I think my husband has been waiting for this edition of SST3K since I came up with the idea of these posts. That’s right, it’s time to tackle string theory!
Mechanical Engineer Husband (MEH): Finally this day has come! It is a good day to demolish string theory!
Me: Don’t you mean unravel?
MEH: …you know, you’re not supposed to write my jokes for me.
Me: But I write the whole…you know what, never mind. What’s your issue with string theory?
MEH: Well, it’s tangled…
Me: Oh, don’t start that.
MEH: All right, all right. Probably my biggest problem with string theory is that there’s no way to test it at this point. I’m not asking for something unreasonable like proving it, but I don’t think it’s too much to ask for a testable idea.
Me: That’s a good point. String theory is one of those theories that makes sense mathematically, but can’t be tested by any experiment we can conduct at this point. It’s a brilliant and cool idea, though!
MEH: I agree, it’s pretty interesting. If I remember right, the basic premise goes down to the smallest units of matter we know, like quarks and electrons. The theory is that these material building blocks are actually tiny one-dimensional constructs, but they’re too small for us to distinguish their structure. Since they’re 1-D, they have only length, and we call them strings because it’s always nice to have a concrete comparison for weird stuff.
Me: Pretty much, yeah. There are different versions of the theory involving different numbers of invisible dimensions and different string structures, but all the plausible versions of string theory we currently have can be treated as specific cases of an overarching model called M-theory.
Whatever version you subscribe to, what matters most is the way the strings vibrate. The frequency and amplitude (strength) of a particular string’s vibration determines what mass, charge, etc., the string has as a result, giving us the various elementary particles shown in this helpful chart. Those include electrons and different types or flavors of quarks, which combine in dozens of ways to give us boring old subatomic particles like protons and neutrons, as well as other, more unusual particles, many of which have been discovered thanks to particle accelerators like the Large Hadron Collider.
MEH: Right. I understand it’s a great problem for mathematicians, since the theory of string-based matter also implies that there are multiple other dimensions, and there’s a lot of theoretical math to explore there.
My problem is that all of it seems incredibly theoretical, but it also seems like it’s one of the rock stars of popular physics. It just seems…I don’t know, unnecessary to focus so much attention on this theory that can’t be tested, when there are so many other things that physicists could wax poetic about to the rest of us.
Me: Well, first of all, there are at least ideas for experiments. One huge issue, though, is that there are a ton of variations of string theory, and even the predictions that all these variants have in common aren’t necessarily unique to string theory. There are some string-specific predictions, like the excitation of strings to higher modes of vibration, but the way that ties to the theory of extra dimensions means that trying to cause or observe excitation is going to take crazy amounts of energy that are way beyond our abilities at this point.
In the meantime, some aspects of string theory have been applied to practical research on exotic states of matter, like superfluids (which have no viscosity and don’t lose kinetic energy as they flow) and the quark-gluon plasma (which mimics some properties of the very early universe). This doesn’t test string theory or rule out the possibility that it just happens to be right in certain situations, but it at least shows that it can be useful for some applications.
I think that even with all the current barriers to testing it, string theory is worth talking about. It isn’t just a cool idea—if it’s right, it ties together gravity and the three other fundamental forces we know of (electromagnetic, weak nuclear, and strong nuclear). Would that be practical enough for you?
MEH: Well, of course! That would be a huge deal!
As I understand it, right now we understand gravity in the realm of classical mechanics (what we think of as “normal” scales of time and space), and the other three forces at the quantum level. We’ve already shown that the electromagnetic and weak forces are really the same force at very high energies, and we have theories that unify those two with the strong nuclear force at even higher energies (quantum field theory).
Unfortunately, this has turned out to be incompatible with our current theory of gravity (general relativity). To get a working model of our universe that includes all four fundamental forces, we’d really need a workable theory of quantum gravity.
Me: Right. There’s no proof that all the fundamental forces can actually be unified, but it’s an intriguing prospect given that we’ve managed to unify the weak and electromagnetic forces. Without proof that it can’t be done, a lot of theorists have spent decades developing mathematical models that explain all our observations in terms of unified forces.
String theory is a model that would give us a consistent theory of quantum gravity. That doesn’t automatically lead us to a complete model linking all four forces (called a Theory of Everything or, charmingly, TOE), but it’s a necessary step and a very helpful one.
MEH: And then we could have antigrav technology!
Me: Well, yeah, that’s one possibility. Unifying the forces would mean we would understand better how all of them played a role in with the expanding universe, helping us understand how our present-day universe works on a level that’s currently a mystery to us. It would also give us the potential to manipulate one interaction or force using another one, like using electromagnetism to manipulate gravity.
MEH: …though when I really think about it, I’m pretty skeptical, since we haven’t managed to do anything like it yet. Look at electromagnetism—people kind of stumbled on the link between electricity and magnetism nearly two centuries ago, instead of going looking for it. I just don’t think our efforts are going to achieve much if we haven’t done it yet by accident with our current capabilities.
Me: Hmm. I agree that from our current standpoint, we can’t predict whether we’re going to make the leaps in technology that will let us work with high enough energies to unify the forces. On the other hand, I definitely don’t agree that we might as well rule it out already. I don’t know if antigrav specifically is within our grasp, but there are plenty of things we can do now that were unimaginable with the energies we were limited to just a few decades ago. I’m not writing off future developments.
Anyway, why did you even bring up antigravity in the first place? You just shot it down yourself a few seconds later!
MEH: Well, yeah, but that doesn’t mean I can’t still get excited about it!
Me: Engineers are strange.
[Just a note: These posts always get vetted by my husband, the real-life MEH, and that last part with being simultaneously excited and skeptical about antigravity was a nearly verbatim transcript of our conversation this morning. Thought you might like to know I don’t make this stuff up! Engineers really are strange.
Obviously, I just scraped the very surface of string theory here, and there’s a lot more you can read for yourself. I try to avoid just giving Wikipedia links, but for broad topics, it’s a great way to find a lot of information in one place. Here’s their page on string theory. There are various links throughout this post, too. Enjoy!]