[pa7x1]

Quantum Gravity is relevant at an energy scale that is completely out of our current technical capabilities. That's hardly the fault of string theory or any other theory of quantum gravity. It's just a consequence of the fundamental constants of our universe and our technical capabilities at this point in time.

String theory makes testable predictions, it is falsifiable. It's just not something we can falsify with our current technical means. That doesn't make the theory unscientific.

Would Quantum Field Theory be unscientific if it had fallen in the hands of the ancient Greeks? The theory falsifiability is an intrinsic property of the theory, irrespective of our technical ability.

[scotty79]

I just read that string theories are all supersymmetric and attempts at formulating non supersymmetric ones pretty much failed to the point noone is interested in attempting to make them.

And LHC experiments ruled out super symmetry in our universe. Does that mean quantum string theory pretty much got falsified?

[pa7x1]

> I just read that string theories are all supersymmetric and attempts at formulating non supersymmetric ones pretty much failed to the point noone is interested in attempting to make them.

This is correct.

> And LHC experiments ruled out super symmetry in our universe. Does that mean quantum string theory pretty much got falsified?

This is not correct. LHC hasn't ruled out the existence of supersymmetry because it doesn't have a sufficiently high energy to do so. It's like having a ladder to look for socks in the drawers of a very tall closet. With the LHC we get to look at the first 5 drawers, we haven't seen the socks in there, we found some panties (the Higgs boson) but there are still drawers higher were perhaps there are some socks.

Nevertheless, the idea stands, we can use accelerators to test the theory and falsify it which comes to show that the theory makes predictions and is falsifiable.

[scotty79]

Ah, so LHC ruled out some supersymmetries, but not all of them?

Is there a finite number of possible supersymmetries? So that with high enough energy you can rule out all of them? Or can you just make up new ones with ever increasing energy needed to rule them out?

[pa7x1]

Supersymmetry must be broken in some way. The way it's broken is just an accident of this universe, in much the same way the radius of the orbits of our Solar System cannot be derived from first principles but they are just an accident of the evolution of our galaxy.

This means that the masses at which supersymmetric particles are found cannot be predicted from first principles, although we can constrain the range based on other observations. Like the mass of the Higgs, for example, as supersymmetric particles should interact with it and if they were too light or too massive this would have consequences that we could see.

To summarize, we cannot get from first principles the masses at which the supersymmetric particles are found exactly, as this is just an accidental feature of our universe. We do have some bounds. The LHC is not enough to discard the entire range. ncmncm is talking his ass off.

[ncmncm]

> Supersymmetry must be broken in some way. The way it's broken is just an accident of this universe

This is religion, until we have evidence, or at least a research program to obtain evidence. We don't.

High-energy physics has abdicated science, and is training up a priesthood who will reliably echo String Church doctrine. Successfully, by evidence seen here.

Fortunately we still have solid-state, fluid, low-temperature, high-Rydberg, plasma, and other physicists to take up the mantle and religiously-disinclined students. There is still plenty of physical science to do. Apostates could join in.

[scotty79]

So is there an energy level that we could use to rule out any supersymmetry?

[ncmncm]

The latter. There will never be a point when we can say we have ruled out supersymmetry. They can always say, oh, you just haven't looked hard enough yet—keep looking!

[scotty79]

So supersymmetry is unfalsifiable in practice. Is there any part of string theory that you can't just bump up to higher energies when it fails at lower energy predictions?

[ncmncm]

Not within this Solar System.

[ncmncm]

> Would Quantum Field Theory be unscientific if it had fallen in the hands of the ancient Greeks?

Yes. Falsifiability is not an abstract property. Something not, today, might be tomorrow. Today, idle speculation; tomorrow, maybe science.

Gravitational frame dragging and gravitational waves were both speculative until recently. Both are implied by General Relativity, which had been tested in many other ways already, so they were far less speculative than strings. That one does not, to my knowledge, predict literally anything at all, and cannot, because the mathematics is still wholly intractable.

Warp fields are speculation, but could become science or even engineering someday. I won't be holding my breath.

[pa7x1]

String theory predicts the existence of supersymmetric particles, predicts the exact number of dimensions of our universe (something that by the way no other previous theory is able to do). Correctly predicts the Black Hole thermodynamics (Hawking radiation, BH temperature and BH entropy) from microstate counting and gives quantum corrections to the semi-classical formula. Predicts a bunch of higher and higher energy particles in a tower whose mass ratios are well defined. They are essentially higher energy excitations of the fundamental modes of the string. So on and so forth.

All this is testable and falsifiable, if we had the technical means to access the energies that are relevant for Quantum Gravity. In the meanwhile, scientists can keep trying to derive lower-energy consequences of the theory or devise smarter experiments that can allow us to test the consequences of the theory at the energy scales that are applicable to us.

[ncmncm]

If, every time we look where the theory says and don't find any supersymmetric particles there, you say, oh well they must be somewhere else, that is no sort of prediction at all.

It cannot be said to predict black hole numbers if those were known, and dictated which subset of the 1e500 possible string theories are still under consideration.

Predicting a number of dimensions is no good if there is no slight indication of any extras at all, and no conceivable way to discover any.

There is no circumstance in which no doubletalk can be conjured to prop up the empty tent. Deliver actually measurable consequences, or GTFO.