Question: This is about the other question i asked about reaching the speed of light and time slowing down. One of the websites that you gave me said that light has no mass, and that it travels instantly in different places. I have 2 questions 1. How is that possible?? 2. If it is possible how did scientists measure the speed of light in the first place if its instantly travels in a vacuum.! Also how can it then take 2million years to reach us from Andromeda galaxy, if has no mass an travels instantly.

Jon Copley answered on 21 Jun 2010:

(1) is a great question! The short answer is that it’s the only possible solution to our observations of how the universe works, so far. Photons can’t have any mass, because otherwise they could not travel at the speed of light. And they have to travel at the speed of light, because otherwise an infinite number of them with zero energy could be created spontaneously.

I *hate* to have to say this, but you’ll just have to “take my word” on that last bit there for now. But you should never just take anyone’s word for anything in science! It would take pages and pages to explain, so if you find ideas like that fascinating, then please keep studying physics as a subject, because it will make it all clear!

(2) We measure light as having a speed because we are not travelling at the speed of light ourselves.

It’s easiest to grasp if we imagine a spaceship that travels from Earth to the Andromeda galaxy and back again at just a fraction less than the speed of light. Now if we had two clocks, which we synchronised exactly, and put one on the spaceship, and kept one here on Earth, when the spaceship came back, just over 4 million years would have passed on the clock left on Earth. But for the clock on the spaceship, only a few moments would have passed.

That’s because time doesn’t actually pass at the same rate for objects that are moving relative to each other. Moving clocks run slow, relative to clocks that are stationary. But to see that effect, you have to be moving very, very fast (or have a very, very accurate clock).

You can see it, however, if you use something called an atomic clock (which is accurate to tiny fractions of a second). If you synchronise two atomic clocks exactly, then send one to Australia on a 747 airliner and back, and then compare it with the other clock that keep you with you in the UK, you can see that the clock that was moving on the plane is running slightly behind the clock that stayed still. It’s bizarre, but it’s true!

Now, back to the Andromeda galaxy… imagine if instead of a spaceship, we sent a beam of light there and back again, at the speed of light. And imagine if we could put a clock on one of the photons in that light, instead of on our spaceship. That clock would show no time had passed for the photon. But 4 million years during its there-and-back journey would have passed for us here on Earth.

Of course, that doesn’t make sense, so physicists say that “the photon has no frame of reference”, because there is no such thing as time or distance for a photon under Einstein’s Special Relativity. That sounds bizarre, but his Special Relativity does seem to explain how the Universe works, at least as we have explored it with experiments and observations so far.