# Size of electron and photon

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Please let me know if the sizes of electron and photon are point like or extended in nature and what evidences are there for the agreed upon size .Is there any way to compare the sizes?

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The electron is a point particle. Scattering experiments have put a small limit on its size.

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The size of a fundamental particle is a tricky idea. It depends essentially on the way you want to measure it.

If using a slow atom to measure the size of an electron on other atom's outer shell, the answer is "the electron has the size of the atom".

But if using a smaller tool to measure the electron's size, the answer gets smaller as well. Use a GeV electron to measure an other electron, for instance the one that was as big as the atom, then the measured electron will give the result expected from a particle that is at one small location - but the chances of being in one particular location decreases with the observed volume.

Up to now, all particles used as a tool, even the "smallest" ones (which often means the highest energetic ones) have seen the electron in a location small enough to look like a point. I that sense, it's a point particle. But:

- Some day an even smaller tool may find a size to the electron

- If the tool is big, so can be the electron

The difference is rather with a composite particle like a nucleus or a proton. Small tools like high-energy protons "feel" the nucleus as a collection of smaller objects spread over a definite volume which keeps its size as the "tool" gets finer.

It's essentially the same story with the photon. A photon can be big or even so huge that one can't imagine it. Take a photon emitted by a star 1 light-year away, in a cone of 1 steradian and lasting 1ps: its volume is 1028m3. But through a telescope, it can be detected by one pixel of a camera, say 5µm*5µm, and then we may decided that the photon was on that area. The chances of catching it were just small.

So the size is not a fixed property of an elementary particle. If a particle can be as local as our tools distinguish, we may call it "elementary". And the idea of "particle", especially the photon in the history, has much to do with this capacity to act locally if the "tool" (here the camera pixel) needs it.

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yes enthalpy is right

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The electron is a point particle, but it does have an effective cross sectional area and a classical radius.

Edited by ajb

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