# Is LET an Interpretation of SR?

1. Jun 18, 2013

### bobc2

There has been a narrative that has run through a number of posts on special relaivity. It establishes the idea that LET (Lorentz Aether Theory) is an interpretation of SR (Special Relativity).

I personally believe this is a false narrative and that there is at least one fact that must be established here: LET is not an interpretation of special relativity.

It is a competing theory. If you wish to use the term, interpretation, you could say that LET and SR are two different interpretations of the results of the Michelson and Morely experiment, Fizeau experiment, life time of high velocity muons, accounting of electromagnetic phenomena associated with Maxwell’s equations, etc.

We can cite extensive text from Einstein’s book, “Relativity” to support this view, but the prominent members of this forum should all be in agreement on this point.

TheBC has already made this point in his last post, and for some reason his statement was ignored. TheBC was very explicit (and quite correct): “…and thus the ether definitely not an interpretation of SR.”

https://www.physicsforums.com/showthread.php?t=692720&page=4 post #62 And Fredrik has emphasiz...g for (compatible with) experimental results.

Last edited: Jun 18, 2013
2. Jun 18, 2013

### PAllen

Do you believe QED is consistent with SR? Do you believe that QED is deterministic? I believe the answers to these questions are yes and no. Therefore SR as conventionally interpreted does not require belief in the block universe.

A simpler argument is that empirically, SR provides a geometric model of given observer's causal past, and SR consistent dynamical theories allow predictions outside of an observer's causal past. These predictions are limited by incomplete information and non-determinism.

As to whether LET is an interpretation or an alternate theory, what criteria do you want to use? One is you have different theories only if there are, in principle, different predictions for observables. Another is different theories can have the same predictions, but be different if their explanatory model is substantively different. But then, do you consider the different interpretations of QM different theories? What about path integral versus S-matrix? Note, I don't think you can say LET is analogous to epicycles because its explanatory model does not get more complex as you add more bodies to your scenario.

3. Jun 18, 2013

### Staff: Mentor

For appropriate definitions of those terms, yes. One of the continuing issues in this whole discussion is that different people (including me) mean different things by "LET" and "SR", yet we all talk as if we all mean the same things by those terms. In other words, this is an issue of terminology, not physics.

And right after that, he says that SR has two interpretations, the block universe interpretation and the ether interpretation. Many people (including me) have been using the term "LET" to refer to the ether interpretation of SR, which is why we say LET is an interpretation of SR. Of course if you mean something else by "LET", as you do, you will say LET is not an interpretation of SR, because you don't mean that by the term.

I personally would have no problem saying "ether interpretation of SR" (or EI for short) and "block universe interpretation of SR" (or BUI for short) if that will help with clarity. But the fact remains that, as Fredrik said, SR does have an ether interpretation.

I think the consensus among physicists is that no experiment can decide between the ether interpretation of SR and the block universe interpretation of SR (or indeed between those two and any other interpretation that is consistent with the experimental predictions of SR). Insofar as there is a distinct theory "LET" that makes different experimental predictions than SR, obviously one can run the appropriate experiments to see which one is right. (I'm not sure which experiments, precisely, you think these would be--MMX?) But no experiment can decide between alternative interpretations of the same theory, since the experimental predictions are made by the theory, not the interpretation.

4. Jun 18, 2013

### bobc2

Hijacking Einstein's Theory

I personally think you folks are hijacking Einstein's theory so as to evolve it into some generic representation based on the application of Lorentz transforms and experimental results related to both LET and "Einstein's Special Relativity". Thus, the new SR is a generic theory with implications and predictions based on application of principles restricted to those experimental results common to both LET and "Einstein's Special Relativity". So, now having a new definition of SR you can logically pronounce LET and "Einstein's Relativity" as equivalent interpretations of SR.

And I'm trying to leave mention of block universe out of the discussion.

Yes, PAllen, this is the one I had assumed we would agree upon: "...Another is different theories can have the same predictions, but be different if their explanatory model is substantively different." Reading Einstein's book, I personally think he would be appalled at the other definition.

Last edited: Jun 18, 2013
5. Jun 18, 2013

### PAllen

Ok, sorry. It was in a quote you linked, so I assumed it was part of what you wanted to discuss. Henceforth, no more.

However, you didn't respond to my examples of how it seems really hard to distinguish between interpretation and empirically indistinguishable theory. It is not just in SR/LET that different physicists and philosophers have very different opinions on this for some pair of models (to try to use an all encompassing category).

6. Jun 18, 2013

### Staff: Mentor

Do you agree that since there is no experimental distinction between the two there is no empirical support for selecting one over the other? If so, then who cares if you call them "interpretations" or "theories", the substantive facts are unchanged: the evidence does not and cannot favor one over the other.

If you dislike the usual term "interpretations" to refer to different ideas which lead to identical experimental predictions in all cases, then what word would you use?

Last edited: Jun 18, 2013
7. Jun 18, 2013

### WannabeNewton

Very well said. The ol' "Shut up and calculate!".

8. Jun 18, 2013

### pervect

Staff Emeritus
My position is more or less that a theory is defined by its experimental predictions, and that two apparently different theories that yield identical predictions in all cases are equivalent theories.

To the extent that LET gives exactly the same predictions as SR, it's such an equivalent theory. "Hijacking" isn't an issue to me, any more than writing Maxwell's equations in differential form, integral form, or in the original form (which I believe was based on quaternions, but the exact form isn't relevant, as long as it's different, which it was.)

My main reservation about saying that "LET is an interpretation of SR" is the number of people who self-identify as beliving in LET who don't agree that the theory makes the same predictions as SR does. Here we get into an issue I don't know how to answer fully, where people can't agree on what a particular named theory is actually predicting :-(.

9. Jun 18, 2013

### Staff: Mentor

He might well have been, but this, too, is an issue of terminology, not physics. Different people use the word "theory" in different ways.

The best way I know of to handle these types of issues is to taboo the controversial terms. In this case, we would have to taboo the terms "theory", "interpretation", "LET", and "SR", since we can't agree on their definitions. Then we could actually talk about physics and actual mathematical models of physics and what predictions different models make, which models make identical predictions, etc. That, to me, would be a more interesting discussion than arguing about whether "LET" and "SR" are different "theories" or just different "interpretations", what Einstein would have said, etc. Your mileage may vary, of course.

10. Jun 19, 2013

### Fredrik

Staff Emeritus
There is no physics whatsoever in this discussion. It's only about the definitions of the terms "theory", "interpretation", "LET" and "SR". This isn't physics, it's philosophy of physics.

What I mean by those terms is roughly this:

A theory is a minimal falsifiable set of statements about the real world. The word "falsifiable" ensures that the set of statements at least assigns probabilities to possible results of experiments. A set of statements is "minimal" if none of the statements can be removed without changing the predictions. This ensures that something like "QED + an invisible blue giraffe named Leonard" isn't considered a theory.

This means that I consider a theory to be defined by its assumptions. The view that theories are defined by their predictions makes sense too, but it has the problem that a list of predictions would have to be considered a theory, even if it doesn't explain anything at all. This bothers me more than anything negative you can say about the idea that theories are defined by their assumptions.

An interpretation of a theory is an attempt to explain what's "really happening" in more intuitive terms than the theory. Note that when you add a set of statements that defines an interpretation to a set of statements that defines a theory, the new set isn't a minimal falsifiable set, and is therefore not a theory.

Minkowski spacetime defines a mathematical framework in which both classical and quantum theories of matter can be defined. I use the term special relativity mainly for that framework, but sometimes also for some specific theory of classical particles in that framework. Note that by my definitions, the classical theory of a single non-interacting particle in Minkowski spacetime is not the same theory as the classical theory of two non-interacting particles in Minkowski spacetime. This is annoying, but it's the price we have to pay to avoid the other annoying thing I mentioned earlier.

Each theory in the framework is defined by a specification of the particles and fields in spacetime, their interactions, and a set of correspondence rules that tell us how to interpret the mathematics as predictions about results of experiments. The set of correspondence rules can be different in different theories, but there are two correspondence rules that are always included: One about how to measure times, and one about how to measure lengths. Since these two are so fundamental, it makes sense to consider them part of the framework rather than part of each theory.

Each theory in the framework has two interpretations: The block universe interpretation and the ether interpretation. Since this applies to all theories in the framework, I allow myself to be a bit sloppy and describe these interpretations as interpretations of SR, even though I define SR as the framework, not as a theory. The block universe interpretation says that clocks and meter sticks at rest in a global inertial coordinate system have the same properties no matter which global inertial coordinate system we're talking about. In particular, clocks don't "slow down". The ether interpretation says that there's a global inertial coordinate system such that clocks at rest in it are ticking at their maximal rates, and meter sticks at rest in it have their maximal lengths. A clock that's moving with velocity v in these coordinates is slow by a factor of $\gamma$. A meter stick that's moving with velocity v in these coordinates is contracted by a factor of $\gamma$. When we try to use that clock and that meter stick to measure the speed of light, the two errors cancel each other out exactly, so that the result is always c, regardless of what v is.

Now, what is Lorentz ether theory? It's essentially SR plus the ether interpretation, and a statement that there's an undetectable substance called "the ether" that fills up all of space. There's a global inertial coordinate system in which every part of the ether is at rest, and that's the one with the property that clocks and meter sticks at rest in it have their maximum ticking rates and lengths respectively. By my definitions, it's not a theory, because it's not a minimal falsifiable set of statements about the real world. So the term "Lorentz ether theory" doesn't really make sense to me.

Historically, I think what happened was that some version of Lorentz ether theory was found first. Then it was discovered that it wasn't minimal, and that's pretty much the end of that story.

Last edited: Jun 20, 2013
11. Jun 19, 2013

### Staff: Mentor

I agree. And since everyone has had a chance to mention their prefered definitions, and there is nothing useful remaining to do, the thread is closed.