Last post I talked of the massive tank of ultra pure water that is Super Kamiokande. So how do we see things in SK? What are we looking for? I mentioned the side of the tank is covered in inverse light bulbs turning any light produced in the water into electricity, which we read out with our computers and reconstruct what went on.

But where does the light come from? Super K is kept in complete darkness. The only light comes from charged particles travelling faster than light in the water.

Wait a minute.

Faster than the speed of light.

But I thought that nothing could travel faster than light.

This is true in empty space, a vacuum or air (which is a good approximation of empty space). Here light reaches the natural speed limit of the Universe, which we give the symbol c, equal to about 300,000 kilometres or 186,000 miles per second.

Refraction with soda straw
Change in the speed of light can be seen with a straw
in water. From Wikimedia Commons.

In water light is slowed down, so that it can actually only travel around 3/4 of the speed it does in air or empty space. This slowing of light can be seen in action by putting a straw in a glass of water or your favourite clear soda. You will notice that there seems to be kink in the straw where you go from the air to the liquid; this is because the speed of light, and therefore the fastest path for light to reach your eyes, is differnt in air and water. Because of this effect it is not against the laws of nature for use to produce thing travelling faster than light does in water, such as 0.8c (80% the speed of light in empty space).

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When things with an electric charge (such as the electron which is being pumped around your computer or phone to power it right now) travels faster than light in water then we get a very interesting thing happens.

You may have heard of something called a sonic boom. This is a loud explosion of air that happens when anything travels faster than the speed of sound through air. Cutting it’s way through the sky, a jet fighter plane it is constantly pushing air out of it’s way at airs natural speed, the speed of sound. The problem comes as the jet reaches this same speed; the air cannot get out of the way of the plane fast enough and so begins to bunch up and build up in pressure (energy). As the plane crosses the sound barrier, and travels faster than the speed of sound in air, then this pressure (energy) build up is released as a huge explosion.

F18 Fighter Jet breaking the Sound Barrier.From Wikimedia Commons.
Advanced Test Reactor
Cherenkov Radiation in a Nuclear Reactor.From Wikimedia Commons.

This “Sonic Boom” was the reason that Concorde, when flying it’s fast London/Paris to New York run, could only hit top speed when over the Atlantic. If it had done so over populated areas it would have wreaked havoc with windows below.

The same thing occurs when things with electric charge travel faster than light can. When I say a particle has a certain electric charge, take the negatively charged electron for example, all I am saying is that that something is constantly telling the world around it that “I exist and if you come in to contact with me you must treat me in this manner”. This information which is constantly streaming from anything with an electric charge tells other electrically charged things, as well as electric and magnetic fields, that if you come within the personal space then you should treat me like this: bend me this way in a magnetic field or repel me is you too have the same like charge.

Cherenkov radiation animation
Build up of electromagnetic information results in light going outward in a cone.
From Wikimedia Commons.

Instead of air bunching up as it does with fighter jets it is this information, streaming out at the speed of light within water, that bunches up and builds up in energy (pressure). This energy is released as light, a visible (optical) version of the Sonic Boom. Instead of hearing it we see it, with our inverse light bulbs. This effect is called Cherenkov radiation.

The light comes out in a cone and casts great circles or rings on the side of Super K which can be recosntructed from the 1000′s of electronic eyes covering the walls.

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