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When people say string theory makes untestable predictions, do they mean: 1) The ways in which its predictions differ from the standard model cannot be tested,

When people say string theory makes untestable predictions, do they mean: 1) The ways in which its predictions differ from the standard model cannot be tested, while the rest of its predictions match up with all tested physics thus far. 2) or, none of its predictions can be tested? People say it's untestable so it's not science, but that only really makes sense in scenario two. In scenario two, it still provides testable predictions that just happen to match other models too.

blablabla123 7y ago
> People say it's untestable so it's not science In that negative viewpoint it's still science, just another branch: Philosophy (Metaphysics) and still Mathematics of course ;-) No expert in String Theory, but the bottom line is that the strings are really small. (~ Planck Length) So directly measuring anything is not going to work. It's more like 1), but known predictions assumed by Standard Model are baked into the parameters.
contravariant 7y ago
Regarding (1), at this point I believe that String theory is still incomplete. It can make some broad predictions (parts of which might be more or less testable, but most will be in the Planck-energy scale), but so far it's proven difficult to fill in all the gaps and get something equivalent to the Standard Model. Basically you've got a few too many degrees of freedom and most of the predicted behaviour is well beyond the energy scale that we can reach experimentally.
T-A 7y ago
The way I'd put it is 3) In its current state, you can get string theory to "predict" just about anything. The reason is the enormous amount of string theory vacua, each giving rise to different "laws" of low energy physics. You've probably seen the old estimate that 10^500 such vacua exist. That turned out to be a slight underestimate: https://arxiv.org/abs/1511.03209 https://arxiv.org/abs/1511.03209 Even if you try to narrow down your search to vacua with the same rough features as the Standard Model (types of particles and interactions) you end up with numbers like 10^723: https://arxiv.org/abs/1810.00444 https://arxiv.org/abs/1810.00444 (Disclaimer: what follows is a caricature meant to illustrate the magnitude of the problem, not a rigorous argument.) So in principle you can imagine performing lots of precision experiments to measure a bunch of quantities which can be derived from string theory, and then run through all those 10^723 vacua to see which ones match your data. It will take a while. Can you derive precise parameters of low energy physics from string theory vacua at a rate of one per second? The number of seconds since the big bang is less than 10^18. You do the math. And even if each quantity you measure can discriminate between two halves of the remaining search space, narrowing those 10^723 alternatives down to 1 will require more than 2400 such mutually independent quantities to be measured (10^723/2^2401 ~ 1). Alas, once you fix the rough features of the Standard Model, it has only about 25 free parameters. So, even given all the experimental data you need and eons to run your search, you will not be able to nail down one string theory vacuum as the one we are in (assuming that string theory is correct), if all you know about experimentally is the Standard Model. That only reduces the uncertainty about which string theory vacuum we are in to 10^723/2^25 ~ 10^715 different alternatives. You need to discover new physics at higher energy, to bring in more free parameters to use in your search. Something like 2376 (!) new free parameters. Only then will you be in a position to finally say that future discoveries in particle physics will be able to decisively rule out string theory (or not). Until then, whatever comes out of an experiment will just tell you that some vacua are out, but you will have plenty left.
nwallin 7y ago
A better word is "unfalsifiable". String theory predicts numbers other spatial dimensions. (at least 11) You can test for these. However, these extra dimensions can be arbitrarily small. You can prove that no extra dimensions greater than 1e-30m (or whatever) exist. This does not disprove string theory. Well, now you have to test for smaller extra dimensions. Let's say you prove no dimensions larger than 1e-40m exist. Still haven't disproven string theory: they could be smaller. So on and so forth. These extra dimensions have to curl up. The way in which these extra dimensions curl up constrain the masses and properties of the various particles. There are a stupidly large number of different ways these extra dimensions can curl up. Something like 10^500. So there's no way to measure the various properties of physics and the universe in a way that falsifies string theory. Regardless of what experiments show, there are still going to be an immense number of curlings that are consistent with the measured reality. IMHO string theory is still science, but it's depressing, fruitless, dystopian science. It lacks... hope.
compsciphd 7y ago
"rest of its predictions match up with all tested physics" is just curve fitting. i.e. what's the difference between #1 and #2? the only predictions that matter is where it differs.

blablabla123 7y ago

> People say it's untestable so it's not science In that negative viewpoint it's still science, just another branch: Philosophy (Metaphysics) and still Mathematics of course ;-) No expert in String Theory, but the bottom line is that the strings are really small. (~ Planck Length) So directly measuring anything is not going to work. It's more like 1), but known predictions assumed by Standard Model are baked into the parameters.

contravariant 7y ago

Regarding (1), at this point I believe that String theory is still incomplete. It can make some broad predictions (parts of which might be more or less testable, but most will be in the Planck-energy scale), but so far it's proven difficult to fill in all the gaps and get something equivalent to the Standard Model. Basically you've got a few too many degrees of freedom and most of the predicted behaviour is well beyond the energy scale that we can reach experimentally.

T-A 7y ago

The way I'd put it is 3) In its current state, you can get string theory to "predict" just about anything. The reason is the enormous amount of string theory vacua, each giving rise to different "laws" of low energy physics. You've probably seen the old estimate that 10^500 such vacua exist. That turned out to be a slight underestimate: https://arxiv.org/abs/1511.03209 https://arxiv.org/abs/1511.03209 Even if you try to narrow down your search to vacua with the same rough features as the Standard Model (types of particles and interactions) you end up with numbers like 10^723: https://arxiv.org/abs/1810.00444 https://arxiv.org/abs/1810.00444 (Disclaimer: what follows is a caricature meant to illustrate the magnitude of the problem, not a rigorous argument.) So in principle you can imagine performing lots of precision experiments to measure a bunch of quantities which can be derived from string theory, and then run through all those 10^723 vacua to see which ones match your data. It will take a while. Can you derive precise parameters of low energy physics from string theory vacua at a rate of one per second? The number of seconds since the big bang is less than 10^18. You do the math. And even if each quantity you measure can discriminate between two halves of the remaining search space, narrowing those 10^723 alternatives down to 1 will require more than 2400 such mutually independent quantities to be measured (10^723/2^2401 ~ 1). Alas, once you fix the rough features of the Standard Model, it has only about 25 free parameters. So, even given all the experimental data you need and eons to run your search, you will not be able to nail down one string theory vacuum as the one we are in (assuming that string theory is correct), if all you know about experimentally is the Standard Model. That only reduces the uncertainty about which string theory vacuum we are in to 10^723/2^25 ~ 10^715 different alternatives. You need to discover new physics at higher energy, to bring in more free parameters to use in your search. Something like 2376 (!) new free parameters. Only then will you be in a position to finally say that future discoveries in particle physics will be able to decisively rule out string theory (or not). Until then, whatever comes out of an experiment will just tell you that some vacua are out, but you will have plenty left.

nwallin 7y ago

A better word is "unfalsifiable". String theory predicts numbers other spatial dimensions. (at least 11) You can test for these. However, these extra dimensions can be arbitrarily small. You can prove that no extra dimensions greater than 1e-30m (or whatever) exist. This does not disprove string theory. Well, now you have to test for smaller extra dimensions. Let's say you prove no dimensions larger than 1e-40m exist. Still haven't disproven string theory: they could be smaller. So on and so forth. These extra dimensions have to curl up. The way in which these extra dimensions curl up constrain the masses and properties of the various particles. There are a stupidly large number of different ways these extra dimensions can curl up. Something like 10^500. So there's no way to measure the various properties of physics and the universe in a way that falsifies string theory. Regardless of what experiments show, there are still going to be an immense number of curlings that are consistent with the measured reality. IMHO string theory is still science, but it's depressing, fruitless, dystopian science. It lacks... hope.

compsciphd 7y ago

"rest of its predictions match up with all tested physics" is just curve fitting. i.e. what's the difference between #1 and #2? the only predictions that matter is where it differs.