# Observing the Speed of Light

Hi,

I've read that the speed of light is consistent for all observers, no matter where they are. So, for instance, a photon on a train is going at the same speed whether you are on the train or off the train. If it was a football, instead of a photon, it would be different speeds right (the speed of the ball for the observer on the train and the speed of the ball plus the train's speed for the observer not on the train)? When I've questioned this, I was told that time dilates and I think the word 'gamma' was mentioned. Could you please explain fully what actually happens for the light to be moving at the same speed for all observers?

AlfieD

Related Special and General Relativity News on Phys.org
A.T.
the speed of the ball plus the train's speed
That is the Galilean transformation, which fails for high speeds. The correct one is the Lorentz transformation. The difference is visualized here:

So in order for the speed of light to be preserved for all observers, moving objects are shrunk and moving clocks are slowed down?

First, what does it exactly mean 'moving objects are shrunk'? Does it literally mean that one of the observers becomes physically smaller? Probably not but could this be explained further?

Second, when it says moving clocks are slowed down, for whom is time slowed down? And how can you have anything but a moving clock?

Nugatory
Mentor
Hi,

If it was a football, instead of a photon, it would be different speeds right (the speed of the ball for the observer on the train and the speed of the ball plus the train's speed for the observer not on the train)?
No. You're expecting that if the train is moving at speed ##u## relative to the ground and the football is moving at speed ##v## relative to the train, then the football will be moving at speed ##u+v## relative to the ground. It's not - it's actually ##\frac{u+v}{1+\frac{uv}{c^2}}##, and this works for footballs, photons, and everything else.

It's almost impossible to tell the difference for any reasonable ball or train, but it starts to show up if one or both speeds is getting close to ##c##. And if either speed is equal to ##c##, the whole thing comes out to ##c## no matter what the other speed is.

phinds
Gold Member
2019 Award
So in order for the speed of light to be preserved for all observers, moving objects are shrunk and moving clocks are slowed down?

First, what does it exactly mean 'moving objects are shrunk'? Does it literally mean that one of the observers becomes physically smaller? Probably not but could this be explained further?

Second, when it says moving clocks are slowed down, for whom is time slowed down? And how can you have anything but a moving clock?
Time dilation and length contraction are artifacts of remote observers. That is, in your own frame of reference, you are always the same size and your clock always ticks at one second per second. Some other observer, from a different frame of reference, SEES you as being length contracted and with a slow moving clock.

You, right now as you read this, are moving at almost the speed of light from the reference frame of an accelerated particle at CERN. Do you feel any different?

WannabeNewton
So in order for the speed of light to be preserved for all observers, moving objects are shrunk and moving clocks are slowed down?
No you're thinking of this in a backwards manner. Time dilation, length contraction, and the Lorentz transformations in general are a consequence of the invariance of ##c## between inertial frames and the flatness of space-time (the latter of which amounts to a set of "symmetry" assumptions). Time dilation and length contraction don't "enforce" the invariance of ##c## but rather they are simply derivatives of this postulation.

First, what does it exactly mean 'moving objects are shrunk'? Does it literally mean that one of the observers becomes physically smaller? Probably not but could this be explained further?
Imagine we have a rod being carried by an observer ##O##. Furthermore ##O## is moving with some velocity ##v## relative to another observer ##O'## along the line joining ##O## and ##O'##. Now ##O## wants to know the length of the rod he's carrying. How can he determine its length?

Well here's one way: ##O## attaches a mirror to the front end of the rod (he holds the back end of the rod). He then emits a beam of light towards the mirror which bounces off instantaneously from the front mirror and arrives back to him. He uses a clock to record the time for the round-trip and says it is ##\Delta t##. Well if the speed of light in the forward direction is the same as the speed of light in the backwards direction, like we assumed, then surely the light beam reached the mirror when ##O##'s clock read ##\frac{\Delta t}{2}## and hence the distance traveled would obviously be ##L_0 = c \frac{\Delta t}{2}##. This distance spans the length of the rod as measured by ##O##.

Can you see why this "radar echo" method would yield a different length for the rod when a similar measurement is made by ##O'## because of the relative motion between ##O'## and ##O##? ##O'## simply measures the rod to be a different (in fact shorter) length from its length as measured by ##O## (the "rest" length since ##O## is carrying the rod); however neither measurement is any more correct than the other i.e. they are both equally valid after being attributed to the respective observers.

Second, when it says moving clocks are slowed down, for whom is time slowed down?
Switching gears a bit, ##O## will record a time ##\Delta t## for one round-trip of the light-clock he carries with him. However ##O'## will measure a time ##\Delta t' > \Delta t## for one round-trip of the light-clock that O carries.

it's actually ##\frac{u+v}{1+\frac{uv}{c^2}}##, and this works for footballs, photons, and everything else.
Sorry but what does the bottom right of the equation actually say? Please forgive both my poor eyesight and my browser's zoom functionality. :)

You, right now as you read this, are moving at almost the speed of light from the reference frame of an accelerated particle at CERN. Do you feel any different?
I feel OK. :D Funny you should mention CERN because I just went there to visit ATLAS.

DrGreg
Gold Member
it's actually ##\frac{u+v}{1+\frac{uv}{c^2}}##, and this works for footballs, photons, and everything else.
Sorry but what does the bottom right of the equation actually say? Please forgive both my poor eyesight and my browser's zoom functionality. :)
Here it is a bit bigger:$$\frac{u+v}{1+uv/c^2}$$

Here it is a bit bigger:$$\frac{u+v}{1+uv/c^2}$$
Haha thanks, looked like a 'w' and an 'o' to me. Very poor eyesight and guessing skills! :D

Can you see why this "radar echo" method would yield a different length for the rod when a similar measurement is made by ##O'## because of the relative motion between ##O'## and ##O##? ##O'## simply measures the rod to be a different (in fact shorter) length from its length as measured by ##O## (the "rest" length since ##O## is carrying the rod); however neither measurement is any more correct than the other i.e. they are both equally valid after being attributed to the respective observers.
Sorry, but I don't get how ##O'## is even measuring it? Could you just explain a bit further as to how they differ in measurements? Thanks.

Switching gears a bit, ##O## will record a time ##\Delta t## for one round-trip of the light-clock he carries with him. However ##O'## will measure a time ##\Delta t' > \Delta t## for one round-trip of the light-clock that O carries.
I thought you said that ##O'## measures it shorter but ##\Delta t' > \Delta t## would suggest that ##O'## measures a time that is greater than ##O##'s time.

WannabeNewton
Sorry, but I don't get how ##O'## is even measuring it? Could you just explain a bit further as to how they differ in measurements? Thanks.
Read through this, it should be instructive to you especially since it uses space-time diagrams to explain everything: http://people.uncw.edu/hermanr/GR/Minkowski/Minkowski.pdf

I thought you said that ##O'## measures it shorter but ##\Delta t' > \Delta t## would suggest that ##O'## measures a time that is greater than ##O##'s time.
No I said length intervals gets contracted, not temporal intervals. We're talking about two different sets of measurements here so don't conflate them otherwise you might end up confused (hence why I said "switching gears"). The link above will hopefully clarify things via the space-time diagrams.

otherwise you might end up confused

http://people.uncw.edu/hermanr/GR/Mi.../Minkowski.pdf[/QUOTE] [Broken]

Hey that's really good thanks a lot!

Last edited by a moderator:
WannabeNewton
Hey that's really good thanks a lot!
No problem! I hope it helps; space-time diagrams are unequivocally the best way to understand measurements in special relativity especially when the measurements involve radar. Also, given the title of your thread, keep in mind the important difference between the one-way speed of light and the two-way speed of light.

A.T.
Time dilation and length contraction are artifacts of remote observers. That is, in your own frame of reference, you are always the same size and your clock always ticks at one second per second. Some other observer, from a different frame of reference, SEES you as being length contracted and with a slow moving clock.
The terms "artifacts", "remote observer" and "seeing" are potentially misleading. The effects have nothing to do with observing from a distance, and are not visual artifacts due to signal travel time. The effects are still there after you already accounted for distances and signal travel time. The effects depend only on the relative movement, not on the remoteness.

1 person
BruceW
Homework Helper
yeah, I agree. The object really is physically shorter according to some other observer. Although, you could say that only the proper length of the object is important, and any other length is just an artifact. But I think this is quite a 'hardline philosophy'.

A.T.
Although, you could say that only the proper length of the object is important, and any other length is just an artifact. But I think this is quite a 'hardline philosophy'.
Yes, you could call all frame dependent quantities "artifacts". Then the velocity of the object would also be just an artifact. But I don't think this is the common usage of the word, hence misleading. Especially combined with talking about "remoteness" and "seeing".

phinds