My favourite particle: the photon

Also at The Guardian.

The fundamental particles

The fundamental particles we have identified so far.

I was talking to my daughter about a star she saw out of her bedroom window the other morning. Actually it was Venus. She had learned at school that some of the stars we see aren’t there any more, because the light from them takes so long to reach us. So we are looking back in time when we look up to the sky. My dad told me when I was little that the light from the sun takes 8 minutes to reach us, and I haven’t stopped thinking about photons since.

We consider photons to be light, because that is clearly what they are most useful for in our world. They are also wifi and radio and microwave and x-ray. Wonderful things. So we need photons to see, to communicate and to cook disgusting baked potatoes.

Even with the advent of wifi, which I’m not sure I could live without, visible light is probably the photon at its best. We with our eyes are practically superheroes. We can get a good idea of the composition of a material just by turning our head towards it. My desk is brown in daylight because it is made of carbon, hydrogen and oxygen, which tend to spit out not very many (aka low intensity) photons of a mixture of colours at the yellowish end of the spectrum. The atoms spit out photons when they are excited, which happens because they have absorbed ‘daylight photons’, from the sun. I think this is a bit like an atom’s version of having a pick-me-up of some sort, feeling a bit queer and vomiting. What goes out is not exactly what went in, it has its own properties, but there is a relationship between the two that is altered according to the properties of the vomiter.

Daylight photons have in turn been vomited up by the sun, fat and bloated with energy from all its nuclear burning and particle-antiparticle annihilations. Yes, that’s probably enough with the vomiting analogy. Photons are made in stars like our sun. They carry energy off, and they don’t get involved with the interactions. All other fundamental particles are rather difficult in comparison. They all demand conservation of something typical to them, which puts limits on the kinds of things that can happen between them. These limits are what give us the universe we have, with planets and stuff, so I’m not knocking them.


The sun

The sun.


We can only really look back in time as far as the point when photons were set free from the hot mess of plasma that used to be our universe. The photons emitted then have been traveling for billions of years through their expanding universe. This seems incredible, as does the fact we can see and hear some of these same photons as the interference you get when your telly isn’t tuned in properly.

But to me the most remarkable thing about photons is that they don’t do time. A photon is traveling through the vacuum of space at three hundred million meters a second. Relativity tells us that when an object’s speed approaches the speed of light (relative to us), time dilates (slows down) for it (relative to us). For a photon moving at the speed of light, the passage of time slows to nothing. The universe is eternally frozen in mid-sentence. Nothing happening, time not passing, a bit like living in the northwest suburbs of Illinois.

About lilyasquith

I am a particle physicist working on sonification of the data output from the ATLAS detector at CERN. The project I'm working on is called LHCsound and is funded by the STFC. It is based at University College London where I have just finished my PhD on the search prospects for a low mass standard model Higgs Boson. I have now moved to Chicago to start an ATLAS postdoc with Argonne National Laboratory.
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