Question: Is it true that that time would stop for something if it travels with a speed of light?

David Freeborn answered on 22 Jun 2013:

Hi usman100,

Yes, you’re right. In relativity, as we get faster and faster, time slows down (for us). One of the ways we make short-lived particles live for longer is to accelerate them to very fast speeds, so that their time slows down and it takes them longer to decay. The speed of light is the speed at which time stops altogether.

The Universe would look very strange for photons, the particles of light. They never experience time, but if they could, they would see other photons moving away from them at the speed of light. This is because the speed of light looks the same, whatever speed you are moving at.

In relativity, changes in speed are equivalent to rotations, and this is one way of thinking about this.

Imagine we have a graph, with space as one axis and time as the other. Your movement is represented by an arrow in a particular direction.

When you are standing still, your arrow is pointing in the time direction. You are moving through time, but you’re not moving through space. If you could travel at the speed of light, your arrow would be pointing in the space direction. You are moving as fast as it is possible to move through space, but you are no longer moving through time. You’ve stopped experiencing time altogether.
For any other speed, you are somewhere between the two. The faster you are going through space, the slower you are moving through time.

But of course, in reality it’s possible for any object with a mass to move at the speed of light. In order to do so, you’d first need to make yourself completely weightless!