With a headline like that, you probably think this is a parody. But in fact, I’m dead serious. Not only that, the discovery was made in the 1960s. Due to an accident of history, the physicists involved just didn’t realize it back then.
That said, there are profound problems with this headline. But the headlines we’ve seen this week, along the lines that “Physicists create a baby wormhole in the laboratory”, are actually WORSE than this one.
It is more accurate to say that “string theory and extra dimensions were discovered experimentally in the 1960s” than to say that “a baby wormhole was created in a lab in the early 2020s.”
And now I’m going to show you why. As you’ll see in this post and the next, the two claims are related.
The First Gasp of String Theory
In the 1960s, a wide variety of “hadrons” (particles containing quarks, gluons and anti-quarks) were discovered — not just the protons and neutrons from which we’re made, or the pions and their cousins found in the 1940s and 1950s, but a whole host of them, with an alphabet-Greek-salad of names. Study of these hadrons led to the proposal, prior to the discovery of quarks, that maybe hadrons are little strings. There was quite a bit of experimental evidence for this idea! But to make a long story short, the proposal eventually failed when quarks were discovered and confirmed in the 1970s. (Meanwhile string theory was repurposed for a theory of quantum gravity etc. [a “Theory of Everything”], and the rest is history/not even wrong/lost in math/not even close.)
But actually, string theory didn’t fail. It was just string theory in flat four dimensions that failed.
Bear with me. This takes a few steps.
String Theory and Quark/Gluon Theories Meet Again
In 1997, Juan Maldacena, following on old ideas of Gerard ‘t Hooft and Alexander Polyakov, among others, and hinted at by works by many other string theory/black hole researchers (such as Igor Klebanov, Andy Strominger, etc.), uncovered strong evidence for a radical conjecture:
- There are quantum field theories (theories of gluons, quark-like particles, and some additional friends, but with no gravity, in a world with three space dimensions and one time dimension) that are exactly equivalent to supersymmetric string theory (a theory with nine space dimensions and one time dimension, with an infinite set of particles and fields, and with quantum gravity) where the strings are moving on a uniformly 9+1 dimensional curved space.
[[If you don’t know what “supersymmetric” means, don’t worry about it; it won’t be relevant here.]]
This sounds crazy at first. How can a theory with quantum gravity be equivalent to one without quantum gravity? and how can two theories with different numbers of space-dimensions be equivalent? Nevertheless, the conjecture is almost certainly correct. In this post I won’t go into the mountains of evidence here in favor of this “AdS/CFT” or “gauge/string duality” conjecture. [A figure illustrating this relation, and some of the others mentioned below, is located at the end of this post.]
Within a short time, Maldacena’s conjecture was extended to theories that are more similar to the real world — gluon/quark/etc. theories that exhibit remarkably real-world-like behavior. This includes formation of hadrons out of gluons and quark-like particles, for instance, along with many extra hadrons not found in the real world. The conjecture implies that these theories (not necessarily supersymmetric themselves) are also exactly equivalent to a supersymmetric string theory, with quantum gravity, but now on a more complicated curved space.
What makes this equivalence possible? The point is that even though the string theory exists in nine spatial dimensions (plus one time), only three spatial dimensions extend out to infinity and are visible macroscopically. The rest are somewhat curled up microscopically, but in a very clever way that assures that one of those dimensions is particularly long and important. [See the figure at the end of this post for a rough illustration.] That long but finite fifth dimension — let me call it the “radial” dimension (the one that stars in the famous work of Lisa Randall and Raman Sundrum, which came soon after Maldacena’s conjecture) — is the one that assures this string theory has properties similar to the real world. What are they?
- Unlike the string theory first considered in the 1960s, in which the strings moved on flat spatial dimensions, the curved nature of the space on which these strings move assures that none of the hadrons predicted by this new string theory arrangement should be massless [except possibly some pion-like particles.]
- For each hadron of low mass (M) and low “spin” (angular momentum J) there should be an associated set of hadrons of ever-increasing angular momentum and mass, with M growing roughly like the square root of J. [[These sets of hadrons are called Regge trajectories.]]
- For each particle of low M and low J, there should be a “tower” of hadrons of increasing M but the same J. [[These sets of hadrons are called Kaluza-Klein (KK) towers.]]
The precise details depend on the particular theory. But these general properties — no massless hadrons, and hadrons organized into Regge trajectories and KK towers — are the basic predictions that are almost independent of any details.
Well, long before this, when people discovered the hadrons of the real world, they learned that the quark-antiquark hadrons (the “mesons”) of the real world do indeed satisfy all of these criteria. (The baryons — hadrons like protons and neutrons — do too, but their story is more complicated and I won’t cover it now; there’s a little discussion here.) The real world has hadrons in Regge trajectories and KK towers, none of them massless. Nowadays we understand that this is the signature of a string theory with an extra finite radial dimension of space. The details of the hadrons teach us, in principle, the details of this string theory and the space on which the strings move.
And so it’s completely clear, in hindsight, that the particle physicists of the 1960s discovered string theory and at least one extra spatial dimension, though they didn’t know it at the time. (It’s even clear what quarks and gluons are — they are spikes on a string that nearly reach one edge of the radial dimension.) It was only after Maldacena’s breakthrough that this became self-evident.
In short, as physicists at the Large Hadron Collider and its many predecessors have been studying the physics of quarks and gluons and the details of hadrons, they have secretly been studying string theory, extra dimensions, and even (to a more limited extent) quantum gravity.
Surely You’re Joking, Mr. Strassler!
Now, many of you will be screaming bloody murder at the spectacular claims made in the two previous paragraphs. And well you should be!… just as you should be screaming even louder at anyone claiming to have made (or even simulated) a wormhole in a laboratory.
The thing is, though, I’m not joking. The claims made in the previous paragraphs are both
- factually true if Maldacena’s original conjecture is correct, and
- morally/ethically outrageous for having left out all sorts of crucial fine print.
By comparison, the claims made about the “lab baby wormholes”, which also rely on Maldacena’s conjecture, are suggestive rather than factually true, and the fine print is more extensive.
So let’s look at the fine print for the hadrons representing a string theory. I’ll need it when I come to wormholes next time.
The Fine Print About Hadrons, String Theory and the Extra Dimension(s)
I have to emphasize that it is absolutely true — if Maldacena’s conjecture is correct — that a theory of quarks and gluons found in the real world is exactly equivalent to a string theory in extra dimensions. Take the real world and ignore its gravity (that would greatly complicate the story.) Though it would be hard to carry out in practice, you could take one of Maldacena’s examples where the equivalence is well-established, add a few things to it (including the weak nuclear and electromagnetic forces and the Higgs field and electrons etc.) which maintain the equivalence, and then start stripping things away [via mass terms and expectation values] until you are left with the quarks and gluons of the Standard Model, and no remnants from supersymmetry or anything else the real world doesn’t have. None of this messes up the equivalence. There *is* a string theory in extra dimensions that is exactly equivalent to the real world.
Finding exactly the best way to construct this string theory, beginning to end, would be tedious and hard. To my knowledge, no one has even bothered to try. Why not?
The problem is that stripping out all that extra stuff, to move the theory toward the real world, is guaranteed to dramatically and qualitatively change the space in which the string theory travels. It will become so tightly wound up and complex that it’s barely a space at all. We don’t know any details of what this space looks like, except that, for sure, the long finite radial dimension in the cases described earlier becomes a very short radial dimension. [See the Figure at the end of this post.] No one has any idea how to calculate anything about string theory on such a space [especially with “Ramond-Ramond background fields”, which make things infinitely worse], and so, no one can be sure how it actually behaves. It’s not even obvious there should be any objects in the theory that intuitively resemble strings at all!
In fact, the only reason to be confident that this string theory actually has the characteristically stringy and extra dimensional features listed in (1), (2) and (3) above is that we have simulated this theory in a laboratory! In many laboratories, in fact. That’s what our particle physics accelerators that make hadrons have been doing for sixty years. You see, from this perspective, the real world’s quarks and gluons, as observed in real-world particle physics experiments, can be viewed as a natural quantum computer simulation of this equivalent string theory, about which we otherwise know very little.
If theorists knew in the 1960s what we know today, the string theory interpretation of the data wouldn’t have been dropped so quickly. It would have lived on, well into the 1970s and 1980s and beyond. The competing views — quarks/gluons vs strings-in-curved-extra-dimensions — would have been seen as complementary, as they are today. But the required string theory is a heck of complicated beast, while the mathematics of quarks and gluons is, by comparison, very simple. Quarks and gluons are a much better intuitive basis for understanding the world, and allow us to make precise calculations for experiments, while the string theory, though it is of intuitive value in numerous contexts, is useless for precise calculations. (Admittedly, this is a technical problem, not a conceptual one. It’s conceivable that someday a mathematical breakthrough, perhaps one that would allow us to simulate string theories on an artificial computer, will make the string theory viewpoint more practically useful.)
[Extremely Important Caution: none of what I’ve said here implies that the string theory I’m referring to is a “Theory of Everything”. The string theory in question has nothing to do with the quantum physics of the gravity that holds you and me to the floor. Remember, this string theory is equivalent to quarks and gluons without gravity. To extend the story, so that the string theory’s gravity and our familiar gravity are one and the same, joined together in a seamless way, is possible (see here). But there is zero experimental evidence that this extension occurs in nature.]
Where does this leave us? We have learned from natural simulation that, for some reason we don’t understand deeply, the very complicated quantum string theory that’s equivalent to the real world’s quarks, gluon and hadrons has some remarkable, surprising, qualitative, but experimentally relevant similarities with the string theories that show up in the context of Maldacena’s conjecture, which aren’t the real world but whose properties can be calculated. Because of that, one can hope to learn some qualitative lessons about the real world using this equivalence (as many authors have done, including myself here and here.) This is a classic technique: consider a universe similar to ours in which you can draw a clear conclusion, and then hope/pray that you can draw a similar qualitative conclusion about our own universe. It works sometimes, but by no means always. You need more evidence, often from experiment, before you can be sure that your conclusion is valid in the real world. But still, even when you’re not sure of it, a plausible conjecture can occasionally point you to even better ideas.
Final Point for Today; Stay Tuned For More
Now, what about those wormholes? They rely on the same Maldacena equivalence, and they suffer from the same fine print, plus a lot more. (For instance, the wormhole that’s been quasi-simulated exists in only one spatial dimension, not three.) I’ll start to tell you their story in my next post.
In the meantime, let me reiterate: it is less true that wormholes (even baby ones) have been made (or even simulated) in a lab than it is that particle experimentalists of the 1960s discovered string theory and extra dimensions. Theorists in this subject have all known about the string theory viewpoint for the last twenty years or so, and we use it often, but we didn’t make a big deal out of it to the world’s journalists. Why not? Because the quarks/gluons viewpoint on the real world is both intuitive and practically useful, while Maldacena’s equivalent theory of strings on a tightly curved space is often neither, not to mention imprecisely known.
But hey, if physicists and journalists are all collectively going to lower the bar and make an international spectacle about a quasi-simulation of a cartoon of a wormhole, then, well, by that standard, I guess we ought to let everyone know that string theory and extra dimensions are absolutely real and have long been the subject of 20th- and 21st-century particle physics experiments. That’s no parody, no joke, no kidding. But don’t misread it for something more than it is. READING THE FINE PRINT ISN’T OPTIONAL!