Hello good people,
One of the avenues of Science I’ve never understood is string theory as it applies to physics and Cosmology.
Could a doper supply a 101 introduction please?
Peter
Have twenty minutes? Watch Brian Greene talk about it.
Basically it’s quantum physics, except the most basic systems are “1D” strings instead of “0D” point particles. However in order for the strings to behave like the fundamental particles which they describe they to have to oscillate in which dimensions higher than the usual 3 dimensions of space.
Can anybody speak to the lenfth of strings? Are they Planck length? Are they bigger, and if so, by roughly how much? Can they be smaller, and does that even make sense?
Thanks for the replies, the first of which I will watch later and I’m afraid the second just leads me further down the rabbit hole.
If someone can lead me through a QED ‘this, therefore’ type of logic that ends up with string theory as it’s conclusion I’d be a not unhappy bunny.
P
To continue from AF’s post, the idea with string theory, in general, is these “strings” (which are fundamental, 1-dimensional loops of energy, as opposed to zero-dimensional point-particles that might make up the subatomic particles in the current standard model) vibrate/oscillate and manifest themselves as different subatomic particles by vibrating at different frequencies.
Though, to make this model work according to the laws of nature we observe, they must move and weave through, If I recall correctly, at least 10 dimensions of space, plus one of time; 11 all told. The 4 dimensional world we’re used to (3 spatial + 1 temporal) remains, but curled up far beyond the “sight” of the largest “microscopes” like the Large Hadron Collider at CERN, are the remaining dimensions these strings vibrate and pass through, making up our reality.
So you are asking, “How long is a piece of string?” :dubious:
Good timing, I’ve been watchingthe elegant universe and was thinking of starting a thread (string?) of my own.
My understanding is that the evidence behind the theory goes basically like this. If we imagine that particles are strings that vibrate in these n>4 dimensions then the math works out such that the 4 basic forces of gravity look like they appear to in the real world.
However there are at least 5 such formulations that give math that matches our observations equally. Further there is no known experiment that can be used to test the theory, which runs into the problem on non-falsifiability but also by definition means that it can’t be used to make useful prediction of things we were not already aware of. Therefor its hard to say whether there really are such strings of if its just math that is tinkered in such a way to match something totally different that is really going on.
The analogy I would use would be the ptolemaic modelof the solar system, which we now know is totally wrong, but which at the time matched the available data in the same way that string theory does today.
Of course one can argue what if any is the actual difference between a model that is isomorphic to reality and reality itself, but that is more of a great debates question.
A decent crash course and interesting read on this is Brian Greene’s The Elegant Universe.
Also, his TED talk gives a good broad stroke.
It’s a fair question - the whole idea of string theory is that these “strings” are one dimensional objects- they have length, but no width. The length is believed to be at a Planck Length scale.
How the strings vibrate is what determines the property of what they constitute. One type of vibration will produce a photon, another type of vibration produces gravity, and another type of vibration produces matter. All matter and energy is a result of how the strings vibrate across different dimensions.
As scientists began to contemplate string theory, 5 major models appeared. Each model was internally consistent and yet each model was incompatible with the others. In 1994 a new model was proposed called M-Theory which merges all 5 string theories under 1 umbrella - the idea being that each theory is correct if viewed in different ways.
String Theory is called non-falsifiable because we haven’t found an experiment yet that can prove, nor disprove it. It still just lives in theory. But the theory is so well formed and pieces fit together so nicely that many scientists agree that there may be something to the idea. This is why it still gets attention despite no proof that it’s real.
So potentially a form of intellectual self abuse?
Just because you cab etc…
P
Fundamental physics; self abuse… a rose by any other name…
The string length is actually a free parameter in string theory, that is, it’s not predicted by the theory (usually, the free parameter is taken to be something called ‘alpha prime’, the square of the string length). So this length is essentially postulated: the original version of string theory, which was thought to be a theory of how hadrons (particles like the proton, neutron etc.) work, had a different (much larger) length scale than the modern superstring, for which it is thought to be around the Planck scale (~10[sup]-35[/sup]m).
Just to head off a bit of potential confusion, the elementary particles we observe aren’t really different modes of the string, i.e. the string vibrating at different frequencies (even though the claim is sometimes made by people who ought to know better), since those would be much more massive than any particles we observe—the first excited mode of a string would have a mass around the Planck mass, which is around 0.02 mg, or 10[sup]19[/sup] times heavier than the proton. Rather, the particle spectrum we observe is dictated by the ‘shape’ of the extra dimensions, and how the string wraps around them. This also produces the problem of extracting definite predictions from string theory: nobody knows what determines the shape of the extra dimensions, and thus, almost every set of elementary particles appears to be consistent with string theory. (This is sometimes referred to under the name of ‘string landscape’, with each point in this landscape corresponding to a possible shape of the extra dimensions, and thus, a different particle spectrum.)
Well, you get the right kinds of particles and forces to appear: you have gravity, which comes part and parcel with string theory, and which is its biggest selling point, since to this day it’s the only (to the best of my knowledge) quantum theory that can reproduce gravity, and you have non-gravitational forces of the same kind we see in nature, and likewise, matter particles of the right kind—but getting exactly those forces and matter particles that constitute our universe out of string theory remains its biggest challenge.
Well, the different string theories (again, none of which are known to exactly and uniquely reproduce our observations) are related to one another by various connections called dualities: T-duality, for instance, roughly connects large and small scales of the extra dimensions, and S-duality connects strongly and weakly interacting theories; the existence of these dualities led to the conjecture that all those string theories are really just aspects of a larger unifying framework of which we have no exact formulation (which we also don’t really have of string theory) dubbed ‘M-theory’ for reasons nobody but Ed Witten might know.
One thing no one has mentioned: Why have string theory at all?
Elegance. The current standard model of particle physics is a zoo. Several dozen different fundamental particles with widely varying properties that don’t seem to relate to each other in any meaningful way. They just are what they are.
String theory is an attempt to boil the particle zoo down to just one thing – strings. And to explain all the different properties of all the different particles simply in terms of different vibration modes.
Exactly! It’s so pretty, it damn well ought to be true!*
Does anyone remember an earlier attempt – maybe 20 years ago? – to simplify the zoo, using two fundamental particles – they were semi-jokingly referring to them as “form” and “void” – which could fit together to make up all the quarks? It, too, was a very pretty mathematical model, but it never seems to have gone anywhere.
- of course, I said the same thing about Harold Camping’s Bible-based numerology, that indicated the end of the world back in March of 2012. His numerology was actually very elegant, and, deep down inside, I kind of wanted it to be true. If I’d been around a few years earlier, I would have preferred the Steady State cosmos to the Big Bang cosmos. So it goes!
Thanks for coloring in the subtleties there. It’s been quite a while since I really took in much from string theory, as despite its grace in some unification, I’m not putting that much “stock” into the model in general as the be-all in fundamental theories.
I’ve never heard of that particular tongue-in-cheek theory, but even my primitive gut says this is probably close to reality, in that all matter and forces are just manifestations of energy, in some interplay between, well… not-energy. Particles and antiparticles. Forms and voids. Etc.
It’s all in the details of course. String theory offering some insight. Our standard model almost bullet proof, yet its quantum mess doesn’t sit quite right…
We’re gonna need a bigger collider.
If decent evidence of extra small spatial dimensions was established would that be taken by the physics community as evidence that String Theory is fairly likely correct. Or would it be spun as a necessary precondition for the theory to be correct but insufficient to be considered much support?
Minimally failure to identify extra spatial dimensions (which is theoretically testable) would falsify the theory, yes?
String theory isn’t the only contender for a quantum theory of gravity: There’s also something called loop quantum gravity (and no, the name does not refer to loops of string). The main thing that the string model has going for it over loop quantum gravity is that it’s unified: The same model describes gravity and the other particles together (loop quantum gravity is purely just gravity). More than that, even: As I understand it, any version of the string model which is consistent with the familiar Standard Model particles also ends up inevitably including a massless spin-2 particle (the general description of a graviton), whether you intended it to have one or not.
If only. There’s no experiment known that could, point blank, detect extra dimensions. What we can do is try to detect sufficiently-large extra dimensions. But when we fail, all that really means is “If there are extra dimensions, they are smaller than X”. Better experiments can rule out smaller and smaller values of X, but they can never rule out all of them.
Now, if we did discover direct evidence of other dimensions, that would be a point in support of the string model. More to the point, though, if we can detect the extra dimensions, we can probably measure them, too, at least determining their length, and possibly also something about their topology. If we did that, then we could start doing some real tests of the string model.
You’re remembering, I think, the Rishon model developed by Haim Harari and Michael Shupe, which is an instance of a general class of models known as ‘preon models’ that posit that quarks and leptons aren’t fundamental, but themselves again composite particles. In this particular model—which isn’t so much a new theory, like string theory, but uses the familiar formalism of quantum field theory to model elementary particles in a different way; model here has then the same usage as in ‘standard model’, i.e. an explicit use of the clay and blocks provided by the theory to produce something that we hope resembles the world as we observe it—there are two fundamental particles, T for variously ‘third’, referring to its charge, or ‘tohu’, the Hebrew word for ‘unformed’, ad V for either ‘vanishes’, i.e. being charge neutral, or ‘vohu’, Hebrew for ‘void’.
The great problem of such approaches is to get the quark masses to come out right—the proton has a mass of about 938 MeV, while its individual constituents, up and down quarks, have masses around a few MeV. So where does it get its mass? Mostly from the kinetic energy stemming from the momentum uncertainty of the quarks, which are confined to a small ‘box’ with a size of the proton diameter.
The same should be true of the rishons making up a quark; only there, the box is much smaller, so the mass should be much higher, of the order of several hundred thousand MeV. But that’s clearly at variance with what’s observed.
A possible way to remedy this is to introduce a very strong binding force between the rishons. Then, one would be able to cancel out the excess mass with the binding energy—similar to what occurs in atomic nuclei, which, below a certain size, are lighter than their individual constituents, such that you gain energy by combining two to make a larger nucleus that is now lighter than the original two (that’s nuclear fusion in a nutshell).
But then, you’d have to introduce a new interaction with very finely tuned parameters into the already somewhat baroque standard model—and you don’t actually gain all that much by postulating the preonic structure in the first place.
However, few ideas ever die for good, and not too long ago, the rishon model was revived in order to produce a topological model of elementary particles, i.e. one based on bands that can be twisted and braided, by Sundance Bilson-Thompson; nothing much seems to have come from that, however.
Yes, there are lots of contenders for a quantum theory of gravity, however, like loop gravity, none of them are actually known to reproduce general relativity in the appropriate limit (and in the case of LQG, I don’t even know if the quantization works consistently—I seem to recall there being an issue of anomalies, i.e. the failure of the quantum theory to reproduce the symmetries of the classical theory, which may spoil the theory’s viability).
Perhaps the best sellling point of LQG compared to string theory is that the former is formulated as an explicitly background-independent, non-perturbative theory, while we still only know the perturbation series of string theory rather than the full theory, which is explicitly background dependent (and it’s not known, I think, whether the full theory will be background dependent or not, though I think matrix models are a good hint for background freeness).