Monday, June 21, 2010

Ask David Gross

Update July 2010: The answers have been posted (click)

Separate page with the video above...

You may ask him a question - via YouTube or Facebook. If you choose YouTube, you have to record your question as a video and
post your question as a reply to the video above (click).
So far, 17 questions have been asked in this way. Some of them are strange. For example, a primitive idiot from the U.K. Labour Party who pompously calls himself Prof Robert Winston asks how Gross can possibly justify the research of something as useless as string theory. ;-)

Alternatively, you may just write your question on Facebook, at a page created by the FB account of

You may also be inspired by one of the ten videos with David Gross at Closer to Truth (PBS).

The deadline is on Friday, June 25th (explanation).


  1. I'd rather ask you. How can a vibrating string create mass or anything else?

  2. Dear Harlow, it may be nicer if you asked Gross. ;-)

    Your seemingly modest 15-word question is equivalent to asking me to explain "anything" about the Universe.

    I can't do it in one comment.

    Stringy vibrations carry energy, according to a formula for energy (potential plus kinetic, in some sense, among other formulae for the "Hamiltonians" that arise in string theory) and energy is equivalent to mass via Einstein's "E=mc^2".

    So the more violently a string vibrates, the more massive particle it will resemble. Because of quantum mechanics, the allowed masses of a string are discrete - so the spectrum of allowed particles is discrete, too. You only have some types.

    Charges, spins, and similar properties of particles have an analogous origin.

    All forces and interactions ultimately boil down to splitting and joining of strings which is string theory's fancier version of the interaction vertices of Feynman diagrams - points in which several lines ("propagators") meet.

    In string theory, they don't meet in points, but to allow strings to split and join is enough for the topology of the histories to have many forms, and to imitate any interactions of field theory.

    When all these observed things you mentioned and "anything else" are carefully studied, it turns out that all of them follow from string theory, but the details why it is so are described on thousands of pages of papers and books and in thousands of formulae that I can't fully reproduce in one comment.

    Already with this sketch, my answer is several orders of magnitude longer than your question.

    Best wishes

  3. Does it also then imply that strings have discrete lengths?

  4. Dear AntiCitizenOne,

    not really. The length of a particular string in string theory is changing with time and it's not really well-defined. If you actually calculate it "exactly", with the resolution (or "minimum distance", technically known as "cutoff") chosen to be an arbitrarily short distance, the length of a string is infinite because the shape resembles the Brownian motion.

    This divergence has no pathological consequences because strings are simply not straight sticks and the length doesn't directly influence any observable quantities.

    Of course, if you try to calculate some "effective length" that is a function of the string's energy, or something like that, you will get discrete values because the energies are discrete values. But if you talk about the "real length", it is not finite.

    In a similar way, the typical string actually occupies an infinite volume, too. The infinite length of the string isn't even squeezed into a finite volume of space. This has no pathological observational consequences, either. The parts of the string that are "extremely far" from the center are produced by very high-frequency modes of oscillations along the string and these high-frequency modes get averaged out very quickly, influencing no slow processes in the world in an observable way.

    Only the actual energy has to be finite. Energy is conserved, bought for $0.20 per kWh, and money is finite, too. ;-)