Light doesn't travel through time?

[Fat=laziness]

According to the special theory of relativity: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

But light travels through space precisely at the speed of light. Doesn't that imply that light doesn't travel through time? What does that even mean?

 

[curiousasker]

I'm no expert, but I'll take a stab here.

If you were able to completely stand still in space and did not move at all, you'd not be moving through space - only through time.

Now, imagine you're in a spaceship with a speedometer, and you slowly accelerate. As you speed up, you move less through space and more through time. You can easily imagine time dilation taking effect when you're at speeds much greater than normal - you'd be experiencing more time for every unit time an outside observer is looking at you. One year for you could be a second for everybody else.

The closer your speedometer reading gets to c, the slower the time goes for you. When you eventually do reach c, time will be so slow, it has stopped moving completely. An infinite amount of time can pass by for you, but not a millisecond has passed for the rest of the universe.

At that point, you'd be moving through space completely, and not through time. Exactly what a photon does.

 

[Dale]

We have had a rash of threads on this exact topic recently (must be infectious). Please spend a few minutes looking. I am getting tired of this same discussion.

 

[A.T.]

According to the special theory of relativity: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

But light travels through space precisely at the speed of light. Doesn't that imply that light doesn't travel through time? What does that even mean?

This is one possible geometrical interpretation of special relativity, where proper time is a dimension, and aging corresponds to movement along that dimension. Since light doesn't age, it is indeed only traveling through space, in this particular model.

See:
http://www.adamtoons.de/physics/relativity.swf
When you pull the speed slider towards 1c, the rocket becomes light, and the proper time goes towards zero.

 

[Fredrik]

According to the special theory of relativity: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

But light travels through space precisely at the speed of light. Doesn't that imply that light doesn't travel through time? What does that even mean?

I posted a detailed reply in one of the other threads. Link.

 

[jtbell]

We have had a rash of threads on this exact topic recently (must be infectious).

More likely, some TV network has been re-running Brian Greene's programs again.

 

[Dale]

More likely, some TV network has been re-running Brian Greene's programs again.

That didn't occur to me, but it seems likely now that you mention it.

 

[rahulpark]

According to the special theory of relativity: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light.

But light travels through space precisely at the speed of light. Doesn't that imply that light doesn't travel through time? What does that even mean?

Light is special, unlike other matters.If it doesn't travel in time how do you find it moving. Light is only object which has constant velocity in all frames.Special Relativity doesn't apply to light. Can a single photon see another photon moving at velocity of c? Light is the basis of Special Relativity but it can't predict light.

 

[Fredrik]

Light is special, unlike other matters.If it doesn't travel in time how do you find it moving. Light is only object which has constant velocity in all frames.Special Relativity doesn't apply to light. Can a single photon see another photon moving at velocity of c? Light is the basis of Special Relativity but it can't predict light.

There's no inertial coordinate system that's comoving with light, but that doesn't mean that SR doesn't apply to light. It certainly does.

 

[Dale]

Light is the basis of Special Relativity but it can't predict light.

It is true that light is not predicted by SR, it is predicted by Maxwell's equations. However, strictly speaking, light is only the basis of SR in a historical sense. Logically, all that is necessary for SR is for there to be a finite speed which is invariant.

 

[rahulpark]

There's no inertial coordinate system that's comoving with light, but that doesn't mean that SR doesn't apply to light. It certainly does.

note that the time travel or spatial travel is found only by other inertial coordinate system. We find it to be moving at c. Then, how can you say that light obeys SR's second law.
SR doesn't define coordinate system moving at velocity c. These corrections were made so that light travels at the same speed in all coordinate systems.

 

[Dale]

We find it to be moving at c. Then, how can you say that light obeys SR's second law.

The second postulate says it moves at c in all inertial frames. We find it to be moving at c in all inertial frames. Therefore it obeys the second postulate.