How does light move?

[claymore]

If I point a laser beam into the night sky and turn it on for exactly one second there will be a beam of light 186000 miles long hurtling through space . what force is it that keeps this beam moving at light speed for millions of years? Friend of mine argues that as soon as the light source is turned off the beam ceases to exist . p.s. I have ten dollars riding on this .

[eltelioni]

It doesn't need a force, the photons have momentum. According to Newton's 1st Law of Motion:
"A particle will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced net force."
So you are correct and your friend owes you 10 dollars.

[claymore]

Thanks eltelioni, the ten dollars is in my pocket. I know that a photon has momentum but what gives it that initial shove ?

[Tom Braider]

What a great question! When I first read it I thought it was simple. But thinking about it for a minute I realised I hadn't a clue how to answer, really.
I "know" that light doesn't need a constant input of force to carry on travelling because I "knew" stuff like "we continue to se the light from distant stars even if they no longer exist".
I "know" that light consists of photons that have the property of always moving (in a vacuum) at a fixed speed, which they don't need to reach by acceleration by an outside force becasue they have no mass. For the same reason, they don't slow down. The instant they're formed, off they go at 186000 miles a second, or whatever the exact figure is. And they continue in a straight line indefinitely. (Although they're affected by gravity because gravity is not a force but a variation in the shape of the universe.)
But when you think about it, what the hell does all that mean? Surely anything that moves must have been given energy at some point? As you say, what gives it that initial shove?
What do you google to get an explanation of stuff like this? Or do I need a lifetime of study? If so, it's too late!

[eltelioni]

The energy comes from an electron in an atom dropping from one energy state to a lower one. Quantum Mechanics tells us that the electron energy states are discrete, i.e. there are only certain energy levels the electron can be in. That means the electron can't just lose a little bit of energy at a time, it must drop to another state and release the energy difference in one step. It does that by emitting a photon. The photon has no mass so it is not appropriate to think about a force accelerating the photon to light speed.

If anyone can improve on this answer, please do so!

[eltelioni]

Another tricky problem is time. Anything traveling at the speed of light does not experience time. In its own frame of reference the photon gets where it is going instantly, so it is created and destroyed in the same instant. That does my head in.