Tuesday, 9 November 2010

Could I use my mobile phone as a particle accelerator?

We all know the sound well. All is peaceful, driving along in the car or sitting in a room listening to a speaker… and all the sudden “blip bzzzzz blaaaaar bzzzzzz”. Something starts buzzing. We all know why; there’s a mobile phone nearby. This noise has somehow become a part of our everyday lives but why does it happen?
If you were an early mobile phone user you might have noticed that they didn’t have the same effect – so it has to do with the GSM phones that we (mostly) use now. There are three things which need to happen together the cause the buzzing:
1.     A pulsing radio transmitter
To connect to the tower and your network, your mobile phone sends out little bursts or pulses of information around 200 times per second (200 Hz). Each of the pulses have a frequency in the radio range, so the signals themselves oscillate millions of times per second – in the MHz range. So this is a ‘pulsing radio transmitter’.
2.     Relatively strong power
Compared to original analog phones, newer mobile phones give out a stronger signal strength as they don’t give it out continuously – as I said above it is ‘pulsed’. For most GSM networks the peak power of the pulses is about 8 times the average power. So we have a relatively strong power source.
3.     Close to a particular type of electronic element
The particular element in the circuit of a speaker which picks up the signal can vary, but it’s usually some solid state device like a diode or a transistor. When the strong pulsed radio signal is near it, the circuit element can detect the pulses and amplify, then send them to the speaker so we can hear the pulsed signal.
The pulses which are amplified come at 200 Hz, and since Middle C on the piano is 261.63 Hz, it is no surprise that we can hear it!
By now you’re probably wondering, given the title of this post: “Could I use my mobile phone as a particle accelerator??”
Particle accelerators use RF (radio frequency) waves to accelerate particles – similar to the ones that are used by your mobile phone. To give a particle some energy, you need a cavity – which is basically a hollow metal box.
When a wave of the right number of oscillations per second enters the cavity, it bounces back and forth within the cavity, with low loss. In physics we call this a standing wave. As more wave energy enters the cavity, it adds to the standing wave, creating a ‘resonance’.
What happens when we put a particle through the cavity? If we time it just right, the particle will always see a ‘peak’ of the wave and will gain a little bit of energy from it. If we didn’t time it right, the particle would lose energy to the wave and slow down. Fortunately, we’re very good at timing these things precisely!
An LHC RF cavity which works at 400 MHz. Photo courtesy of CERN.
The particle only gains a little bit of energy each time, so it’s useful if we have a circular accelerator where the particles go through the cavity again and again, gaining more energy each time. This is what happens in accelerators like the Diamond Light Source (using electrons) or ISIS (using protons), and even in the LHC. We can also use more than one cavity, the one in the picture has (I think) four cavities in one, and we can place them end-to-end. It’s a game of weighing up the cost of more cavities with how long your particles will take to accelerate.
So is it possible to just connect up a mobile phone to the cavity and accelerate particles? It would be great (and make my life as an accelerator physicist a lot easier!) but unfortunately it’s a question of power.
A mobile phone handset has a peak output power of about 2 Watts. The LHC cavities require a power of around 300 kW per cavity – so you would need the same power as 150,000 mobile phones!
So unfortunately, it looks like it isn’t going to happen any time soon. But in the meantime, I find it fascinating that the LHC and my mobile phone are, at least in some way, based on the same technology.

4 comments:

  1. .. and don't forget the venerable old ISIS injector - three of the linac tanks each pulsing at around 2MW 50 times a second (2 million mobile phones) .. and that is far from cutting edge hardware

    http://twitter.com/@doubledodge

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  2. .. or a million phones at 2W each phone...

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  3. Wow, 2 MW, that's a serious electricity bill!

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  4. True, it would be if it was constant, but as its only ON for around 350us that's actually a duty cycle of around a 1:60. Still more than I would want to pay if I was picking up the bill though.

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