## October 29, 2007

### Fundamental Physics: Where We Stand Today

#### Posted by John Baez

Are most of the entries on this blog too technical for you? Well, try this:

• John Baez, Fundamental physics: where we stand today, Department of Physics and Astronomy, James Madison University, November 2, 2007.

Since the discovery of the W and Z particles over twenty years ago, few truly novel predictions of fundamental theoretical physics have been confirmed by experiment. On the other hand, observations in astronomy have revealed shocking facts that our theories do not really explain: most of our universe consists of "dark matter" and "dark energy". Where does fundamental physics stand today, and why has theory become divorced from experiment?

It’s a talk for anyone interested in physics: a few equations at first, when I explain general relativity, but then just words and pictures!

For more details, try:

I’m flying to Dulles airport on Wednesday, spending a night at my folks, then driving to Harrisonburg Virginia the next day. On Thursday evening I’ll give a public talk on Zooming out in time: a history of climate change — but avid readers of this blog will have seen that already.

On Friday I’m giving a lunchtime chalk talk on quantum gravity, and then the above talk for the physics/astronomy colloquium. If you’ve seen earlier versions of this talk, the only difference is that I’ve added material about the Bullet Cluster, chopped out pictures of the Very Large Telescope in the Atacama Desert, and improved my explanation of why positive vacuum energy makes the expansion of the Universe speed up.

Then I’ll drive back to Great Falls and visit my folks for the weekend!

Whew.

Posted at October 29, 2007 11:48 PM UTC

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### Re: Fundamental Physics: Where We Stand Today

Hi John,

I have a very interesting but exhausting day at Hogwarts, er, I mean at Oxford University behind me. Now I am reading your notes on fundamental physics to relax a little. Here is a first comment.

General relativity is a beautiful work of pure thought. The Standard Model is a baroque mess: we live in an interesting world.

Isn’t Yang-Mills theory a beautiful work of pure thought, too?

We have a bunch of indications that GR and YM are not disconnected from each other, structurewise.

What is xyz in

general relativity : Yang-Mills :: xyz : Standard Model

??

Isn’t it something like: one parameter specifying the size of the cosmological constant?

19:1 is large, but is 19:1 a baroque mess? If so, at what quotient does baroque mess start?

Posted by: Urs Schreiber on October 30, 2007 12:53 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Isn’t Yang–Mills theory a beautiful work of pure thought, too?

Yes, but you wouldn’t have chosen the gauge group $\SU(3) \times SU(2) \times \mathrm{U}(1)$ and coupled it to fermions transforming in the direct sum of three copies of the representation coming from the 6-1 homomorphism $\SU(3) \times \SU(2) \times \mathrm{U}(1) \to \SU(5)$ followed by the obvious action of $\SU(5)$ on $\Lambda \mathbb{C}^5$… and then thrown in a Higgs boson, like a cherry on top.

I just wanted the audience to see there’s a difference in levels of complexity here.

What is xyz in

general relativity : Yang–Mills :: xyz : Standard Model

??

Good question. General relativity plus dark matter and dark energy? Or MOND, maybe?

Have fun at Hogwarts!

Posted by: John Baez on October 30, 2007 2:18 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Urs, in your talk at Hogwarts did you or the audience say anyhing about possible applications of bundle gerbes for physics?

I visited Hogwarts (i.e., Oxford University) twice when I was a young boy when my father was working at Cambridge University. (Funnily enough, I never asked my father why he was working at Cambridge given that he already had appointments in two science departments at Harvard University, but I would guess that at these times he did not have to supervise his postdocs at Harvard and that he wanted to collaborate more closely with his colleagues at Cambridge).

Hogwarts is fun to visit but I would not want to live there year round due to the weather. In this sense, the academics at a place like Berkeley, CA are smart to be where they are. (To attract more attention to mathematics, one could even consider starting a rivalry between West coast mathematicians versus East coast mathematicians somewhat akin to the previous rivalry between West coast and East coast rappers but without all the guns and violence !-)

Speaking of Cambridge University, here are three interesting tidbits I know about Sir Isaac Newton:

1) There was a group of ancient Chinese philosphers (whose name I forget) which believed in Newton’s 1st and 3rd Laws of Motion, but of course they missed the crucial 2nd Law of Motion because they did not even conceive of differential equations. (This is the only thing I know about philosophy).

2) Like Newton did , my father actually performed experiments upon his own eyes. (Hopefully, none of the kids who have watched the TV show “Jackass” are reading this comment and will not try something stupid :-)

3) I work in quantitative finance and to the best of my knowledge, Newton was the first quant to think about finance. After losing a large sum of money in the South Seas bubble, Newton wrote “I can calculate the motions of heavenly bodies, but not the madness of people.” After quitting math and science, Newton worked in finance.

I always keep this quote in the back of my mind so that hopefully I won’t end up fooling myself. (If this quote is too much for one to remember then consider this simpler quote from the old rap group Public Enemy which is simply “Don’t believe the hype!”)

Posted by: Charlie Stromeyer Jr on October 30, 2007 11:27 PM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

i really wonder how the number of 200 trillion stars in the virgo supercluster came into being! Did someone count that?

Posted by: ericv on October 30, 2007 1:04 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Eric wrote:

i really wonder how the number of 200 trillion stars in the virgo supercluster came into being! Did someone count that?

Yes — the world’s oldest grad student.

I’m sure people have estimated the number of stars in many ways, but I’m not an expert, so I don’t know the details.

I’ve read the Virgo Supercluster contains about 200 galaxy clusters, with a total of about 2,500 large galaxies and 25,000 small ones. The Milky Way counts as a large one, I think, and people say it contains about 100 billion ($10^11$) stars. Naively multiplying this by 2,500, we get 250 trillion ($2.5 \cdot 10^14$) stars, which is in the right ball-park if we ignore the small galaxies.

Presumably the experts do the calculation a bit more carefully. Maybe someone can read this and say how they do it:

• Brent Tully: The Local Supercluster, Astrophys. J. 257 (1982), 389–422.

Or maybe an expert will stop by and tell us.

Speaking of big numbers, people estimate the observable Universe has about $10^10$ galaxies in it, and about $10^{21}$ stars.

A sextillion stars!

Posted by: John Baez on October 30, 2007 2:09 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

I didn’t know your folks were in the capitol area. And here we’ve never bumped into each other when visiting respective homes for the holidays.

Posted by: John Armstrong on October 30, 2007 5:34 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Where do you hail from?

My folks live in Southdown Farm, a woodsy place near the Potomac, north of the town of Great Falls.

Posted by: John Baez on October 30, 2007 6:13 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Well, after my father failed his tenure bid at UMN, we moved to Columbia, MD, and he started in at UMBC.

Incidentally, he’s still there, and has been undergrad chair in the math department for about 20 years. It was quite a shock to me to find that in most places people have to be bribed into it with such prizes as getting first pick of the research postdoc ahead of the wishes of the hiring committee :/

Posted by: John Armstrong on October 30, 2007 6:51 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

One correction for your slides: the LHC is 27km in circumference, not diameter.

And I really don’t agree with your characterization of what happened with string theory as people reacting to the fact that it was making incorrect or untestable predictions by focusing on mathematical elegance. There’s some very nice mathematics that has come out of things like the topological string, but that’s always been a minority effort among string theorists. The reaction to the fact that simple, relatively elegant, string theories give wrong predictions was that many string theorists started working on ever more complicated and inelegant “string theory backgrounds”, trying to evade contradiction with experiment, and ending up with something completely unpredictive. The Landscape definitely can’t be blamed on the search for mathematical elegance, quite the opposite…

Posted by: Peter Woit on October 30, 2007 2:40 PM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

Peter wrote:

One correction for your slides: the LHC is 27km in circumference, not diameter.

Whoops! Fixed.

I’ve even been there, but it’s so big I couldn’t tell the difference.

And I really don’t agree with your characterization of what happened with string theory as people reacting to the fact that it was making incorrect or untestable predictions by focusing on mathematical elegance. There’s some very nice mathematics that has come out of things like the topological string, but that’s always been a minority effort among string theorists. The reaction to the fact that simple, relatively elegant, string theories give wrong predictions was that many string theorists started working on ever more complicated and inelegant “string theory backgrounds”, trying to evade contradiction with experiment, and ending up with something completely unpredictive.

Okay, good point. My main contact is with people who do mathematically elegant stuff with no immediate contact to experiment, like my pal Urs here (who has upheld his honor by joining a math department). The inelegant stuff I just ignore. You’re better at keeping up with those complicated and not very successful attempts to fit the actual data. And, I can believe you — and I sort of hope it’s true — that this activity commands more attention among the rank and file of string theorists.

So, I’ve modified a few sentences in my talk.

By the way, those sentences were meant to apply not just to string theorists, but also loop quantum graviters — like me! I always went for “mathematical elegance”, so I was deploring my own sins.

Posted by: John Baez on October 31, 2007 4:35 AM | Permalink | Reply to this

### Re: Fundamental Physics: Where We Stand Today

John,

I just don’t think the search for mathematical elegance is a sin! If experiment is giving no hints, and no one has a good idea about how to make progress based on experimental results, getting ones motivation from what is mathematically elegant may not be very promising, but it might be the best you can do. And even if it doesn’t lead to good physics, it quite possibly will lead to good new mathematics, which is worthwhile (and might someday lead to good new physics).

Urs is definitely a good example of someone pursuing mathematical elegance, I’m just not so convinced what he’s doing has much to do with string theory…

Posted by: Peter Woit on November 1, 2007 3:24 PM | Permalink | Reply to this

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