Tag Archives: virtual_particles

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.

Higgs Symposium: A More Careful Summary

My rather hasty, breathless and inconsistent summaries (#1, #2 and #3) of last week’s talks at the excellent Higgs Symposium (held at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics) clearly had their limitations.  So I thought it might be useful to give a more organized overview, with more careful language appropriate for non-expert readers, of our current knowledge and ignorance concerning the recently discovered Higgs-like particle (which most of us do believe is a Higgs particle of some type, though not necessarily of the simplest, “Standard Model” type.)

I’m therefore writing an article that tries to put the questions about the Higgs-like particle into a sensible order, and then draws upon the talks that were given at the Symposium to provide the current best answers. About half of the article is done, and you’re welcome to read it.  Due to other commitments, I won’t probably get back to finish it until next week.  But “Part 1″ is long enough that it will take some time for most readers to absorb anyway…

Is Supersymmetry Ruled Out Yet?

[A Heads Up: I'm giving a public lecture about the LHC on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.]

Is supersymmetry, as a symmetry that might explain some of the puzzling aspects of particle physics at the energy scales accessible to the Large Hadron Collider [LHC], ruled out yet? If the only thing you’re interested in is the answer to precisely that question, let me not waste your time: the answer is “not yet”. But a more interesting answer is that many simple variants of supersymmetry are either ruled out or near death.

Still, the problem with supersymmetry — and indeed with any really good idea, such as extra dimensions, or a composite Higgs particle — is that such a basic idea typically can be realized in many different ways. Pizza is a great idea too, but there are a million ways to make one, so you can’t conclude that nobody makes pizza in town just because you can’t smell tomatoes. Similarly, to rule out supersymmetry as an idea, you can’t be satisfied by ruling out the most popular forms of supersymmetry that theorists have invented; you have to rule out all its possible variants. This will take a while, probably a decade.

That said, many of the simplest and popular variants of supersymmetry no longer work very well or at all. This is because of two things: (click here to read the rest of the article.)

Another Speed Bump for Superluminal Neutrinos

Here’s another major strike against the OPERA experiment’s claim of superluminal neutrinos, in addition to the Cohen-Glashow argument I described last week. It comes from a very natural place: the weak nuclear force. The theory (i.e. the equations) that we use, with great success, to predict the behavior of the weak nuclear force inextricably links some of the properties of neutrinos and charged leptons (electrons, muons and taus.) Because of this linkage, you simply can’t make neutrinos travel faster than light without making electrons do it too — by a smaller amount, to be sure, but still bigger than a part in a billion. And it turns out the effect is large enough that it should already have been detected by existing experiments, putting OPERA’s result further in doubt. Continue reading

Virtual Particles: Not Particles At All

Among the many tricky concepts which the layperson has to grapple with when learning about particle physics is something called “virtual particles”, which show up in cute pictures called “Feynman diagrams” along with “real particles”.  In most books for the public, some words are mumbled about the uncertainty principle and how virtual particles are particles that exist for a short time and then disappear.  Well, this isn’t really wrong, and it is the language that physicists use.  But I have found that the language is so misleading for non-experts that it leads to many confusions among my readers.   For this reason, I’ve taken a different tack in this pedagogical article, with the thinking that the most important thing for a layperson to understand about virtual particles is that they really are not particles at all, despite the name, and they don’t behave that much like them.  They are more of a generalized disturbance in a field, while a real particle, a nice ripple in a field, is a special one.

Why would you want to read this article?  Because in addition to the Cohen-Glashow argument against the OPERA experiment’s result on superluminal neutrinos, which appeals to a process similar to Cerenkov radiation, there’s a more powerful, but more subtle, argument that involves virtual particles.  I’ll explain that in my next post.