Cherenkov Radiation: When Things Go Faster Than Light

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Cherenkov radiation from a nuclear reactor core300,000 kilometres per second is the limit, the speed of light, nothing can exceed that speed. This is one of the most famous results of Einstein’s famous e = mc². That is nice, and convenient, except for that things can go faster than light. Why? The speed of light, the ultimate limit, is only reached in the vacuum of space, otherwise, we can slow light down, and overtake it.

Light always slows down when it passes through matter, and when objects overtake it we get Cherenkov radiation. It is a visual sonic boom, and it works in the same way. Sonic booms are the result of reaching the speed of sound; sound ahead of the object can’t dissipate or get away, so pressure builds. When the speed is exceeded the pressure is released as a terribly loud shock wave that bludgeons the ears of anything within a few kilometres. Cherenkov radiation is similar, but quieter. To observe it, you will need a nuclear reactor core and a lot of water.

In certain nuclear reactor cores water is used as a coolant. Due to the fact that water is, to be simple, made of stuff, it slows the speed of light that passes through it to a rather zippy 224,910 km/s. This is quick, but less than the ultimate limit, so it can be beaten.

The reactor core releases charged electrons which disrupt the electromagnetic field in the water, a field that includes light. They race at various speeds, but many travel faster than light. The electromagnetic charge in front, which is limited by the speed of light in the water, cannot escape. The waves propagate in front and then the science happens. A photonic shock wave. Light is released.

If we managed this feat with an electrically charged jet fighter the result would be even more blindingly brilliant than a sonic boom, unfortunately it is woefully impractical. Similarly if I were to accelerate a bowl of petunias beyond the speed of sound, you would be able to hear it, but it would be much less impressive. So, the result of Cherenkov radiation is nothing but a pleasing, if rather unsettling blue glow from the water.

The discovery of Cherenkov Radiation won Pavel Alekseyevich Cherenkov the Nobel Prize in 1958, it shows that light slows down when it interacts.  There are many speeds of light, but in space, at 300,000km/s light is unbeatable, that we know.

Unless of course we don’t understand the Universe at all. However the chances of that are rather slim.

Further Reading:

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Deployed on by Alexandre Coates in Macro Oddities 7 Current Replies

7 Responses to Cherenkov Radiation: When Things Go Faster Than Light

  1. Andreas

    I have a question. Could this mean, that the early universe was glowing dark blue? Or that there may be a star glowing dark blue somewhere? Or are atomic reactors and icecube-experiments the only areas where this radiation can be observed in the whole universe?

    • Alexandre Coates

      It can be observed anywhere where particles are travelling faster than the speed of light in that medium. It could happen in metal, air or even a bowl of strawberry jam. All you need is to make some particles go faster than the light can in that substance (though that is rather difficult).
      As for the early universe it is very difficult to say. During the first parts of the universe it was filled with subatomic particles in what is called a ‘gluon soup.’ Whether it was thick enough to slow down light more than particles I can’t even guess at. I’m learning more physics all the time, but I don’t know it all yet 🙂

      Thanks for the question.

  2. noname

    your last sentence is conceited jibberish. we don’t know a tenth of what we think we do… try humility sometime

    • Guillermo

      He meant the chances that the speed of light in space are beatable are slim.

    • leeswindell

      What’s your name, ‘noname’?

  3. Pingback: Why do nuclear power reactors glow? – JFSC Physics

  4. john

    Great article! Really enjoyed it. I was looking for an explanation of Cherenkov radiation and this greatly helped me. When I was reading the article, I was wondering if you were going to refer to tachyons since they, theoretically, are faster than light.


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