Question: Why does time change when you are travelling at the speed of light?

Anna Scaife answered on 13 Nov 2014:

Hi Rebecca – the best way to picture this is to think about things moving relative to each other. For example, when you’re in a car travelling down the motorway at 70 mph, if there’s another car next to you going at the same speed then neither car is moving *relative* to the other because the difference in velocity is zero. Likewise, if there’s a car coming towards you at 70 mph then their speed *relative* to you is 140 mph (70 – (-70)).

So if you were travelling down the motorway at the speed of light (“c”) then anything coming towards you would theoretically have a relative speed greater than c – BUT it’s not possible for something to have a speed greater than c because c is a constant! So something else has to change… and that thing has to be either time or distance because that’s how you work out speed: v = d/t.

So if something is moving at the speed of light, or close to the speed of light, then either time or distance can appear altered. We call this “time dilation” and “length contraction” in special relativity.