Speed of light is measured by all observers

[protonman]

How is it that the speed of light is measured by all observers regardless of their relative motion?

 

[EL]

How is it that the speed of light is measured by all observers regardless of their relative motion?

Think you mean:
Why is the speed of light in vacuum allways measured to c, regardless of the relative motion between the light source and the observer?

Since the speed of light in vacuum is allways measured to c, regardless of the relative motion between the light source and the observer!

 

[protonman]

Think you mean:
Why is the speed of light in vacuum allways measured to c, regardless of the relative motion between the light source and the observer?

Since the speed of light in vacuum is allways measured to c, regardless of the relative motion between the light source and the observer!

That is not a serious answer but the way you re-phrased the question is correct.

 

[turin]

One suggestion is that space-time is not "real." Therefore, labelling points in the space-time manifold is somewhat arbitrary, including how the axes of the coordinate system are oriented. It is, of course, found that there is a preferred type of motion (constant v as opposed to d²x/dt² /= 0). Assuming a coordinate system that does not induce a nontrivial d²x/dt² on free particles, then Maxwell's equations give c, regardless of the further particulars of the coordinate system, including the orientation of the space-time axes.

This is all somewhat philosophical, and, regarding the physics, the popular answer, "that is just the way it is," is valid.

 

[protonman]

One suggestion is that space-time is not "real." Therefore, labelling points in the space-time manifold is somewhat arbitrary, including how the axes of the coordinate system are oriented. It is, of course, found that there is a preferred type of motion (constant v as opposed to d²x/dt² /= 0). Assuming a coordinate system that does not induce a nontrivial d²x/dt² on free particles, then Maxwell's equations give c, regardless of the further particulars of the coordinate system, including the orientation of the space-time axes.

This is all somewhat philosophical, and, regarding the physics, the popular answer, "that is just the way it is," is valid.

This is not a valid answer. This is something that needs to be understood. Anyone serious about physics should be deeply concerned that this is not understood. If you are just interested in solving equations and answering trivial questions then it is not an issue though.

 

[jdavel]

protonman said: "This is not a valid answer. This is something that needs to be understood. Anyone serious about physics should be deeply concerned that this is not understood. If you are just interested in solving equations and answering trivial questions then it is not an issue though."

The postulates for theories in physics can never be explained. If they could, then those explanations would be based on something even deeper in physics that couldn't be explained, and they'd be the postulates. Some branch of physical science has to be the one that can't say, "Our postulates are based on the theories of a more fundamental branch". That branch is physics.

If having to just accept things like "the speed of light is constant because that's how nature is", seems invalid or unsatisfactory then physics probably isn't for you. Because, trust me, there are a lot more things you'll have to accept, and some of them will make constant light speed seem easy!

Accepting things like constant light speed, hardly relegates physics to just "solving equations and answering trivial questions." Unless you think questions like: "How did the universe begin? How long ago did it happen? What was the universe like back then? What will happen to it?" are trivial. Again, if you do, physics probably isn't for you.

Incidentally, if physicists had spent the past 99 years fretting about why light speed is constant instead of thinking about what it implies, we'd be no closer to answering the questions in the last paragraph than we were in 1905. And where were we then? One example should give you an idea: We had no idea what made the sun hot!

 

[protonman]

First off I am not asking if physics is right for me. You have no idea what my understanding of physics is. Second, my question is important and useful. What is special about light that observers measurements of space and time must always be such that they ensure that light is measured the same for all observers? There is obviously something special about light. I have my own understanding and explanation but am interested in what other have to say.

 

[master_coda]

First off I am not asking if physics is right for me. You have no idea what my understanding of physics is. Second, my question is important and useful. What is special about light that observers measurements of space and time must always be such that they ensure that light is measured the same for all observers? There is obviously something special about light. I have my own understanding and explanation but am interested in what other have to say.

Whatever your explanation is, it will depend upon assuming something else is true "because that's just the way it is".

In general such assumptions are accepted until some simpler set of assumptions is found, but there will always be assumptions. So demanding a theory that does not make "that's the way it is" assumptions is pointless.


Of course, if you have a way of deriving the current assumptions of physics from a simpler set of assumptions, that's great. But most scientists will be very skeptical of such a claim, and for good reason. Almost invariably such proposals turn out to be philosophical conjectures that are impossible to test, instead of actual scientific theories.

 

[outandbeyond2004]

I agree with master_coda. In science as in virtually any other branch of philosophy, you have to start with assumptions.

protonman, why don't you just show your assumptions and let us see if they are better than Einstein's?

 

[AntiMagicMan]

Experiment shows that the speed of light is measured the same for all inertial observers. The laws of classical electrodynamics give a classical view to why it happens, because of the symmetry of those laws under lorentz transformations.

Why it happens is not a question for physicists.

 

[Scott Sieger]

In my unqualified way I find Photonmans question very interesting and more to the point the response so far also.

Relativity if I'm not mistaken suggests that time slows according to velocity. If time is slower for a mass traveling at velocity how can that mass ( or someone traveling on or in that mass) measure the speed of light as 'c' for surely the time dilation would ruin any chance of that?

protonman, why don't you just show your assumptions and let us see if they are better than Einstein's?

There is no doubt about Einstiens Brilliance and I don't think this is in dispute. However it is worth remembering that high school physics teaches Einstiens work and the work of others quite thoroughly. What was once frontier physics is now almost common knowledge.

Therefore, it isn't suprising that some one wants to and eventually is bound to improve on it all.

 

[chroot]

Relativity if I'm not mistaken suggests that time slows according to velocity. If time is slower for a mass traveling at velocity how can that mass ( or someone traveling on or in that mass) measure the speed of light as 'c' for surely the time dilation would ruin any chance of that?

You are mistaken. Relativity says that if someone flies away from you in a spaceship at a significant fraction of the speed of light, it will appear to you as though the events on the spaceship are running slow. To the people on the spaceship, everything looks the same as it always does. One of the central ideas of relativity is that you cannot measure the speed of your spaceship without looking out the window.

- Warren

 

[protonman]

I agree with master_coda. In science as in virtually any other branch of philosophy, you have to start with assumptions.

protonman, why don't you just show your assumptions and let us see if they are better than Einstein's?

I never said I had any assumption.

 

[protonman]

Your attempts to dismiss my question are total bull****. Science is about explaining why things in the physical world are the way they are. Before Quantum Mechanics no one could explain why the sky was blue. QM gave an answer to this. Yes eventually you get to the point where the answer is just "that's the way it is" but science has not reached that point with light.

 

[master_coda]

I never said I had any assumption.

You claimed to have an explanation. This explanation must require assumptions, or else it is not in fact an explanation. They may be assumptions you consider to be self-evident or obvious, but they still are assumptions.

I agree that there's nothing wrong with asking "why is that?" It's actually a fundamental question in science. But since you haven't actually put forth an explanation, you shouldn't be offended that people are dismissing you.


You come here, state that physics isn't doing a good enough job explaining because it assumes things, claim to have an explanation that fixes everything (an explanation you don't provide) and then get upset when you're dismissed?

 

[selfAdjoint]

Einstein discovered that he could account for the Lorentz transformations in electrodynamics (which had already been discovered by Lorentz, but which wee not well motivated) by assuming the two postulates: Galilean relativity of inertial frames and constancy of the speed of electromagnetic radiation for all inertial frames. So the "reason" for the constant speed is to provide a consistent account of the observed relationships in electrodynamics.

More modern explanations are like this. When you work out minkowski space you have to have a constant to multiply your time units by to convert them to length units, so the all the units of your spacetime vector will be the same. Call this constant c (c for "constant" or "conversion" now, instead of "celeritas"). Then continuing to work through the math you find two things about this constant. First, it has the dimensions of a speed: length over time ("so many length units per time unit"). Second, it turns out to be a Lorentz scalar, meaning it is preserved under Lorentz transformations, meaning in turn that it's the same in all inertial frames. Well that's good news, you wouldn't want your basic definition of what a vector is to vary among different viewpoints!

So now you have the Lorentz transformations with this new constant c in them, and you can easily prove that IF a body travels at that speed THEN it has to be massless. And conversely if a massless body isn't prevented from doing so, it WILL travel at c.

Then you have the word from quantum mechanics that the mass of the photon is zero, and you conclude that it travels at c, which we already found to be a Lorentz scalar, which means that all inertial observers will measure that speed the same. Other massless bodies, like the gluons, are also assumed to move at c, and they also would have their speed invariant between inertial observers.

Does this help at all?

 

[jdavel]

protonman said:

(post #5) "This is not a valid answer. This is something that needs to be understood. Anyone serious about physics should be deeply concerned that this is not understood. If you are just interested in solving equations and answering trivial questions then it is not an issue though."

(post #7) "You have no idea what my understanding of physics is."

Yes I do. Based on what you said in #5, your understanding of physics is negligible. What's worse, with your attitude, it will always be negligible.

But here's a chance for a fresh start. At the end of your post #7 you say: "I have my own understanding and explanation but am interested in what others have to say."

Ok, now you've seen what we have to say; it's your turn. What's your explanation for constant light speed?

 

[DrChinese]

Theories operate in a competitive environment. We go from one useful description of reality to a better one over time. The role of science is to help us compare theories and move in a positive direction.

protonman seems to want to skip the preliminaries and go to the end. Good try, but his criticism is a red herring. How can you criticize a theory that works perfectly in its domain? No theory claims to have all of the answers to all questions. Yet, anyway. :)

In the meantime, if protonman has something positive to offer, let's have it.

 

[protonman]

I would tell you my ideas but I am concerned that you will take then as your own and perhaps publish then in a journal of intellectual thought.

 

[jdavel]

selfadjoint said: "Einstein discovered that he could account for the Lorentz transformations in electrodynamics (which had already been discovered by Lorentz, but which wee not well motivated) by assuming the two postulates: Galilean relativity of inertial frames and constancy of the speed of electromagnetic radiation for all inertial frames."

Einstein assumed "Galilean relativity of inertial frames"? Is that really what you meant?

 

[jdavel]

I would tell you my ideas but I am concerned that you will take then as your own and perhaps publish then in a journal of intellectual thought.

Ok, let us know when you get them published, and I'll read them then.

 

[quantumdude]

selfadjoint said: "Einstein discovered that he could account for the Lorentz transformations in electrodynamics (which had already been discovered by Lorentz, but which wee not well motivated) by assuming the two postulates: Galilean relativity of inertial frames and constancy of the speed of electromagnetic radiation for all inertial frames."

Einstein assumed "Galilean relativity of inertial frames"? Is that really what you meant?

I think it is what he meant, but just to clarify: He did not say that Einstein assumed the Galilean transformation. He said that Einstein assumed Galilean relativity (meaning the postulate that it should not be possible to differentiate inertial frames by doing any experiment that does not make reference to the outside world), which is correct.

 

[Scott Sieger]

You are mistaken. Relativity says that if someone flies away from you in a spaceship at a significant fraction of the speed of light, it will appear to you as though the events on the spaceship are running slow. To the people on the spaceship, everything looks the same as it always does. One of the central ideas of relativity is that you cannot measure the speed of your spaceship without looking out the window.

- Warren

I am sorry,
I was under the impression that time dilations where a reality and not a just a perception of light speeds...

 

[protonman]

Ok, let us know when you get them published, and I'll read them then.

I don't care about getting them published. Besides the world is probably not ready for my ideas and would most likely reject them due to their ignorance.

 

[master_coda]

I don't care about getting them published. Besides the world is probably not ready for my ideas and would most likely reject them due to their ignorance.

Of course. Because when faced with the possibility that either:

A) You are wrong.

- or -

B) You are smarter than everyone else in the world and so nobody else can comprehend your brilliant ideas.

then the answer is clearly B. Possibility A is clearly impossible.

 

[Nereid]

I am sorry,
I was under the impression that time dilations where a reality and not a just a perception of light speeds...

Pions 'at rest' are observed to decay in x microseconds; pions produced by cosmic ray air showers - moving at speeds of z m/s - are observed by Scott, Nereid, Tom, protonman, etc to decay in y microseconds. Hmm, seems pretty 'real' to me.

 

[brookstimtimtim]

yesterday after having a beer or 12, I wrote a mean post on another thread about aether. It closed quickly. Sorry. I have not been drinking today, So let's see if I can ask a good question (that will not close the thread). If time slow downs as something travels closer and closer to "c",would this suggest that the atomic motion of matter slows, and can it be that something is causing that? Space having some kind of resitance to it. It's not that I believe it is aether, but like the first post, it is hard to accept the answer "it just is".

 

[russ_watters]

I am sorry,
I was under the impression that time dilations where a reality and not a just a perception of light speeds...

They are a reality, but like velocity, you can only measure it in relation to someone else.

If time slow downs as something travels closer and closer to "c",would this suggest that the atomic motion of matter slows, and can it be that something is causing that? Space having some kind of resitance to it. It's not that I believe it is aether, but like the first post, it is hard to accept the answer "it just is".

To date, there is no evidence of this "something" causing the motion of matter to slow. It is certainly possible, but without any evidence, it can't be assumed or even theorized. And it works perfectly well to use the current explanation: that time itself is slowing.

All of the laws of the universe 'just are' - either that or they were made by God for a reason only he knows. Either way, you do have to live with that answer.

 

[Scott Sieger]

Russ this is interesting...

Say you are traveling at a velocity that creates time dilations for you the traveller relative to another frame...and as you are traveling you decide to measure the speed of light even though you are in a time dilated state...how could you measure "c" accurately when your tools to measure with are time dilated? Wouldn't the measurement be in error due to time dilation?

 

[PRyckman]

Laymans terms to topic question.

If you travel at nearly the speed of light. Let's say 99.99 Percent of it Light still travels ahead of you at light speed. This is because time is distorted to you, Although you felt as if traveling a light year in just a few days, to Earth it still took you a year. 0.001% more than a year to be exact)

Therefor the greater speed you travel the more time is distorted making up the difference.
(fits into equation;)

 

[chroot]

Say you are traveling at a velocity that creates time dilations for you the traveller relative to another frame...and as you are traveling you decide to measure the speed of light even though you are in a time dilated state...how could you measure "c" accurately when your tools to measure with are time dilated? Wouldn't the measurement be in error due to time dilation?

No. Let's say you're the captain of a starship, and you take off on a trip at 0.99c, relative to your buddy back on Earth. Your large relative velocity will make your clock appear to tick slowly to your buddy. On the bridge of the ship, you will never notice anything funny going on, no matter how fast you go. Your starship will always look its normal size, and your watch will always tick its normal rate.

This is a consequence of the principle of relativity: if you're in a spaceship, and you close all the windows, there's no way you can tell how fast you're going. The ship cannot be said to have a velocity unless you reference that velocity to another object, like the Earth. You cannot say "the ship is going 0.99c." You must say "the ship is going 0.99c relative to the Earth."

- Warren

 

[Scott Sieger]

Chroot I accept what you are saying...can i try another angle to my question...

youare the starships captain and you are traveling at .99'c'...you check the speed of light and it comes in aas youwould expect...say 300,000kspsec.

back on Earth they do the same check and as well it comes in at 300,000kspersec.

However the starships second is slower than the Earth's by let us guess...10%

so form Earth's reference frame the starships measurement of light is 10 % faster...compared to that of Earth's...
which makes it 330,000 kspsec from Earth's frame of reference...

**** I hope I got the math right...ha ...do I make sense.?...

 

[chroot]

Scott,

You can't measure the speed of light that doesn't go through your machine, can you?

- Warren

 

[Scott Sieger]

but you can measure the speed of light inside your machine...
as in source and reflections inside your ship...not from outside the ship...

 

[chroot]

Right. Relativity says you'll measure the speed of light going through your machine as c. It says nothing about the speed of light that doesn't go through your machine. Since you can't measure the speed of light that can't go through your machine, it's rather moot to discuss what value you'd get.

- Warren