cityofgates (![[info]](http://l-stat.livejournal.com/img/userinfo.gif){kind=small} cityofgates) wrote,@ 2006-10-03 21:56:00 |
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A lot of people have been asking me lately about the deal with modern theoretical physics. Is strings wrong? What's the Standard Model? Why are people so excited about quantum gravity. So I thought I would write a post to explain what I understand of the situation. This might be a multipart series.
The basic deal is this: We know a hell of a lot about the way the world works. We have some comparatively simple laws that explain every sort of physical happening we have identified. If this seems like a strange qualification, it is. We have some very strong indications that there are other phenomena out there that we have never encountered. In fact, by weight, most of the universe seems to be composed of something other than the kinds of matter we know. More about this later.
Most of what we know, we learned during the last century or so. The exponential growth in human technological resources made it possible to pursue experiments that previous generations never dreamed of. We've figured out how to focus electron beams like light, making it possible to see the inner details of human cells. (Ordinary microscopes break down at about 500x magnification. An electron microscope can get up to 10,000x.) We've learned to launch telescopes into space, getting a clear view of the stars for the first time. And we've built telescopes that can see other wavelengths of light, like radio and X-ray bands. (Here's a funny story: Back in 1965, in New Jersey, at Bell Labs, Arno Penzias and Robert Wilson were trying to tune a new radio telescope, one of the more sensitive ones that had been built by that day. They kept getting a rather peculiar signal, a constant hum somewhere in the low infrared. They tried everything. They checked all the wires. They cleared the pigeonshit out of the detector. They tried aiming the telescope in different directions. No difference. They got the same weird signal no matter which way they pointed the detector. Eventually, the realized that what they were seeing was the dim warped echo of the moment the Moment The Lights Came On. This wasn't the Beginning; in fact, our best guess is that it's about 300,000 years after the Big Bang. But it was the first time that the early universe was cool enough that ordinary radiant light could form. Penzias and Wilson were seeing the remanant glow of the first moment the universe was cool enough to merely glow.)
And we've built x-ray spectroscopes and nuclear reactors, and photo multiplier tubes, and particle accelerators, which have let us study the internal structure of molecules, which turn out to be made of atoms, and of atoms, which turn out to be made of a nucleus of protons and neutrons surrounded by a cloud of electrons. We've looked inside protons and neutrons and discovered that they are made of these funny little things called quarks. Protons, and neutrons, and many of the more stable nuclear particles are made of various combinations of up and down quarks. There are other kinds of quarks, too; more later. We've also looked for substructure in the electrons, but haven't, to date, found any. We've also discovered other kinds of particles. There are neutrinos. Maybe you wondered at some point if a neutron is really made of an electron and a proton...well, in some sense it is. It's a proton, an electron, and a neutrino, in a way. More precisely, a neutron is it's own thing, but it can decay. It's not stable, and sometimes when you're not watching, a neutron will transform into a proton, an electron, and a neutrino. There are also muons. The muon is just like an electron, but 200x heavier. It's also unstable; most muons will decay within microseconds of being spotted, usually into an electron and some neutrinos. If you feel a bit overwhelmed, imagine how particle physicists felt in the 1950s, 60s, and 70s. Their job was to explain the elementary constituents of matter, and every time they turned around, they were finding new particles.
Things turned around for them by the mid 70s though. Most of the particles they found weren't fundamental particles, just complicated combinations of quarks. In the end, they found that there are 3 generations of particles, each generation composed of 1 lepton (an electron-like particle), two kinds of quark (e.g. up & down), and a neutrino. There are also force carrier particles, the 8 gluons, 3 weak force particles, and the photon. And there's probably something like a Higgs particle. I'll talk about this some other time. The main thing right now you want to know is that, in the mid 1970s, particle physicists had put into place something called the Standard Model, which describes every physical phenomenon we've seen on terrestrial scales.
We also have, thanks to Einstein, a theory called general relativity which describes the nature of gravity. That's a separate post, also for some other time. There are some serious theoretical conflicts between general relativity and the Standard Model; the two theories use descriptions of the world which simply don't seem to be compatible. It's not English & French; it's Angelic & Demonic. So theorists have a serious puzzle on their hands. But they aren't making particularly good progress on it. The reason is basically this: We can't test general relativity in high gravity fields very easily, because there aren't very many available near here. On an absolute scale, the gravitational force of the Sun and the Earth and stars is actually incredibly weak. Spacetime barely curves at all here. We've found some clever indirect tests in cosmology and astrophysics, but we don't have much direct experience. And without direct experience, without a microscope so good that it lets us see spacetime the way an electron microscope lets us see bacteria guts, we don't have much of an idea how gravity has to behave on these scales. So we don't know how to modify general relativity, except in the sense that the theorists have some vague guesses abou how the theory has to look. And we don't know how to modify the Standard Model; we've never actually seen a particle of matter or an interaction besides gravity, which can't be described by it.
So we're in an odd situation. The flow of experimental data which led us to general relativity and the Standard Model has slowed down somewhat, and our guesses 30 years ago are still holding good today. (Congress is partly to blame for this; they cut funding for the SSC in the early 1990s, in an attempt to show how budget conscious they were.) We know GR and the SM don't fit together, so there's clearly still work to do. But we don't have any data that the theories can't explain*. So we don't know what to do.
* except Dark Matter, which is in fact, what most of the universe is made of. Unfortunately, Dark Matter barely interacts with ordinary matter. It doesn't reflect light, and doesn't bounce off any instruments we have available. So we can only see it through its gravitational influence, which unfortunately doesn't tell us much about it.
This is basically the problem with string theory right now. It is an accumulation of clever guesses, peculiar coincidences, and wishful thinking. We don't know if it matches our world. We can't test it out very easily. We have some theoretical prejudices that make it look attractive in some ways, and highly problematic in others. But the basic fact about string theory -- and about any other extension of the Standard Model or GR that you might hear about, like supersymmetry, or Grand Unified Theories, or loop quantum gravity, or preons, or whatever -- is that it has not been tested. We haven't gotten a good look at any physics that can't be described by the Standard Model or GR, so we don't know which of these models, if any, is actually correct. My guess is that none of them are actually right. But I don't have anything better to propose, so I do mathematics.
For the record, I think strings is interesting to study in at least one way: It forces us to think about old ideas in new ways, and even if it's not much good for describing our world, it gives us a nice venue for testing our ideas about going beyond the physical theories we have. In this sense, it's superior to most of the other theories out there. But it's not remotely clear that string theory is anything but a playground for theorists to learn from, for the foreseeable future. (We're going to need to go way beyond what any terrestrial particle accelerator can provide to see the phenomena that we might need strings to describe.) My best guess, my hope, is that we'll find some interesting new phenomena at the LHC, maybe superparticles, maybe a complicated Higgs sector, who knows, maybe some Rarita-Schwinger fields. (The last situation would actually be the most exciting: Rarita-Schwinger fields are expected to be tied in some way to gravity. Unfortunately, it's also the least likely.) Then you'll probably stop hearing about string theory for a while. A few people will probably work on it, and a few on other forms of quantum gravity, and other weird kinds of speculations. But the dry spell of experimental data seems to be ending, and there's going to be a lot of interesting work in the next few decades, sorting out what Dark Matter is, and the details of neutrino masses and the Higgs sector and electroweak symmetry breaking. So don't pay too much attention to these public arguments in theoretical physics. Some of the complaints are legitimate, but most of the troubles are probably going to fade as we get new data and people focus more on more solvable puzzles.
The basic deal is this: We know a hell of a lot about the way the world works. We have some comparatively simple laws that explain every sort of physical happening we have identified. If this seems like a strange qualification, it is. We have some very strong indications that there are other phenomena out there that we have never encountered. In fact, by weight, most of the universe seems to be composed of something other than the kinds of matter we know. More about this later.
Most of what we know, we learned during the last century or so. The exponential growth in human technological resources made it possible to pursue experiments that previous generations never dreamed of. We've figured out how to focus electron beams like light, making it possible to see the inner details of human cells. (Ordinary microscopes break down at about 500x magnification. An electron microscope can get up to 10,000x.) We've learned to launch telescopes into space, getting a clear view of the stars for the first time. And we've built telescopes that can see other wavelengths of light, like radio and X-ray bands. (Here's a funny story: Back in 1965, in New Jersey, at Bell Labs, Arno Penzias and Robert Wilson were trying to tune a new radio telescope, one of the more sensitive ones that had been built by that day. They kept getting a rather peculiar signal, a constant hum somewhere in the low infrared. They tried everything. They checked all the wires. They cleared the pigeonshit out of the detector. They tried aiming the telescope in different directions. No difference. They got the same weird signal no matter which way they pointed the detector. Eventually, the realized that what they were seeing was the dim warped echo of the moment the Moment The Lights Came On. This wasn't the Beginning; in fact, our best guess is that it's about 300,000 years after the Big Bang. But it was the first time that the early universe was cool enough that ordinary radiant light could form. Penzias and Wilson were seeing the remanant glow of the first moment the universe was cool enough to merely glow.)
And we've built x-ray spectroscopes and nuclear reactors, and photo multiplier tubes, and particle accelerators, which have let us study the internal structure of molecules, which turn out to be made of atoms, and of atoms, which turn out to be made of a nucleus of protons and neutrons surrounded by a cloud of electrons. We've looked inside protons and neutrons and discovered that they are made of these funny little things called quarks. Protons, and neutrons, and many of the more stable nuclear particles are made of various combinations of up and down quarks. There are other kinds of quarks, too; more later. We've also looked for substructure in the electrons, but haven't, to date, found any. We've also discovered other kinds of particles. There are neutrinos. Maybe you wondered at some point if a neutron is really made of an electron and a proton...well, in some sense it is. It's a proton, an electron, and a neutrino, in a way. More precisely, a neutron is it's own thing, but it can decay. It's not stable, and sometimes when you're not watching, a neutron will transform into a proton, an electron, and a neutrino. There are also muons. The muon is just like an electron, but 200x heavier. It's also unstable; most muons will decay within microseconds of being spotted, usually into an electron and some neutrinos. If you feel a bit overwhelmed, imagine how particle physicists felt in the 1950s, 60s, and 70s. Their job was to explain the elementary constituents of matter, and every time they turned around, they were finding new particles.
Things turned around for them by the mid 70s though. Most of the particles they found weren't fundamental particles, just complicated combinations of quarks. In the end, they found that there are 3 generations of particles, each generation composed of 1 lepton (an electron-like particle), two kinds of quark (e.g. up & down), and a neutrino. There are also force carrier particles, the 8 gluons, 3 weak force particles, and the photon. And there's probably something like a Higgs particle. I'll talk about this some other time. The main thing right now you want to know is that, in the mid 1970s, particle physicists had put into place something called the Standard Model, which describes every physical phenomenon we've seen on terrestrial scales.
We also have, thanks to Einstein, a theory called general relativity which describes the nature of gravity. That's a separate post, also for some other time. There are some serious theoretical conflicts between general relativity and the Standard Model; the two theories use descriptions of the world which simply don't seem to be compatible. It's not English & French; it's Angelic & Demonic. So theorists have a serious puzzle on their hands. But they aren't making particularly good progress on it. The reason is basically this: We can't test general relativity in high gravity fields very easily, because there aren't very many available near here. On an absolute scale, the gravitational force of the Sun and the Earth and stars is actually incredibly weak. Spacetime barely curves at all here. We've found some clever indirect tests in cosmology and astrophysics, but we don't have much direct experience. And without direct experience, without a microscope so good that it lets us see spacetime the way an electron microscope lets us see bacteria guts, we don't have much of an idea how gravity has to behave on these scales. So we don't know how to modify general relativity, except in the sense that the theorists have some vague guesses abou how the theory has to look. And we don't know how to modify the Standard Model; we've never actually seen a particle of matter or an interaction besides gravity, which can't be described by it.
So we're in an odd situation. The flow of experimental data which led us to general relativity and the Standard Model has slowed down somewhat, and our guesses 30 years ago are still holding good today. (Congress is partly to blame for this; they cut funding for the SSC in the early 1990s, in an attempt to show how budget conscious they were.) We know GR and the SM don't fit together, so there's clearly still work to do. But we don't have any data that the theories can't explain*. So we don't know what to do.
* except Dark Matter, which is in fact, what most of the universe is made of. Unfortunately, Dark Matter barely interacts with ordinary matter. It doesn't reflect light, and doesn't bounce off any instruments we have available. So we can only see it through its gravitational influence, which unfortunately doesn't tell us much about it.
This is basically the problem with string theory right now. It is an accumulation of clever guesses, peculiar coincidences, and wishful thinking. We don't know if it matches our world. We can't test it out very easily. We have some theoretical prejudices that make it look attractive in some ways, and highly problematic in others. But the basic fact about string theory -- and about any other extension of the Standard Model or GR that you might hear about, like supersymmetry, or Grand Unified Theories, or loop quantum gravity, or preons, or whatever -- is that it has not been tested. We haven't gotten a good look at any physics that can't be described by the Standard Model or GR, so we don't know which of these models, if any, is actually correct. My guess is that none of them are actually right. But I don't have anything better to propose, so I do mathematics.
For the record, I think strings is interesting to study in at least one way: It forces us to think about old ideas in new ways, and even if it's not much good for describing our world, it gives us a nice venue for testing our ideas about going beyond the physical theories we have. In this sense, it's superior to most of the other theories out there. But it's not remotely clear that string theory is anything but a playground for theorists to learn from, for the foreseeable future. (We're going to need to go way beyond what any terrestrial particle accelerator can provide to see the phenomena that we might need strings to describe.) My best guess, my hope, is that we'll find some interesting new phenomena at the LHC, maybe superparticles, maybe a complicated Higgs sector, who knows, maybe some Rarita-Schwinger fields. (The last situation would actually be the most exciting: Rarita-Schwinger fields are expected to be tied in some way to gravity. Unfortunately, it's also the least likely.) Then you'll probably stop hearing about string theory for a while. A few people will probably work on it, and a few on other forms of quantum gravity, and other weird kinds of speculations. But the dry spell of experimental data seems to be ending, and there's going to be a lot of interesting work in the next few decades, sorting out what Dark Matter is, and the details of neutrino masses and the Higgs sector and electroweak symmetry breaking. So don't pay too much attention to these public arguments in theoretical physics. Some of the complaints are legitimate, but most of the troubles are probably going to fade as we get new data and people focus more on more solvable puzzles.
