# How do electromagnetic waves/light travel at the speed of light?

When something reaches the speed of light, then time is said to stop, or at least is perceived to have stopped. The reason is that at that speed, the object become infinitely heavy and requires infinite energy to move. My question is, why does light itself (electromagnetic wave) continue to move, or move at all, if it is travelling at the speed of light, and should be frozen in time? My own best guess is because light doesn't have mass, and so does is not affected by $E=mc^2$. But then why aren't other energy-only waves able to travel at light-speed? As for the implications, would it be possible to travel at the speed of light if an object could be converted into light and then back somehow?

Lots of questions! I'll try to get each of them:

the object become infinitely heavy and requires infinite energy to move

Light (photons) have no rest mass - ten, a hundred or a million times nothing is still nothing, so the (non-existent) mass isn't relevant - as you say further down. (Light does, counter-intuitively, have momentum, but that's another story).

why does light itself [electromagnetic wave] continue to move, or move at all, if it is travelling at the speed of light, and should be frozen in time?

This depends on your perspective - if you're "riding on the photon" you'd find that it doesn't travel - no time passes - and the point where it "starts" and "finishes" are effectively the same. In technical terms, "the spacetime interval for any light-like path is always zero". From the perspective of us, sitting on Earth, and looking up at the Sun, yes, the light takes ~8 minutes to get here.

But then why aren't other energy-only waves able to travel at light-speed?

They are...probably. Both theory and current experimental evidence show that gravitational waves, for example, propagate at c. ("Speed of light" is a slightly misleading term in that regard.)

As for the implications, would it be possible to travel at the speed of light if an object could be converted into light and then back somehow?

Yes, absolutely. If I was able to read every last detail about what made up "you" and then email it down a laser beam to the Andromeda galaxy, where it's then recreated by a magic box, then you'd experience zero time between your data leaving Earth and arriving at Andromeda, 2.5 million lightyears away. There's good reasons why this is impractical/impossible in practice, but if you were to write the idea into a SF story it'd be considered accurate, relativity wise.

• So let's see: If light has momentum, it also has mass? Second, shouldn't light be stationary for the observer, and moving for the one riding it [I am inferring this based on what scientists have said about blackholes] May 14, 2016 at 20:12
• Third, give me the five minute list of why it is impossible to, say, convert people or at least computer data into electromagnetic waves, and 'email' to mars by laser. And I meant to ask if light has inertia. May 14, 2016 at 20:20
• 1: physics.stackexchange.com/questions/2229/… 2: The idea of an "observer riding the photon" is a bit lies-to-children and handwavy. Remember, no time passes, so an observer couldn't actually observe. 3: Quantum mechanics is the main problem, on top of being way beyond our current tech anyway. And we do convert computer data to light (inc radio etc) for lightspeed communication. May 15, 2016 at 0:05