Time Dilation: A Philosophical Debate or Scientific Reality?

[JesseM]

No of course not, we are not talking about light here but about relative motion of mass objects, something entirely different.
With light it is different.

So you're using a definition of "relative speed" which somehow only makes sense when applied to light, the definition cannot be applied to anything else? Under your definition, there is only one possible "relative speed", and it is 0c?

You say it has something to do with frames, but a frame is simply a concept.

Yes, but it is a concept defined in terms of actual measurements.

There is nothing physical about frames, and in GR they are next to completely useless.

Not really, you still use coordinate systems in GR, and for them to have physical meaning you still need some notion of a measurement procedure which can tell you what coordinates a given event should be defined.

All we have is objects of mass that are or are not in relative motion with other obects of mass

The words "relative motion" are meaningless unless you define it in terms of some measurement procedure.

inertial v.s. accelerated state and the speed of light, and the fact that light always is emitted and always escapes at c and that that is independent of relative motion.

Speed is meaningless unless you define it in terms of some measurement procedure. You don't need "frames" per se for your statement to be correct, it is also correct if you just define "speed" in terms of measurements on rulers and clocks moving inertially (and if you're measuring one-way speed as opposed to round-trip speed, you also need to specify that clocks at different points on the ruler are synchronized according to the Einstein synchronization convention). But if you don't define speed in terms of either coordinate systems or measurements on physical rulers and clocks, your statement is meaningless.

If your objection is just to frames and not to the idea that we need to measure speed physically, what physical measurement procedure are you imagining that will lead us to the conclusion that a sublight object is moving at 0c "for light"? How does a light beam measure the speed of another object relative to itself? You can't get a ruler or clock moving at that speed, and again, if you consider the observations of an observer moving away from the object in the limit as his speed relative to the object approaches c, then according to measurements on his own rulers and clocks the object's velocity approaches c in this limit, not 0.

 

[MeJennifer]

If your objection is just to frames and not to the idea that we need to measure speed physically, what physical measurement procedure are you imagining that will lead us to the conclusion that a sublight object is moving at 0c "for light"?

I have not objection to frames, unless they are viewed as some hidden reality of space or space-time.

what physical measurement procedure are you imagining that will lead us to the conclusion that a sublight object is moving at 0c "for light"?

There is no movement relative to the speed of light Jesse, if there were we would not be able to state that light always gets emitted or absorbed at the speed c. We would instead measure that if something moved at say 0.2c that light would be emitted at 0.8c in the direction of movement. This is obviously not the case. So it is clear, at least to me, that relative movement between objects of mass is entirely irrelevant to light.

For light there is no movement.
Consider space-time with the Minkowski metric. Observer the angle between the worldline of an accelerated mass particle and photon, does the "speed" of the mass particle make any difference whatsoever? No it does not, the angle remains the same.

Lorentz effects are caused by relative movements of mass objects.
But here we also should be a bit more selective.
An effect can only be observed once there is change in the rate of change of the distance between the observer and the measured object. If the rate of change remains constant one could at most infer that the rate of change in the past was different, but one cannot actually observe length contraction, time dilation or increase of relativistic mass.
So it is more accurate to say that the Lorentz effect can only be observed if the rate of change of the distance between an observer and a mass object changes.

 

[JesseM]

There is no movement relative to the speed of light

What does it mean to talk about movement relative to a speed rather than relative to an object or particle (such as a photon)?

Jesse, if there were we would not be able to state that light always gets emitted or absorbed at the speed c.

Physically, the statement that light alwasy gets emitted or absorbed at the speed of c just means that whenever you measure its speed using inertial rulers and clocks, you find its speed is c. Speed is not a theological or philosophical concept--do you agree that all statements about speeds must be ultimately be justified in terms of some set of physical measurements? If so, please explain what you mean by "the speed of a mass object relative to light is always 0c" in terms of actual physical measurements.

For light there is no movement.

I have no idea what the physical meaning of this statement is supposed to be. If it is not a theological or philosophical claim, then please explain what it means in terms of physical measurements.

 

[MeJennifer]

I have no idea what the physical meaning of this statement is supposed to be. If it is not a theological or philosophical claim, then please explain what it means in terms of physical measurements.

Did you ever study a space-time diagram for a photon, a so-called null interval? I suppose you did. How much "movement" of mass objects does a light beam encounter from one spacetime event to another? I hope you will see that it is in fact none whatsoever. So why is it that you have trouble with my statement?

 

[JesseM]

Did you ever study a space-time diagram for a photon, a so-called null interval? I suppose you did. How much "movement" of mass objects does a light beam encounter from one spacetime event to another? I hope you will see that it is in fact none whatsoever. So why is it that you have trouble with my statement?

I don't know what this means either. When you talk about spacetime events "encountered" by a light beam, are you talking solely about events on the light beam's own worldline? If so, what does it mean for anyone (whether a sublight observer or a light beam) to measure the "movement" of any other object based only on the intersections between their own worldline and the object's worldline? Normally measuring movement depends on recording two different positions of an object at different times, if you only measure the position once you don't conclude the object is at rest, you just don't have enough information to measure its speed.

 

[eiapeteides]

OK Guys. Now you really confused the hell out of me! The only remaining question I actually had was:

Is the Lorentz contraction a real physical effect or is it a mere optical illusion due to the relative motion between the one who measures the length of an object and the measured object?

 

[pervect]

What sort of experiment would you perform to tell if it is "real" or an "optical illusion"?

 

[eiapeteides]

I can't think of anything. But just because "I" can not distinguish between them doesn't make "Real" and "Illusion" the same?

 

[JesseM]

I can't think of anything. But just because "I" can not distinguish between them doesn't make "Real" and "Illusion" the same?

If it helps, it's not an optical effect, length contraction is what you get when you factor out light signal delays, or when you measure "length" using only local measurements. For example, suppose there was a rod that was 10 meters long in its own rest frame, and it was moving past you at 0.6c. If you held out a ruler which had a bunch of synchronized clocks attached to each marking, then if the back end of the rod was passing the 0-meter mark when the clock there read exactly 12:00, then the front end would be passing the 8-meter mark when the clock at that marking also read exactly 12:00, so you could use these simultaneous local measurements to judge that the length of the rod in your frame had shrunk to 8 meters, just as the length contraction equation predicts.

Of course, it's important to understand that the concept of what it means for a pair of clocks to be "synchronized" differs from frame to frame, so that even though in your frame both clocks tick 12:00 "at the same time", other frames would assign these events different time-coordinates. If you're interested, on this thread I provided an illustration of how two rulers moving past each other at relativistic speeds, with clocks at each marking, would view one another in their own rest frame, and how the fact that they define simultaneity differently can explain why each one measures the other to be shrunken.

Since the question of which ruler is shrunken also depends on your choice of frames, this makes it ambiguous whether length contraction should be called a "real physical effect"--like I said, it isn't optical, but it isn't frame-independent either, and usually physicists would only define a quantity or observation as truly "physical" if it would be agreed upon by all observers.

 

[pervect]

I can't think of anything. But just because "I" can not distinguish between them doesn't make "Real" and "Illusion" the same?

Nope - but it does make the answer to your question a matter of philosphy, rather than science, unless you can pin your question down better.

There generally isn't any end or resolution to philospohical arguments, but a scientific argument can ultimately be resolved by the court of experiment.
A corollary to this is that most (though perhaps not all) scientists stop arguing when they realize they only disagree about things that can't be tested, i.e. that they are arguing about philosophy rather than science.

For what it's worth, I tend to regard oberver independent quantites as being more "real" or at least more fundamental than observer dependent quantities. This POV leads to the view that the Lorentz interval, being independent of the observer, is a more fundamental property of nature than length. But you won't necessarily find total agreement on this point, though I personally think the physics is easier when one views things in this way.