Hello to everyone! Today, simultaneously to the opening of the movie dedicated to him, I’d like to talk to you about Stephen Hawking and his research work about the Theory of Everything ! Maybe you noticed a few days ago in France the opening of this movie called “Une merveilleuse histoire du temps” [literally: A wonderful history of Time] and this movie recounts the life of the famous astrophysicist Stephen Hawking. You may know that Stephen Hanwking dedicated a great part of his life as a researcher to the pursuit of what we call “the Theory of Everything” This is in fact the English title of the movie, called “the Theory of Everything”. So, today I’d like to explain :
What is the Theory of Everything? Why do we call it like that? Why are we researching it? And…what is Hawking’s contribution in it? To understand better what is the Theory of Everything, we have to go back to the first half of the 20th Century. You may already know that at this period there was 2 very big revolutions in Fundamental Physics. The first one was the discovery of the General Relativity theory by Einstein. The General Relativity theory is a theory about the force of Gravitation (Gravity). In fact, at the time Einstein proposed his theory, we already had a theory of Gravity. It was Newton’s Theory : the law of Universal Attraction. The one we are taught during high school. Einstein started with a different postulate. He proposed that if massive objects are attracted to each other, it is not because of a force between them, but because they curve the space-time continuum. Einstein’s theory doesn’t actually bring many new things compared to Newton’s theory … except when the objects are very massive. There is in particular 2 very special cases. The first one is the existence of Black Holes. Einstein’s theory indicates: “there are regions of space from which nothing can escape… not even light !” … that’s what we call Black Holes. And today we know that Black Holes exist.There is actually one quietly sitting in the middle of our Galaxy. and you can even look in its direction if, during summer, you aim for the Sagittarius constellation. The other big discovery coming from Einstein’s Theory is the idea of an expanding Universe. Einstein’s Theory tells us that the Universe is expanding and that if we go back in time, if we rewind the movie of the Universe’s history, until 13.8 billion years, the Universe was in a very dense state, completely condensed on itself, and very hot, and this period is what we call the Big Bang. Big Bang is directly deduced from the General Relativity theory. The other big revolution that occured more or less at the same time, in parrallel, is the Quantum Theory. Quantum Mechanics are a bit on the opposite side of General Relativity, because it is a theory about matter at the microscopic level. By microscopic, what are we talking about? We talk about atoms… and also everything underneath, everything smaller, protons, electrons… every elementary particles. And Quantum Mechanics describes a world that, at this microscopic level, is very different from the world we are used to, at the macroscopic level. The best way to understand this is to think about an atom. You know, we used to represent an atom as a kind of small solar system, with electrons orbiting around protons, a little in the same way as planets orbit around a star. In fact, Quantum Mechanics tells us that things happen in quite a different way. For planets orbiting a star, they usually are at a specific distance from the star but they can be, in theory, at any distance there is no mandatory distance. At the microscopic level, for electrons, there are mandatory orbits.This means that an electron can only be at a specific distance from the proton. It can be on an orbit, or on the next one, but it can’t be in-beetwen the two. Things are discontinuous, and we sometimes say that things are “quantified”. In fact, that is what gives its name to Quantum Mechanics. The other big news at the microscopic level is that particles can be in several states simultaneously. You know, the usual image we give is that of Schrödinger’s Cat, the cat that can be simultaneously dead and alive. For an electron orbiting around a proton in an atom, this is exactly what happens. The electron is … in every positions of its orbit at the same time. It is everywhere on its orbit. So it is a very different thing compared to what we are used to ourselves, at the macroscopic level. Quatum Mechanics can appear to be quite weird, but you have to know that this theory works very well. First of all, this is the basis of all our current understanding of Particles Physics … and not only in (particle) accelerators, but Quantum Mechanics is also what allows us to understand how semiconductors work and thus how to make transistors, microprocessors, and without Quantum Mechanics, you may not be watching this video. Ok, all this is good and well, but very quickly, physicists understood that Quantum Mechanics, and General Relativity, were 2 completely incompatible theory. This can be seen already…in General Relativity, everything is continuous, and every thing is in a well determined state. In Quantum Mechanics, things are discontinous, and objects can be in several states at the same time. So those two theories are fundamentally incompatible. And you know Physicists… They really like trying to unify things. If they could have only one Theory that encompasses those two, that would suit them very well. And so this is that hypothetical Theory that would encompass both that we call – a little pompously – the Theory of Everything. Here, I can hear you tell me: “Why do we care to have a Theory of Everything?” I told you: “Quantum Mechanics is for microscopic objects… …and General Relativity is for very heavy objects “. In general, very massive objects are not microscopic, and microscopic objects are not very heavy. Well, this is not entirely true… we know at least 2 cases where we need a theory that regroups Quantum Mechanics and General Relativity. The first one is to understand what happens inside Black Holes. You know, a Black Hole is only accumulating matter All this matter falls in the center of the Black Hole and so at the center of the Black Hole, we have a huge amount of matter concentrated in a very small volume. To understand what really happens at the center of a Black Hole, we need to have a Theory that allows to do both General Relativity and Quantum Mechanics We need a Theory of Everything. The other case is … about the very first moments of the Big Bang. In fact, in the first moments of the Big Bang, the Universe was in a very, very concentrated state. It was very heavy… but it was also so contracted that to really understand what happened then, we need a Theory of Everything. And this is very important because, you know, we often say “the Universe was born 13.8 billion years ago” In fact, it isn’t true, we don’t really know. If we really want to understand what happened at that time, we need to have a Theory that allows one to do both General Relativity and Quantum Mechanics, at the same time. So, what has Hawking done in all this? Hawking was the first, in the seventies, to manage to mix a little bit both Quantum Mechanics and General Relativity. More precisely, Hawking worked on the physics of Black Holes, and he showed that if we put a little bit of Quantum Gravity in Black Holes, we find out that they actually emit a radiation. We used to say “A black hole is something that can only swallow things. It can not emit anything.” And Hawking showed that no : if we take Quantum Mechanics into account, that picture changes, and Black Holes emit a radiation. That radiation is energy and thus, if Black Holes emit energy : its mass diminished. So Black Holes can diminish, and they actually can completely disappear. That’s what we call the Black Hole Evaporation. And this was a little revolution as, for the first time, someone managed to mix a bit of Quantum Mechanics and General Relativity. Moreover, by doing this, this changed completely the image we had about Black Holes. And so this is what remains Hawking’s biggest find, his big scientific discovery. His name is even attached to this because we call the Black Hole radiation the “Hawking Radiation”. So today, where are we in this research of a Theory of Everything? Well, in fact, we are not really much further than where we were at the time of Hawking’s contribution. The most elaborate attempt to this day – at least, the one that has the most researchers and also the most publicity – is called the String Theory. String Theory’s idea is to reset everything to zero, and to postulate: ” Actually, the work is not made of particles, but it is made of little uni-dimensionnal objects, which are a kind of little strings, and that can, like particles, move, interact, disintegrate, merge, etc. String Theory allows us to solve quite a number of problems that we had before while trying to mix a little too simply General Gravity and Quantum Mechanics. There is just a price to pay, which is that String Theory only works in ten dimensions. You can not go a String Theory in Four dimensions or in 18 dimensions…It has to be in 10 dimensions. One dimension of time, and nine dimensions of space. This is a problem, because until proven otherwise, today, space dimensions, we only saw 3 of them. So we have to explain where are the other six. Today, String Theory is the most developped approach, but it is still far from being called “the Theory of Everything”. And you have to know that there is not only String Theory in life, there are also other approaches and in particular one, those last twenty years, we saw a new approach that was developped, called Loop Quantum Gravity. Don’t be mistaken by the term of loop. The loops in Loop Quantum Gravity have almost nothing in common with the Strings in String Theory and this approach is very interresting because, for once, it works in four dimensions. This is, one dimension of time, and three dimensions of space. In particular, the Physicist that are working on that subject have looked into what we can comprehend of the very first times of the Big Bang when using that Theory. And what they saw is that it was possible that our Universe is not actually born 13.8 billion years ago, but that it is the result… of a resurging of a previous Universe! So you have to imagine a previous Univese that has collapsed on itself and that, just before it reached the size of a point, had bounced upon iself and gave our own Universe. So, I have to tell you something about this approach of Loop Quantum Gravity, I have a slight preference for it, as it was my research subject a few years ago. I promise you : one day I’ll make a video dedicated to Loop Quantum Gravity. Thank you for watching this video, I hope you liked it. You should know that you can subscribe to this channel. You can also find me on my blog: Science étonnante (Surprising Science) as well as on Facebook and Twitter. See you next time!