In Science we use a set of units called the Standard International Units. The kilogram, the meter, the second, the ampere, the mole, the Kelvin and the candela. All of our other measurements are taken by combining and defining these units. But in quantum mechanics we use a set of units called the planck units. There's a planck distance, a planck time, even a planck energy, mass and so on. The planck units are based on actual physical features of the universe. The planck distance for instance is the smallest distance across which the laws of physics are known to definitely work (anything smaller and we don't know). So why not switch to these units, why keep the SI ones?
The answer is mainly historical. In 1875 a meeting of Scientists took place to agree on International units to use, and it was called "The Meter Convention". It was here that they decided on the meter as the unit of measurement, the kilogram as a unit of mass and so on. They got their units largely from measuring the Earth and doing calculations on it. There is, in fact, an original kilogram made of platinum sitting in a vault at the Pavillon de Breteuil, likewise an original meter. The current SI units were largely decided at a meetings between 1875 and 1960, but the General Conference of Weights and Measures still meets every few years to review things.
While the planck system of units is, in a sense, a more fundamental property of the universe, the reason the GCWM use the current system is because it must be used by all Scientists all over the world, not just quantum Scientists. Furthermore, the quantum system is a lot newer and less familiar to people.
For me, the only SI unit which actually bugs me is the unit of ampere to measure electrical current. Because an ampere is a coulomb per second, I personally would have made a coulomb the fundamental unit of charge and made ampere the derived unit. But there are more people in the world who deal in current than actually deal with the property of charge itself, so the ampere sticks. So the reason we use the scales we do is mainly out of international agreement and "what's best for everyone to communicate" rather than "what's the Universe fundamentally doing". I can see arguments for both ways of doing things.
It's an interesting question to ask. Electrons and the nucleus of an atom have opposite charges and one of the important rules of our universe is that opposite charges attract. Yet electrons "orbit" the nucleus at set distances - how can this be so?
The answer is actually best illustrated by flipping the question on its head: why doesn't an electron just escape the atom and fly off into space? Answer: the opposite charge of the nucleus holds it in place. Electrons don't just have charge, they have all sorts of other properties, one of them being energy. The more energy an electron has, the more it can spread out its location i.e. the further away from a nucleus it can get. High energy electrons break free of their atom and go zipping off into the Universe. So really, think of it as an electron's tendency to leave the nucleus. It's constantly trying to escape, but the positive nuclear charge is holding it in place so that it can't escape.
The answer is "yes" and "no" depending on how far underground you are. The key is that all masses in the universe (every particle except light in other words) has a gravitational pull to it. The further you are from the mass, the weaker the gravitational force. Newton figured out that the relationship between two objects is their masses multiplied, divided by the distance between them squared, all multiplied by a constant number. So the further you are from something, the less gravitational force you experience. Obviously as you go into space, you feel less of a pull toward the Earth's gravity, but if you go in the other way, things get a little bit strange.
As you start digging into the Earth you'll find yourself getting closer and closer to the most massive and dense part (the outer core). So the gravity will gradually increase as you go deeper and deeper. But something else is happening at the same time. As you dig down, you end up with more of the Earth "above you". Say you've dug 12 km into the ground, you now have 12 km worth of planet above your head and that's going to have a gravitational effect, pulling you upward.
Up until you get to the outer core of the Earth, the "downward" direction wins and you will feel more and more gravity. But once you go past it into the inner core, the effect starts reversing. You find that you're now approaching a point where all the gravity is pulling you outward in all directions. At the very centre of the planet, there's nothing pulling you inward anymore, but you have an entire planet's gravity pulling you outward. The effect is that, rather surprisingly, at the centre of the Earth you become weightless.
So as you dig, Earth's gravity increases up to a point, then tails off to zero. The only problem now is surviving the insane heat of a planet's core!
In my video on gravitational waves I mention that they might be something we could use to communicate with and detect parallel universes, if such things exist. Question is, what makes gravity so special?
Firstly, there are two common types of parallel universe theory, one is the many worlds interpretation of quantum mechanics suggested by Hugh Everett in which Universes are constantly diverging and decohering from one another. In this model the Universes are utterly separate and no communication would be allowed between them. The other common parallel universe idea is the multiverse hypothesis of M-theory which suggests that there are many, many universes all inhabiting a higher-dimensional space called "the bulk".
OK, so where does gravity come in? Well, the problem with gravity is that it's remarkably weak. It seems strong when we're standing on the Earth because there's a lot of mass there, so a lot of gravitational force. But when you rub a balloon and stick it to a wall, you've just beaten the entire Earth's gravitational pull with a bit of static. Gravity is actually the weakest of the known forces in nature, of which there are four: Strong, Electromagnetic, Weak and Gravity.
But it's not just the fact that it's the weakest force, it's weakest by a very peculiar amount. If we took the Strong force to be 1, the electromagnetic would be 0.01, the weak would be 0.0000001 and the gravity force would be 0.00000000000000000000000000000000000001. That's not just weaker, that's a bizarre amount of weaker.
The solution some physicists have proposed is that the reason gravity appears so much weaker is that it is the only force with the ability to leak out of our universe. And, by contrast, gravity can leak into our universe from other universes. If this were true then any time we tried to measure gravity it would seem a lot weaker than it should be (a lot is missing) and we would also find a mysterious amount of apparent extra gravity in the universe with us (which we do).
To be explicit: we don't know for definite if gravity can escape our universe, we don't even know if there is an "outside" for it to escape into. But, now that we know gravity waves are a real thing, we can start studying them and probing to see if these other universes do exist. It's kind of like when we ask if there's extra-terrestrial life in our solar system. We don't know, but if we had to place bets on where to look, Europa and Mars would be good candidates. Likewise, we don't know if parallel worlds really do exist but if so, gravity waves are our best chance of detecting them!
Infinity is the word we use to represent something that has no limit, edge, boundary or final value. It is something which doesn't finish or lasts forever. The best example is numbers. The biggest number you can think of can still be multiplied by 2, or have another number added to it. The scale of numbers doesn't come to an end so we just keep counting and counting and counting forever. Infinity isn't really a number because you can't do simple maths with it. For instance, if you take an infinite set of things and add one, you have gone from an unmeasurable number to another unmeasurable number i.e. infinity + 1 = infinity again. This makes it more of a concept than an actual number.
Where it gets really interesting is when we consider different types of infinity. Take the set of whole numbers (natural numbers) 1, 2, 3, 4... we end up with a set that goes to infinity. But now imagine we did it with only the even natural numbers 2, 4, 6, 8... we would still get to infinity. Thing is, the normal numbers are (even numbers + odd numbers). There are half as many even numbers as normal numbers. So the infinity we count to using the even numbers is half the size of the other infinity.
Or, simpler version, imagine we started at 2 and then "counted to infinity". 2, 3, 4, 5 etc. etc. This will still get to infinity, but we haven't included the number 1, so this infinity should be smaller than the other infinity by a whole number. Or imagine we counted all the decimal points. 1, 1.1, 1.2, 1.3 and so on...this infinity would be bigger than the normal infinity.
A man called George Cantor set out some rules for dealing with infinities in the late 19th Century. He referred to the full set of natural numbers as "aleph null" and then other sets of infinities as "aleph one", "aleph two" and so forth. There's a whole branch of mathematics dedicating to dealing with infinities called "transfinite mathematics" which deals with these alephs.
Interestingly, some mathematicians don't believe in infinity. Doron Zeilberger, for instance, is convinced that infinity is a mistake and that if you count up the numbers and keep going you just loop back around to a smaller number eventually...there really is a biggest number out there somewhere!
From a Scientist's perspective infinities are very useful because there isn't a way we can see for them to exist. Infinity contains all sorts of logical paradoxes and impossibilities which can be written on paper but translate to impossibilities in reality. So we use infinity as a kind of test: if a hypothesis contains an infinity somewhere, it's incomplete or we've made a mistake. Only once we've removed the infinities, can we be confident we've got a theory that works.
This is a great question because absolutely nobody on Earth has a clue. You ask psychologists, psychiatrists, neurologists and neuroscientists and none of them can give us a solid answer. There are lots of possible explanations based on educated guesswork and some evidence (hypotheses) but nobody's really been able to give us anything concrete yet. My personal favourite explanation, the one which makes the most sense, is that the brain needs to remove a toxic chemical called amyloid beta from the brain. It's a chemical which gets produced as we think and process stuff during the day, but once it builds up too much the brain doesn't work as well. So the brain needs to put everything on hold while it gets rid of the toxin. I like this hypothesis because it's straightforward and easy to understand. But I don't know if it's true, in fact I doubt it's the full explanation. After all, if we need to undergo this process, how come there are plenty of animals which don't seem to need sleep at all? The mystery of Science continues!
Jupiter, the largest known planet in our solar system, is a big mystery to us. We can see it in our telescopes, even the naked eye sometimes, and we've sent probes right past it and around its moons, but we don't really know much about it at all. From analysing its mass and size we can make some guesses about what it's made of and how it formed, but until we send a probe into it we can only make educated guesses. However, what our educated guesses tell us is pretty cool.
Our current estimate is that the core of Jupiter, probably about 60,000 km across, is made of tightly compacted elements, probably similar to rock, although a lot denser. A bit like the solid core of the Earth, the pressure and gravity of Jupiter's core is likely to be an unknown substance (in the sense that we've never seen anything like it directly on Earth so have no idea what it would look like).
Surrounding that is probably my favourite layer, a sphere of liquid metal Hydrogen. The idea of liquid metal Hydrogen is so strange and unlike anything we've ever seen that my mind starts bending at the thought of it. If you've ever seen a zeppelin or a Hydrogen balloon you know that Hydrogen is normally a very sparse gas, lighter than air. Now imagine it somehow turning into a metal? Just mind-blowing. Not only that, but it's a rotating core of liquid metal which means the entire planet probably conducts electricity.
Coating this is a layer of liquid Hydrogen (a bit like liquid Nitrogen, just less dense) and then finally a thin layer of Hydrogen gas. The inner cores of Jupiter are probably rotating and grinding against each other which leads to changes in temperature and pressure. These changes affect what happens on the next layer out and so on, until what we see on the surface of Jupiter (the atmosphere) is one constant lump of storm. A storm-ball floating in space surrounding layers of liquid metal hydrogen and hyper-dense rock. So what happens inside Jupiter's atmosphere? Weird, weird things!
Amid all the excitement about gravitational waves (check my video for more on that) another fascinating discovery may have just been made in Tokyo by Kisamori and Shimoura, whose latest findings are published in Physical Review Letters (3 February, 2016). This new discovery concerns a hypothetical particle that's been debated for over half a century: the tetraneutron.
Inside the nucleus of an atom the smallest particles are things called quarks. Quarks have charge (just like protons and neutrons do) but they have other properties as well which we need to consider, one of these properties is called "spin". The theory gets very weird, very quick but the key features of spin are as follows.
1) It doesn't mean the particle is literally rotating around an axis.
2) It is, however, related to the possible rotations a particle can have.
3) Only certain values of this property seem to be present in the Universe.
The same way there's no such thing as half an electron-charge, there's no such thing as "a quarter spin" for instance. It's a property the Universe uses and there are restrictions imposed on it. Precisely why there are restrictions on particle spin is unclear. It might just be a feature of the Universe (like the fact that matter exists at all) or there could be some underlying principle we don't know about yet. At the moment spin seems to obey certain rules and we sum up these rules rather neatly using what's called the Pauli Exclusion Principle. This principle states that no two Fermions (particles with half a "spin") can have all the same exact features...if they did, they would just be the same particle.
For instance, if there was a copy of me, it would have the exact same mass, height, appearance, charge, colour etc. etc. and for some reason the Universe refuses to let two identical things like this exist. If there is a deeper reason, we haven't got to it yet. We just know it's seemingly true. If your particles have a whole spin, they can be identical clones of one another, but if we're talking fermions, the Universe seems to obey Pauli.
In the core of a nucleus, quarks are lumped together in groups of three and groups of two. The groups of three are what we call protons and neutrons and they obey the Pauli Exclusion Principle. The way protons and neutrons bind to each other is via something called "The Strong Nuclear Force" which, rather confusingly, is not the same as "The Strong Force" - one of the four fundamental forces of nature. The Strong Nuclear Force is actually a pseudo-Force, it's the result of protons and neutrons exchanging quarks, so it should really be called the Strong Nuclear Interaction (and it sometimes is). As protons and neutrons hand quarks back and forth between each other, they are held together and, in this process, briefly have lots of properties in common. But never all of them, never violating Pauli.
There are a lot of fiddly rules called "Quantum Chromodynamics" which list what protons and neutrons can and can't do, but one of the important outcomes is that, due to the Pauli Exclusion Principle, a proton and a neutron can bind to each other because they're different enough. If you had two neutrons, say, and tried to bind them, they would have to have identical energies as well as having identical charge, spin etc. etc. In other words, they won't do it.
Two neutrons and a proton can bind together comfortably (it's called Tritium) because the two neutrons are interacting with the proton slightly differently at any one time, so they never violate Pauli, but two neutrons together, or three, or four or five doesn't seem to be possible to bind as one stable particle. You could attract lots of neutrons together via gravity (as is proposed in a neutron star) but they don't actually bind to one another.
So, if our understanding of the Pauli Exclusion Principle is right, and our understanding of how neutrons and protons bind to each other is also right, dineutrons, trineutrons, tetraneutrons and so on should be impossible. For them to bind, they'd have to be undergoing the Strong Nuclear Interaction/Force which would at some point, violate Pauli.
The recent discovery in Tokyo provides some scant evidence of what might, possibly, maybe be a tetraneutron briefly existing. More research is needed obviously, but if it's validated over the coming years then either Pauli is wrong or we need to change our model of how protons and neutrons bind together (Strong Nuclear Interaction). If I was a betting man I'd say this discovery will probably turn out to be something else. But, if tetraneutrons do turn out to be real, I'd bet on our model of particle-bindings being at fault rather than the Pauli Exclusion Principle.
This is a fantastic question and one which nobody can fully answer yet. Generally speaking, questions to do with taste i.e. why we like one thing and not another, are still in their infancy in terms of getting good answers. Nevertheless, small steps have been made to answering these questions. Here's what we do know:
Around 4 months into pregnancy, the ears are mostly developed and sound can be detected. The foetus will most oven hear the sound of it's mother's voice therefore and will likely find this type of sound (and any voice similar) will unconciously remind you of the most familiar (and therefore safe) sound you heard during development. There's a wealth of studies indicating that newborn babies respond more favourably to the sound of a mother's voice than to other voices, most recently in a 2012 study by Lesley Peltzer where it was found that girls engaged in stressful activities (the study involved solving math questions in front of an audience) the girls were calmed down better by hearing their mother speak over the phone, than by reading the same message sent from their mother in a text.
Similarly, a 2010 study carried out at the University of Minnesota found that students who played instruments tended to respond better to music that had similar frequencies to their instrument. In other words, our fondness for sound does seem to have a strong cultural influence. We tend to like sounds and music we're familiar with.
However, it's also worth noting that animals often have specific noises that are used to signify danger, food and a desire to breed. These calls are often the same, or similar, for different animals of the same species in different areas. Meaning there does also seem to be some evolutionary bias to like some sounds and not others. Most obviously, we don't like noises which gradually get louder and louder because it will give the impression of something approaching us (which immediately sets our instincts on alert for a predator). So ultimately, it's probably going to be a mixture of evolutionary preference for certain sounds, mixed in with what we were exposed to when we were young.
This is a question which plagued Scientists for a very, very long time. Initially posed by a man called Olbers (I can't remember his first name, I've got a feeling it was William?) The idea is that because there are so many stars, in all directions, all giving out light...why isn't the sky glowing with star light, why is it mostly dark?
The quick answer is that there are a finite number of stars and the Universe hasn't been around forever. So we can see all the stars which have formed, but only a certain number have formed and, given how small we are compared to the Universe, we're not going to see most of the stars because they're so spread out and far away. But this doesn't get to the real heart of the question: In the past, the Universe must have been filled with light from the big bang's initial expansion, the formation of atoms and the formation of early stars...where has all this light gone?
The answer to this question, rather surprisingly, is that the night sky IS filled with starlight, it's just our eyes which can't pick it up. Our Universe seems to have started existing in its present state around 13.7 billion years ago. Stars don't form instantly, in fact they take millions of years to grow and start glowing. So in the initial Universe there were no stars, but then they started forming and giving light out.
That light still exists in the Universe and it's flying about everywhere. The only problem is that the Universe is stretching. For reasons which aren't understood yet, our Universe is expanding at such a rate, that beams of light get stretched out with it. What we know is that light is, in a sense, made of ripples in the electromagnetic field all around us. As this field gets stretched by the Universe (that's the bit we don't get) the beams of light are stretched too.
As the beams of light get stretched, their colour changes. Suppose a Sun is glowing yellow, that beam of light is yellow when it emerges from the Sun, but is gradually stretched to orange, then red, then infra-red, then microwaves and so on. This means that the really old stars gave their light out so long ago, and it's been stretched so much, that by the time it reaches us, it's been stretched beyond the visible spectrum.
The more recent stars' light hasn't been stretched as much, so we see it visibly as (usually) white light. If we could see infra-red light, then the sky really would be glowing because we'd be able to see billions and billions of stars in all directions. So ultimately, the reason the sky appears dark is down to three things: 1) The Universe is finite in time. 2) The Universe is stretching. 3) Stars take time to form.