# Speed of light is measured by all observers

Nereid
Staff Emeritus
Gold Member
Scott Sieger said:
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.

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 lets 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
Mentor
Scott Sieger said:
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.

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 travelling 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?

Laymans terms to topic question.

If you travel at nearly the speed of light. Lets 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 travelling 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
Staff Emeritus
Gold Member
Scott Sieger said:
Say you are traveling at a velocity that creates time dilations for you the traveller relative to another frame....and as you are travelling 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

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 earths by let us guess.....10%

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

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

chroot
Staff Emeritus
Gold Member
Scott,

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

- Warren

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
Staff Emeritus
Gold Member
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

yea i think he's got you there. And if shining a light from that .99 ship ahead of you, to an observer on earth seeing that light shone on them, it should be significantly blue shifted correct?

chroot
Staff Emeritus
Gold Member
It would be redshifted if the spaceship is receding away from you, and blueshifted if it is approaching you.

- Warren

Thanks Chroot for your indulgence.....the point that I wanted to explore was that light measured inside the ship would be (10%) faster due to dilation than same measured on Earth....I know it makes no real value out of it but would suggest that light can be effected by dilation as well as the mass that may carry that light within it......

If this is correct then light projected within the ship is travelling faster than 'c' measured on earth but not in any sense other than that of dilated time.....for relative to the ship 'c' is still 300,000kpsec......( inside the ship )

Is this a fair conclusion?

If not what speed would light be travelling WITHIN the ship when the ship is traveling at say 0.8 'c'?

chroot
Staff Emeritus
Gold Member
It's a total moot point. You can only speak of measurements that you can physically make. Measuring the speed of light in a spaceship with an apparatus based on Earth is not possible. There's nothing else I can say on this topic.

- Warren

master_coda said:
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.
Exactly, you are almost as smart as me.

If I was looking down on 2 ships and saw one travling at 1/4 of "c" and the other travling a "c". How would the ship travling at 1/4 of "c" see the 2nd ship. Would he say that it went a shorter distance for a shorter time? Also what are the exceptions to the rules, for example if I look at the moon then trun 180 deg. to look at a star behind me, with relativity could I just say that I did not move that everything else did? This movement would faster that "c", what is the catch that allows this. I know this sounds stupid but what does allow it.

chroot
Staff Emeritus
Gold Member
Velocities do not add linearly in special relativity.

Rotation is not relative.

- Warren

I thought so, with the Rotation is not relative, and asked that stupid question to lead up to the next stupid question. Before I ask it though, I still would like to know what would the slower ship would see. Ok. now for the next stupid question. Is it safe to say that is just not just speed that cause a time dilation, but speed and distance traveled away from you. If you take the old example of the train moving at the speed of light, and put a twist on that saying it is traveling on a circle track with you in the center, would a time dilation be there, or could you consider this rotation?

chroot
Staff Emeritus
Gold Member
Time dilation and length contraction act along the position vector connecting the observer and the object. On a circular train track, there is never any velocity along the position vector, only orthogonal to it. Therefore, there will be no time dilation or length contraction observed.

Also, please note that the train could not go to the speed of light, only very close to it.

- Warren

Centrifugal forces on the train would create a time dilation effect like gravitational redshift. Also, the train would not be perfectly rigid; it would pancake somewhat. Moreover, I am not sure what the train would look like to the observer, going that fast around the track. Detailed and careful raytracing might turn up some interesting effect?

I believe chroot is wrong about the time dilation of the train.

The proper time of the train is related to the coordinate time of the observer's irf by this equation:

$$(ds)^2 = (dt)^2 - (dx)^2 - (dy)^2$$

( no dz term because the train does not go up/down.)

The train is located by the observer in polar coordinates by these equations:

$$x = r cos\phi$$
$$y = r sin\phi$$
where r is the distance from the observer to the track, and phi is the angular position of the train = $\omega t + \phi_0$

Divide the first equation by $(dt)^2$ and do some calculus to obtain the time dilation factor:

$$(ds/dt) = \sqrt{1-(\omega r)^2}$$

Ok, I'm getting feed back on the train, but of the two ships, and what the fisrt ship would see (about 4 post back)? I just had another thought no the train. Isn't time dilation part of mass going to infinity, if so, would on time dilation mean the train could go the speed of light? Then If the train did have time dilation then what is the diffrences in this example and the one where I spin my head.

Last edited:
Why is the speed of light invariant in today's view?
(paraphrased)

... 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. ...

[2clockdude replies:]
Thinking people would want to know why light's one-way speed
is Lorentz invariant.

And the answer is simple, as follows:
It's because Einstein forced it to be by definition, so this
case of Lorentz invariance has nothing to do with physics (or
with the nature of nature).

Further explanation:
If I decide to force two clocks to obtain one-way light speed
invariance, then of course they are going to obtain it, and of
course my math (in this case, the Lorentz transformations) will
faithfully (if stupidly) reflect this; however, this clearly
has nothing to do with physical science (or with the nature of
nature) because it is a mere convention (just as is the length
of an inch).

Even further explanation:
Forget about the Lorentz transformation math, it merely reflects
Einstein's definition of clock synchronization, which has no basis
in either theory or fact. Forget about Einstein's definition of
clock synchronization because it is not physics, it is only a
convention. Also, it produces absolutely asynchronous clocks.
Forget about the so-called theory of special relativity - it is
not really a scientific theory but is merely a definition of
clock synchronization. (SR is based solely on Einstein's definition
of synchronization.)

Moreover:
The important thing in physics is experiment, not theory, not a
mere definition of synchronization, not a postulate, not a
principle, not some math based on a definition, etc., etc.
Therefore, the question in this case becomes Who has ever
experimentally used two non-rotating, relatively-at-rest clocks
to measure the one-way speed of light? And the answer is No one.
So no one really has proved experimentally that light's one-way
speed is indeed invariant (or isotropic). In fact, no one has
shown that light's one-way, two-clock speed can even be an
experimentally-found law of nature, as Einstein claimed it to
be. You might begin by asking yourself Why has no one ever used
two same-frame clocks to measure the one-way speed of light?
In fact, this has never even been done on paper, so we don't
even have the excuse of the lack of technology, as if this
could even be a real excuse today. The real reason for the lack
of the performance (if only on paper) of the one-way version of
the Michelson-Morley experiment is simple: No such experiment
exists. Why is this? It is because nature cannot synchronize
clocks in order to give us an experimental result in the one-way
case. (Only man can synchronize clocks, so the only result we
can obtain in the one-way case is a man-given one, and man does
not give us the laws of nature, man gives only definitions and
conventions, such as Einstein's definition of synchronization.)

In conclusion:
To answer my paraphrased version of 'protonman's' important
question, I can say that the one-way light speed invariance in
today's view is irrelevant because it was not given by nature,
but was merely forced by man via a synchronization definition.
Since there can be no natural value for light's one-way, two-clock
speed, there can be no postulate, hypothesis, or theory which
pertains to such a value (and which claims to predict it). Even
though Nature cannot give us a one-way law, we can still use two
clocks to correctly measure light's one-way speed, as long as the
two clocks are correctly related (or correctly synchronized).
This means that the most important goal of (flat) space-time
physics is two absolutely synchronous clocks. (And such clocks
will find a variable one-way light speed. Indeed, if we were to
correct for clock slowing and rod contraction, we would obtain
a variable round-trip speed of light.)

Tom Mattson
Staff Emeritus
Gold Member
2clockdude said:
Why is the speed of light invariant in today's view?
(paraphrased)

... 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. ...

[2clockdude replies:]
Thinking people would want to know why light's one-way speed
is Lorentz invariant.

You can think about it all you want, the question has no known answer. And as has been noted, even if we did have the answer, that explanation would have some unexplained phenomenon behind it.

And the answer is simple, as follows:
It's because Einstein forced it to be by definition, so this
case of Lorentz invariance has nothing to do with physics (or
with the nature of nature).

Wrong. Einstein did not have the power to force the speed of light to be absolute, "by definition", or any other means. The speed of light is a Lorentz scalar because that's what it is measured to be.

Further explanation:
If I decide to force two clocks to obtain one-way light speed
invariance, then of course they are going to obtain it, and of
course my math (in this case, the Lorentz transformations) will
faithfully (if stupidly) reflect this; however, this clearly
has nothing to do with physical science (or with the nature of
nature) because it is a mere convention (just as is the length
of an inch).

???

You can't "force" two clocks to do anything that would violate the laws of physics. Anyway, the one-way speed of light has already been measured by decaying pions in flight. Even when the pions are moving at over 99% of the speed of light, guess what the speed of the pions is? You guessed it: c.

Or do you suppose that we can force pions to emit photons that only travel as fast as we want them to?

Even further explanation:
Forget about the Lorentz transformation math, it merely reflects
Einstein's definition of clock synchronization, which has no basis
in either theory or fact. Forget about Einstein's definition of
clock synchronization because it is not physics, it is only a
convention. Also, it produces absolutely asynchronous clocks.
Forget about the so-called theory of special relativity - it is
not really a scientific theory but is merely a definition of
clock synchronization. (SR is based solely on Einstein's definition
of synchronization.)

No, it isn't based on that. You have to read beyond the initial thought experiment. He develops relativity from scratch from the two postulates (neither one has to do with clock synchronization). The thought experiment at the beginning of the paper is presented to prepare the minds of the readers for one of the remarkable conclusions of the paper, namely that temporal measurements are not absolute.

In any case, the discussion at the beginning of the paper is most certainly grounded in reality, as time dilation has been well confirmed, as have other consequences of relativity.

In conclusion:
To answer my paraphrased version of 'protonman's' important
question, I can say that the one-way light speed invariance in
today's view is irrelevant because it was not given by nature,
but was merely forced by man via a synchronization definition.

Nope. Whether you measure the speed of light one-way or two-way, you get the same result, namely that the speed of light is a Lorentz invariant.

2clockdude's post #48
https://www.physicsforums.com/showpost.php?p=192224&postcount=48

looks remarkably like what Martin Miller would write. You know the fella who got mad and called some people meatheads or something like that in a thread called 'The SR Question of the Century.'

2clockdude is all experimentalist and no theorist, seems like. If he is Martin Miller, what can we do to make him understand that we cannot begin to comprehend the universe without some initial set of assumptions = the start of some theory? We need both theory and experiment. Without a theory, an experiment is just fumbling in the dark = no planning. Without experiment, theory is just wild guessing. We need to do both theorizing and experimenting at the same time.

I still could not comprehend Martin Miller's objections = also 2clockdude's - either that or he/they are just completely wrongheaded or in emergency need of some philosophical head-cleaning.

Let me concede that nobody has measured the one-way speed of light directly by means of a non-rotating set of two relatively-at-rest clocks that we know are synchronized. He predicts that (oh, my honey, THEORY!!) we will find the speed of light to be variable, contradicting SR. Well, shrug, until people do the experiment and convince the rest of the science world that indeed light propagation is locally anisotropic, I'll just side with SR, thanks, against the likes of what Martin /or & 2clockdude offers, (which is not much more than an objection, as far as I can see). I just don't know of any actual experiment that SR could not explain, as long as we can neglect gravitational effects and we can add quantum theory if need be.

2clocky, you are welcome to present a complete set of postulates/principles to replace SR. If they are indeed in some way better than those of SR, I would be happy to switch to them, provided they pass all experimental tests as well as SR has to date.