# Relativity, the flow of time and speed

1. Nov 14, 2007

### atymtrvlr

Question:

According to Einstein an object traveling at relativistic speeds would not experience the passage of time (time stands still).

Speaking from the vantage point of the relativistic object:

Let's consider a photon. At 1C time stands still for the photon (T=0).

1. With the absence of time, isn’t the relative velocity of the photon actually infinite (again, from the vantage point of the photon)?

By this I mean V=D/T where V=velocity, D=distance and T=time

Speed is relative to distance traveled in a given unit of time. If T reaches 0 (zero) the calculation cannot be performed & speed cannot be determined.

In another example, if you were in a car traveling at 1C, your speedometer wouldnt function. It requires space and time to calulate speed.

Is this right? Does anyone have any thoughts on this?

I also have a follow-up question.

Thanks!
atymtrvlr

2. Nov 14, 2007

### Chris Hillman

You went off the rails at the very beginning

No, "relativistic velocities" means "fast enough that relativistic formulas must be used". You are thinking of taking the limit $v \rightarrow c=1$ in the Lorentz transformation, but this fails as you can check. Physically speaking the reason is easy to understand: if you try to accelerate a particle of mass m to the speed of light you will need an infinite amount of energy, which is impossible.

Several items from the sci.physics FAQ should help: see http://www.math.ucr.edu/home/baez/physics/

3. Nov 14, 2007

### atymtrvlr

Thanks

For the response.