Chris -   There’s some suggestion that if you have big, seismic events in one place that you can encourage faults to be triggered in other places. On a global scale the answer is probably not because there are thousands of earthquakes happening around the world all the time. By the time the energy reaches an individual fault the chances are it’s probably largely diffused so it’s not sufficiently intense to trigger anything.

Dave - I think it’s possible for one earth quake to trigger another earthquake which probably would have happened in the next year anyway. as one piece of rock is pulled across another at some point is is just about to slip, at this point quite a small vibration can make it slip.

Chris - Presumably it’s geographically quite close.

Dave - Yeah the closer you are the more likely it’s going to happen. I think there’s evidence it can happen the other side of the world but only on earthquakes which would have happened very soon anyway.

Chris - You’re right. HIV, when you become infected with it, it infects a class of cells that have markers on their surface called CD4 cells. That can include white blood cells, macrophages and a whole other class of immune cells. Some of those cells become what’s called productively infected. So the virus goes in, hijacks a cell and turns it into a virus factory. Not all cells have that happening in them. In some cells the virus goes in, it doesn’t turn it into an immediate virus factory. What it does is it makes a DNA copy of the virus RNA and integrates that DNA copy or inserts the DNA copy of the virus inside your own DNA. Then it just turns off and so you have cells wondering around your body that contain HIV and they can turn on that HIV when they want to or when the signals are right for that to happen. To all intents and purposes they’re just a cell going about their daily business. How do you know that person’s got HIV? There are tests that we do in the laboratory to detect HIV are what’s called serological tests. One test will look for antibodies because although people don’t seem to become immune to HIV they nonetheless make huge numbers of antibodies against different parts of the virus. We run a blood test in which you take a sample of the patient’s blood and you present that blood sample with various proteins which are made synthetically but they’re based on what’s on the surface of a virus. They look for whether antibodies in the patient’s blood can bind or lock on to that surface with the viral coat on it. If that happens it means that it’s a reaction test and it goes positive and we can tell. Another way to do it is if people are just acutely infected, they’ve only just been infected with HIV they may not have made any antibodies by that stage. That sort of test would miss that. There’s another kind of test which looks instead for virus antigen. When the virus is growing in cells it’s producing lots of virus proteins which get spat out by cells and they go round the bloodstream. You can do various tests which do the reverse of the test I just described. They have antibodies on the surface of the test plate. Those antibodies grab out of the blood any virus proteins. You can then detect that they’ve been picked up and that gives you a positive. There’s two ways to do it. It’s all done indirectly by markers. There’s a third way which is actually doing it by DNA tests. You can take a sample of a patient’s blood and you can then do PCR – polymerase chain reaction – and you can try to amplify or copy virus DNA and it only copies if the virus is present. You can detect how much virus is there and you can detect virus that’s lurking inside your own genome.

Dave - You certainly can conduct electricity in a liquid. Whether it’s going to move fast enough to overwhelm the flow is basically you need a voltage. If you’re trying to make current flow opposite to the liquid you need more voltage to get the same current because there’s going to be more resistance. It’s effectively moving a lot faster. You certainly can get an electric current moving in a moving liquid.

Chris - How fast does an electric current flow through a liquid?

Dave - Normally in a metal it’s very slowly because you’ve got an awful lot of carriers. On average it’s moving a few millimetres a second.

Chris - The effect is instantaneous because it’s like Newton’s cradle: you put charge in at one end and it knocks everything along and charge comes out the other end.

Dave -  The actual signal is moving at near the speed of light. In a liquid you should certainly get a Newton’s cradle push-along. You might need a little more voltage to get the same current though.

Helen - They contain something called oxalic acid. The reason it’s in the leaves is because it’s there to put predators (herbivores who come along and munch them) off. It’s not good for them and it’s not very good for us as well. You would have to eat an awful lot of it to actually kill yourself. The lethal dose: LD50 which is enough to kill off 50% of the rats that are given the dose of 375mg per kg of rat. That would equate in humans to around 5kg of leaves which anyone would believe is rather a lot. Yes. It’a problem. It acts through your kidneys. It’s a compound that actually interacts with metal ions and can form crystals and trigger kidney stones. That can be a problem. Symptoms include weakness, burning of the throat and mouth, difficulty breathing and if you’re really unlucky – a coma. Stear away from those rhubarb leaves I think is the answer.

Chris - What I want to know is who discovered that you could eat the bit in the middle but not the bit at each end.

Helen - So many questions on food. There’s the weirdest foods in the world that you’d never imagine. Someone, I think, tries anything.

Dave - This is a mangling of quite a famous experiment. What the actual thing they should have told you is that if you fire a rifle horizontally and drop the bullet from the same place at the same time then they’ll both hit the ground at the same moment.

Chris - So you’ve got a bullet in one hand, rifle in the other. You fire the gun at the same moment you let go of another bullet from your hand. The two bullets should actually hit the ground...

Dave - At exactly the same time. If there’s no air resistance basically how fast you’re going horizontally has no relationship to how fast you accelerate downwards. The bullet which is moving and the one which is accelerating downwards both move at exactly the same speed. They’ll both hit the ground at the same time. If you take a gun and fire it straight downwards then the bullet’s going to come out of the gun at several hundred metres per second. It’s going to hit the ground far quicker than the one you fire horizontally.

Chris - We did an amazing experiment at school which I remember to this day which shows how good this experiment was: the monkey and hunter experiment. We had a sort of blow-pipe with tinfoil across the end of the blow pipe which was making an electrical contact to an electro-magnet that was holding a tin can at a distance. A ball bearing was put into the glass tube. You blow down the glass tube so the ball bearing leaves the blow pipe, breaking the piece of foil in the process. Therefore it cuts the supply to the electromagnet. The tin can starts to fall at exactly the same rate as the ball bearing leaves the tube. If all things are equal, i.e. the can is being accelerated down by gravity at the same rate as the ball bearing is being accelerated down by gravity the two will hit each other. They always do. It’s called the monkey and hunter experiment because the idea is that the monkey is dangling in the distance on a tree and the person fires the gun. Assuming it takes no time for the gun discharge to reach the monkey. The monkey lets go of the tree and starts to drop at the same minute the bullet leaves the gun. Therefore the bullet’s falling and the monkey’s falling and they should still reach each other. It’s a very elegant way of explaining it.

Helen - Underwater animals glow a lot; much more than on land. That comes down to the fact there isn’t any light once you get only not very far down into the sea. They do tend to glow and it’s a chemical reaction. It happens in all sorts of different ways. Some creatures use a type of bacteria. Some creatures use their own chemicals which are then put together. It’s essentially something that a general group of compounds that we’ll call luciferins which naturally undergo a reaction with an enzyme that catalyses that reaction. That’s called luciferase. It turns luciferin into something called oxyluciferin which, at the same time, produces light. One of the reasons why light is important: it’s a good way of communicating in the dark .It’s also a good way of snooping in the dark. One of the most clever fish is something called a loosejaw fish which is a malacostade. They actually cheat because most of the light in the ocean that are created by creatures are blue or green. That is actually the light that can be seen down there mostly. The red lights get absorbed much earlier higher up in the water column. This creature called the loosejaw fish creates its own red light. Nothing else can see it because no fish really bother trying  to see red light because there isn’t any red light down there. If you make your own red light and you can see it yourself then you’ve got your own secret sensor.

Chris - It’s a spotlight so you can go searching underwater.

Helen - It’s ingenious because they can see things, the prey they’re after don’t know they’re being looked at and they can see it. It’s really ingenious. Another thing, one of the glowing creatures is the jelly fish. One of the Nobel prizes this year went to three guys: Osamu Shimomura, Martin Chalfie and Roger Tsien who jointly were involved with the green fluorescent protein, GFP, which I think Chris you know all sort about in terms of the molecular world and what’s going on inside us. By attaching this glowing thing onto parts of anything you can put it under a microscope and see what you’re looking at. It’s a really ingenious thing which has been recognized by this Nobel prize.

Chris - GFP is just a fluorescent protein that some animals make but they can grab ultraviolet rays and turn those ultraviolet rays into green light that we can see.  It’s a clever protein that’s capable of doing that.

Dave - I think there’s a couple of reasons why it might make a subtle difference. One of them is that there’s a lot of oxygen in air that we breathe. Oxygen’s quite bad for rubber. It will cause it to break down and get brittle and crack. That’s going to reduce the elasticity of the tire and make it last less long and make it slightly less efficient. The other thing is that ideally you want a gas inside your tire which if it compresses then expands again it doesn’t absorb any energy. The ideal gas for that is what are called ideal gases. The best ones are mono-atomic gases like..

Chris - Argon, xenon…

Dave - Helium would work probably because it would be quite cheap. It would escape very easily because it…

Chris - It’s very small and would escape through the gaps in the rubber molecules.

Dave - If you’ve got things like diatomic gases, things like nitrogen and oxygen then that’s slightly worse because they take more heat to heat up. They can’t just move – they can actually spin as well which is another way of absorbing the energy. Triatomic gases, things like carbon dioxide and water take even more energy to heat up. They are less elastic.

Chris - Just on a more pedantic scale, Dave. Isn’t it that a nitrogen molecule weighs slightly more than an oxygen molecule also worthy of mention? If you pump just nitrogen into the tire the actual gas inside will weigh slightly less?

Dave - It might have a minute effect. I think fundamentally that unless you’re doing a huge number of miles and really stressed about the efficiency increasing the pressure of your tires a bit more is going to have much more effect than filling them with nitrogen.

Chris - It’s a very good question. The answer is that bacteria are single-celled organisms. They are living, alive and have a metabolism. Viruses are the ultimate parasite. They’re not really alive. They are an infectious bag of genes which are absolutely tiny. A flu virus is one ten-thousandth of a millimeter across and they’re so tiny that they do not have any f the machinery inside them to make new copies of themselves. They have to infect a cell in order to do that. That means you’ve got a problem because bacteria look totally different to our own cells so it’s fairly easy to make drugs and chemicals that will roger a bacterial metabolism and which will not affect our own metabolism. Because viruses have to prey on our own metabolism and they have to use our own cells to make copies of themselves it’s very difficult to find ways to discriminate between the virus and a healthy cell and therefore avoid side-effects. There are some drugs that can do that. The most famous is a drug called acyclovir, most people will know it as zovirax which you put on cold sores. This works because the drug is a special chemical which is activated only in the viral infected cell. The virus makes an enzyme which locks onto the drug molecule and it switches it on. It will not get switched on in any other cell and once the drug is switched on it forms a special DNA letter which when the virus incorporates it into its DNA it cannot make the DNA chain of the virus grow any more. The chain terminates the virus and stops the virus growing its own DNA. It’s very difficult to do this. This thing that researchers are now looking at is the possibility of something like RNA interference. This is where you make short pieces of genetic material which are the mirror image of the viruses own genes. By putting those into the cell they lock onto the viral genes and make what’s called double stranded RNA. The cells usually associate this with rubbish, junk or viral infection and they target them to the cellular equivalent of the wastepaper basket and It gets ditched. That’s how you can switch off viruses and cells. Our own immune system uses that strategy as well sometimes. It’s a key problem that we’ve been grappling with for a long time. That’s why we haven’t got a cure for the common cold, I’m afraid.

Chris - There would be if you were to breathe in all the smoke. The way in which fireworks make their nice pretty colours is by exploiting an effect that Bunsen of Bunsen burner fame discovered about 150 years ago. The science of spectroscopy: he realized that when you look at something, say a distant star, you can work out what the chemicals are in the star because different chemicals absorb light and they produce light at specific fingerprint wavelengths. You can exploit that fact. If you heat up an element it will emit light in one wavelength. You can get different colours from each chemical. Each chemical has its own unique flame colour. You give the elements some energy by heating it up in a firework and it glows a pretty colour. To get those nice colours in your fireworks you have to put lots of metals in. Strontium’s a good choice for making red colours. Barium can make pretty green colours and so can copper salts. They can make greeny-blue colours. That’s how you get the colours. The problem is that all those things can be toxic in big doses. The reassuring thing is that Disney have done some studies where they have Disney World in Florida and Disney Land in California where they jettison thousands of pounds worth of fireworks in their displays. Over 20 years of doing this they have never detected any significant heavy-metal poisoning in their waters in the water features where they let their fireworks off. Cynics would say that’s because all the people at the fireworks displays have gone home with the heavy metals inside them. I didn’t say that – I just heard it!

Dave - Absolute zero is because temperature is because temperature is effectively how much energy each individual molecule has. If you give them more energy it gets hot if take energy out it gets cooler. Because something can have effectively zero energy you can’t take any more energy out of it. Then you can have a minimum temperature. You can put in as much energy as you like but it becomes faster. There isn’t really a maximum temperature.

Chris - Well let’s see if someone tries it in the future. We know that the temperature inside the sun is probably about 15,000,000 degrees with lasers to create fusion on the surface of the Earth some scientists are achieving something in the region of 100,000,000 degrees.