String theory has always seemed intuitively wrong to me. From Wikipedia:

>In theories of particle physics based on string theory, the characteristic length scale of strings is assumed to be on the order of the Planck length, or 10E−35 meters

Yet electrons repel each other over distances of many meters by I think the virtual exchange of photons. How on earth would that work? How does your photo string know to head to an electron string trillions and trillions of times it's length away?

As far as I can tell the field became popular for sociological reasons that you could get grants for it and the like rather than any connection to reality(?)

▲

ajkjk a day ago | parent | next [-]

whether or not string theory is at this point grifty and weird, the theoretical basis for it is far stronger than you would think based only on reading critics / pop sci explainers. It is not like, missing any obvious physical facts in its foundation. Rather it is trying to say: look, we have this zoo of particles with seemingly random masses and properties; is there same lower-level framework which can produce the zoo that we see according to a simpler list of rules? The obvious choice for this, especially given some of the "hierarchies" of particles that are observed, is that they are in some way resonant modes of some kind of underlying object. Which is where you get the strings from. (Which might sound like a weird justification if you are not aware of all the other aspects of physics which get explained as resonances of fields; this is a standard sort of justification which there's a lot of good reasons to be interested in, at least initially.)

▲

jiggawatts a day ago | parent | prev | next [-]

You're being somewhat unfairly voted down, it's a legitimate questions because the popular media so grossly misrepresents what string theory is, especially in their visuals.

It's hard to visualise in 3D, but if you cut down the spatial dimensions to just 1D (a line), then theories like string theory just turn the infinitely thin mathematical line into a tube. You can picture a tube that vibrates, or has waves in its cross-section. Don't think of the the "strings" as actual little loops moving around in space, they're a modification of what space is.

You can even do the same kind of line->tube extension of a space with even more extra "loop" dimensions than the number of base dimensions. AFAIK the current theories have 10 total, of which 3 are the usual "large" dimensions of space, the rest are "small" and rolled up like the tube example.

▲

tim333 a day ago | parent [-]

Even so I don't get how electrons a meter apart interact through that stuff, as opposed to the electrical fields which spread out through space and so interact with other electrons as featured in quantum field theory which is what physicists use to actually calculate physical results, as opposed to string theory which fails to calculate actual physical results.

"if you cut down the spatial dimensions to just 1D" doesn't sound very physical to me.

I'm maybe being downvoted fairly. I studied physics and it don't think it's a misunderstanding of popularisation or that string theory is untestable, I just think it's straight wrong and not how the physical universe works.

▲

ajkjk a day ago | parent | next [-]

The string theory model is structured from the start to reproduce existing theories as a consequence, so the problem here is a lack of understanding on your part rather than a mistake in the theory. I do suspect that most people who are working on this stuff (or any stuff for that matter) don't think very hard about the basic phenomenological claims to the point where they can explain them well.

In particular your model of electric fields isn't very good. An electric field's flux around a volume reveals the presence of a particle in that volume. That's not because the field, a bunch of vectors in space, "happens" to integrate to something nonzero if there's a particle in that volume: it's because in some sense the presence of a field with a nonzero flux and the presence of a charged particle are the same thing; the particle is the existence of a divergence in the field within that volume.

Moreover in QFT (and this part is handwavey as I only learned enough to vaguely understand this, but it's better than nothing) the presence of the "field" ends up looking like the sum of what you get if you integrate over every possible way of a emitting or receiving a photon at that point; the accumulated integrals destructively interfere in such a way as to produce a value which reflects where the particles are. So very roughly idea of a field existing at a point and having a "value" is like saying: there are a bunch of things out there that I (a charged particle) can detect by exchanging photons, and the accumulated effect, when you consider all the different quantum superpositions of ways of doing that, is a single vector which induces a force on me. Other fields add up to more complicated objects than vectors.

Once you look at things like that, there should be no objection to how strings and electrons might interact. Whatever's going on at that string level averages out over larger timespans to just look like electron field. Not dissimilar from how all the individual charges in an atom average out to look like a single charge (but are perhaps detectable if you get up really close, in dipole and higher moments, or in how the atom deforms / reacts to nearby charges).

It might help to be aware of the concept of a topological defect (https://en.wikipedia.org/wiki/Topological_defect; there's a great explainer article somewhere that I can't seem to find) as a reductive picture of what a particle "is". I've heard this doesn't work for fermions for some technical reason, but in any case it's very useful as an illustration of the sort of thing that a particle "can be": a vortex in a material can act like a particle and even exhibit attractive/repulsive forces. So I picture the string theory model as answering the question: what kind of substrate could produce vortexes and other "defects" that act like the particles we see? Dunno if that's accurate but it seems like a natural question to me, anyway.

▲

tim333 17 hours ago | parent [-]

I can definitely buy a lack of understanding on my part - my understanding of all this is quite fuzzy. But I'm not sure string theories reproduce existing theories so much a say this is compatible with the other stuff in a handwavy way? I don't think there's any "we assume strings and thus can deduce quantum mechanics and general relativity" stuff? I don't think anyone can deduce quantum mechanics and Einstein's deductions with relativity are based on simple observations like the speed of light being observed to be constant and acceleration seeming similar to gravity?

	▲	ajkjk 12 hours ago \| parent [-]
		It's not "we assume strings and deduce QM" it's "strings are a model which is defined so as to produce QM and also...". It is, by design, a theory of QM and GR. That's the point. It's a claim about what lies "below" the level of the individual fields in QFT which gives rise to them (and also gravity).

▲

jiggawatts a day ago | parent | prev [-]

> Even so I don't get how electrons a meter apart interact through that stuff

Very roughly: It's possible for point-like (or tiny looped) particles to interact as long as they take every possible path instead of just the one path that would cause a collision. How you interpret this is... up for debate. I prefer the many-world interpretation (MWI), but not everyone agrees.

> "if you cut down the spatial dimensions to just 1D" doesn't sound very physical to me

That's just a simplification to aid understanding, it's now how the theory actually works.

▲

tim333 a day ago | parent [-]

Yeah maybe. My flatmate of some years was doing a PhD in string theory at UT Austin along the lines of "if you cut down the spatial dimensions to just 1D" but he was a mathematician, not really a physicist and was ok with that if it produced interesting mathematics. For real physical things like wiring the lighting system I'd do it because he wasn't so good with that.

I think he went into string research because he was good at maths and there was grant money available for that rather than a deep belief that that was the nature of reality, which is kind of what I mean by sociological factors.

I think much string theory may be like that. Interesting maths but not good at figuring where electrons go.

▲

XorNot a day ago | parent [-]

Except this is how electrons actually go, and it has real testable consequences. The question I'm aware of (because it related to my degree in nanotechnology) was: are metals conductive at different dimensionalities?

Because at the nanoscale, you in fact can have 1D, 2D and 3D metals. 3D metals are bulk solids - like we're familiar with. 2D metals are planes of single (or very few) atoms. 1D metals are lines - think placing individual metal atoms down in a row - nanotubes are a practical example.

All real, possible structures to build.

When you do measurements on all these structures you get...weird answers. Like is a nanotube a superconductor? And the answer is...yes, but also no. Yes because you'll in fact view superconductivity like behavior, but no because actually it's a ballistic electron conductor - at the right energy level an electron bounces through the thing without hitting it, but not all electrons can do that at all energy levels, so you still measure a voltage across a nanotube between two conductors.

But a nanotube is 1D - we only have 1 dimension things move in (from one end to the other). So - conductive, not a superconductor, but you can kind of use it like one sometimes. And we know 3D metals are conductors - that's obvious right...so what are 2D metals? Presumably conductors right...?

And the answer is...nope, insulators - at least sometimes. And the reason is because the sum of all possible electron paths in a 2D metal is the electron always returns back to where it started - and those grow much faster mathematically then paths where the electron ends up somewhere else.

But only in 2D: in the 1D case most paths take you out of the conductor. And in the 3D case, the number of paths which land you somewhere else grows much faster then those which loop back, due to the extra dimension of freedom. But 2D metals are constrained - for any given path elsewhere, there are mathematically far more that land back where you started. This is observable, measurable behavior which is a topic of research for future semiconductors. Yet it's almost entirely quantum probability based behavior.

	▲	tim333 a day ago \| parent [-]
		I think the 1D mentioned in jiggawatts comment is a different thing to a confining electrons to a nanotube type of setup.

▲

CubicLettuce a day ago | parent | prev [-]

[dead]