# Where do photons get their energy from?

If energy required to accelerate a particle to the speed of light is infinite then where do they get it from?

But first if photon's are massless, then why do they collide to some other thing and get reflected as in our daily life or get deflected after colliding to an atom in the sun and just move inside the sun for a long time before escaping it?

• Possible duplicates: physics.stackexchange.com/q/3541/2451, physics.stackexchange.com/q/20289/2451 and links therein. – Qmechanic Feb 26 '13 at 18:06
• This asks more about their source of energy than acceleration, which could also be a duplicate. – askewchan Feb 26 '13 at 18:09
• It's rather clear that it asks about the source of energy required to accelerate a photon, then asks about why do they interact if they are massless etc. Anyway, hopefully we've covered both bases :-) – alexarvanitakis Feb 26 '13 at 18:13
• At some point you just have to let go the pop-sci heuristics and dig into the math. If you don't you end up asking a endless series of question like this that are founded on fundamental misunderstandings of the physics. Light has no mass (in the proper meaning of rest mass) and so (1) always moves at c and (2) doesn't have to worry about the apparent magnification of that mass. – dmckee Feb 26 '13 at 18:52

Photons have always been moving at the speed of light, they were not accelerated to that speed; you will see that it's going to be difficult to even conceptualise photons accelerating once you learn about special relativity.

In addition, the relation of velocity to momentum is different in special relativity in comparison to Newtonian mechanics so that it is in fact possible for a zero mass particle to carry momentum, and thus collide etc

EDIT: You can see this using the energy-momentum relation: If you set m=0 the formula still makes sense.

Photos get their energy from all sorts of events, but that energy goes into creating the photon, rather than accelerating it. Photons can be created, or radiated, by other energy sources, for example electricity (like a light bulb), nuclear reactions (like the sun or radioactivity), or a particle collision.

When photons collide or reflect they are actually in a sense being absorbed (annihilated) and a new photon is emitted (created). In this case the first photon provides the energy to create the second one.

Your first sentence is correct. You said acceleration to light speed takes infinite energy - yes its true. We don't know for sure about photons but WE think they are not accelerated! They are created at this speed so no problem with infinity arises.

The energy required to accelerate a massive particle to the speed of light is infinite, yes. Photons are massless, though, so they travel at the speed of light regardless of what amount of energy you put into them. (It doesn’t really make sense to talk about accelerating a photon, since it will always travel at the speed of light.)

I see that none of the answers really cover it why photons always travel at the speed of light and where they get the energy from.

1. When the universe was created there was first a sea of photons, they were all created as pure energy, and thus they were all moving at that only speed, the speed of light. Time did not exist, or at least photons did not move in the time dimension, their speed in the time dimension is 0 or not comprehensible.

2. You think of this the wrong way, you think you need to speed up to that spatial speed c, but that is not right. Energy moves at the speed of light, and everything else is relative to that.

3. You need energy to slow anything down from the speed of light, and that energy comes from rest mass. Rest mass is the thing that slows particles down from the speed of light to a slower spatial speed.

4. Photons always move in empty space (although EM waves can move in mediums), and they do not have a rest mass, nor can they be brought to rest.

5. In empty space, you do not need energy to keep moving.

6. The only way to slow down photons is by gravity. If you look at a photon near the sun, it will move slower then c (measured from the Earth), it is called the Shappiro effect.

7. The only way to bring a photon to spatial speed 0 is to absorb it.

8. But in this case the photon does not exist any more so its speed is not comprehensible.

## protected by Qmechanic♦Nov 7 '13 at 0:49

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