# Speed of light is measured by all observers

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

EL
protonman said:
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!

EL said:
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
Homework Helper
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 prefered 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.

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turin said:
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 prefered 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.

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!

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.

protonman said:
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 explaination 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.

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?

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.

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 travelling 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 highschool 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
Staff Emeritus
Gold Member
Scott Sieger said:
Relativity if I'm not mistaken suggests that time slows according to velocity. If time is slower for a mass travelling 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

outandbeyond2004 said:
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.

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.

protonman said:
I never said I had any assumption.

You claimed to have an explaination. This explaination must require assumptions, or else it is not in fact an explaination. 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 explaination, 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 explaination that fixes everything (an explaination you don't provide) and then get upset when you're dismissed?

Staff Emeritus
Gold Member
Dearly Missed
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?

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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
Gold Member
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.

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.

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?

protonman said:
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.

Tom Mattson
Staff Emeritus
Gold Member
jdavel said:
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.

chroot said:
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.......

jdavel said:
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.

protonman said:
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.