What's the Healthiest Way to Eat an Entire Cake?

Roger - Well, the first thing to say here is that the human eye and the camera see things very differently. The eyes got about 160 million rods and cones, which are the light sensitive elements in the retina but they're not spread evenly and the central part of the retina, which is known as the 'macula,' which is the bit we look with when we're looking at something and we want to see something as clearly as possible. Here the cones are packed in very tightly and they give the eye it's best resolving power, and they also provide it with colour vision. This area of very accurate sight only accounts for about two degrees of our whole visual field.

Andrew - It's kind of interesting, if I understand what you're saying correctly, that at the resolution of our eyes is tailing off into our peripheral vision and yet the experience of looking at things, I don't really feel like everything's sort of fuzzy on the edges of my vision, I feel like I can just see things. So how do those things fit together?

Roger - Well that's because you've always seen like that.

Chris - We don't know any better.

Roger - But the digital camera doesn't have any of that sophistication. It sees equally sharply at every point on the sensor or the film, whichever it may be, the periphery and the centre. If it were to see as well as we see with our macular vision, our central vision, we wouldn't be talking about millions of pixels, we'd be talking about billions of pixels.

Chris - Giles.

Giles - You know our eyes are always scanning so when we're actually looking we're only looking at a very, very, tiny portion of it. So what we're actually seeing is really a result of our eyes scanning the entire time.

Roger - You're absolutely right. We're scanning all the time and the brain is filtering out the scanning so everything appears to be still. And more than that, we can actually see in three dimensions because we happen to have a pair of eyes at the front of our heads, looking forward and getting two slightly different images which the brain, once again, can compare, put together, and allow us to appreciate distance and depth. In other words, see in 3D.

Chris - I suppose the main consideration here is that when you take a digital picture, that is literally a snapshot in time. Whereas our eyes, there's about a million nerve fibres in each optic nerve feeding a train of information in through a series of relay stations and onto the back of the brain where we decode what we see.

And, actually, there's an enormous amount of processing going on in what's being fed to our consciousness that we are totally unaware of, and so the visual world we live in is a complete creation, complete artifice made up for us by the way our brain works. And what I call for instance blue, may not be what Kirsten calls blue or red at all, it's just someone's told me that this neurological experience that I call blue happens to look blue to me, but that might be seen by Kirsten's brain as a completely different experience. But we can't compare, we can never find out.

We put Alan's question to cosmologist Andrew Pontzen... Andrew - Hawking radiation is really one of these breakthroughs. It happened in the mid 1970s named after Stephen Hawking, of course, and it was a breakthrough in our understanding of the Universe. Because according to Einstein's famous theory of general relativity, which is our best understanding of the way that gravity works, it's possible to cram so much stuff into such a tiny space that nothing can get out at all from that region of space. It's just trapped in there by the sheer strength of gravity and that's what we call a black hole.

What Hawking radiation is all about is showing that actually, when you combine that theory of general relativity with the other major theory of the twentieth century which is quantum mechanics, which is all about the way that very tiny bits of matter behave in the universe, you get a really remarkable result. Which is that, actually, very, very slowly things can trickle back out of a black hole, and we call that trickle of stuff coming back out of a black hole Hawking radiation.

So with this question saying, can a black hole start shining? In a sense, what Hawking radiation is saying is it already is shining, it's just shining so very, very faintly that we can't really see it. But one of the interesting predictions of Hawking radiation is the rate at which stuff drifts back out of black hole. In other words the rate at which it's shining, increases as a black hole gets less massive. So you can imagine what happens is, if you leave black hole for long enough, very slowly there's this trickle of stuff coming out so slowly it's losing it's mass is just seeping away into the Universe. So it get's smaller and smaller and, as the black hole gets smaller and smaller of course, the brightness, the rate at which all of this stuff is coming out is going up. And so, actually, it gets brighter, and brighter as time goes on.

Now for this to really happen, you need to leave the black hole well alone. If you just chuck a bit of extra stuff in the black hole, of course you'll easily cancel this out. So you need to take a black hole, isolate it for many, many times the age of the Universe, in fact, something like ten to the fifty-three times the age of the Universe.

Chris - It's not going to happen any time soon then?

Andrew - It's not happening any time soon. Then eventually, you will see it. You will see it literally light up and at the last 0.1 seconds of it's life it will actually pretty much explode. Because this process of stuff leaking out and generating energy becomes so efficient it explodes with something like a million H bombs. We're measuring everything in H bombs today. It will be something like a million H bombs..

Chris - Only a million. So the Neutron star thimble can knock this into next week then?

Andrew - That's absolutely right. In astronomical terms this is a tiny, tiny amount of energy but still, it's a million H bombs in the last tenth of a second in the life of a black hole.

Chris - Thank you Andrew. Ed Wilson on twitter @nakedscientists has tweeted in. He's actually thinking about the practicalities of, going back to the Neutron star, of getting the thimble full of said Neutron star material to Earth. And says, well that's going to take a huge amount of energy in and of itself, so that's another tricky aspect of that.

Andrew - Yeah. And you would also need to make a very, very, well reinforced thimble. You go and scoop this thing up and then somehow you actually have to keep it together until you get it to Earth in order to destroy the Earth. It would be the worst Bond movie ever.

Chris - Kerstin.

Kerstin - I was wondering, can you actually measure this? Do we have any chance of observing the black holes radiating?

Andrew - It would be extremely hard to observe an actual black hole radiating for the reason that real black hole in the Universe, and they do exist, but the real ones tend to be surrounded by gas, and dust, and stars, and things that will be putting out radiation much, much, much faster than this tiny trickle. But, people are trying to create in a laboratory sort of analogues to black holes where they make very, very small systems which behave in a very similar way to black holes and, in that context, people have actually seen this behaviour.

Eagle-eyed expert Roger Buckley had the answer to Korochkina's question... Roger - Well short-sighted people usually see better with contact lenses than with glasses and the reason is that there's no frame in the way. If you have an occupation which requires use of optical instruments like a microscope, in my case the operating microscope for example, it's much easier to get close to where you need to be if you're wearing contact lenses rather than glasses. And the other thing is, if you're short-sighted, the contact lenses will produce a slightly larger image on the retina so you will actually see better.

But against that, you have to bear in mind that glasses never did any eye any damage, whereas contact lenses can. The can cause infection, particularly if they're worn overnight. You're twenty times more likely to develop a corneal infection if you wear your lenses overnight than if you take them out every day.

Chris - Now if I go around struggling and I don't correct my vision because of vanity or whatever reason, are there any consequences apart from obviously walking into walls and having car accidents?

Roger - Yeah. Those are the main ones probably. We're talking about now in adults, in children it may be different. There may be reasons why children have got to wear glasses to correct their sight but in adults with their sight established, you're not going to damage your eyes if you don't wear your glasses. You could wear an out of date prescription, you could even wear someone else's glasses. You wouldn't see very well but it's not going to cause any damage.

Chris - Giles has just waved his hand. Are you wearing someone else's glasses?

Giles - I'm not wearing because of vanity. Because of how I look I don't wear glasses. But my question is: short-sightedness - is it reversible at all without laser surgery I mean? Does it go away or once you've established your eyesight, that's what it is?

Roger - Yes, in a word. It's all to do with the shape of the cornea but, more usually, with the total length of the eyeball. If the eyeball is too long you're short-sighted and there's nothing you can do to shorten it.

Chris - It's because the light is falling in front of the retina rather than on the retina and then it's passing through it's focal point and spreading out again, so you get a blurred image. You're not seeing spots of light you're seeing bigger blobs of light so things look blurry.

Roger - Exactly right.

Chris - Where do you stand on this whole idea of reading in the dark because people used to say to me when I was a kiddie - Oh don't read in the dark in poor light, you'll strain your eyes. Is that true?

Roger - I don't think there's any real evidence of that. The only thing I would say that the modern lifestyle which involves reading and using tablets, and telephones, and gadgets, is producing a race of short-sighted people. It's an epidemic. We don't go out and play games in the open air. We watch television, we work close too, if you see children walking in the street they're very often their nose is almost on the telephone or the pad, whatever it may be. And this is causing them to accommodate grossly, much more than is necessary.

Chris - Adapt their vision for close work?

Roger - Yes. Chris - Giles.

Giles - I didn't know this. In other words during development obviously you have brain plasticity, you know how everything is actually wiring themselves up, this is also true for your eyesight? You need to develop your eyesight to actually get to twenty-twenty, six-six vision? Roger - Yeah. The visual system when a child is born is really quite primitive, but also the eye is small, it's got to grow. What you don't want it to do is to grow too much, become too long and, therefore, short-sighted. So this is why it's better for children to be outside playing, kicking a football around, than it is to be concentrating on something just a few inches from their nose.

Chris - And last one Roger, very quickly. Laser surgery, where do you stand on that?

Roger - I think if the cases are very carefully chosen by experts, it can be very useful and it does have certain medical uses. For example, if one is very different from the other they can be evened out. But for purely cosmetic purposes, I have my doubts.

We asked Kerstin Goepfrich how big a solar panel would have to be to charge a phone... Kersten - Well I guess this depends on where you are. I brought with me my phone charger because I think we can assume we want to charge our phone as fast as we can do. With this thing which plugs into the socket on a wall. So Chris, can you read off the output - it says on here which power output this can give us?

Chris - I reckon I need glasses or laser surgery or something to see. Oh dear, it's really indistinct. It's 1500 milliamp at 5 volt DC. And I almost needed the Hubble space telescope to see that. It's nearly as small as Georgia Mill's mince pie that we got before the programme!

Kerstin - 5 volt / 1.5 amp, if you multiply the two values that gives you the power output, which is 7.5 watts. So if we want a solar cell to produce 7.5 watts, how large would it have to be? Well, the best solar panels that have been made on a sort of laboratory scale had efficiencies up to 46 percent, so that's pretty good. However, the ones you would put on your roof, they typically have efficiencies of about 15 percent. So what does that mean? 15 percent of the energy that comes from the Sun is actually converted into electricity and in the areas of Northern Europe, we can assume that about 1,000 watts per square metre of sunlight hit the Earth. So, 1,000 watts per square metre, 15 percent, that means 150 watts per square metre.

So, we don't quite need 150 watts to charge a smartphone. We said we'd need 7.5 watts, that means we don't need a full square metre to charge our phone, but about the size of an A4 paper would be enough.

Chris - There you go. So that seems eminently doable. Andrew...

Andrew - Although, presumably, that would be at midday in full sun. When it's cloudy or dusk or something you're.

Chris - What about at nighttime?

Kerstin - You've exactly got the point there. I mean, there's not always sun. About 1,500 hours of sunshine is what we get here in the UK, for instance, out of 8,000 hours. So, yes, that's assuming perfect conditions so it might not be as practical. However, if you want to make this last vital phone call, it would save your day. There are these gadgets out there if you're still looking for Christmas presents.

Giles Yeo crunched down on Pius' question... Giles - Ah, that's an interesting question. I think what a lot of people think is as you're eating all the blood rushes from your head to your stomach in order to begin the digestion process.

Chris - You speak for yourself, Giles.

Giles - That's a myth. So the reason why you get tired after, that's not a myth, that actually does happen. It's not because of blood rushing from all parts of you body to your stomach. But I think the best way we can understand it is when you have a large mass of food within you gut and gastrointestinal tract, it turns on something called the 'parasympathetic nervous system.' So there is a nervous system which when you smack your face - ouch! - that kind of nervous system, but the autonomic nervous system which govern your unconscious bits of the things that happen.

Now there are two portions to that. There's the sympathetic nervous system the 'fight or flight' syndrome and there is the parasympathetic system which is actually the 'feed or breed' take a chill pill, sit down and slow down.

Chris - I heard it called 'rest and digest.' I hadn't heard it called 'feed or breed,' that's a new one on me but I like it.

Giles - So what happens is, as far as we understand it, this large mass of food, postprandially, after you eat, tilts your autonomic nervous system towards the parasympathetic so, therefore, causing you to feel tired. Now the reasons why, like once again we don't actually now, but evolutionarily you can imagine you need some time to sit and actually digest stuff before you can actually get up and run again. I don't know but that's probably the reason why.

We asked Giles Yeo about whether saturated fats really are so bad for us... Giles - You know what - it depends on who you are. If your genetics allows you, and there are some people out there who are lucky enough to be able to eat as many eggs as they want, as much cholesterol as they want and actually have no damage to them at all, then it is no harm to you at all.

Saturated fats, depending once again your genetics. Having too much of it will always make you fat, but if susceptible to damage from it, for cardiovascular reasons and other reasons, then it is going to be bad for you.

This is against a background of people saying, well fat is actually great for you now you should be giving up sugar. At some point, fat was very bad for you, sugar's good for you. You've got to eat enough of everything.

Chris - I think one other point to bear in mind that was an epiphany to me is that actually your body makes cholesterol in it's own cells. It needs it because it's very important for the integrity of the membranes in our cells. If you don't actually eat cholesterol, you will just make more in your cells and, therefore, cutting out cholesterol isn't going to make any difference really to your health. It's the saturated fat that then affects how much cholesterol and other lipids are in your bloodstream because you absorb the fat and then distribute it round your body in your bloodstream.

Giles - Exactly. So it's going to be down to your biology and how you actually handle that. If you're able to sustain that kind of diet then you're going to be fine, if not then you're not going to be fine.

We asked Kerstin Goepfrich Mike's burning question...

Kerstin - Yes, we do actually have meteorites here from Mars, even though it is only a small fraction of the meteorites we find on Earth. We've found about 60 thousand meteorites and only just over a hundred of them have come from Mars. And we know they have come from Mars because we have Mars rovers which collected samples from the atmosphere from Mars, and these gases, they have been found enclosed in the meteorites we have found here on Earth. And they can escape the gravity of Mars by being hit by an asteroid that sort of knocks out a meteorite from Mars.

Chris - So something big slams into Mars, ejects material off the surface of Mars, which happens to have trapped in it tiny bubbles of martian atmosphere. Delivers the meteorite over time to Earth and, obviously, because we've got these rovers out there and we know what the Mars atmosphere is made of we can compare the two and say that must have come from Mars?

Kirsten - Precisely.

Giles has some good news for any (moderate) red wine drinkers...

Giles - Yes, wine is part of the mediterranean diet, drunk responsibly and within reason. I think what they're saying is a couple of glasses with the meal is part of the mediterranean diet, and it's red wine we're talking about here is part of the mediterranean diet. As far as I understand, studies that have been done comparing red wine to an equivalent amount of grape juice, it is the actual process of converting those grapes into the wine that provides whatever goodness is there. And I think that people still don't know exactly what it is in the red wine that is actually beneficial.

Chris - So we know that it's beneficial but we don't know exactly why. There's something in the process. But don't overdo it but there does appear to be evidence to support having a glass of wine as part of a mediterranean diet. Is that what you're saying?

Giles - That's absolutely correct

Andrew Pontzen had the answer to Sreejith's question...

Andrew - Quantum entanglement is one of the most beautiful and bizarre phenomena in all of physics quite frankly. It comes from this world of quantum physics that we've been discussing quite a lot already on the programme, the physics of the very small. And it's this phenomenon where, seemingly, what you do to one tiny little chunk of matter, one tiny particle, in one place can instantaneously change how another particle in a completely different place is behaving. And when I say instantaneously, bizarrely, I actually mean instantaneously.

We have a kind of idea in physics that nothing can go faster than the speed of light, but there's no way that one part of the universe could influence another part of the universe without there having been time for some light at least to get from point A to point B. This seems to break that idea.

So it's actually a very sensible question to say - okay, sure, if I drop something into a black hole I'll never see it again, at least not in the form that I dropped it in. But could I use this bizarre effect of quantum entanglement to find out something about what's happened to it?

Chris - Is this what they dubbed "spooky action at a distance?"

Andrew - Yeah. It's not just they, it's Einstein that dubbed it "spooky action at a distance," because he hated it. It's so weird, it's so cutting against the grain of everything else we know about in physics.

Chris - But Niels Bohr said "if you're not baffled by quantum you didn't understand it."

Andrew - Absolutely. And I think that's a very fair way of putting it. However, what we do know about this phenomenon of quantum entanglement is you cannot actually use it to transmit information faster than the speed of light, which seems to contradict what I just said. So although one chunk of stuff can influence another chunk of stuff at a great distance seemingly fast than the speed of light, that can't be used to transmit a message.

So suppose I gave you one half of my entangled pair of particles and you tried to use it to transmit me a message.

Chris - You shared your quantum entangled mascara with me?

Andrew - Absolutely. I would only share it with you Chris.

Chris - I'm touched.

Andrew - Suppose we did that. Suppose you tried to figure out how can I send Andrew a message. Unfortunately, it turns out that there's actually a mathematical theorem showing you cannot use this effect to send me a message. If you can send me a beam of light at the same time, then you can kind of boost the amount of information that gets sent. But because the way that one of the pair affects the other is extremely subtle, it can't straightforwardly be used to send a message as you would with say a beam of light.

So if you can't put things in contact with a beam of light, you also can't put them in contact with quantum entanglement. So the answer is no.