Why is it that we like drinks with gas, like beer or coke? If we drink these beverages without it, they taste bad, but gas is tasteless. What effect do bubbles have? Is it just down to the sensation? Ricardo in Brazil

I think it's the texture. The bubbles in the drink actually make very little contribution to the acidity. Acids generally taste a little bit lemony. However, if you look at drinks like Coca Cola for example, they're so acidic, they'll rot your teeth away. In fact, Coca Cola is on the same level as stomach acid, so the carbonic acid effect from the dissolved carbon dioxide makes very little difference. What the carbon dioxide does do is make a very interesting sensation in your mouth. Firstly, if you think of a glass, you often see a stream of bubbles all coming from one place. This is an imperfection in the glass that makes it very easy for bubbles to form, and is called a nucleation point. Your tongue has hundreds of tiny imperfections, and therefore nucleation points. This makes hundreds of tiny bubbles start pinging off all over your tongue. This is like having your tongue massaged by bubbles. I think this added dimension is what makes fizzy drinks taste good.

November 2005

What's gravity made of? Tara, via email

That question would probably fox most scientists! There are various theories of gravity. The most straightforward one was put forward by Isaac Newton. He said that it was the force between two bodies that attracts bodies together, and the strength of the force is proportional to the masses of the two bodies over the distance between the two bodies times itself. So the bigger something is, the more it attracts something. There is also the Einstein theory, which is about gravity being the curvature of space time. This is often illustrated by a rubber sheet with a big ball bearing in the middle, which makes a dip. If you then take a smaller ball bearing and ping it off at 90 degrees, it will move round and round the big ball bearing, like an orbiting planet. There are also gravity waves, which are ripples in space time that propagate away from a massive body.

November 2005

Is it true that someone's head would explode if you went out into space without a helmet on? Tanna in Montana, US

If you go into space, there is essentially no pressure. The pressure of any gases you have in your head at the time would expand and potentially split it apart. However, I've never done the sums to see if it could break apart bone. Your body fluids would also boil instantly and evaporate.

November 2005

Is it possible to see a gamma ray burst in the sky? I saw something in the sky flash on and off one night. John in Clacton

The retina in the back of your eye is sensitive to a very narrow range of what is called the electromagnetic spectrum. The electromagnetic spectrum contains all of the different wave lengths of light. This includes microwaves, x-rays and gamma rays, but you can't see any of them. Therefore, what you saw wasn't a gamma ray burst.

November 2005

We're told that photons can't escape from a black hole, and I was wondering if gravitons can escape from a black hole? If so, what are the consequences of that? Clive in Suffolk

That's a very good question. Photons are light particles, and you're quite right in saying that they can't escape from black holes. This is why they look black. Gravitons are thought to mediate gravity, and are massless spin-2 particles. If gravitons couldn't escape from a black hole, you'd have a problem because the black hole wouldn't be able to mediate the gravitational force and you wouldn't feel a gravitational force around it, which we believe isn't the case. There are some quite technical reasons for why particles can mediate a force even though they're in a black hole across the event horizon, but it would take me longer than the rest of this show to explain it.

November 2005

How far is the nearest galaxy? Sarah in Hardwick

The nearest galaxy to our galaxy, the Milky Way, is called Andromeda. It's about 2 million light years away, and we think it looks very similar to our own galaxy. It's difficult to know what our own galaxy looks like because we're in it, but calculations show that it's a flat spiral disc shape. For a long time, people thought that our galaxy was the only galaxy, and that any smudges they saw in the sky were nebulae, which are clouds of gas that form new stars. It was only after some more looking and calculations that people realised that these smudges were too far away to be in our own galaxy. Even Einstein fell foul of misconceptions of that sort. When he was developing his theory of relativity, he had to put in a term that made the universe nice and stable. He thought it consisted of these fixed stars.

November 2005

How do scientists check the distances between planets, and how accurate are their methods? Brian in Singapore

In recent times, the methods have been very accurate because we've sent spacecraft to various planets and can do timing measurements. This is very accurate. In the old days, people had to do incredibly painstaking measurements to see if things were moving a tiny bit. What surprises many people is that Pluto was only discovered in the 1930s, so it's only fairly recently that we've had any idea about the outer reaches of our own solar system.

November 2005

From a scientific rather than a religious point of view, why should there be a universe? Why isn't there just nothing? Bob, via email

Well it's thought that most of the matter we have in the universe today did come from nothing. By nothing, I mean the vacuum. There is a favoured mechanism for creating the matter that fills our universe, and this is the convergence of the energy density of the vacuum. In simple terms, one would think of this as nothing, or no material, into matter. So in many ways, everything we see did come from nothing. The way energy density is converted into matter is quite complicated, but it's thought that that's how it occurred, through inflation.

November 2005

How do scientists measure the life of the sun? Helen in Norwich

We can't actually observe any single star evolving because the time scales involved are far too long. Our sun is sort of middle aged at the moment, and is about 5 billion years old. It should be kicking around for another 5 billion years or so. The way that people understand stars is really by modelling their structure and then observing lots of examples of different stars in different stages of their evolution and seeing if the collection of observations you make fits the theory. We believe that we understand the structure of these stars and so it's really a theoretical construct. We have a model for these stars, it matches all the stars that we see. When we look at the sun, we know what kind of star it is and its properties, and we can say that it's about 5 billion years old.

November 2005

How many human years are there in a light year? Susie in Lincolnshire

A light year is a measure of distance. It's the distance light travels in a year, so it's not a measure of time. Calling them light years s rather confusing I must admit! So it's not the same as converting human years to dog years. They are two different measurements completely.

November 2005

If our weight is the difference between gravity and the centrifugal force of the Earth's rotation, why are we not crushed at either pole area? E.M. Morgan, via email

It's true that there are both gravitational forces and centrifugal forces acting on us. We're not crushed at the pole because the force of gravity is not strong enough to crush us. In fact, the very small change in gravity with change in latitude is tiny. However, you're right to point this out as it is different between the poles and the equator, however small its effect.

November 2005