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Comment by OneTimePetes | Hacker News Reader

So string theory has become the "Getting high on abstraction"-enterprise software department - of physics?

That is… fairly accurate.

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.

> 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?

That's not a prediction. That's a known fact that any purported theory must explain, to avoid being instantly falsified.

A prediction is a fact that is falsifiable in principle, but is not known to be true when the theory is expounded.

Strange definition of prediction. It sounds like you are using the word fact like it could somehow still be false.

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

I think the point was more that it is comparatively easy to develop your new theory to fit known 'facts', so that testing against them should usually be regarded only as a sanity check rather than as additional evidence. To be a bit pithy, prediction is worth more than postdiction.

(Of course, if your theory fails to comport with known 'facts' but successfully predicts the result of new observations, then that is deeply exciting!)

Here's another way to explain it:

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.

> you are using the word fact li ke it could somehow still be false.

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.

In the context of physics (of which I'm not an expert), I believe you are using the term 'facts incorrectly. What you are calling a fact, in physics would be a prediction produced by our existing models, that is supported by observation.

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.

We don't know if the rules of physics will ever change, so I'd say "apple moving downwards" is a prediction, and also an experiment that could falsify the theory.

String theory specifically fails to predict/explain gravity. Most versions aren't even background-independent, so they just assume spacetime is a thing.

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.

Just wondering why would it be disappointing if it implies the anthropic principle?

"Well, in most universes, there is one law of gravity that works on red objects, and a totally different law of gravity that works on blue objects. But by a random coincidence with a very small probability, in some tiny fraction of the universes, the law of gravity happens to be same for both red and blue objects. These are the universes where gravity can hold planets together, so these are the only universes where life can appear."

Do you think such would be a philosophically satisfactory explanation on why red and blue objects obey the same law of gravity?

Hmm. To me, "too many tunable parameters" sounds very much like "not a fundamental explanation". It's a philosophical bias, not an experimentally verified fact, but... the fundamental theory should have very few parameters/constants in it.

For example, how many constants does General Relativity have? G and c, and isn't that all?

c is usually set to 1 if you’re doing relativity. It’s more a scale factor, and more a statement about the rest of physics than about relativity.

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.

Well, sure, we can set c and G to 1 if we want to, because it makes the math come out easier. But when we do that, we're really picking units where c and G have those values. But if we don't do that, if we stick with MKS or whatever, then c and G have values that have to be measured.

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.

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