One-Way Speed of Light: Is it Possible?

In summary, the discussion on the one way speed of light suggests that it is not possible to measure it directly. Instead, we must reflect it off a mirror and divide its travel time by two, giving us its round trip average speed. Time dilation also makes it impossible to synchronize two separated clocks, leading us to assume that light travels at the same speed in all directions. However, some have questioned whether light could have a superposition of all speeds, indicating something profound about light and spacetime. While this has been explored by physicists, it is generally considered a trivial curiosity.
  • #1
kochanskij
44
4
TL;DR Summary
If light travels at C+V in one direction and C-V in the other, could we detect that? How? If not, could this indicate something profound about light and spacetime?
From what I've read, it is not possible to measure the one way speed of light. We must reflect it off a mirror and then divide its travel time by 2, giving us its round trip average speed. Time dilation makes synchronizing two separated clocks impossible. We just assume light goes at C in all directions. (I'm not talking about the speed of its source or detector. Assume everything is in the same inertial frame) Please correct me if I'm wrong.

If it goes at C+V in one direction, it must go at C-V in the opposite direction. We always measure an average speed of C. But we can never know what V is. Could this lead to something profound about light and space and time? Maybe light has no definite one way speed. Maybe it is in a superposition of all speeds. V is every velocity 0 < V < C. Is this possible? Has any physicist explored this? Or is the one way speed of light just a trivial curiosity?
 
Physics news on Phys.org
  • #2
 
  • Love
  • Like
Likes malawi_glenn and FactChecker
  • #3
kochanskij said:
TL;DR Summary: If light travels at C+V in one direction and C-V in the other, could we detect that? How? If not, could this indicate something profound about light and spacetime?

From what I've read, it is not possible to measure the one way speed of light. We must reflect it off a mirror and then divide its travel time by 2, giving us its round trip average speed. Time dilation makes synchronizing two separated clocks impossible. We just assume light goes at C in all directions. (I'm not talking about the speed of its source or detector. Assume everything is in the same inertial frame) Please correct me if I'm wrong.

If it goes at C+V in one direction, it must go at C-V in the opposite direction. We always measure an average speed of C. But we can never know what V is. Could this lead to something profound about light and space and time? Maybe light has no definite one way speed. Maybe it is in a superposition of all speeds. V is every velocity 0 < V < C. Is this possible? Has any physicist explored this? Or is the one way speed of light just a trivial curiosity?
The first thing to realize is that measuring the one-way speed of light requires that the clocks at both ends are perfectly and undeniably recording identical times. That opens a can of worms that is not possible to solve.
The next thing to realize is that Einstein's theory of Special Relativity solved some problems for the first time and made predictions (including atomic energy, physics happening slower for fast-moving objects, etc.) that have been convincingly confirmed. So, whatever he needed to assume for that theory is strongly supported.
 
  • Like
Likes dextercioby
  • #4
kochanskij said:
TL;DR Summary: If light travels at C+V in one direction and C-V in the other, could we detect that?
No.

kochanskij said:
If not, could this indicate something profound about light and spacetime?
It indicates that simultaneity is relative. I think that's pretty profound.

kochanskij said:
Time dilation makes synchronizing two separated clocks impossible.
That doesn't sound right to me. Do you have a reference for this claim?

kochanskij said:
Has any physicist explored this?
Of course. It's a well-understood phenomenon. For at least 100 years, I think.
 
  • Like
Likes Vanadium 50 and Dale
  • #5
kochanskij said:
Or is the one way speed of light just a trivial curiosity?
I'd go with this option. You can choose to use coordinates in which the speed of light is anisotropic. In SR they just make all the maths messier so nobody does so, but you could. However, it's quite common to do in GR because it tends to be more convenient to pick coordinate systems adapted to the geometry of spacetime than worry about whether they're everywhere orthonormal.
 
  • Like
Likes Sagittarius A-Star and Dale
  • #6
kochanskij said:
If it goes at C+V in one direction, it must go at C-V in the opposite direction.
No. The two-way-speed of light must be ##c##.

If the distance is ##L## and the light moves with ##c+v## in one direction, because the 4D-reference coordinate-system was defined this way, then the time for both directions is
##t=\frac{L}{c+v}+\frac{L}{u}= \frac{2L}{c}##

Then light must go at
##u= \frac{c+v}{1+2v/c}## in the opposite direction.

Calculation:
https://www.wolframalpha.com/input?i2d=true&i=Divide[L,\(40)c+V\(41)]+Divide[L,u]=2Divide[L,c]
 
  • Like
Likes Vanadium 50 and Ibix
  • #7
Sagittarius A-Star said:
No. The two-way-speed of light must be ##c##.

If the distance is ##L## and the light moves with ##c+v## in one direction, because the 4D-reference coordinate-system was defined this way, then the time for both directions is
##t=\frac{L}{c+v}+\frac{L}{u}= \frac{2L}{c}##

Then light must go at
##u= \frac{c+v}{1+2v/c}## in the opposite direction.

Calculation:
https://www.wolframalpha.com/input?i2d=true&i=Divide[L,\(40)c+V\(41)]+Divide[L,u]=2Divide[L,c]

Thank you for your correction. I feel silly that I forgot that you don't find an average velocity by adding the two velocities and dividing by 2. A very basic error on my part.
 
  • Like
Likes Sagittarius A-Star and jbriggs444
  • #8
Sorry to resume this thread. Many times in PF has been discussed that the invariance w.r.t. inertial frames of one-way speed of light is just a matter of simultaneity convention employed to define it. In particular in SR inertial frames are defined in a such way that the one-way speed of light is actually the invariant ##c## (based on the Einstein synchronization convention).

However the two-way speed of light is actually measurable and experimentally it is the invariant ##c## as well.

A problem I see is that it is implicitely assumed that Einstein's synchronization convention can be always applied consistently.

What do you think about ?
 
  • #9
cianfa72 said:
it is implicitely assumed that Einstein's synchronization convention can be always applied consistently.
No. In flat spacetime it is known that you can always apply Einstein synchronization between clocks at rest relative to each other.

In curved spacetime it is known that you can't always do that, and nobody assumes that you can.
 
  • #10
PeterDonis said:
No. In flat spacetime it is known that you can always apply Einstein synchronization between clocks at rest relative to each other.
Is it actually an experimental result ?
 
  • #11
cianfa72 said:
Is it actually an experimental result ?
Where would you go to find a flat spacetime to experiment in?

It's a mathematical result using the known geometric properties of flat spacetime and inertial worldlines.
 
  • Like
Likes cianfa72
  • #12
PeterDonis said:
It's a mathematical result using the known geometric properties of flat spacetime and inertial worldlines.
Ok, so using for instance families of mutually "at rest" inertial worldlines in flat spacetime we can check that Einstein's synchronization convention holds consistently.
 
  • #13
cianfa72 said:
Ok, so using for instance families of mutually "at rest" inertial worldlines in flat spacetime we can check that Einstein's synchronization convention holds consistently.
Yes.
 

Similar threads

I Is the concept of a one-way speed of light meaningful and measurable?
    • Special and General Relativity
Replies
25
Views
2K
I Has anyone measured the speed of light in one direction?
    • Special and General Relativity
2
Replies
45
Views
3K
I Can we determine the one way speed of light by combined measurements?
    • Special and General Relativity
5
Replies
146
Views
6K
B The One-Way Speed of Light: Speculation of a Tired Mind
    • Special and General Relativity
Replies
8
Views
1K
B Speed of Light Reflection: Einstein's Train & Mirrors
    • Special and General Relativity
2
Replies
38
Views
2K
B Speed of Light Unaffected by Moving Emitter
    • Special and General Relativity
Replies
25
Views
1K
B Measuring Speed of Light: Transit Time & Interference
    • Special and General Relativity
Replies
18
Views
1K
B Problem about postulate of the invariance of the speed of light
    • Special and General Relativity
Replies
33
Views
2K
I Is it feasible to measure one way speed of light this way?
    • Special and General Relativity
2
Replies
52
Views
3K
B Speed of light and this Accelerating Rocket scenario
    • Special and General Relativity
2
Replies
45
Views
3K

Hot Threads

Recent Insights

Back
Top