Over time, it has happened that some solution in one area of physics could also be used in a quite different area. Or, at least, inspired the solution. Unfortunately, this does not always work. Even quite often it happened that when reaching the finer points it turns out that something promising did in the end not work. Thus, it pays off to be always careful with such a transfer, and never believe a hype. Still, in some cases it worked, and even lead to brilliant triumphs. And so it is always worthwhile to try.
Such an attempt is precisely the content of my latest paper. In it, I try to transfer ideas from my research on electroweak physics and the Brout-Englert-Higgs effect to quantum gravity. Quantum gravity is first and foremost still an unsolved issue. We know that mathematical consistency demands that there is some unification of quantum physics and gravity. We expect that this will be by having a quantum theory of gravity. Though we are yet lacking any experimental evidence for this assumption. Still, I also make the assumption for now that quantum gravity exists.
Based on this assumption, I take a candidate for such a quantum gravity theory and pose the question what are its observable consequences. This is a question which has driven me since a long time in particle physics. I think that by now I have an understanding of how it works. But last year, I was challenged whether these ideas can still be right if there is gravity in the game. And this new paper is essentially my first step towards an answerhttps://arxiv.org/abs/1908.02140. Much of this answer is still rough, and especially mathematically will require much work. But at least it provides a first consistent picture. And, as advertised above, it draws from a different field.
The starting point is that the simplest version of quantum gravity currently considered is actually not that different from other theories in particle physics. It is a so-called gauge theory. As such, many of its fundamental objects, like the structure of space and time, are not really observable. Just like most of the elementary particles of the standard model, which is also a gauge theory, are not. Thus, we cannot see them directly in an experiment. In the standard model case, it was possible to construct observable particles by combining the elementary ones. In a sense, the particles we observe are bound states of the elementary particles. However, in electroweak physics one of the bound elementary particles totally dominates the rest, and so the whole object looks very similar to the elementary one, but not quite.
This works, because the Brout-Englert-Higgs effect makes it possible. The reason is that there is a dominating kind of not observable structure, the so-called Higgs condensate, which creates this effect. This is something coincidental. If the parameters of the standard model would be different, it would not work. But, luckily, our standard model has just the right parameter values.
Now, when looking at gravity around us, there is a very similar feature. While we have the powerful theory of general relativity, which describes how matter warps space, we rarely see this. Most of our universe behaves much simpler, because there is so little matter in it. And because the parameters of gravity are such that this warping is very, very small. Thus, we have again a dominating structure: A vacuum which is almost not warped.
Using this analogy and the properties of gauge theories, I figured out the following: We can use something like the Brout-Englert-Higgs effect in quantum gravity. And all observable particles must still be some kind of bound states. But they may now also include gravitons, the elementary particles of quantum gravity. But just like in the standard model, these bound states are dominated by just one of its components. And if there is a standard model component it is this one. Hence, the particles we see at LHC will essentially look like there is no gravity. And this is very consistent with experiment. Detecting the deviations will be so hard in comparison to those which come from the standard model, we can pretty much forget about it for earthbound experiments. At least for the next couple of decades.
However, there are now also some combinations of gravitons without standard model particles involved. Such objects have been long speculated about, and are called geons, or gravity balls. But in contrast to the standard model case, they are not stable classically. But they may be stabilized due to quantum effects. The bound state structure strongly suggests that there is at least one stable one. Still, this is pure speculation at the moment. But if they are, these objects could have dramatic consequences. E.g., they could be part of the dark matter we are searching for. Or, they could make up black holes very much like neutrons make a neutron star. I have no idea, whether any of these speculations could be true. But if there is only a tiny amount of truth in it, this could be spectacular.
Thus, some master students and I will set out to have a look at these ideas. To this end, we will need to some hard calculations. And, eventually, the results should be tested against observation. These will be coming form the universe, and from astronomy. Especially from the astronomy of black holes, where recently there have been many interesting and exciting developments, like observing two black holes merge, or the first direct image of a black hole (obviously just black inside a kind of halo). These are exciting times, and I am looking forward to see whether any of these ideas work out. Stay tuned!
Showing posts with label Gravity. Show all posts
Showing posts with label Gravity. Show all posts
Wednesday, August 7, 2019
Tuesday, May 14, 2019
Acquiring a new field
I have recently started to look into a new field: Quantum gravity. In this entry, I would like to write a bit about how this happens, acquiring a new field. Such that you can get an idea what can lead a scientist to do such a thing. Of course, in future entries I will also write more about what I am doing, but it would be a bit early to do so right now.
Acquiring a new field in science is not something done lightly. One has always not enough time for the things one does already. And when you enter a new field, stuff is slow. You have to learn a lot of basics, need to get an overview of what has been done, and what is still open. Not to mention that you have to get used to a different jargon. Thus, one rarely does so lightly.
I have in the past written already one entry about how I came to do Higgs physics. This entry was written after the fact. I was looking back, and discussed my motivation how I saw it at that time. It will be an interesting thing to look back at this entry in a few years, and judge what is left of my original motivation. And how I feel about this knowing what happened since then. But for now, I only know the present. So, lets get to it.
Quantum gravity is the hypothetical quantum version of the ordinary theory of gravity, so-called general relativity. However, it has withstood quantization for a quite a while, though there has been huge progress in the last 25 years or so. If we could quantize it, its combination with the standard model and the simplest version of dark matter would likely be able to explain almost everything we can observe. Though even then a few open questions appear to remain.
But my interest in quantum gravity comes not from the promise of such a possibility. It has rather a quite different motivation. My interest started with the Higgs.
I have written many times that we work on an improvement in the way we look at the Higgs. And, by now, in fact of the standard model. In what we get, we see a clear distinction between two concepts: So-called gauge symmetries and global symmetries. As far as we understand the standard model, it appears that global symmetries determine how many particles of a certain type exists, and into which particles they can decay or be combined. Gauge symmetries, however, seem to be just auxiliary symmetries, which we use to make calculations feasible, and they do not have a direct impact on observations. They have, of course, an indirect impact. After all, in which theory which gauge symmetry can be used to facilitate things is different, and thus the kind of gauge symmetry is more a statement about which theory we work on.
Now, if you add gravity, the distinction between both appears to blur. The reason is that in gravity space itself is different. Especially, you can deform space. Now, the original distinction of global symmetries and gauge symmetries is their relation to space. A global symmetry is something which is the same from point to point. A gauge symmetry allows changes from point to point. Loosely speaking, of course.
In gravity, space is no longer fixed. It can itself be deformed from point to point. But if space itself can be deformed, then nothing can stay the same from point to point. Does then the concept of global symmetry still make sense? Or does all symmetries become just 'like' local symmetries? Or is there still a distinction? And what about general relativity itself? In a particular sense, it can be seen as a theory with a gauge symmetry of space. Makes this everything which lives on space automatically a gauge symmetry? If we want to understand the results of what we did in the standard model, where there is no gravity, in the real world, where there is gravity, then this needs to be resolved. How? Well, my research will hopefully answer this question. But I cannot do it yet.
These questions were already for some time in the back of my mind. A few years, I actually do not know how many exactly. As quantum gravity pops up in particle physics occasionally, and I have contact with several people working on it, I was exposed to this again and again. I knew, eventually, I will need to address it, if nobody else does. So far, nobody did.
But why now? What prompted me to start now with it? As so often in science, it were other scientists.
Last year at the end of November/beginning of December, I took part in a conference in Vienna. I had been invited to talk about our research. The meeting has a quite wide scope, and also present were several people, who work on black holes and quantum physics. In this area, one goes, in a sense, halfway towards quantum gravity: One has quantum particles, but they life in a classical gravity theory, but with strong gravitational effects. Which is usually a black hole. In such a setup, the deformations of space are fixed. And also non-quantum black holes can swallow stuff. This combination appears to make the following thing: Global symmetries appear to become meaningless, because everything associated with them can vanish in the black hole. However, keeping space deformations fixed means that local symmetries are also fixed. So they appear to become real, instead of auxiliary. Thus, this seems to be quite opposite to our result. And this, and the people doing this kind of research, challenged my view of symmetries. In fact, in such a half-way case, this effect seems to be there.
However, in a full quantum gravity theory, the game changes. Then also space deformations become dynamical. At the same time, black holes need no longer to have the characteristic to swallow stuff forever, because they become dynamical, too. They develop. Thus, to answer what happens really requires full quantum gravity. And because of this situation, I decided to start to work actively on quantum gravity. Because I needed to answer whether our picture of symmetries survive, at least approximately, when there is quantum gravity. And to be able to answer such challenges. And so it began.
Within the last six months, I have now worked through a lot of the basic stuff. I have now a rough idea of what is going on, and what needs to be done. And I think, I see a way how everything can be reconciled, and make sense. It will still need a long time to complete this, but I am very optimistic right now. So optimistic, in fact, that a few days back I gave my first talk, in which I discussed this issues including quantum gravity. It will still need time, before I have a first real result. But I am quite happy how thing progress.
And that is the story how I started to look at quantum gravity in earnest. If you want to join me in this endeavor: I am always looking for collaboration partners and, of course, students who want to do their thesis work on this subject 😁
Acquiring a new field in science is not something done lightly. One has always not enough time for the things one does already. And when you enter a new field, stuff is slow. You have to learn a lot of basics, need to get an overview of what has been done, and what is still open. Not to mention that you have to get used to a different jargon. Thus, one rarely does so lightly.
I have in the past written already one entry about how I came to do Higgs physics. This entry was written after the fact. I was looking back, and discussed my motivation how I saw it at that time. It will be an interesting thing to look back at this entry in a few years, and judge what is left of my original motivation. And how I feel about this knowing what happened since then. But for now, I only know the present. So, lets get to it.
Quantum gravity is the hypothetical quantum version of the ordinary theory of gravity, so-called general relativity. However, it has withstood quantization for a quite a while, though there has been huge progress in the last 25 years or so. If we could quantize it, its combination with the standard model and the simplest version of dark matter would likely be able to explain almost everything we can observe. Though even then a few open questions appear to remain.
But my interest in quantum gravity comes not from the promise of such a possibility. It has rather a quite different motivation. My interest started with the Higgs.
I have written many times that we work on an improvement in the way we look at the Higgs. And, by now, in fact of the standard model. In what we get, we see a clear distinction between two concepts: So-called gauge symmetries and global symmetries. As far as we understand the standard model, it appears that global symmetries determine how many particles of a certain type exists, and into which particles they can decay or be combined. Gauge symmetries, however, seem to be just auxiliary symmetries, which we use to make calculations feasible, and they do not have a direct impact on observations. They have, of course, an indirect impact. After all, in which theory which gauge symmetry can be used to facilitate things is different, and thus the kind of gauge symmetry is more a statement about which theory we work on.
Now, if you add gravity, the distinction between both appears to blur. The reason is that in gravity space itself is different. Especially, you can deform space. Now, the original distinction of global symmetries and gauge symmetries is their relation to space. A global symmetry is something which is the same from point to point. A gauge symmetry allows changes from point to point. Loosely speaking, of course.
In gravity, space is no longer fixed. It can itself be deformed from point to point. But if space itself can be deformed, then nothing can stay the same from point to point. Does then the concept of global symmetry still make sense? Or does all symmetries become just 'like' local symmetries? Or is there still a distinction? And what about general relativity itself? In a particular sense, it can be seen as a theory with a gauge symmetry of space. Makes this everything which lives on space automatically a gauge symmetry? If we want to understand the results of what we did in the standard model, where there is no gravity, in the real world, where there is gravity, then this needs to be resolved. How? Well, my research will hopefully answer this question. But I cannot do it yet.
These questions were already for some time in the back of my mind. A few years, I actually do not know how many exactly. As quantum gravity pops up in particle physics occasionally, and I have contact with several people working on it, I was exposed to this again and again. I knew, eventually, I will need to address it, if nobody else does. So far, nobody did.
But why now? What prompted me to start now with it? As so often in science, it were other scientists.
Last year at the end of November/beginning of December, I took part in a conference in Vienna. I had been invited to talk about our research. The meeting has a quite wide scope, and also present were several people, who work on black holes and quantum physics. In this area, one goes, in a sense, halfway towards quantum gravity: One has quantum particles, but they life in a classical gravity theory, but with strong gravitational effects. Which is usually a black hole. In such a setup, the deformations of space are fixed. And also non-quantum black holes can swallow stuff. This combination appears to make the following thing: Global symmetries appear to become meaningless, because everything associated with them can vanish in the black hole. However, keeping space deformations fixed means that local symmetries are also fixed. So they appear to become real, instead of auxiliary. Thus, this seems to be quite opposite to our result. And this, and the people doing this kind of research, challenged my view of symmetries. In fact, in such a half-way case, this effect seems to be there.
However, in a full quantum gravity theory, the game changes. Then also space deformations become dynamical. At the same time, black holes need no longer to have the characteristic to swallow stuff forever, because they become dynamical, too. They develop. Thus, to answer what happens really requires full quantum gravity. And because of this situation, I decided to start to work actively on quantum gravity. Because I needed to answer whether our picture of symmetries survive, at least approximately, when there is quantum gravity. And to be able to answer such challenges. And so it began.
Within the last six months, I have now worked through a lot of the basic stuff. I have now a rough idea of what is going on, and what needs to be done. And I think, I see a way how everything can be reconciled, and make sense. It will still need a long time to complete this, but I am very optimistic right now. So optimistic, in fact, that a few days back I gave my first talk, in which I discussed this issues including quantum gravity. It will still need time, before I have a first real result. But I am quite happy how thing progress.
And that is the story how I started to look at quantum gravity in earnest. If you want to join me in this endeavor: I am always looking for collaboration partners and, of course, students who want to do their thesis work on this subject 😁
Friday, November 13, 2009
The forces of nature I - Gravity
After illustrating last time how a force can be created by the exchange of particle, it is about time to make a list of which forces there are in nature, and which of them are included in the standard model.
Actually, there is only one force in nature which we currently know and which is not included in the standard model of particle physics. This is gravity. That is the force which pulls one inevitably to the ground, as long as one is not actively working against it. And the one which makes it so hard to get up in the morning. Or so.
It is actually not only the ground, and thus the earth, that is pulling at you, but actually also the earth is pulled by you. However, since the earth is much heavier than you are, it is rather ignorant of your presence. However, it cannot ignore the pull of the moon, to which it reacts with the tides. Nor can it ignore the sun, and this makes earth orbiting around it. On a larger scale, the solar system feels the pull of the milky way, making the solar system orbiting the center of it. And our galaxy the center of the local cluster of galaxies.
In fact, any object which has mass pulls any other object towards it, which has also mass. Actually, this is not entirely correct: Mass is not necessary, it suffices if there is energy in the game. This will lead a bit too far astray now, as it is necessary to delve into the theory of relativity for why this is the case, and I will leave this to later.
However, the generic concept that some objects act a force on each other because they both have a certain property is far more general. It is the simplest example of a charge. Gravity is simple in that everything pulls everything else to itself. In other cases, which will be encountered next time, this is not always the case: Some charges pushes away other charges.
So, why is gravity not included in the standard model (yet)? The simple answer is that we do not yet know how to really do it. There are quite a number of ideas, going by the fancy names of string theory, quantum loop gravity, and many others. However, none of these ideas could have been yet made so precise that it would actually explain how gravity quantitatively fits into the standard model.
The major problem encountered is that it is very hard to make gravity a quantum theory. That has rather technical reasons, and there are some hot leads how we can possibly circumvent this in the future. But not yet. The basic problem is essentially that we do not yet know how to cope with a pileup of gravitons, the (hypothetical) particles carrying the gravitational force, which inevitable always occurs in a quantum theory. That is actually an involved technical problem. For that reason gravity is not yet part of the standard model of particle physics, but instead described by a classical theory, general relativity.
The question is whether this matters when we want to talk about particle physics. The fortunate answer is that it does not, in most cases. The reason is that gravity is a very weak forces. Compared to those described by the standard model, it is about 10000000000000000000000000000000000000 times weaker than the weakest other force of the standard model. Therefore, only if there is a large charge - thus mass or energy - gravity becomes important. That happens only at energy scales which are more than 10000000000000 times larger than accessible in any experiment so far. In nature, it only occurs very close to a black hole or very, very early in the history of the universe. So, for most purposes, and in particular the ones of this blog, gravity can be neglected.
However, there are a number of open questions related to our limited understanding of gravity which have to do with large scales rather than particles: E.g., why is the universe expanding today? Also these questions will not be discussed for the moment in this blog.
Actually, there is only one force in nature which we currently know and which is not included in the standard model of particle physics. This is gravity. That is the force which pulls one inevitably to the ground, as long as one is not actively working against it. And the one which makes it so hard to get up in the morning. Or so.
It is actually not only the ground, and thus the earth, that is pulling at you, but actually also the earth is pulled by you. However, since the earth is much heavier than you are, it is rather ignorant of your presence. However, it cannot ignore the pull of the moon, to which it reacts with the tides. Nor can it ignore the sun, and this makes earth orbiting around it. On a larger scale, the solar system feels the pull of the milky way, making the solar system orbiting the center of it. And our galaxy the center of the local cluster of galaxies.
In fact, any object which has mass pulls any other object towards it, which has also mass. Actually, this is not entirely correct: Mass is not necessary, it suffices if there is energy in the game. This will lead a bit too far astray now, as it is necessary to delve into the theory of relativity for why this is the case, and I will leave this to later.
However, the generic concept that some objects act a force on each other because they both have a certain property is far more general. It is the simplest example of a charge. Gravity is simple in that everything pulls everything else to itself. In other cases, which will be encountered next time, this is not always the case: Some charges pushes away other charges.
So, why is gravity not included in the standard model (yet)? The simple answer is that we do not yet know how to really do it. There are quite a number of ideas, going by the fancy names of string theory, quantum loop gravity, and many others. However, none of these ideas could have been yet made so precise that it would actually explain how gravity quantitatively fits into the standard model.
The major problem encountered is that it is very hard to make gravity a quantum theory. That has rather technical reasons, and there are some hot leads how we can possibly circumvent this in the future. But not yet. The basic problem is essentially that we do not yet know how to cope with a pileup of gravitons, the (hypothetical) particles carrying the gravitational force, which inevitable always occurs in a quantum theory. That is actually an involved technical problem. For that reason gravity is not yet part of the standard model of particle physics, but instead described by a classical theory, general relativity.
The question is whether this matters when we want to talk about particle physics. The fortunate answer is that it does not, in most cases. The reason is that gravity is a very weak forces. Compared to those described by the standard model, it is about 10000000000000000000000000000000000000 times weaker than the weakest other force of the standard model. Therefore, only if there is a large charge - thus mass or energy - gravity becomes important. That happens only at energy scales which are more than 10000000000000 times larger than accessible in any experiment so far. In nature, it only occurs very close to a black hole or very, very early in the history of the universe. So, for most purposes, and in particular the ones of this blog, gravity can be neglected.
However, there are a number of open questions related to our limited understanding of gravity which have to do with large scales rather than particles: E.g., why is the universe expanding today? Also these questions will not be discussed for the moment in this blog.
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