The Standard Model More Deeply: Gluons and the Math of Quark “Color”

For readers who want to dig deeper; this is the second post of two, so you should read the previous one if you haven’t already. (Readers who would rather avoid the math may prefer this post.)

In a recent post I described, for the general reader and without using anything more than elementary fractions, how we know that each type of quark comes in three “colors” — a name which refers not to something that you can see by eye, but rather to the three “versions” of strong nuclear charge. In the post previous to today’s, I went into more detail about how the math of “color” works; you’ll need to read that post first, and since I will sometimes refer to its figures, you may want to keep in handy in another tab.

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A Prediction from String Theory

(An advanced particle physics topic today…)

There have been various intellectual wars over string theory since before I was a graduate student. (Many people in my generation got caught in the crossfire.) But I’ve always taken the point of view that string theory is first and foremost a tool for understanding the universe, and it should be applied just like any other tool: as best as one can, to the widest variety of situations in which it is applicable. 

And it is a powerful tool, one that most certainly makes experimental predictions… even ones for the Large Hadron Collider (LHC).

These predictions have nothing to do with whether string theory will someday turn out to be the “theory of everything.” (That’s a grandiose term that means something far less grand, namely a “complete set of equations that captures the behavior of spacetime and all its types of particles and fields,” or something like that; it’s certainly not a theory of biology or economics, or even of semiconductors or proteins.)  Such a theory would, presumably, resolve the conceptual divide between quantum physics and general relativity, Einstein’s theory of gravity, and explain a number of other features of the world. But to focus only on this possible application of string theory is to take an unjustifiably narrow view of its value and role.

The issue for today involves the behavior of particles in an unfamiliar context, one which might someday show up (or may already have shown up and been missed) at the LHC or elsewhere. It’s a context that, until 1998 or so, no one had ever thought to ask about, and even if someone had, they’d have been stymied because traditional methods are useless. But then string theory drew our attention to this regime, and showed us that it has unusual features. There are entirely unexpected phenomena that occur there, ones that we can look for in experiments.

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In Memory of Joe Polchinski, the Brane Master

This week, the community of high-energy physicists — of those of us fascinated by particles, fields, strings, black holes, and the universe at large — is mourning the loss of one of the great theoretical physicists of our time, Joe Polchinski. It pains me deeply to write these words. Everyone who knew him personally will … Read more

Modern Physics: Increasingly Vacuous

One of the concepts that’s playing a big role in contemporary discussions of the laws of nature is the notion of “vacua”, the plural of the word “vacuum”. I’ve just completed an article about what vacua are, and what it means for a universe to have multiple vacua, or for a theory that purports to describe … Read more

Which Parts of the Big Bang Theory are Reliable, and Why?

Familiar throughout our international culture, the “Big Bang” is well-known as the theory that scientists use to describe and explain the history of the universe. But the theory is not a single conceptual unit, and there are parts that are more reliable than others.

It’s important to understand that the theory — a set of equations describing how the universe (more precisely, the observable patch of our universe, which may be a tiny fraction of the universe) changes over time, and leading to sometimes precise predictions for what should, if the theory is right, be observed by humans in the sky — actually consists of different periods, some of which are far more speculative than others.  In the more speculative early periods, we must use equations in which we have limited confidence at best; moreover, data relevant to these periods, from observations of the cosmos and from particle physics experiments, is slim to none. In more recent periods, our confidence is very, very strong.

In my “History of the Universe” article [see also my related articles on cosmic inflation, on the Hot Big Bang, and on the pre-inflation period; also a comment that the Big Bang is an expansion, not an explosion!], the following figure appears, though without the colored zones, which I’ve added for this post. The colored zones emphasize what we know, what we suspect, and what we don’t know at all.

History of the Universe, taken from my article with the same title, with added color-coded measures of how confident we can be in its accuracy.  In each colored zone, the degree of confidence and the observational/experimental source of that confidence is indicated. Three different possible starting points for the "Big Bang" are noted at the bottom; different scientists may mean different things by the term.
History of the Universe, taken from my article with the same title, with added color-coded measures of how confident we can be in our understanding. In each colored zone, the degree of confidence and the observational/experimental source of that confidence is indicated. Three different possible starting points for the “Big Bang” are noted at the bottom; note that individual scientists may mean different things by the term.  (Caution: there is a subtlety in the use of the words “Extremely Cold”; there are subtle quantum effects that I haven’t yet written about that complicate this notion.)

Notice that in the figure, I don’t measure time from the start of the universe.  That’s because I don’t know how or when the universe started (and in particular, the notion that it started from a singularity, or worse, an exploding “cosmic egg”, is simply an over-extrapolation to the past and a misunderstanding of what the theory actually says.) Instead I measure time from the start of the Hot Big Bang in the observable patch of the universe.  I also don’t even know precisely when the Hot Big Bang started, but the uncertainty on that initial time (relative to other events) is less than one second — so all the times I’ll mention, which are much longer than that, aren’t affected by this uncertainty.

I’ll now take you through the different confidence zones of the Big Bang, from the latest to the earliest, as indicated in the figure above.

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Brane Waves

The first day of the conference celebrating theoretical physicist Joe Polchinski (see also yesterday’s post) emphasized the broad impact of his research career.  Thursday’s talks, some on quantum gravity and others on quantum field theory, were given by Juan Maldacena, on his latest thinking on the relation between gravity, geometry and the entropy of quantum … Read more

Celebrating a Great Brane

Today and tomorrow I’m at the Kavli Institute for Theoretical Physics, on the campus of the University of California at Santa Barbara, attending a conference celebrating the career of one of the world’s great theoretical physicists, Joe Polchinski. Polchinski has shown up on this website a couple of times already (here, here and here).  And … Read more

Quantum Field Theory, String Theory and Predictions (Part 9)

Today I continue with my series of posts on fields, strings and predictions. During the 1980s, as I discussed in the previous post in this series, string theorists learned that of all the possible string theories that one could imagine, there were only five that were mathematically consistent. What they learned in the first half of the … Read more

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