# Particles and forces

1. Mar 22, 2006

### skywolf

if a photon carries energy from one electron to another?
then what carries energy from the electron to the photon?
or is the photon just formed from that energy?

2. Mar 22, 2006

### vaishakh

What do you mean by carrying energy. Photon transports energy from one place to another place. But electron is stationary in an atom. Moreover Photon is an amount of energy, a packet in which energy exist. So there is nothing like carrying. If energy is radiated back, that will happen only in the form of photons.

3. May 12, 2011

### stonehaven

It is thought to be massless but how can you have energy without mass?

We can create llight or photons by heating metal untill it glows and starts to release photons or the philiment in a lightbulb right? well where are the photons comming from? Can they really be just massless packets of energy?

I dont think so, they must indeed have mass which come from other particles from which the photons are beiing emmitted.

4. May 12, 2011

### mathman

The photon "mass" is given as its energy equivalent.

5. May 12, 2011

### phinds

Einstein showed a long time back that mass and energy are equivalent, as Mathman said. If you don't believe that, you'll need to look into the basics a bit more.

6. May 12, 2011

### WannabeNewton

A photon does not have rest mass which is the conventional mass you think of.

7. May 12, 2011

### DaveC426913

Yes. That is pretty much the definition of a photon.

Energy and mass are two sides of the same coin, and can be converted back and forth.

8. May 12, 2011

### henry_m

There is a lot of confusion regarding the relationship between mass and energy which comes from the old-fashioned distinction between 'rest mass' and 'relativistic mass'. The modern point of view is that the only useful sort of mass to talk about is the 'rest mass'.

The famous equation E=mc2 isn't quite complete and this causes a lot of confusion. The full thing is E2-p2c2=m2c4, where p is the momentum. This equation essentially just says that the LHS is an invariant quantity for any particle (or indeed any isolated system), and it's value defines the mass. Photons are massless, but they carry energy since they have momentum: E=pc. Ideas such as 'photons have mass by virtue of their motion/energy/momentum' and 'stuff gets heavier as you speed it up' are outdated and quite misleading I think.

Think of it like this: mass is the energy a system has merely by virtue of its existence.

Sorry to go off on a bit of a tangent, but hopefully this will help to clear up some confusion. I think if anyone wants to ask more or disagree, best start a new thread to avoid going too off topic on here.

9. May 13, 2011

### Drakkith

Staff Emeritus
The photon and the electron interact through the electromagnetic force. You could say that is what "carries" the energy, though I don't think that phrase is very accurate. Remember that an electron and another electron can easily transfer energy between themselves. Simply shoving one at another one transfers the kinetic energy to the other electron.