We all know that irritating feeling when your neat computer cables get turned into a thick weave of plastic dreadlocks. Ultimately, the reason this happens is due to one of the most important laws in all of Science: the second law of thermodynamics. The second law states that "the change in entropy of a closed system is greater than or equal to zero with respect to time". It's a beautiful law. I have a tie with the equation on it, and Ludwig Boltzmann has it on his tombstone. There's a lot of subtley going on in that definition but for our purposes we can state it pretty simply: things are more likely to get disordered.
A pile of sand will spread out rather than jump into a neat column, bedrooms become messier, cars get rusty and a pencil balanced on its tip will fall over. The same thing applies to cables and strings. Given a tiny amount of movement energy, they will gradually get tangled up (if you think about it this is quite logical because if they start out in perfect alignment and you move them about...there's only one way for them to rearrange...into a messier configuration!)
That's why a bunch of cables given even a small amount of agitation will quickly become knotted together. But DNA doesn't. And this is actually a pretty deep puzzle. A single thread of DNA is somehwere between 1 and 3 meters long when stretched out (depending on which chromosome you pick) and you have enough of those strands in your body to reach the Sun and back several times. That's a lot of thread, and it doesn't get knotted up. At the moment, nobody has a clue why, but mathematicians and biologists are hot on the case.
There is a particular branch of mathematics called topology which deals with the shapes of objects and how deforming them changes their properties. And, within topology, there are three sub-sets of ideas called (I'm not making this up) braid theory, knot theory and tangle theory. The idea is that if you solve some simple equations for a knot made of string, the same equations can be modified to describe a knot in a piece of hair, or a piece of marshmallow. The same mathematics should underpin everything. So if we can find out what makes something more/less lilkely to tangle, we might be able to answer the DNA question.
A study conducted by Robert Matthews at Aston University in 2004 (who also carried out research on whether flipping buttered toast is more likely to land butter side down) was able to show that having loops in your thread makes it less likely to get completely tangled. His research even involved recruiting students from a school in my hometown of Coventry and getting them to tie over 12,000 knots in different types of looped string. It turns out that strings with loops in them are less likely to get tangled.
Then there's the research of Dorian Raymer from Chicago Univestiry in 2006, who measured how likely a string is to get tangled depending on how much it's being jiggled about. It turns out that the length of string actually has more to do with whether it gets tangled than how much it's being agitated.
Some people have suggested that by having loops in the DNA, nature has found the perfect way to minimise tanlging, while others have suggested that DNA is made of chemicals which are less likely to fold around each other. The simple answer is that we just don't know. A helix of string or wire would get horribly knotted (slinkies anyone?) but DNA is somehow able to avoid this curse. Answer unknown...for now at least.
P.S. This has nothing to do with string theory.
This is a wonderful question - but quite difficult to answer. Modern Biology tells us that human beings are fairly new to the planet. There's been life on Earth for around 3.5 billion years, but that life has constantly changed form through the process of natural selection and human beings weren't around for most of it.
In a very condensed nutshell it goes something like this: imagine you have a creature representing a tree trunk. Over time, the children of these creatures are born slightly different to their parents, giving rise to new varieties of creature, and so the species splits into several categories, like the boughs of a tree. Each of these new species then does the same, giving us lots of new creatures again (the branches) and so on (twigs) and so on (twiglets). Give this process a few billion years and what began as one species of simple bacteria has turned into the millions of different species we see today, one of which is the human race.
Imagine if you began at the present day, with the human race, and traced back along the family tree, going right the way back to the first creature on the planet. You could set up a timeline showing the gradual evolution of humans. But at what point did we become the way we are? If our oldest ancestor was some slimy bacteria floating in a rock-pool, when do we draw the line and say "this is when the human race began?"
It's surprisingly tricky to give a good answer because the creatures which came just before humans were about 99.999% identical. In fact, within the last few thousand years, we've seen changes in the shapes of human skeletons, so are we a different species to the humans who lived in ancient Greece? We are slightly different after all. But if you want to get technical, I am sliiiiiightly different to even my own parents. My genetics are not quite the same as theirs due to tiny mutations, would I be classed as a separate species? This is the problem: which tiny change was the one which made us "human". It would be like taking a grain of sand, adding another one, adding another one, and then asking when it became a pile. There is no obvious point, and different people would draw the line in different places.
After all, do we consider human to be when we began walking on our hind legs? When we began developping language? When we invented culture? When we began creating art? It's a good issue to debate, but most Biologists would agree that the human race (under any of the reasonable definitions) emerged somewhere between 1 million and 500,000 years ago.
In 1995 a woman named Nancy Lieder claimed she had been contacted by aliens and warned of an imminent collision between the Earth and another planet named either Nibiru or Planet X. The name Planet X comes from Percival Lowell, the astronomer who suggested an extra planet beyond Neptune (which he nicknamed X for short).
This claim from Nancy Lieder is the sum total of the so-called "Planet X theory". It's really a bad name, because a theory is an idea which has lots of evidence to support it, but Lieder's claim has none. In fact, Lieder's original claim was that the Planet-X collision would take place in 2003...which I'm pretty sure was proven false. There are still some people who claim that the Planet X event is going to happen, but these claims aren't based on any actual data so there's little reason to be alarmed by them. I could just as easily claim that a giant strawberry is headed for Earth. Technically you can't prove that false, so I could refer to it as the Stawberry-event-theory, but there's no reason you'd believe it. So, what is Planet X "theory"? Nothing to worry about.
This question has been, by far, the trickiest one I've answered since I launched the website. It's also been the most interesting because it's really stretched my teaching ability to the limit. I've been thinking about how best to explain it for days and I think I've finally hit upon something which just about explains it, although I'm sure someone else could do it better. Firstly, let's talk about what shell theorem's second postulate is.
Imagine a perfectly spherical but hollow object. A ball-shell in other words. The wall of this shell has even thickness and the hollowed out centre is a perfect sphere also. Now imagine the shell has a strong gravitational pull because the shell-wall is thick enough. Essentially, we're imagining a perfectly hollowed out planet. The first postulate of the shell theorem is fairly simple. It says that if you're outside this shell, its gravity field will attract you from every angle and pull you in. You can imagine a point dead centre of the sphere which is what you're being pulled toward. This idea of a "centre of gravity" (the point through which weight appears to act) isn't too strange. Even though there is actually nothing in the centre of the sphere, you're still pulled as if there were. So far, so good. Where things get weird is with the second postulate.
The second postulate says that if you are inside the sphere, you float freely no matter where you are. If you're in the centre, you'd be equally attracted to the shell in all directions so you'd float. But the second postulate says that the same thing happens anywhere. Even if you're right over by one wall of the sphere, you still float freely and feel no attraction to the wall a few meters away from you. This doesn't seem right. After all, if you're right next to one side of the sphere and far away from the other side, shouldn't the force near you be stronger and ultimately win? Second postulate says no. How can that be right?
The reason this question has been so interesting is because all the conventional answers are just a bunch of equations and, as I've said before, equations are not explanations. If a students asks me what voltage is, I don't just write Voltage = Current x Resistance and say "there you go". Even though that's mathematically correct, this gives you no actual understanding. Equations are notations and language short-cuts for complicated ideas, but every equation is still describing something which could otherwise be expressed in words. The trick is to try and get to the bottom of what the equation really means and that's what I'm going to attempt to do. If I can't explain it in simple non-mathematical form, then I don't really understand it myself, I just "recognise the equation." Equations without explanations is lazy Science and lazy teaching. So here goes...
Let's imagine you are floating right over on the "east" side of the sphere (a sphere is non-directional and has no East, but I need to pick words to use). The east-most point is closest to you, so you feel its force very strongly. The western point of the sphere (the point most opposite to you) is far away, so it's effect is weak. But there's something else we need to factor in. There's a North and South point on the sphere too which are pulling you AWAY from the East point, half as strong as the West. These North and South points are half as far away, but there's two of them, so you have an equal force pulling you AWAY from East point as well as toward it.
And it doesn't stop there. Imagine the sphere having a North-East point. That point is also trying to pull you away from the East. And the same for the South-East point. In fact, and this is the crucial bit, there are more points on the inside of the sphere pulling you AWAY from East, and only one point on the East pulling you TOWARD it. These effects ultimately cancel out.
Imagine standing near the edge of a circle of people. The person nearest you is offering you £100 to go and shake their hand. But the two people either side of them are each offering you £99 to approach them instead. There's actually £198 worth of attraction trying to persuade you away from the £100 person. But you can't move toward either of them, because they're evenly matched - the amount is perfectly equal, so you're not actually attracted to either of them, you're attracted to both.
Inside the sphere, you end up with a balance of "lots of points attracting you weakly" verses "one point attracting you strongly" and these two effects cancel out perfectly. Because the inside of the is spherical, there are an infinite number of points trying to pull you away from anywhere you approach. Even when you're right beside one wall of the sphere, the infinte other walls are pulling you as well and they end up equalling the point directly next to you. As a result, you feel zero effect and you float there, freely.
It's a good question but the simplest and best answer is that it doesn't. Hot air only rises when there's cold air and gravity acting on it as well. Here's how it works: in cold air the particles are moving around without much energy. They take up a small volume because they're only tootling from A to B, so a cold gas is a small gas. Heat things up and the particles move around more, which means they spread out and the gas takes up more volume i.e. it expands. This always happens, no matter where you are, hotter gases take up more space.
But now imagine you've got some hot gas in a room and some cold gas surrounding it. Both gases are being pulled down by gravity, so they're both "falling". However, the hot gas is expanding as well, meaning it can fight against gravity a bit. The cold gas, by contrast, isn't expanding, it's just falling under gravity. So the cold gas will drop to the floor. The hot gas will drop as well BUT it's also got kinetic energy that can partially overcome gravity. The effect is that the cold air will, on average, fall better and the hot gas will get squeezed upward. If there's no cold gas surrounding the hot gas, it doesn't rise, it just sits there.
What's more, if you remove the constant effect of gravity, hot air no longer rises, it just expands in all directions. In fact, flames in a space station are perfectly spherical (google "spherical flame in microgravity") So really, hot air doesn't rise. It's actually that cold air falls better.
The sky on Earth is mostly blue during the day. The reasons for this are either very simple or very complicated, depending how far down the rabbit hole you want to go. In one sense, the colour of the sky is because the chemicals IN the sky are partly coloured, it's just a very weak effect (one in every few trillion trillion molecules responds to visible light by emitting colour). Another way of saying it is to talk about the fact that high energy light, such as blue, gets scattered about the sky more, so you're more likely to see blue light being thrown out away from the Sun, making the rest of the sky appear blue. In a sense, I'm really saying the same thing twice there. Any object's colour is a result of how much it's scattering visible light, so in a sense the colour of the sky is simply because the chemicals are blue (this is massively simplifying it, but honestly the complex explanation doesn't add much extra detail).
An even deeper answer is to admit that we don't really know 100% why the sky is blue. It's a result of the way light interacts with certain types of magnetic molecules and the complete mechanism isn't understood. There is also the fact that dust and vapour particles in the air will colour it. Venus, for example, has a mostly CO2 atmosphere (CO2 is completely colourless under any definition of the word) but there's a huge amount of particles floating in the air, giving it a colour. We don't yet have a full understanding of how different mixtures of gases and chemicals produce different colours.
What we do know, however, is that different chemical mixtures on different worlds respond differently to light. Some planets and moons we don't know about because we've not been there and taken a photograph (we've only done that on Venus, Mars and Titan so we know those with confidence) so most of the assumed sky colours are guesswork. Here, mostly for fun, is a rough guide to the colour of the sky in different parts of our solar system based on the chemicals in their atmosphere.
Mercury Hydrogen/Helium Colourless
Venus Carbon Dioxide/Nitrogen/Sulfur Dioxide Orange
Earth Nitrogen/Oxygen/Carbon Dioxide/Argon/Ozone Blue
Mars Carbon Dioxide/Dust Orange/red
Jupiter Hydrogen/Helium/Methane Blue
Saturn Hydrogen/Helium/Methane Blue high up, yellow low down
Titan Nitrogen/Methane Yellow
Uranus Hydrogen/Helium/Ammonia/Methane Turqoise
Neptune Hydrogen/Helium/Ammonia/Methane Blue
Imagine a boat sitting in the water creeping forward at a very slow rate. It will send a ripple of water out in front of it, albeit a slowly moving one. You've got to imagine the boat moving ar around a couple of centimeters per second for this. But if the boat picks up any speed, it crashed into the water in front of it faster than the ripples can escape. It no longer creates a swell of water in front, all the water-wave ends up trailing out behind it in a V shape, what's called the "wake" of the boat. If you watch the boat going past you, the water triangle will follow soon after and splash you. This is an aquatic boom. A sonic boom works the same way except with air instead of water.
As a plane moves through the air, it smashes into air particles and scatters them out in all directions, including in front of it. The speed of sound is, on average, about 333 m/s (varying slightly with temperature and altitude). If the plane is travelling at 100 m/s, then the air particles will go flying away, leaving the plane behind. But let's say the plane is moving at 400 m/s. At this point the plane is bumping into air particles, but also catching up before they escape. The air-build up in front of the nose can't get away.
The air will end up with lots of movement energy (kinetic energy) but it can't move forwards. Instead, the air ends up forming a cone around and behind the plane, trailing behind it like ripples on the lake behind a boat. The plane is moving so fast it drags a vortex of air along with it. When this enormous cone of air hits your eardrums, you hear the sonic boom.
This image demonstrates it beautifull:
The most basic, nitty-gritty theory physicists have for explaining the Universe is called Quantum Field Theory - it's the deep-down set of laws we assume everything else is based on. Like learning the basic moves of a game of chess, if we understand the laws of QFT we can explain any phenomenon you care to mention.
There are just two problems. QFT doesn't explain the existence of gravity and QFT leaves an enormous mystery open: why is nature so messy? What I mean is that the raw ingredients of our Universe don't follow any logic, they're a higgeldy piggeldy mess of particles whose properties seem to be random.
It's like discovering that the ingredients of the Universe are a bag of revels: some large, some small, some with caramel centres, some Malteser etc. There's no reason to it but it definitely seems to be true. But since QFT can't explain this wierdness, QFT must be incomplete and we're on the lookout for a new theory to explain reality. One of the possible avenues is an idea called string theory.
In the 1970s a group of theoretical physicsts began tackling the QFT problems in a casual way - mainly out of curiosity and for fun, they began imagining that the deep structure of the world wasn't made of fields and their particles, but made of strings. Everything in the Universe was really made of a tiny string-object and all the particles we can see are just these strings coiled up and vibrating in different ways.
The first string theories were only used to deal with a small part of QFT however, what's called the bosonic aspect, so the theory's full name was Bosonic String Theory. It was never intended to be rigorous though and was mainly developped as a way to keep the equations simple. In fact, Leonard Susskind (the theory's main inventor) has even described it as something "we were just playing around with", but then something rather weird happened - bosonic string theory turned out to accidentally explain the existence of gravity.
Suddenly, people got interested and wanted to see if they could use this strange new idea to explain all of QFT, not just the bosonic part of it. Could we use string theory to maybe explain all the particles in the Universe? One of the tricks used was to assume that strings could act in symmetrical ways - vibrating left, but also vibrating right - spinning clockwise, but also spinning anticlockwise (or something similar to that, the reality of what the strings are doing is too hard for humans to visualise).
By imagining that all these string-like objects had a property called "supersymmetry" Physicists discovered they could explain quantum field theory and include gravity, as well as giving us an idea of where the randomness of particles comes from. So the strings were renamed "Superstrings" and the idea is still being worked on today!
Let's assume we really could dig a tunnel all the way through the Earth's core, right to the other side. If we dropped a rock at one end, the first thing that would happen would be an acceleration toward the centre. It would get pulled by the gravity of an entire planet and hurtle down at 9.81 ms-2. But here's where it gets interesting.
The deeper you got into the Earth, the more of the planet would be above you. As the rock falls, it gradually finds more and more of the Earth is now above it (rather than below it), and all of this plant-stuff has a gravitational attraction of its own. This means as it started to approach the centre of the Earth it would begin to slow down, it starts to get pulled "up" as well as "down".
It's going to have a lot of momentum however, which will carry it through the centre of the tunnel, but it's already begun slowing down, so it won't go shooting to the other side of the planet. What will happen is that it will begin to yo-yo around the centre of the Earth, being pulled back and forth for several minutes until eventually it reaches an equilibrium and will then sit in the centre, hovering perfectly. At this point, it is equally attracted outwards (because all the gravity is surrounding it) so it will become weightless and will sit there, floating, until we drop another rock down in an attempt to knock it out.
The Universe is mostly empty space, with a few stars floating around. Steafan's suggested we take all the heat energy from all the suns, plus all the emptiness, and average it out. How cold would it end up being? The question sounds a bit unusual but there's a good reason to ask it; this is one of the ways Scientists think the Universe might end. It's called the "Heat death of the Universe" hypothesis and the idea is that the Universe is gradually stretching out, so everything will eventually become so far apart that nothing will heat anything up and the Universe will become cold. So, how cold will the Universe be at this point?
The answer is - pretty much the same temperature as it is now! Thing is, the suns and planets account for very little of the Universe's total volume and mass. Most of space is pretty empty, hovering at -270 degrees Celsius (about 3 Kelvin). If we averaged out all of the suns, it would barely make any difference. It would be like taking a handful of matches to Antarctica, lighting them all and working out what the average temperature of Antarctica would now be. While technically we have increased the average heat of Antarctica, it's by such a small amount as to be barely noticeable. Same principle with the Suns and the Universe. In other words, the thermal energy of the Universe already is pretty well distributed.
There are pockets of heat (galaxies) but once they've cooled down, space's temperature won't have changed much. There will still be lone particles, plus empty space has an energy value associated with it, so the temperature will never drop to absolute zero, but it will probably hover around -270 degrees.