Is the Speed of Light Constant in All Reference Frames?

[LURCH]

Some confusion has come up ni this thread about the fact that all motion must be relative. The statement itself is true, but what we mean by the speed of light being absolute is that its speed relative to any observer is allways the same. This was discovered by actually measuring it.

I think this latest post by Russ is the most direct answer to your original question. Time dilation and length contraction are opposite sides of the same coin, you see. Eintsein's great contribution was the fancy bit of reasoning that says that if speed is distance over time, and the speed of something remains constant to any observer, then either the distance or the time must very. And as it turns out, the correct answer is "both". So, as one's frame of refference changes, and the speed of light remains 300,000 kps, one can logically deduce that either a different killometer or a different second is being used to make that measurement. With this realisation, suddenly all meaurements of light's speed made sense.

 

[NanakiXIII]

This was discovered by actually measuring it.

But I'm trying to understand it.

a different killometer or a different second

'scuse me?

Maybe I'm not reading this right, but I didn't know there were different types of kilometers and seconds.

 

[Kurdt]

It is I take it undisputed that you believe that light is the ultimate speed limit. This comes from the fact that the particle is massless and therefore the perfect candidate to go at an 'infinite' speed but the fact it does not is rather handy for the existence of the universe and an underlying factor in Einstein's other famous equation that mass and energy are equivalent.

The big question I guess you are trying to understand is why is this the speed that is always measured? Well that question is most tricky indeed. I do not know if it can be explained in any other manner other than the fact that we have done experiments and it turns out to be that way. For myself I've always thought that the answer lay in the nature of light and the way we, humans, and instruments interpret it. But alas I am not famous for it yet.

 

[NanakiXIII]

I suppose it would make sense that the speed of light is the ultimate speed limit, but then again, you never know. That's not my question though.

It's the result of experiment, but surely it is understood why it is as such?

I don't think I really understand your explenation (about the nature of light and how we interpret it).

 

[Kurdt]

It wasn't really an explanation just indulging myself in a random thought. Light is electromagnetic. It may just be that there is something we are missing about how a photon behaves, that is making the question of how it is always determined to have the same speed a very difficult one to answer. Mathematics can only model what we 'see', it can't tell us the mechanism behind what is happening. It is experimentation that let's us know what the mechanism is. All I am saying is that there may be some phenomena of light we may not yet have come across through experimentation that holds the key to the answer of your question.

 

[NanakiXIII]

So you're saying that this question hasn't been answered before?

 

[Kurdt]

Well I was on the lines of deductive reasoning from the lack of response to your "How does the mechanism work?" question. I do not know for sure but I have done a little research with the resources I have since you posted this and the theory of relativity is concerned with how inertial observers view different events. The postulate Einstein assumes is "All inertial observers measure the same speed for light". Hence from this postulate alone the mechanism of how light is determined to be the same for all inertial observers is lost, in the fact that it is just assumed to be so.

 

[NanakiXIII]

Umm...should I take that as a yes?

So people just assumed it is true, because it works, but why and how exactly it works, they don't know?

Also, what does "inertial" mean? I'm sorry, English is not my native toungue.

 

[Kurdt]

I wouldn't like to put down a firm yes because I would not confess to knowing everything there is to know about it, but from my experience it has always been assumed as just being so and has not gone into the mechanics of the process (I invite someone to contradict me here).

An inertial observer is one who observes an event with no external force applied to it. So basically the inertial bit just means limited to cases with constant velocity.

 

[NanakiXIII]

Oh, okay. Let's see what others reply.

I see.

Well I don't think 0105 is a good time to be thinking of Relativity, so I'll check for replies tomorrow. Thanks for all the replies so far.

 

[NanakiXIII]

Anyone care to help?

 

[russ_watters]

I think you formulated that wrong, or I'm misinterperting (I can never spell that word) it. He's walking 1m/s relative to the train. How could someone on the platform not agree? He may not be walking 1m/s relative to the platform, but we're talking about relative to the train.

You more or less have it: the only speed you can measure directly from your reference frame is the speed relative to you. Yes, the person on the platform can find the speed of the man relative to the train, but it requires two speed measurements (the speed of the walking man and the speed of the train relative to the platform) and a transformation equation (just subtract). This is analagous to the transformations done in SR.

a different killometer or a different second

'scuse me?

Maybe I'm not reading this right, but I didn't know there were different types of kilometers and seconds.

What you measure to be 1km (or 1sec) may very well be something altogether different to someone else. Keeping the train analogy, if those people (on the train and on the platform) in my thought experimen have a way to measure the speed of the walking man to a precision where Relativity matters, they will in fact disagree over how far, fast and long the man walks. The one thing they will always agree on (if they bounce lasers off of each other) is the speed of light.

Have I mentioned GPS yet (I always do)? GPS works by combining high precision time signals from several satellites in one reciever. Because of the precision required, Relativity comes into play. The clock rates are adjusted prior to launch according to the predictions of Relativity so that they will remain in sync when in orbit: when sitting on the ground, the clocks don't keep accurate time! This is one of the best examples of Einstein's Relativity in action.

Also, what does "inertial" mean? I'm sorry, English is not my native toungue.

An inertial frame of reference is one that is not accerating. And I'm shocked to hear English isn't your native tongue - you are quite fluent.

As for "why?" - I don't know. How far do you want to take that question? You only have to ask "why?" about 3 times before the answer becomes "it just is."

 

[NanakiXIII]

What exactly is "analogy" ?

What you measure to be 1km (or 1sec) may very well be something altogether different to someone else.

they will in fact disagree over how far, fast and long the man walks.

How could that be?

Have I mentioned GPS yet (I always do)?

I believe you have (or someone else did).

"it just is."

That's all I needed to know. Thanks.

And I'm shocked to hear English isn't your native tongue - you are quite fluent.

I watch too much TV.

 

[russ_watters]

What exactly is "analogy" ?

A comparison.

How could that be?

Time/space dilation.

RE: why?
Q: Why is the sky blue?
A: Well, nitrogen scatters the blue light in the atmosphere...
Q: Yeah, but why blue?
A: God likes blue.

"Why?" questions quickly lead to philosophy. Sometimes you just have to accept that that's the way the universe works. We have observed these things to be true.

 

[NanakiXIII]

I see. Thank you very much, everyone that replied.

 

[turin]

I don't think that's true. If the Michelson Morley epxeriement had detected a medium for light waves, why would the Galilean transfomations have needed to be replaced? What would have replaced them? Surely not the Lorentz transformations!

I read your question in too much haste. I think you meant, "if the MM exp. had found anisotropy (shifting of the interference pattern)." I was for some reason just reading it as, "if there were an ether." Well, I haven't thought about the "what if shifting interference" too much, so I don't know, but I suppose you're right, there wouldn't be anything immediate to suggest that the Galilean transformations were incorrect as an epistemic device.

 

[manikandan_physics]

don't get more confused

Now I'm not absolutely sure this is the right forum, but here goes.

I've been told that the relative speed of light is supposed to be the same in any reference frame. Is that true, and if so, how is that possible?

did u ever doubt the concepts of absolute space abnd absolute time when the Newton's laws were taught to u in u'r school.since every motion is relative in this universe these concepts were thrown out by modern physicsts.einstein replaced them with absolute speed and that is the speed of light.thus the speed of the beam emitted from a moving aircraft is not (vel of light +vel of aircraft) but only that of light. this is a factand have been proved by experiments.