Tag Archives: gravity

Did the LHC Just Rule Out String Theory?!

Over the weekend, someone said to me, breathlessly, that they’d read that “Results from the Large Hadron Collider [LHC] have blown string theory out of the water.”

Good Heavens! I replied. Who fed you that line of rubbish?!

Well, I’m not sure how this silliness got started, but it’s completely wrong. Just in case some of you or your friends have heard the same thing, let me explain why it’s wrong.

First, a distinction — one that is rarely made, especially by the more rabid bloggers, both those who are string lovers and those that are string haters. [Both types mystify me.] String theory has several applications, and you need to keep them straight. Let me mention two.

  1. Application number 1: this is the one you’ve heard about. String theory is a candidate (and only a candidate) for a “theory of everything” — a silly term, if you ask me, for what it really means is “a theory of all of nature’s particles, forces and space-time”. It’s not a theory of genetics or a theory of cooking or a theory of how to write a good blog post. But it’s still a pretty cool thing. This is the theory (i.e. a set of consistent equations and methods that describes relativistic quantum strings) that’s supposed to explain quantum gravity and all of particle physics, and if it succeeded, that would be fantastic.
  2. Application number 2: String theory can serve as a tool. You can use its mathematics, and/or the physical insights that you can gain by thinking about and calculating how strings behave, to solve or partially solve problems in other subjects. (Here’s an example.) These subjects include quantum field theory and advanced mathematics, and if you work in these areas, you may really not care much about application number 1. Even if application number 1 were ruled out by data, we’d still continue to use string theory as a tool. Consider this: if you grew up learning that a hammer was a religious idol to be worshipped, and later you decided you didn’t believe that anymore, would you throw out all your hammers? No. They’re still useful even if you don’t worship them.

BUT: today we are talking about Application Number 1: string theory as a candidate theory of all particles, etc. Continue reading

A Quantum Gravity and Cosmology Conference

I attended a conference this past week celebrating two great physicists (Steve Shenker and Renata Kallosh) whom I got to know pretty well during the early part of my career. Unlike most of the conferences I’ve attended in recent years, there were no talks at all about the Large Hadron Collider; the community of speakers was largely drawn from experts on quantum field theory, quantum gravity, string theory and cosmology.

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Any one of the talks would require an extensive article, especially since the required background material isn’t currently explained on this website. So rather than get bogged down in details, I thought I’d try to give you more of the general flavor — reflective, perhaps, of the tenor of the field at the moment. I’ll cover a couple of the talks later, if time permits (though I’m a bit under the gun at the moment…)

If you like to put labels on people, you’d probably call most of the speakers “string theorists.” This is a useful label if you’re in a hurry and not very interested, or if you want to abuse people, but not so useful if you want to actually understand their research. Indeed, out of about 21 talks, there were 3 on string theory.  That said, many of the speakers have in the past done some research in string theory, and many of the talks owe a debt to lessons that have been learned from string theory.

So what were the talks about?  What are these people actually doing? Continue reading

A Celebration of Two Careers

This week I’m at Stanford University, where I went to graduate school, attending a conference celebrating the illustrious careers of two great physicists, Renata Kallosh and Steve Shenker.

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Kallosh is one of the world’s experts on black holes, supersymmetry,  cosmic inflation (that period, still conjectural but gaining acceptance, during which the universe is suspected to have expanded at an unbelievable rapid rate), and “quantization” (i.e., on how to define quantum field theories and quantum gravity theories so that they actually make mathematical sense — which is not easy to do correctly). Much of her work concerns supersymmetry and its application to quantum gravity and to superstring theory. Her technical expertise and her inventiveness are legendary, as is her friendly enthusiasm. I’ve known her since I was a graduate student; she was one of a number of famous scientists from the former Soviet Union who came to the United States around 1988-1989.  Aside from just interacting with her within Stanford’s small community of theoretical physics students, postdocs and faculty, I also attended two courses that she taught on advanced topics, one on supersymmetry and one on quantization.

Shenker is famous for a number of papers that significantly changed our understanding of quantum field theory, quantum gravity and string theory. His fame derives in part from his ability to extract deep insights about physics from just a few mathematical clues — often ones that only he recognizes as being clues in the first place. Shenker was a faculty member at Rutgers when I was a postdoctoral researcher there in the mid-90s. (He later moved to Stanford.)  I cannot count the profound lessons that I learned during those years from him (and the other Rutgers faculty), both at our daily group lunch, and in the lounge where several of the faculty and other postdocs would regularly gather in the mornings. And I was even fortunate enough to write a paper (on black holes and their entropy) together with him and another then-postdoc, Dan Kabat. Aside from his down-to-earth no-nonsense style, and his strong support of young people and their ideas, one thing I remember well about Shenker is that it was perilous to say anything interesting to him while walking back from lunch on a bitterly cold day. He would stop and think… and the rest of us would freeze.

In the wider world of the public, and especially the blogosphere, Kallosh and Shenker would probably be labelled as “string theorists.” Such terminology would be somewhat crude, for it would fail to capture the range and depth of their careers. Appropriately, the talks at the conference so far have ranged widely, including general attempts to make some sense of quantum gravity,  discussion of the information-loss problem of black holes (the so-called “firewall” problem), unexpected subtleties in how quantum field theory works (yes, we are still learning!), new ways of thinking about the physics of electrical conductors and insulators, and advances in our understanding of cosmic inflation. And there were even a couple of talks on string theory.  (That said, the long shadow of string theory, and its direct and indirect influence on many other subfields, can be palpably felt at this conference.  More on that subject another day.)

Since I’ve been so busy with Large Hadron Collider physics in recent years, and haven’t been following these subfields closely, it’s been a very educational conference for me.  I’ll describe some of the talks later in the week.

Strings: History, Development, Impact

Done: All three parts of my lecture for a general audience on String Theory are up now…

Beyond the Hype: The Weird World of String Theory (Science on Tap, Seattle, WA, September 25, 2006). Though a few years old, this talk is still very topical; it covers the history, development, context and impact of string theory from its earliest beginnings to the (then) present.

Be forewarned: although the audio is pretty good, this was an amateur video taken by one of the organizers of the talk, and because the place was small and totally packed with people, it’s not great quality… but good enough to follow, I think, so I’ve posted it.

  1. Part 1 (10 mins.): String theory’s beginnings in hadron physics and the early attempts to use it as a theory of quantum gravity.
  2. Part 2 (10 mins.): String theory was shown to be a mathematically consistent candidate for a theory of all of quantum gravity and particle physics, and became a really popular idea.
  3. Part 3 (9 mins.): How string theory evolved through the major technical and conceptual advances of the 1990s.

By the way, if you’re interested in other talks I’ve given for a general audience, you can check out my video clips, which include a recent hour-long talk on the Quest for the Higgs Boson.

Why You Can’t Easily Dismiss the Cosmological Constant Problem

I’m still early on in my attempts to explain the “naturalness problem of the Standard Model” and its implications.  A couple of days ago I explained what particle physicists mean by the term “natural” — it means “typical” or “generic”.  And I described how, at least from one naive point of view, the Standard Model (the equations we use to describe the known elementary particles and forces) is unnatural.  Indeed any theory is unnatural that has a

  • a spin-zero particle (in the Standard Model, the newly discovered Higgs particle), which
  • is very lightweight in the following sense: it has a very very low mass-energy compared to the energy at which gravity becomes a strong force, and which
  • isn’t accompanied (in the Standard Model specifically) by other related particles that also have small masses.

But I didn’t actually explain any of this yet; I just described it.

Specifically, I didn’t start yet to explain what causes the Standard Model to be unnatural.  This is important to do, because, as many attentive readers naturally complained, my statements about the unnatural aspect of the Standard Model was based on a rather arbitrary-sounding statistical argument, and story-telling, which is hardly enough for scientific discussion.  Patience; I’ll get there, not today but probably the next installment after today’s.

To see why the argument I gave is actually legitimate (which doesn’t mean it is right, but if it’s wrong it’s not for a simple reason you’ll think of in five minutes), it is necessary to look in a little bit more detail at one of the most fundamental aspects of quantum field theory: quantum fluctuations, and the energy they carry.  So for today I have written an article about this, reasonably complete.

Be prepared — the article runs headlong into the only naturalness problem in particle physics that is worse than the naturalness problem of the Standard Model (the one I wrote about on Tuesday)!  I am referring to the “cosmological constant problem”.  In a nutshell:

  • we can calculate that, in any typical quantum field theory with gravity, the amount of energy in empty space (often called `dark energy’) should be huge, and we know of no way to avoid having it in a typical somewhat-realistic theory of the universe,
  • yet measurements of the cosmos — in fact, the very existence of a large and old universe — assure that, if Einstein’s theory of gravity is basically right, then instead of a huge amount of `dark energy’, there’s just a very small amount — not much more than the total amount of mass-energy [E=mc² energy] found in all the matter that’s scattered thinly throughout the universe.

After you’ve read about quantum fluctuations and the cosmological constant problem, and have a bit of a sense as to why it is so hard to make it go away, we can go back to the Standard Model, and try to understand the naturalness problem that is associated with the Higgs particle and field.  It all has to do with another aspect of quantum fluctuations — the fact that their energy depends on, and therefore helps determine, the average value of the Higgs field.

Floating in Space … But Why?

Have you perhaps wanted to go into space someday, so you could float around and do somersaults like the astronauts you see on TV? You know, out in space, where everything floats, because … because

because there’s no gravity in outer space???

  • Hmm… If there were no gravity in space, what would keep the Earth orbiting the Sun? Or the Moon orbiting the Earth?

umm… because…

Courses, Forces, and (w)Einstein

This week and next, I’m very busy preparing and delivering a new class (four lectures, 1.5 hours each), for a non-technical audience, on the importance of and the discovery of the Higgs particle.  I’ll be giving it in Western Massachusetts (my old stomping grounds).  If it goes well I may try to give these lectures elsewhere (and please let me know if you know of an institution that might be interested to host them.)   Teaching a new class for a non-technical audience requires a lot of concentration, so I probably won’t get too much writing in over that period.

Still, as many of you requested, I do hope soon to follow up last week’s article (on how particle physicists talk about the strength of the different forces) with an article explaining how both particles and forces arise from fields — a topic I already addressed to some extent in this article, which you may find useful.

Now — a few words on the flap over the suggestion that math Ph.D. and finance expert Eric Weinstein, in his mid-40s, may be the new Albert Einstein.  I’ve kept my mouth shut about this because, simply, how can I comment usefully on something I know absolutely nothing about?  (Admittedly, the modern media, blogosphere and Twitter seem to encourage people to make such comments. Not On This Blog.) There’s no scientific paper for me to read.  There’s no technical scientific talk for me to listen to.  I know nothing about this person’s research.  All I know so far is hearsay.  That’s all almost anyone knows, except for a few of my colleagues at Oxford — trustworthy and experienced physicists, who sound quite skeptical, and certainly asked questions that Weinstein couldn’t answer... which doesn’t mean Weinstein is necessarily wrong, only that his theory clearly isn’t finished yet.  (However, I must admit my expert eye is worried that he didn’t have ready answers to such basic questions.)

What I do know is that the probability that Weinstein is the new Einstein is very low.  Why?  Because I do know a lot about how very smart people with very good ideas fail to be Einstein.  It’s not because they’re dumb or foolish. Continue reading

What is the “Strength” of a Force?

Particle physicists, cataloging the fundamental forces of nature, have named two of them the strong nuclear force and the weak nuclear force. [A force is simply any phenomenon that pushes or pulls on objects.] More generally they talk about strong and weak forces, speaking of electromagnetism as rather weak and gravity as extremely weak.  What do the words “strong” and “weak” mean here?  Don’t electric forces become strong at short distances? Isn’t gravity a pretty strong force, given that it makes it hard to lift a bar of gold?

Well, these words don’t mean what you think.  Yes, the electric force between two electrons becomes stronger (in absolute terms) as you bring them closer together; the force grows as one over the square of the distance between them.  Yet physicists, when speaking their own language to each other, will view this behavior as what is expected of a typical force, and so will say that “electromagnetism’s strength is unchanging with distance — and it is rather weak at all distances.

And the strength of gravity between the Earth and a bar of gold isn’t relevant either; physicists are interested in the strength of forces between individual elementary (or at least small) particles, not between large objects containing enormous numbers of particles.

Clearly there is a language difference here… as is often the case with words in English and words in Physics-ese.  It requires translation.  So I have now written an article explaining the language of “strong” and “weak” forces used by particle physicists, describing how it works, why it is useful, and what it teaches us about the known forces: gravity, electromagnetism, the strong nuclear force, the weak nuclear force, and the (still unobserved but surely present) Higgs force. Continue reading