photography by mark waugh
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Faster Than Light

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.

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).

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.

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.

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.

Dr Ben Still

The Super K in Super K Sonic Booooum

Super Kamiokande (Super K for short) is a real piece of scientific equipment, a particle detector, located underneath 1km of rock at the heart of Mount Ikenoyama in Gifu prefecture in western Japan. It is a massive cylinder as tall as a 14 story building (40m) and the same in diameter. Looking into this cathedral sized cylinder are 11,146 electronic eyes called photo multiplier tubes (PMTs) attached to the walls. Essentially inverse light bulbs, turning light into electricity rather than the other way around, each of these PMTs are whopping 50cm (20″) in diameter.


This vast space is filled with over 33 Olympic sized swimming pools worth (50,000 tonnes) of ultra pure water. The water is so pure in fact that if you were to put your hand in it you would lose all of the naturally occurring salts and oils in your skin leaving it bone dry. The reason for this is we require light to be able to travel the entire distance through the water to reach our electronics on the walls. Any impurities dissolved, like the stuff you see on the back of your bottle of mineral water, would scatter the light on it’s journey to the walls and reduce the amount we see.

So light from particles in this huge amount of ultra pure water enter our inverse light bulbs and is turned into electricity which we then channel to our computers to build up a picture of what is going on in Super K.

Dr Ben Still

Next Blog: Faster than light.


Tomorrow is our final day and I don’t want it to end. Each day I have lost track of time in either the boat or lecture room of the darkened John Dalton workshop. Each hour racing by as if minutes. Time flying as I get into discussions, answer questions, talk about physics and give science boat tours. Not my normal 9 ’til 5, not that it even exists for physicists.

Once more to the boat, once more to the talk. Once more to tentative audiences hanging on my words, surprised and amazed with the booooum and the light. Once more I will miss my lunch, forget my hunger and push on through. Once more I will have the privilege to talk of ghosts, faster than light and the creation of it all.

Once more I will wear my white Tyvek suit. Come and join us before we are gone.

Dr Ben Still

Build it and they will come…

We have had over 250 visitors through the doors of Super K Sonic Booooum this weekend and with a busy start to the day today, when we open at 2pm, we hope to continue string right through to Wednesday. If you have been one of the lucky through our doors we would love to hear about your experience. Leave us comments or questions on this website or contact us directly. If you have not visited yet it is not too late to buy tickets through this site or just drop by to see us in the John Dalton building, M1 5GD; we are open from 2pm-9pm today (Mon), 11am-4pm tomorrow (Tues) and 2pm-9pm in our grand finale on Wednesday.

We have had a brilliant team of volunteers helping us out and I would like to thank you all for your hard work, managing all the people visiting and joining in with boat and rope pulling duty. The photo attached to this blog was taken after our triumphant first day with just some of these volunteers alongside the scientists, technical crew and the design director Nelly.

I look forward to seeing and talking with many more people, answering questions and transporting them to Japan to talk about particle physics.

Dr Ben Still

The Weekend

And so ends a long weekend. Long but rewarding. I have had an incredible amount of fun talking about science with many different people this weekend. I have received some brilliant questions. Taken members of the public to a mine underneath a mountain in the West of Japan. Discussed the life and death of stars, the creation of the entire visible Universe and the symmetrical laws that govern it. All of this from a disused workshop in Manchester.

From a girlfriend surprising their boyfriend to families on a day out, all seem enthralled by the sights, sounds, atmosphere and science of Super K Sonic Booooum. Wowed by the experience all seem to discover something new about the Universe in which they live. Maybe the fact that it is all made from just twelve building blocks or that light travels slower in water than in air. Whatever the message taken away I hope it raises questions, questions that were not there before.

Three more days and I hope many more questions and intrigued people to follow.

Dr Ben Still