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I am a biochemistry and molecular biology major. If photons can be absorbed by electrons, wouldn't that mean light has a charge? Electrons only attract positive charges. Isn't it?

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It means exactly the opposite. It means that electron has a charge - the photon interacts with those particles that have a charge because the photon is the messenger of the electromagnetic force that is excited by this charge. But because the "gauge group" behind electromagnetism is Abelian, the messenger - photon - is neutral itself. –  Luboš Motl Apr 14 '13 at 4:43
    
Abelian, neutral? I knew electrons are negatively charge but how could something that's not charged be attracted to something that is charged? Negative attracts positive. –  Jose Apr 14 '13 at 19:44
    
@Jose: That's an oversimplified version of how electromagnetism works. There's a lot going on behind the scenes for "negative attracts positive" to work, and it's not the whole story when it comes to E&M. –  Dan Apr 14 '13 at 23:05
    
@Jose: Also, electrons aren't "attracted to" or "repelled by" the kinds of EM fields we call light. The interaction is more complicated. –  Dan Apr 14 '13 at 23:06
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marked as duplicate by Waffle's Crazy Peanut, Kostya, Manishearth Apr 16 '13 at 6:16

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3 Answers

The simple answer is "no". Absorption and attraction are two different things.

The simplest "absorption" of part of the photon's energy comes in Compton Scattering: A photon scatters on an electron and gives up part of its energy, changing wavelength in the meantime.

Scattering means two trajectories meeting and an exchange of energy kinetically takes place modifying the two participants' four vectors. They were not attracted to each other, their trajectories overlapped quantum mechanically and they interacted.

Absorption is more complicated because it needs all the tool kit of quantum mechanical equations and bound state solutions.

The electrons are in bound states about the molecules and atoms, moving in orbitals. This happens because the attraction of the negative charges of the electrons with the positive charges of the protons in the nuclei of the atoms quantum mechanically is represented by a potential well whose solutions are the orbitals of the electrons. In second quantization attraction is the exchange of virtual photons between the electrons and the positive charges to create the effective potential that binds them to each other.

What may be you call absorption in a photon impinging on atoms/molecules is the total absorption of the photon by kicking an electron to a higher orbital. It is not "attracted" by the orbital. It just happens that the photon's energy is exact enough so instead of compton scattering it is totally absorbed its energy kicking up the electron.

Alternatively, the photon is the carrier of the electromagnetic force that sets up attractive fields in bound systems. It does not itself carry a charge.

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That seems like a great answer, but it's a little difficult to understand =). Thank you! –  Jose Apr 14 '13 at 19:49
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Light does not have a charge. "Charge" is just what we call the tendency of something to affect and be affected by electromagnetic fields, which are what light is made of.

The reason positive and negative charges are attracted to each other is that they both give off these fields. An electron creates an electric field that attracts positively charged things, and a proton creates an electric field that attracts negatively charged things. Moving charges create magnetic fields. Also, a changing electric field creates a magnetic field and vice versa.

If a charge is accelerated, it creates a combination of changing electric and magnetic fields that sustain each other, this is called an electromagnetic wave. Light, radio waves, and X-rays are all just different frequencies of these waves.

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Based on what you just said light does have a charge. –  Jose Apr 14 '13 at 19:42
    
@Jose: The definition of charge you gave in your question is not what people mean when they talk about electric charge. Light does affect electrons, but that's not what electric charge is. –  Dan Apr 14 '13 at 22:59
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No. Photons (of electromagnetic radiation) are produced by accelerating electric charges. This means that an excited electron (at a higher energy state) emits energy in the form of electromagnetic radiation proportional to the difference between the two energy levels (orbitals) between which the transition of electron take place.

If such an excited electron can emit energy as EM radiation and return to a lower energy state, then the symmetry which is it can absorb a photon an get to an excited state (depending on the energy of photon) is also possible. Sometimes, an energetic photon can also knock off electrons from metal surfaces (photoelectric effect).

This doesn't mean that the photons should have charge. Instead, they interact with charged particles and transfer their energy. In short, they're just the electromagnetic force carriers.

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