A Particle Physicist's Perspective

By JoAnne Hewett | November 18, 2005 2:14 am

Over the last week, there has been intense discussion on the topic of string theory. It was sparked by the NYT op-ed piece by Lawrence Krauss, and the discussion has taken place here, elsewhere, and in the hallowed hallways of who knows where – I can vouch that the blogosphere has sparked hot debate in my local hallways. So now that the debate has momentarily died down, it’s time to spark it up again. I always was a troublemaker….

In particular, there is, to my view, an important aspect missing from the debate: the perspective of a particle physicist. That is, a physicist who deals with the production of particles in experiment, the experimental determination of their properties, and what that experimental information reveals about the underlying laws of nature. Yes, I used the word experiment three times in that sentence. That’s because physics is an experimental science.

So, people have asked, is string theory worth studying? I would answer yes, for two reasons. One has already been given, namely, that string theory is currently our best idea about how to quantize gravity. We obviously need to explore that link further, no matter how tenuous. The second is more subtle in that ideas/techniques developed from string theory pursuits have benefited other fields such as mathematics, cosmology, and particle physics. Since my expertise is in the latter, I will only speak to that. There are two concrete examples that have led to important advances in particle theory:

1) Techniques applied to higher order calculations in perturbative Quantum Chromodynamics. In order to definitively detect new physics beyond the Standard Model at high energy colliders, we need to understand the production of Standard Model processes to a very high degree of accuracy. This is a complicated, nearly impossible, task to calculate via brute force. But people have applied ideas and techniques, borrowed from string theory, to render these horrendous calculations tractable. This has resulted in significant advances in the understanding of the Standard Model “background” and will be used extensively at the Large Hadron Collider.

2) Branes. The concept of an n-dimensional membrane upon which certain particles can be confined was developed within string theory, but has revolutionized ideas on the form that particle physics beyond the Standard Model may take. We now have numerous theories, each with their own particle physics motivation, which make use of branes and extra dimensions, and, most importantly, are testable at particle accelerators. Numerous calculations and experimental searches have already been carried out. Because of advances in string theory, particle physicists are literally thinking in new dimensions.

However, there are other aspects of string theory which I, as a particle physicist, tend not to find as pleasant or rewarding.

Arrogance #1: I find the arrogance of some string theorists astounding, even by physicist’s standards. Some truly believe that all non-stringy theorists are inferior scientists. It’s all over their letters of recommendation for each other, and I’ve actually had some of them tell me this to my face. Obviously it is complete nonsense. There are some extremely bright and some not-so-bright and some average folks in both fields.

Arrogance #2: I personally find the attitude that people actually think we know enough at this point to define the `theory of everything’ to be quite conceited. To me, it is equivalent to claiming that the universe rotates around the earth. I would bet everything I own that nature has many, many surprises in store for us as we delve deeper experimentally.

Arrogance #3: String theory is so important that it must be practised at the expense of all other theory. There are two manifestations of this: string theorists have been hired into faculty positions at a disproportionally high level not necessarily commensurate with ability in all cases, and the younger string theorists are usually not well educated in particle physics. Some literally have a hard time naming the fundamental particles of nature. Both of these manifestations are worrying for the long-term future of our field.

Do I think string theory is the ultimate theory of nature? No. I believe it is much more likely that we are having trouble quantizing gravity because that’s not the right thing to do. Perhaps quantum theory is only an effective theory, valid at currently accessible energy scales, but not valid at scales where gravity becomes important.

Does this mean I’m against string theory? No! I only wish for it to be practised in reasonable proportion to other endeavors, sans the grandiose claims (which have yet to be realized).

Do I think we should continue to popularize string theory to the public? Yes. The public is fascinated with this endeavor, and as long as it is presented as an endeavor, ultimately falsifiable by experiment, it is fine with me.

Do I think this fashion trend will continue? No. My bet is that the LHC will produce physics so exciting that physcists will drop whatever they are doing and flock to explain it.

So, that is my perspective. I am clicking the publish button and donning my hardhat, awaiting the furious comments to be tossed at me…


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