Question: If times slows down as you approach the speed of light. why doesn\'t this affect light itself??

Jon Copley answered on 21 Jun 2010:

As Louise says, one way of looking at this is to consider that you need to have mass to experience time dilation, so we can then say photons are therefore not affected. But the “time dilation” effect does apply to anything with even just a tiny mass if it is travelling near the speed of light. One of the ways that we can tell this is because we can detect particles called muons arriving at the surface of the Earth.

Muons are created in the upper atmosphere, when cosmic rays hit the atmosphere. Muons have a very short lifespan – they only last a couple of millionths of a second (and they also have a tiny mass). Although they travel at nearly the speed of light, their short lifespan should mean that they could only travel 600 metres or so (if they did not experience time dilation). But we can detect them arriving at the Earth’s surface, 100 kilometres below.

So how can the muons reach the surface of the Earth, if they have such a short lifespan? Well, because they travel at nearly the speed of light, they experience that “time dilation” effect. To us as stationary observers, this gives the muon time to reach the surface of the Earth. But if there were no time dilation effect for the muons, they would decay long before reaching us.

However, here comes the fun part (at least I think it’s the fun part!)… Ok, a photon has no mass (or actually “rest mass”, which is the mass of a photon if we could stop one whizzing about to measure its mass) – every cunning attempt to measure the mass of a photon has come up with a measurement of zero. This means a photon can travel at the speed of light.

Now, I think that another way of looking at your question (and I have to be careful here, because I’m not a physicist!) is to consider that a photon travelling at the speed of light is actually subject to *infinite* time dilation (and it can only be subject to infinite time dilation because it has no mass). Consequently (and here’s the good bit!) ***time does not exist*** for the photon. So from the point of view of a photon, they don’t actually travel anywhere (because travelling involves covering a distance in a time – and there is no time for them. And actually, there is no such thing as distance in their frame of reference either, but that’s another story…).

Anyway, here are what I think are two very nice webpages about time dilation, and Einstein’s Special Relativity, for more info if you are interested:
http://www.btinternet.com/~j.doyle/SR/sr3/sr3.htm

and

http://www.btinternet.com/~j.doyle/SR/sr2.htm