How would you explain string theory to non-physicists such as myself? I'm especially interested in how plausible is it and what is needed to successfully prove it?
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2$\begingroup$ I'm tempted to answer: with much difficulty, in a highly qualitative way, and only by reading a fair-sized book. There are many decent pop-sci books on string theory; I can't remember the names of any I read, but I'm sure someone can recommend one or two. $\endgroup$– NoldorinCommented Nov 2, 2010 at 19:36
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2$\begingroup$ I think it's safe to say that a person won't really get anything useful out of such an explanation if he or she doesn't have some background in (quantum) field theory. $\endgroup$– j.c.Commented Nov 2, 2010 at 20:28
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$\begingroup$ Greene's book has already been mentioned, but Nova's The Elegant Universe is a somewhat informative 3-hour video series. Obviously, to truly understand the theory's progress, years of mathematics and physics must be studied, preferably at a university. $\endgroup$– VorticoCommented Nov 2, 2010 at 21:20
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6$\begingroup$ Obligatory xkcd: xkcd.com/171 $\endgroup$– ManishearthCommented Feb 9, 2012 at 3:20
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4 Answers
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I've noticed that none of these answers actually answer the question.
The simplest explanation of string theory I can think of:
Particles we currently consider "point particles" (electrons, quarks, photons, etc.) are actually tiny pieces of string with each a characteristic vibration. They interact in a sort of harmony that results in/manifests as the physical laws we observe.
If anyone with more knowledge in the field can correct me, I ask for improvements. This is just how I personally explain it to people who ask, and I'd hate to give out false information.
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5$\begingroup$ actually even QFT already does not consider matter as point particles but as fields (for details see e.g. here) $\endgroup$ Commented Mar 10, 2011 at 9:30
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1$\begingroup$ @Tobias: the modern definition of point particle is a field with renormlizable interaction and simple short-distance propagator. $\endgroup$ Commented Oct 22, 2011 at 6:42
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$\begingroup$ Dear Justin L. Where have you learnt String Theory? I am just curious. During a Msc degree? Your profile still says you are an undergrad. I don`t know how are university degrees in other countries anyway. $\endgroup$ Commented Nov 17, 2012 at 17:51
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$\begingroup$ ...But if you were in my country, it would be nearly impossible to have had Strings (and diff geometry, GR, QM, QFT and so on, before) in a simple physics degree, here they are simply too short. $\endgroup$ Commented Nov 17, 2012 at 18:03
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4$\begingroup$ Don't you believe it. It doesn't work, and the answer shows (well, actually flashes) that it is merely the B Green documentary bullshit. Every single physics thing I have learnt, has turned out to be a completely different thing from the idea I had acquired in pop science sources. It is simply a different thing. I could try to explain why this happens in physics, but Feynman explains it much better, when talking about (the impossibility of) learning physics without maths, in The Character of Physical Law. $\endgroup$ Commented Jul 14, 2013 at 23:11
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String theory is the theory of gravity which starts out by postulating that only things at the boundary of space-time make sense. The local space and time have to be reconstructed from the boundary description. This is called the holographic principle. In the 1960s, a primitive vesion of this idea was called the S-matrix principle.
There are two kinds of boundaries, those far away from everything, called "cosmological" and those which are on top of matter, called "black hole horizons". The description of these two horizons are similar. The oscillations of these boundaries describe the entire space-time nearby.
The detailed form of string theory begins with the postulate that there are black holes that make extended, light, 1 dimensional lines. The vibrations of these black holes then must account for all the particles in the theory, because the vibrations of a black hole encodes anything that can fall through.
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$\begingroup$ Ron, does that cosmological boundary have anything to do with any of the horizons defined in plain $\lambda cdm$ cosmology, or it is a different concept? $\endgroup$ Commented Nov 17, 2012 at 0:03
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$\begingroup$ (I mean FLRW, sorry) $\endgroup$ Commented Nov 17, 2012 at 1:39
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1$\begingroup$ @Eduardo: It's heuristic, so it has something to do with it, but it's only rigorously understood in AdS spaces and for certain black holes in certain limits, so the general correspondence between boundary states and interior states is not something one can pontificate about in general. The FLRW horizon is growing with time, that makes it very difficult to understand the state space of quantum gravity on this background, or even whether the universe is a pure state or mixed state, and what the Hilbert space is supposed to be. Realistic cosmologies are unfortunately an open question in ST. $\endgroup$ Commented Nov 17, 2012 at 16:18
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2$\begingroup$ Ok, thanks! I don't know about strings yet, but it seemed strange since FLRW cosmology is merely a particular aplication of 'classical' GR. $\endgroup$ Commented Nov 17, 2012 at 18:45
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Some of the leaders in the field are Brian Greene and Michio Kaku. Both have made some Sci channel or Nova series that seemed appropriate for non-physicists.
I haven't been following the latest in physics for a while, but I thought that M-Theory had supplanted string theory some time ago. If so, then Kaku would be the guy. Greene's series and book "The Elegant Universe" also goes into some Quantum mechanics, which might help as well.
These are really the only two players I know of that simplify string theory. Does anyone else have some ideas?
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$\begingroup$ Those are certainly the two most well-known string theorists, though it could be argued that physicists such as Edward Witten are equally important in the field. $\endgroup$– NoldorinCommented Nov 2, 2010 at 19:40
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7$\begingroup$ Witten is much more important to the field than Greene or Kaku. $\endgroup$– j.c.Commented Nov 2, 2010 at 20:30
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$\begingroup$ @j.c.: Yes, but we save his credit for M-Theory. ;) $\endgroup$– VorticoCommented Nov 3, 2010 at 0:39
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3$\begingroup$ I don't think Witten is known for his communication with non-physicists, though. (Maybe he's good at it, I don't know) But I happen to like The Elegant Universe as a popular description of string theory so +1 for that. $\endgroup$– David ZCommented Nov 3, 2010 at 22:55
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$\begingroup$ Just to complement "The Elegant Universe", a quickstart. A talk in TED by Brian Green "The Universe On A String" youtube.com/watch?v=YtdE662eY_M $\endgroup$– pablassoCommented Nov 4, 2010 at 16:34
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Great question! Classical physics is known as the physics of the very large, objects that you can see with the eye. I can deterministically tell you exactly where something will stop, or how it will move if I have it’s velocity , and direction of travel. These things move like particles.
Quantum physics is known as the physics of the very small, stuff smaller than atoms. We’ve discovered that when you have mass , it’s easy to make super accurate guesses of how you will move in gravity but when you barely have any mass at all, like an electron, it becomes very difficult to guess how it will move. These particles do not obey the laws that rule the world we see around us. These things have properties of waves and particles.
There are big mathematical , and theoretical problems, when trying to put these two types of physics theories together. String theory tries to explain that if the universes smallest possible form was a string, it could satisfy being a particle, and a wave.
String theory goes beyond what is physically provable , to mathematically supported , hence it’s usually lumped in the theoretical bucket. Like religion, however much you want to have faith in it, no one can prove or disprove it. The particles are too small to see , just like the push, and pull of a magnet, we can only feel around in the dark. We know something is going on , but not enough to say it’s definitely an apple. Only that it feels curvy.
We need another Einstein and Newton. Big break throughs will happen, but you need a once in every several generations genius , thousands of years of scientific work, and technology we don’t have, and will not have for a long time.
I wrote a simple intro piece on string theory , which can be found here. Referencing string theory experts
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2$\begingroup$ This doesn't attempt to explain string theory at all, it just talks about the difficulties of it (which seems to be suggesting that we cannot test string theory, which AFAIK there would be testable hypothesis from string theory). $\endgroup$– JMacCommented Dec 17, 2018 at 13:49
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$\begingroup$ The question is 3 parts 1. explain string theory “specially interested” on 2. how plausible is it -plausible and 3. what is needed to successfully prove it? Be a genius that comes around 1 in several generations. That is the order of magnitude of effort required. I should say: if you are not familiar with everything in my listed paper, you would need to know all of that from classical physics , to quantum, to QFT, to super Riemann, to Calibri-Yau space , and much much more, far beyond my understanding toward the very edge of research in pure mathematics. $\endgroup$– melpCommented Dec 17, 2018 at 14:12
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2$\begingroup$ I don’t think this post attempts to explain string theory either. This is a pity as indeed this is a good question. The linked paper appears relevants so maybe there’s a way to shorten its 18 pages to make this answer self contained. $\endgroup$ Commented Dec 17, 2018 at 14:56
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1$\begingroup$ @melp I don't know how you could remotely go about proving that it would require a genius of the order of magnitude that comes around once in several generations. Science is rarely if ever resting on any individuals ideas alone. It also requires enough research to be done in a field to have enough background information to reach those conclusions. Again, this also seems to suggest that string theory could not be proven physically, which depending on what you mean, doesn't make a lot of sense. This asserts a lot of things about how difficult it is without any real support. $\endgroup$– JMacCommented Dec 17, 2018 at 15:17