# Why is the invariance of light a problem?

1. Jan 1, 2014

### bobie

Surely I am missing something, can you explain what?

If we shoot a gun while travelling on a train, the speed of the bullet is its usual speed plus the speed of the train vt, because the bullet inside is already travelling at vt.

If we produce an EMR on the train (or on the earth in the case of MMX) the "bullet" is created on the spot and has no previous speed.

So, why should the speed of light vary in any frame of reference? why look for the aether, why thw MMX? the speed of light depends only on properties of the medium and not on the speed of the emitter or the receiver.

2. Jan 1, 2014

### bp_psy

In a medium the speed of mechanical waves is dependent on the speed of the observer.Imagine that you are moving at a constant velocity over a sea with waves .The speed of the waves that you measure will depend on your speed with respect to the bottom of the sea.The same is true for sound. The idea was that Galilean relativity should apply to light also. The fact that it doesn't was indeed very strange and the special theory of relativity required some very deep changes in our understanding of physics such as time dilation, length contraction and the idea of space time.

ps. What is EMR and MMX? Endoscopic mucosal resection and Matrix math extension doesn't make much sense to me.

3. Jan 1, 2014

4. Jan 1, 2014

### bp_psy

Yes but at the time there was no reason to think that light should not obey Galilean relativity or that electromagnetic waves didn't really need a medium in which to propagate.They only knew that light could be described as a wave with a very high velocity. The difference in velocity between reference frames in which the speed of light could be measured at the time was also small so one could also not say that light didn't indeed obey Galilean relativity. The Michelson experiment showed that it didn't and special relativity showed how and why.The idea of an aether propagating wave was the best model at the time considering what was known.

5. Jan 1, 2014

### Saw

The fact that the speed of light is independent of the speed of the light source means that it is going to "behave in the same way" no matter whether it is flashed from the train or from the platform. Thus two beams projected from train and platform at the same time will always travel together.

But another thing is that "the same speed is measured" from the two reference frames. Speed is distance divided by time. Suppose that a measurement of the speed of any of those beams is carried out from the train and the platform. In the Galilean mindset, time is absolute, so the two frames will measure the same time. But one one is moving with regard to the other, so they *cannot" measure that light travels, in such time interval, the same distance. Hence it was expected that the measurements of light speed should be frame-dependent.

6. Jan 1, 2014

### bobie

Thanks, you are all speaking in the past. I was not referring only to Galileo and MMX I am referring to relativity, too: why do we need it to justify the invariance of light. It is invariant because nothing can affect its speed.The way it appears to observers is a problem of the observer.
Where is the problem?

7. Jan 1, 2014

### Staff: Mentor

Yes, it really is that simple... in hindsight.
It took close to a half-century (roughly between 1860 and 1905) for physicists to figure this out, in part because the implications - relativity of simultaneity, differential aging, length contraction and time dilation - are so counterintuitive.

Even today, probably 90% of the traffic in the relativity forum is about helping people understand this stuff. So the problem isn't the invariance of the speed of light, it's understanding all the consequences of that invariance.

8. Jan 1, 2014

### bobie

Why are they a problem? why do we need relativity is my question, if problems arise when different observers get different result isn't just their problem?We suffer all kinds of illusions and distortions in this world.

9. Jan 1, 2014

### chingel

Relativity is there to explain how the results will change for different observers, so that you can predict and understand what will happen in different scenarios for different people.

The constant speed of light can be a problem to believe because it means that time can change speed, e.g. one twin can age twice as fast as the other and so on. It was much easier to believe that if you move towards the light it would appear to be moving faster than believing that the space-time fabric distorts and time changes speed. On an everyday basis things don't seem to work that way, if you run after some moving object it appears to be moving slower compared to you.

10. Jan 1, 2014

### bobie

Why must Time change speed? And if we speak of time changing anythind we are giving it an ontological status it has not. That is what I do not understand. Time is not an entity with powers or qualities, time is the sum of billions of physics principles not related to one another. Relativity treats time as an omnipotent entity.

11. Jan 1, 2014

### Pythagorean

The results aren't arbitrary. There's a systematic way in which observers get different results that can be predicted. So it's a meaningful difference, not a subjective one.

12. Jan 1, 2014

### bobie

Even the distortion of a stick in a liquid is systematic and predictable, but we do not conclude that space has changed, is warped. Where is really the problem? why can't it be the observer's problem?

13. Jan 1, 2014

### Bandersnatch

Because these are not illusions but real physical effects.
The thickness of the atmosphere IS contracted for fast muons created in its upper parts, so that they reach Earth despite seemingly too short lifetime.
The time DOES pass at a different rate for communication and positioning satellites in Earth orbit, so it has to be corrected for.

14. Jan 1, 2014

### chingel

Yes that is why people had a hard time getting a grasp of relativity, because among other things it says that the speed of time is something that can change. If one twin stays on Earth and the other goes on a round trip at a fast enough speed, it is possible that for one twin 10 years have passed but for the other 50 years have passed. And it is not that years just seem to have passed, they have really aged that much, one has more wrinkles. If they both had an animal with a lifespan of 5 years, one would be dead for a long time.

Such tests have been made with extremely accurate clocks on airplanes.

To get a sense that something must change, consider a spaceship moving past you at 0.5c. Once it is next to you, it emits a photon in the forward and another in the backward direction. You see these photons move at c, but the spaceship moves toward one photon at 0.5c. So it is a lot closer to that photon. However a guy on the spaceship sees both photons move away at the speed c, so that he is exactly in the middle of the two photons. So you think what is going on, the photons and the spaceship must be at a certain position and the spaceship is either in the middle of the photons or it is not. Therefore something must be changed. And it is not only that they somehow see it differently, they can measure the positions with all sorts of machines, for example the spaceship could have some detectors on really long sticks catching the photons and the results would be the same.

15. Jan 1, 2014

### Staff: Mentor

The problem is that the invariance of c is incompatible with the Galilean transform. So we had effectively been using the wrong transform for quite some time, and we needed to develop a new transform that was compatible with the invariance of c, the Lorentz transform.

16. Jan 1, 2014

### bobie

All effects can have many explanations. If I put a chiken in the fridge and another in the freezer, I might conclude that time has changed, as well, but that is not correct, of course.
I am not discussing what relativity says, I would like to understand what is the problem that cannot be solved without getting in the trouble of time changing.
Do you realize that if Time is considered as an absolute entity, all the phisical fenomena in a space ship are altered? and that includes gravity and everything all the rules would change and not the age of the twins: the electrons in the atoms wouldn't get enough electrostatic energy and would collapse etc...?. Wouldn't that be catastrophic?

Thank you all for your patience

Last edited: Jan 1, 2014
17. Jan 1, 2014

### bobie

Is that the main, real problem, Dalespam? You usually go to the core of the problem, could you expand on that? what does that mean?

Last edited: Jan 1, 2014
18. Jan 1, 2014

### Staff: Mentor

Ever since Galileo it was understood that the same laws of physics work in multiple reference frames. These reference frames, called inertial frames, were assumed to be related by the Galilean transform. However, the Galilean transform says that there is no invariant speed, so the discovery that the speed of light was invariant was a direct contradiction of the Galilean transform. That meant that we needed to rethink our understanding of inertial frames and the laws of physics.

19. Jan 1, 2014

### bobie

I understand that, DaleSpam, but why can't we find other explanations not touchin time, which is a very tricky issue? could you respond to my doubts in post #17?

20. Jan 1, 2014

### PAllen

There are a set of phenomena predicted by SR that have been observed. There do exist alternate explanatory model, but they are considered by most highly contrived compared to SR (e.g. LET). SR is accepted because it accounts for all observations in the simplest way known.

A few of many phenomena, that all fall out with a couple of assumptions and trivial math from SR:

1) If two labs separate and meet after following different patterns of motion, all physical processes will have evolved differently between them, but remain consistent with within each lab. That is, the relative motion has affected mechanical clocks, radioactive decay, biological processes, etc. all identically. This universality of rate of change is simply given the name time.

2) Muons created in the upper atmosphere will reach the ground even thought their decay rate at rest suggests half should be decayed every 600 meters of travel.

So, the question is, do you look for separate explanations for a bunch of observed phenomena, or accept SR which derives all of these from a couple of trivial assumptions?

21. Jan 1, 2014

### Staff: Mentor

Nobody has found such an alternative explanation which is experimentally distinct from relativity. Obviously, not finding something doesn't prove it's non existence, however the existing experimental evidence puts severe constraints on any alternative explanations. Any alternative explanations must reduce to the Lorentz transform in the relativistic limit, just as the Lorentz transform must reduce to the Galilean transform in the Newtonian limit.

A "chicken decay" clock is sensitive to temperature, but there are other clocks (e.g. Radioactive decay) which are not. In contrast, all clocks, regardless of their mechanism, are sensitive to time dilation. Experimentally, what is the difference between all clocks of every mechanism slowing down equally and time slowing down?

22. Jan 1, 2014

### bobie

Of course yes, if it were so simple. But if you accept the assumption that all processes in the world can be altered in a tiny fraction of the world you are opening up a n endless Pandora-box of trouble.

What happens to processes (nearly all) that are time-dependent? forces like gravity and electric?
A ship orbiting the Earth at 9.9999.. c vould have no gravity, no atomic bonds, what would happen to radio communications? etc...

23. Jan 1, 2014

### bobie

I didn't mean that to be a positive example but just a fallacy, my doubts regard the fact that all processes slow down , that supposes omnipotence, see post #23

24. Jan 1, 2014

### Staff: Mentor

That is not an assumption of relativity. There are only two assumptions: the principle of relativity and the invariance of c. Everything else is derived, not assumed.

But also I don't recognize any derived result corresponding to what you describe here. Perhaps it is just a matter of wording, but I don't know what you are referring to.

25. Jan 1, 2014

### Staff: Mentor

The experimental evidence is clear. EM processes, strong and weak nuclear processes, and gravitational processes all display time dilation. There is no room for doubt concerning the observed experimental facts. All processes slow down as predicted by SR.

I don't know where you get this strange comment. No omnipotence is assumed, only the two postulates.

Please keep your comments reasonable or I will have to close the thread. There is no suggestion in the scientific literature which supports this very strange comment about omnipotence.