Monday, March 06, 2006

John Preskill: weirdness at work

John Preskill - yes, that's a new picture of him on the Wikipedia page that our fellow blogger just took - has given the first of his three prestigious Loeb lectures here at Harvard. It was an introduction to quantum computation. This fact makes it clear that the content of the lecture largely overlapped with the lectures number 23 and especially 24 of my quantum mechanical course in the fall; see the course website where the PDF files are available.

However, Preskill's presentation was completely smooth, comprehensible, and filled with very entertaining animations in his PowerPoint presentation. And in many respects, it went well beyond my lecture.

He explained things like
  • entanglement (this word comes from Schrödinger, by the way)
  • decoherence
  • history of these EPR things, including Schrödinger's reply to the EPR 1935 paper (described as the last Einstein's paper whose relevance survived to the present days): did you know that the "spooky action at a distance" is even spookier in German? It is "spukhafte Fernwirkungen" :-)
  • difference between bits and qubits, and the difference between (red) boxes and (red) soxes :-)
  • explanations why the quantum computers can be faster, with the factoring algorithms as examples
  • quantum cryptography and a safe transmission of the key to encode information - in a detailed way that makes all the necessary steps transparent; Alice and Bob were exchanging secret data while Eve (...) was eavesdropping ;-)
  • history of quantum computation
  • the nature of basic operations of quantum computers
  • methods of error corrections - with a beaver trying to undo the errors introduced by a dragon
  • basics of technical realizations of the quantum computers with ion traps (if I remember well) as the main example
Preskill started in our field - with various monopole solutions in GUT theories and other discoveries that were helpful to create the right atmosphere for cosmic inflation to be born - and gradually evolved into one of the leaders of the field of quantum information and quantum computation.

He has also become famous for having won the bet against Stephen Hawking; a well-deserved victory. Hawking used to argue that the information must be lost after a black hole evaporates. This followed from Hawking's semiclassical calculations but the rules of quantum mechanics - namely unitarity - dictated otherwise and John Preskill correctly decided that the postulates of quantum mechanics are primary.

Preskill is also a poet. Howard Georgi who introduced the speaker mentioned that Preskill's poem could also be thrown into a black hole. Fortunately, the information is preserved anyway, as argued below, which is why Georgi's efforts won't lead anywhere. You can compare Preskill's poem with a poem by a Nobel prize winner who will speak in the same room at the same time next week.

Mostly because of the convincing developments in string theory where black holes are found as objects within a completely unitary, quantum mechanical framework (and the case of the AdS/CFT correspondence may be even more convincing than Matrix theory and other approaches), Hawking decided that the main argument for the information loss, namely that it should be impossible to construct a consistent unitary theory which agrees with black hole physics in the semiclassical approximation, has been shown incorrect, and his bet is effectively lost. Hawking has bought Preskill an encyclopedia of Preskill's choice, namely an encyclopedia of baseball.

Note that this bet could only be ended because Hawking turned out to be very reasonable. If Preskill had made a bet against Peter Woit or someone like that, he would be told that no arguments for unitarity exist and the arguments based on string theory do not exist either because the existence and insights of string theory can simply be denied and pictured as an uncertain source of knowledge about quantum gravity. And Peter Woit would undoubtedly use even stronger and sillier words. ;-) In reality, string theory has shown its ability to authoritatively answer deep questions about quantum gravity such as the information loss paradox, and only complete outsiders misunderstand that these general insights are established despite our ignorance about the detailed choice of the vacuum. Yes, they are established by pure thought, if you need to hear it.

Finally, I want to say that without any exaggeration, John Preskill belongs to the top five best physics communicators in this world. Be sure that I haven't written this sentence more than five times in the last 10 years. He is a fast thinker who knows how to analyze things in details, present them in a completely comprehensible fashion, and answer the questions that appear along the way.


  1. Lubos:

    I did not know that you actually BELIEVED in the QM unitary condition. If you believe in the unitary rule you should have no problem accepting my GUITAR theory. The fundamental of GUITAR is merely applying the unitary condition to the whole universe, not just a sub system. That's all and no more and no less. I make no other postulation. And I have clearly demonstrated that it naturally leads to both QM and GR, and leads to specific values of the related physics constant. With all the detailed and very specific and accurate numerical results it is hard for any one to say anything against my theory. Although it is not finished yet.


  2. did you know preskill subscribes to mwi,
    though he doesn't know (like no-one does) how this intelligibly gives rise to probability,
    and he doesn't seem to be very interested in his (according to his believes) infinite doppelgängers, because he never has written one paper about mwi?

  3. Berry, nope, I didn't know. Now I kind of know. Another interesting question, aside from the question whether I knew or whether I know, is whether I care. ;-)

    This blog entry was about a particular constructive topic, quantum computation, that Preskill *did* write about. His opinions about a question he's never written about may be diverse and thousands of physicists are totally confused about those topics and it's great that they don't try to write about things they don't know.

    The belief in MWI is somewhat widespread among physicists. In polls, a big percentage would answer MWI, even among the sensible experts. In most cases, it doesn't mean much. They don't have any particular "MWI theory" in mind; the answer is just an attack against various straw men, like the straw man claiming that the Copenhagen interpretation doesn't want the laws of QM to hold for macroscopic systems; or against various "collapse is real", or on the contrary spiritual misinterpretations, etc.

    They mean various things but it's still true that nothing such as a "meaningful MWI framework underlying QM and allowing one to extract the same probabilistic observables in a more controllable way" doesn't exist. It's a chimera. That's also why it's reasonable for reasonable people not to write about it.