# Photon Travel: Is Our Universe a Single Point?

• B
• tworitdash
In summary, the popular misconception is that time dilation and length contraction do not apply to photons because their reference frame is at rest. This is incorrect because all inertial frames must move at the speed of light.f

#### tworitdash

I have come across a questions which reads "Why does even photon travel?". After reading special and general theory of relativity, this one bugs me all over. For a particle moving closer to speed of light, clock slows down and the space around it contracts. I see a photon travel and I see things around me, because my velocity is much low and and I experience time. However, for a photon, clock doesn't move at all and it doesn't really know what is space (because it is contracted to the maximum). I see light generated from a source and reaching a destination, but a photon doesn't even experience all that. Is the entire universe is a point for a photon? If that's so, where does it come from? Is there any higher dimension, where someone is generating it for us, because for me it is very peculiar and nothing else travels exactly at the speed of light. How's it so?

for a photon, clock doesn't move at all and it doesn't really know what is space (because it is contracted to the maximum)

This is a common pop science misconception. The correct statement is that the concept of "reference frame" with clocks and rulers does not apply to a photon. The Lorentz transformation is not well-defined for ##v = c## so the concepts of "length contraction" and "time dilation" cannot be applied to photons.

• tworitdash
However, for a photon, clock doesn't move at all and it doesn't really know what is space (because it is contracted to the maximum).
This statement is based on the misapprehension that length contraction and time dilation formulae apply to an inertial frame moving at the speed of light. This is wrong. There is no such frame in relativity - it is self-contradictory, since light would have to be both at rest and doing 3×108m/s.

Any attempt to describe "the perspective of a photon" will therefore fail, and any conclusions drawn from this line of thinking are nonsense.

• tworitdash
Moderator's note: Thread level changed to "B".

• nitsuj
Well, to start with, "for a photon" is a statement meaning that you can transform to its rest frame and observe. But you can't. Also, length contraction is only in the direction of movement, not in all directions. So if you approach c the universe is only contracted in 1 direction.

• tworitdash
This statement is based on the misapprehension that length contraction and time dilation formulae apply to an inertial frame moving at the speed of light. This is wrong. There is no such frame in relativity - it is self-contradictory, since light would have to be both at rest and doing 3×108m/s.

Any attempt to describe "the perspective of a photon" will therefore fail, and any conclusions drawn from this line of thinking are nonsense.
So, as in all reference frames, light travels with the speed c, a reference frame which is at rest with respect to it is simply not possible as far as I understand from this answer and all other answers as well. Is my understanding correct?

• nitsuj
So, as in all reference frames, light travels with the speed c, a reference frame which is at rest with respect to it is simply not possible as far as I understand from this answer and all other answers as well. Is my understanding correct?

Yes. The fundamental issue is that the equations for length contraction and time dilation are not defined for light (or anything traveling at or beyond the speed of light). These phenomena apply to sub-light velocities only.

The examples of "all lengths being 0 at the speed of light" or "time stopping at the speed of light" are invalid extrapolations of the equations. These are rife in popular science books and videos but are not found in undergraduate textbooks on SR.

• Ibix
So, as in all reference frames, light travels with the speed c, a reference frame which is at rest with respect to it is simply not possible as far as I understand from this answer and all other answers as well. Is my understanding correct?

Note that if you had a frame of reference in which a light beam were at rest, the light beam would would not be moving at speed ##c## in that frame. But a beam of light must travel at speed ##c## in all inertial reference frames, that is the basis upon which the theory describing time dilation and length contraction is constructed.

No there are not. You can construct theories in which photons have mass and therefore do not travel at a defined speed. The classical version is the Proca potentials, from which you recover Maxwell's equations if you let the mass be zero.

There are implications to a non-zero photon mass, which allow us to measure it. Results are consistent with zero mass, and the upper bound is something like 10-54kg, I believe.

It's worth noting that something that does travel at c must travel at c in all frames in relativity, by hypothesis. Howevern we are not, and cannot be, 100% certain that anything travels at c.

• tworitdash
No there are not. You can construct theories in which photons have mass and therefore do not travel at a defined speed. The classical version is the Proca potentials, from which you recover Maxwell's equations if you let the mass be zero.

There are implications to a non-zero photon mass, which allow us to measure it. Results are consistent with zero mass, and the upper bound is something like 10-54kg, I believe.

It's worth noting that something that does travel at c must travel at c in all frames in relativity, by hypothesis. Howevern we are not, and cannot be, 100% certain that anything travels at c.
Quite interesting though. Thank you!