we just know that our models fit well. I think you will not get a good answer to your question.

As too all often in philosophy, it depends a bit on what associations are triggered by a word. Dictionary: "having to do with machinery". Surely not!

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But Minkowski and relativity only address part of the story.

The do not address granularity v continuity , stochastic processes, scale invariance, nor do they form a complete set when assembling the constitutive relations and the conditions of compatibility.

I am OK thanks. I understand that Minkowski space-time and relativity do not necessarily explain the whole caboodle.

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No, I didn't overlook it. I ignored it.

Science theory is verified or disproved using calculations.

[..]

Not without hijacking your thread, and running into what mods would call speculation.[..]

You are indeed hijacking this thread with off-topic comments. However, you raised the interesting subtopic of understanding the reality of kinetic energy, and which I intend to clarify.

Allow me to pose the question: Is it reasonable to assume that Minkowski space-time is the best explanation for what nature "uses", or is the scientific jury still out?

One may ask if red is better than sweet; the scientific jury will hand it over to the philosophical jury.

Emphasis in bold mine:

I don't really understand the question. Is there a "physical" model of anything? Ultimately all physics has is a set of mathematical models that seem to work. Why is space-time any different than, say, the electromagnetic field or quark color charge? They are all just mathematical abstractions that describe how things work.

Indeed physics is, ultimately, just math. Except for die-hard mathematicians, most people like to understand the "why" of mathematical relationships in physics. For example Newton's theory did not only improve the mathematical predictions, it also increased physical understanding. Similar atomic models no doubt improved physical understanding long before atoms could be observed.

Edited by Tim88
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Indeed physics is, ultimately, just math. Except for die-hard mathematicians, most people like to understand the "why" of mathematical relationships in physics. For example Newton's theory did not only improve the mathematical predictions, it also increased physical understanding. Similar atomic models no doubt improved physical understanding long before atoms could be observed.

It sounds like you are trying to understand what is "really happening" with space-time, rather than "just the math". But space[-time] is no more or less mysterious than any other aspect of our knowledge of the world, so I am just curious about the motivation.

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Indeed physics is, ultimately, just math. Except for die-hard mathematicians, most people like to understand the "why" of mathematical relationships in physics. For example Newton's theory did not only improve the mathematical predictions, it also increased physical understanding. Similar atomic models no doubt improved physical understanding long before atoms could be observed.

I'm confused by what you mean by physical understanding. Is this the "why" you refer to, as it seems to me (since you separate it from mathematical predictions) or is it something else?

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But Minkowski and relativity only address part of the story.

Sure, for now I assume we are really speaking of classical notions. At some level one may wish to have some more complete quantum theory of 'space-time' and then show how the classical notions follow. But right now we don't have much of a clue on this.

As too all often in philosophy, it depends a bit on what associations are triggered by a word. Dictionary: "having to do with machinery". Surely not!

Mechanical here refers to the classical notion of space-time being filled by some very rigid fluid that we canot for various reasons detect. For example in the Lorentz theory length contraction is used to explain why we cannot see this aether.

...most people like to understand the "why" of mathematical relationships in physics.

I am - like the others - not really sure what you mean by this and so what answer you are looking for. Well, this is a philosophy thread so maybe there is no clear answer anyway!

Edited by ajb
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[..] Mechanical here refers to the classical notion of space-time being filled by some very rigid fluid that we canot for various reasons detect. For example in the Lorentz theory length contraction is used to explain why we cannot see this aether. [..]

That's even more wrong than I suspected: as clarified in the cited reference, Lorentz's ether is not mechanical in that sense as it does not propose a material ether; and I I'm pretty sure that also Minkowski's block universe doesn't match that description.

[edit:] Note that, if one wants to make any sense of the popular odometer illustration, a block universe does require something that can be compared to the "ground" that is required for the odometer to function. But it would be a mistake to think that the block universe therefore is made of some kind of rigid fluid!

Edited by Tim88
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...Lorentz's ether is not mechanical in that sense as it does not propose a material ether;

The model was thrown out in 1911 - you have to forgive me if I am not familiar with details. I am not sure if the nature of he aether is really explained in this and related models. But for sure if you have a aether then you must be implying that the Universe is filled with 'something'. The moden understanding would be in terms of fields, and we now know about the electromagnetic field.

As you are an expert, what do the old paper say about the nature of the Lorentz(-Poincare) aether?

and neither does Minkowski's block universe, if I'm not totally mistaken.

If you are talking about understanding the Universe as 3+1 dimensonal then you are right, this has nothing to do with a aether.

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I'm confused by what you mean by physical understanding. Is this the "why" you refer to, as it seems to me (since you separate it from mathematical predictions) or is it something else?

Perhaps the clearest is to relate to the conceptual model of atoms, see http://link.springer.com/article/10.1007/s10838-009-9109-x :

Even without hard positive evidence, atoms had explanatory power as to the "why" of certain mathematical predictions. Perhaps you can agree that such a concept can be useful to make for example sense of the mathematical formula's of vacuum pumps.

The model was thrown out in 1911 - you have to forgive me if I am not familiar with details. [..] But for sure if you have a aether then you must be implying that the Universe is filled with 'something'. The moden understanding would be in terms of fields, and we now know about the electromagnetic field.

As you are an expert, what do the old paper say about the nature of the Lorentz(-Poincare) aether?

[..]

If you are talking about understanding the Universe as 3+1 dimensonal then you are right, this has nothing to do with a aether.

The references I provided tell me a very different story than you give here [edit: what else happened in 1911?], and I'm hardly an expert on the history but I did a lot of reading after I discovered that it's a mistake to trust textbooks. Lorentz was not a philosopher, as far as I recall he merely assumed, following Maxwell, the existence of a medium that serves as reference for EM wave propagation - but by extension, also the existence and motion of matter. Such an ether model can itself not be material.

If you wish, I can first present for example mutual time dilation as explained with the Lorentz ether, and next I or someone else can do the same with the block universe. However it may be interesting to first follow up on the subtopic started by Sensei in post #6, as an elaboration of that includes mass, energy as well as the magnetic field concept.

And I'm afraid that different people may understand different things with understanding the Universe as 3+1 dimensional; but block universe strongly suggests "4 dimensional". Maybe that's the starting point of the Great Debate.

Edited by Tim88
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Perhaps the clearest is to relate to the conceptual model of atoms, see http://link.springer.com/article/10.1007/s10838-009-9109-x :

Even without hard positive evidence, atoms had explanatory power as to the "why" of certain mathematical predictions. Perhaps you can agree that such concepts can be useful to make for example sense of the mathematical formula's of vacuum pumps.

I was asking in the context of your statement of Newton's theory improving mathematical predictions, as well as increasing physical understanding.

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The references I provided tell me a very different story than you give here [edit: what else happened in 1911?], and I'm hardly an expert on the history but I did a lot of reading.

1911-ish seems to be about the time that everyone in physics accepted special relativity over aether models - though the idea has never gone completely.

Lorentz was not a philosopher, as far as I recall he merely assumed, following Maxwell, the existence of a medium that serves as reference for EM wave propagation - but by extension, also the extension and motion of matter. Such an ether model can itself not be material.

From what I can gather, Maxwell first sort a mechanical aether to explain his electromagnetic theory. He then changed to a more abstract notion of an 'electromagnetic aether'

- I am not sure how 'non-mechanical' this really is.

Lorentz took this more abstract idea and made modifications, as did Poincare. The theory is more a theory of electrons and how they interact, this is not quite written into Maxwell's equations.

Today, special relativity and field theory (both classical and quantum) form the basis of our understanding.

If you wish, I can first present for example mutual time dilation as explained with the Lorentz ether, and next I or someone else can do the same with the block universe. However it may be interesting to first follow up on the subtopic started by Sensei in post #6, as an elaboration of that includes mass, energy as well as the magnetic field concept.

My understanding is that the rather ad-hoc theory of Lorentz and Poincare can be better and easier explained using special relativity. Also, the mathematical elegance of special relativity should not be overlooked.

And I'm afraid that different people may understand different things with understanding the Universe as 3+1 dimensional; but block universe strongly suggests "4 dimensional". Maybe that's the starting point of the Great Debate.

Still, the 3+1 dimensional notion of Minkowski space-time has little bearing on the aether - or really it shows that the idea of using Galilean/Newtonian notion of space and time together with some other 'medium' is probably wrong. By passing to Minkowski space-time we really get to grips with the Poincare invariants of Maxwell's theory and remove auxiliary notions.

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I was asking in the context of your statement of Newton's theory improving mathematical predictions, as well as increasing physical understanding.

Obviously my illustration of atoms was clear for you, as you did not ask about that. Let's not deviate here from the topic which already has a wide scope. [edit:] But I now recall that I summarized Einstein's reference to Newton in the mother thread, maybe you would like to challenge their argument there.

1911-ish seems to be about the time that everyone in physics accepted special relativity over aether models - though the idea has never gone completely.

[..]

My understanding is that the rather ad-hoc theory of Lorentz and Poincare can be better and easier explained using special relativity. Also, the mathematical elegance of special relativity should not be overlooked.

It is clear that you did not even look at the references that form the basis of this discussion, nor the summary of some of those references that I gave in two posts in the "mother" of this thread.

As for mathematical elegance, there can be no difference between the same mathematics - that is just nonsense! See: https://en.wikisource.org/wiki/Translation:On_the_Dynamics_of_the_Electron_(June)and https://en.wikisource.org/wiki/Translation:On_the_Dynamics_of_the_Electron_(July)

Anyway, nature can't care less about our taste of elegance and the discussion here focuses on physical understanding.

[..]

Still, the 3+1 dimensional notion of Minkowski space-time has little bearing on the aether - or really it shows that the idea of using Galilean/Newtonian notion of space and time together with some other 'medium' is probably wrong. By passing to Minkowski space-time we really get to grips with the Poincare invariants of Maxwell's theory and remove auxiliary notions.

That is somewhat mixed up, I would say. Decomposing:

The Newtonian notion of space is not very different from the Lorentzian notion of "ether" (perhaps he should have used another word?); no additional "medium" is required. Minkowski's notion of space-time is 4 dimensional spacetime, as expressed by him in the reference I gave; it's nowadays called "block universe". What you call "the 3+1 dimensional notion of Minkowski space-time" is probably the 3+1 dimensional notion of Poincare space-time; you can read about it in the link I provided. That's also the notion of Langevin, who explained in great detail Minkowski space-time equations and the effects on our conceptual understanding of space and time. And he explained, as I summarized already in the other thread, how they have a bearing on the ether (perhaps you would say space or space-time). I agree that Poincare's invariant space-time interval can be nicely illustrated by means of Minkowski space-time diagrams.

You probably did not know that in 1907 Einstein published a paper in which he presented the preceding work of Lorentz and his own as one and the same theory; and that was so because he considered the mathematical predictions, and not differences in philosophy. His own philosophy changed several times anyway.

PS.a historical discussion is not the aim of this thread; nevertheless providing a bit more context was perhaps useful.

Edited by Tim88
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As I understand it, the mechanical aether was due to Fresnel.

A.J. Fresnel, Ann de chimie, at de physique. 9, 57 (1818)

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I think that the reason why Relativity is more questioned than other physics subjects, is because it handles about things we can relate to our every-day life, and confronts us with the way we imagine the world.

For me, both models (3D space and spacetime) are complementary. I wonder why physicists so vehemently reject the first.
I understand that physicists feel more confortable with the 4D mathematical representation.
But as a layman, I don't have the impression that I live a 4D reality. Time, space and motion are physical notions for me. When I'm driving my car, I know I am in motion, not the landscape. And if I hit a tree, I don't think it's the kinetic energy of the tree that crashes my car.
Relativistic effects are much more understandable by thinking of a theoretical rest frame (maybe the frame wherein the vectorial sum of the velocity of all the particles of the universe is zero?). No ether is needed.
For me, nature is not weird at all (as far as relativity is concerned). c-invariance, time dilation, length contraction etc are all very understandable.

But, of course, I also see that SR, with the 4D Minkowski spacetime, is a much more powerfull model and must be prefered for physics work.

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