3 ms·

> We don’t actually know what string theory is, just that it’s a “framework” that encompasses QFT and much more. We can’t predict anything with it now and don’t

> We don’t actually know what string theory is, just that it’s a “framework” that encompasses QFT and much more. We can’t predict anything with it now and don’t see any plausible way of predicting anything in the future, > 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.

denton-scratch 5y ago
That contradiction jumped out at me too. But note that these are utterances by two different people. 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.
- cout 5y ago
  My naive understanding is that there are so many tunable parameters in string theories that you can use them to model our universe but not to make predictions outside what our existing models already predict. So you can falsify a given set of parameters, but if you find a set of parameters that fits our universe there's no way to know if those are the right parameters or why these parameters here and not those parameters there that also fit.
mjreacher 5y ago
The context of this is that proponents of string theory say it predicts gravity and hence is a successful theory of quantum gravity. However it also can predict many other things that wouldn't make sense in our universe depending on what parameters you use. The key point is that it doesn't really make any new predictions we can currently test at energy scales we can reach (from what I understand).
pontus 5y ago
There's a subtle distinction between these two statements. 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.
- sampo 5y ago
  > 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. 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.

denton-scratch 5y ago

That contradiction jumped out at me too. But note that these are utterances by two different people. 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.

cout 5y ago
My naive understanding is that there are so many tunable parameters in string theories that you can use them to model our universe but not to make predictions outside what our existing models already predict. So you can falsify a given set of parameters, but if you find a set of parameters that fits our universe there's no way to know if those are the right parameters or why these parameters here and not those parameters there that also fit.

mjreacher 5y ago

The context of this is that proponents of string theory say it predicts gravity and hence is a successful theory of quantum gravity. However it also can predict many other things that wouldn't make sense in our universe depending on what parameters you use. The key point is that it doesn't really make any new predictions we can currently test at energy scales we can reach (from what I understand).

pontus 5y ago

There's a subtle distinction between these two statements. 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.

sampo 5y ago
> 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. 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.