6. The Human Body and how everything stays together

Up until now, I've only described photons going from place to place and their interaction with electrons at the surfaces of mirrors and lenses. QED, however, plays an integral part in describing how stuff stays together in the first place.

Within the human body, the electrons and protons in our atoms are held together by an electric field. But what is an electric field? Who tells the electrons on the carbon atoms on your chromosome 12, "Stick around! There are protons over here!"

QED says the answer is a virtual photon. A proton passes one of these "particles" to an electron, and the virtual photon gives the electron some momentum, keeping it within a certain range of the nucleus. The direction of this momentum (the time-integral of force) corresponds to the direction in which the particle is "pushed". If an electron were to exchange virtual photons with another electron, the force would push them apart.

How these virtual photons determine which direction to push is a difficult question, however. These particles aren't like most anything else because they occupy everywhere at once, so pushing doesn't have the same meaning. (For an explanation, try this Virtual Particle FAQ.) With virtual photons, exchanges can occur instantaneously, regardless of distance - like faster-than-light travel, except you can't send any information along with them.

The net effect, however, is quite important for you and me. Thanks to proton-electron virtual photon exchanges, our bodies (and everything else for that matter) do not disintegrate. Thanks to electron-electron virtual photon exchanges, I am able to type this text without my fingers passing through the keyboard. Finally, thanks to virtual photons being created from vacuum, black holes are able to radiate energy! (This last benefit doesn't have much relevance to anyone living on a normal time-scale, however.)

This has been (only) a qualitative introduction to how QED affects many of the basic things we take for granted. While many of its features were explained by older physics (such as the wave and superposition model of light), QED provides a quantum framework to describe many of these events more accurately and on a more basic level.

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"After all, it doesn't make sense..."