Conversations on Quantum Gravity
math.columbia.edu
math.columbia.edu
> String theory proposes that the universe is composed of minuscule objects which vibrate in the manner of the strings of a musical instrument. Like such strings, they have resonant frequencies and harmonics.
Nima Arkani-Hamed has pointed out that the explanation is a completely outdated viewpoint that permeates every popular description of the theory starting from Wikipedia. Brian Greene wrote a very influential book for the public: "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory (1999)" that came out when the second revolution of string theory was happening and the book was outdated already when it was published.
What is now known about string theory is that strings are not a core concept in the framework.
String theory is a framework that has number of dualities, like:
- Strings theories and quantum field theories are different way to talk about the exact same thing.
- AdS/CFT correspondence. What happens in the volume of space can have equal description from different theory that happens in the region that surrounds that volume.
While I've heard about dualities between different string theories, this is news to me. Could you elaborate in how far QFT (as a meta theory) "contains" string theory?
It leaves so much to parameters, you could describe practically any universe with it. It’s closer to a a generic virtual machine for universes than an actually useful theory.
That doesn’t mean it can’t be right, but it would be really disappointing if it is. It would imply the anthropic principle really is the only reason our universe looks the way it does. And no, you can’t use string theory to predict anything.
Not even that an apple moves in a downward direction if I drop it?
A prediction is a fact that is falsifiable in principle, but is not known to be true when the theory is expounded.
There are no facts in science^. There are observations and falsifiable models. Also these models don't try to explain as much as they are useful to predict an observation. If it fails to predict accurately it is falsified.
^ unless you mean fact = observation. but then I don't see how it can be wrong
(Of course, if your theory fails to comport with known 'facts' but successfully predicts the result of new observations, then that is deeply exciting!)
Since string theory is extremely tunable, you can just overfit it to all the data we already have about how the universe works.
So the only way to get some certainty would be to have it make a prediction that we don't know the answer to, and then test it. At this point in time, we don't have the means to conduct such an experiment.
Quite so: whether my sister's name is "Andrea" is a fact. It might or might not be true that's her name.
Perhaps I'm using the word "fact" incorrectly, but I think it's correct: for example, whether the earth is flat or curved is a fact; the proposition that it is flat is false. But the word "proposition" seems rather abstract, in a context like this.
Our current model of gravity predicts what happens when an apple falls, and that's what we observe. That is what you're calling a 'fact'.
String theory can produce all of these same predictions. If it only predicted the same stuff we can already predict, then it's useless. If it can make a new prediction, then its potentially valuable. So if it can make a definitive statement that will be true or false that says when you do X, Y will happen, and right now we can't predict what will happen after X, or our current model says Z will happen then we're very close to adding value. But Y could still be wrong.
So finally if string theory saying X will cause Y, our current theory doesn't know what will happen after X, or thing something other than Y will happen we can run and experiment and falsify one of them. If we observe Y happens then great, string theory is useful in modeling our universe. If anything else happens, then it hasn't provided modeling/predictive value.
The current complaint against string theory is it's not about to make a solid prediction and let us run any test to see that it's true or false. So essentially if it's useless.
It like a theory that says an alligator is ornery because it has a lot of teeth and no toothbrush. That's falsifiable. If I give it a tooth brush then it should stop being so angry. If it one and it stops my theory is right if I give it one and its still angry my theory is wrong.
Instead imagine a theory. When asked "why are alligators so angry?" Answers "because i said so". Well that's not very useful. You can use that to "explain" everything, but predict nothing. That's why falsifiable predictions are the test of value for a model/theory.
So no.
This seems weird because LQG is at least trying to be background independent. And spacetime isn't just a thing, it's the thing that needs to be explained in quantum terms.
Concentrating on particle-ising (or operator-ising) gravity seems like missing the point to me.
But I'm not remotely a pro, so I could easily be missing something important.
Do you think such would be a philosophically satisfactory explanation on why red and blue objects obey the same law of gravity?
For example, how many constants does General Relativity have? G and c, and isn't that all?
The same is true for G. It’s an important constant of course, but it’s an… adapter. I’m not sure if I’d say relativity has any free parameters at all, but really “relativity” is just “perspective, when one of the dimensions involved is time.”
…special relativity anyway.
For a universe with massive particles, massless particles and four dimensions, there are only about three ways you can build its geometry and have it make sense. Of the two others, 4-Euclidean spacetime had trouble with causality, and Galilean relativity (infinite speed of light) has trouble with everything else.
So, in a sense, relativity is super-determined. There certainly are other possible universes that work out to be consistent, but they’re further away and fewer in number than you’d think.
Einstein was able to do a lot of work in his head, with no experiments, because he figured out that every other option was inconsistent. We’d very much like a final theory of everything to be similar.
And if you do set c and G to be 1, then you have to use the measured values to know what your units are within that system.
But maybe the problem is with my use of "free". By "free" parameter, I mean one that has to be measured, that isn't determined by the theory.
What am I missing? What are the priors for trusting the author of this blog more than the seemingly universal voice of these authorities?
Certainly one shouldn't believe everything an expert says uncritically, and anyone is entitled to investigate even the most authoritative pronouncement, but I think believing that pure reason, without the benefit of specialist training, is the equal if not the better of expertise and experience can lead one all too quickly down the wrong path.
(For example, pure reason definitely rebels at first against quantum mechanics--at least, mine does, and I'm a mathematician!)
Of course if the authority has critical information that can not be communicated then no compelling argument can be made and the age of enlightenment was but an illusion.
There's a number of physicians who are worried about the way modern physics operates and particularly about string theory. Peter Woidt is an influencial voice in this debate and has written a book about it, but he's far from alone. The criticism is that string theory has failed in a lot of the things it was supposed to solve, that the LHC hasn't supported the idea of supersymmetry particles, which was a key concept within string theory and was expected by its proponents. Also a key criticism seems to be that there's no real reason to believe string theory is true and other attempts are false, but there seems to be a strong opinion by many string theorists that alternatives should not be explored.
Admittedly I have mostly read one side of this debate.
But you don't really have to worry about the distinction here. Woit is not advocating for any of the alternative approaches to quantum gravity. He's aware that they are basically crap. His complaint is that he thinks the string theorists are claiming to have solved the problem.
> but the theory is a successful theory of quantum gravity, unlike our competition.
These can't both be true... Either it's capable of making predictions, or it's not a successful theory.
At any rate, a "theory" that can't make predictions is no theory at all, in my book. And a theory that encompasses quantum gravity should make predictions that are more precise than any previously made.
I'd expect such predictions to be untestable in practice, using contemporary methods - the theory as expounded should encompass any test that can currently be performed, without the need for predictions. Otherwise the theory is falsified at birth.
Any theory that diverges from our currently accepted theories (like general relativity) will only start differing at sufficiently high energy. There's usually a dimensional analysis type of argument that will tell you roughly when that will happen. For gravity that's the Planck scale. Now, this is true for any theory of quantum gravity, including loop quantum gravity, string theory, or any other theory that you'd consider. There is just no way to distinguish the modified theory from standard GR below the Planck scale.
Now, this is the sense in which people say that string theory is untestable. But again, so is any other theory of QG. So, if your argument for not studying string theory is that it doesn't make testable predictions, you are actually saying that you should not study quantum gravity at all.
Interestingly, though, string theory (unlike things like LQG) is actually a theory of more than gravity. So, unlike these other versions, string theory at least has a hope of being testable. The problem that we're currently facing within string theory is that while there's essentially a unique theory at high energy, at low energies like the ones at the LHC, the theory has multiple solutions (sort of like a quadratic has two solutions). The issue is that the number of these solutions to string theory is too large to go through and check one by one (people throw around the number 10^500). I think this is why people sometimes say that string theory is more of a framework than a fundamental theory. It's possible that someday wed figure out which the correct solution is, who knows.
At the same time, constructing a quantum mechanically consistent theory of gravity is very very hard and, in fact, the only example we have of such a theory is string theory.
That's the sense in which people say that string theory is a successful theory of QR but still isn't able to make any definite predictions.
A different version of the argument: After X amount of work and effort, string theory has not managed to make falsifiable predictions. Here X is much larger (by a factor of 100 maybe?) than the work and effort spend so far to study quantum gravity. Who knows, if quantum gravity might proceed to give falsifiable predictions after some more work.