We don't really understand the Earth's magnetic field. We think it's produced by something to do with lots of molten iron flying around in the Earth's core. It's really difficult to model and on average it seems to flip about every quarter of a million years. It's been a million years since the last one flipped, so we think we're probably about due for one. This magnetic field does all sorts of things. It protects the atmosphere from the solar wind which is lots of particles being thrown off from the sun. They get caught by the magnetic field and get thrown off to the North and South Poles. This is what causes the Aurora Borealis or the Northern Lights. If the Earth's magnetic field flipped, it would probably almost disappear and start changing round, so we'd all get Northern Lights. It would have some negative effects though. It might have some effects on the climate because it would heat up the top of the Earth's atmosphere and it would do things like stop radio communications.

I have a bottle hear so I can demonstrate this for you. First of all, I'm going to blow a note. Now we all know that if we put a bit of water in the bottle, we expect the note to go up because there is less air in the bottle to vibrate. If there is less to vibrate, then you get a higher note. Now the question is, if I squeeze the bottle and make it smaller, surely the note should also go up. But what happens is that it goes down. If I take the squeezed bottle and lower it into a tub of water, it goes up again, and this gives us a clue. When the bottle is perfectly round, we know that round things are stiff and solid. That's why pressure vessels are round and why a coke bottle with pressure in it is round. That's the strongest shape. But when you squeeze it you make the sides flat and that makes them weak. So actually you can tell when you put your finger on the side of the bottle, you'll actually be able to feel the flat bits vibrating a lot. That means that the bottle has an effective size that is bigger than it really is because it's moving some of the space around it. So the note goes down.

There are two questions here. One is if we had a very loud screaming baby on the Earth, would somebody on Mars in theory hear the baby screaming, or would you need to have some interplanetary baby monitor? The answer is no, you wouldn't be able to hear the sound from Earth on Mars because the space in between Earth and Mars is a vacuum. There's a wonderful experiment you can do where you put an alarm clock under a bell jar and you start a vacuum pump going. You then stop hearing the bell even though the bell is still going. But then the question is, well what if your baby was in the bedroom in your Mars planetary home? Would you be able to hear the baby crying from downstairs in your bedsit? The answer would depend very much on the atmosphere on Mars and my understanding is that there's not very much atmosphere: about 1% of the atmosphere on Earth, so it's getting close to a vacuum. So there are two things: the amount of energy that you can pump into the atmosphere is less; and also the speed of sound would be a lot faster.

Instruments are quirky things. It's a bit like cars; you might be driving along and when you go 57.5 miles per hour, the steering wheel starts to wobble. That's just a particular characteristic of your car. Some things you can fix and other things you can't. Some of you who play the cello or others stringed instruments might know about the wolf note: there's a certain note that just does funny things. It could be that it's an interaction between the acoustic mode or the note you're playing which is all to do with the air, and the actual structure of the flute such as it being made of metal. That's very important. You can think of a saxophone and a clarinet: a clarinet is made of wood and a saxophone is made of metal. That's essentially what makes the instruments sound quite different. So there is an interaction between the sound, the air and the metal and maybe that's why you pay an extra £500 for an ultra good flute.

If I play a nice pure tone, then that's got a particular frequency. But if I were to play all notes together, then that starts to become what you might call noise. Let's imagine that you played every single note that you can possibly imagine, which would not only be the white and black notes on the piano, but all the notes in between. In acoustics we would call that white noise. It's not just sound where you find vibration. You find vibration and waves in the surface of water, light is a wave, you'll find waves in slinky springs, and all over the place. You may have heard of things like the cosmic background radiation. Actually these are all electromagnetic waves of various kinds and light can be as much white noise as anything. So the reason it's called white noise is because we know that white light is made up of all the colours of the rainbow; lots of frequencies all at once. So white noise is taken from white light. When there was the big bang all sorts of frequencies were emitted. The noise from the big bang had one peak frequency so it's not all the frequencies at once and so it probably isn't really white noise.

One of the reasons is that not all instruments are harmonic. If you were to take the example of a Javanese gamelan, they use a lot of metallophones. Because they are metal bars, they don't strictly have harmonics. In fact they're what we call partials. Rather than having a frequency at which they vibrate and harmonics above that of one, two three or four times, the modes of vibration are no longer integer multiples, so 1.54, 1.76. Hence they make a wonderfully rich sound. But when we play them with other instruments that are harmonics, the harmonics and the partials don't coincide with each other and that causes a very confusing pattern of vibration inside the cochlea and the inner ear. It's this which leads to problems with tuning between these two different instruments.

Ultrasound is sound but it's got such a high frequency that you can't hear it. It's so high and the wavelength's so small that you just can't hear it.