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Magueijo: laserweapon shortcomings (QG limit on coherent pulse energy)

  1. Mar 30, 2006 #1

    marcus

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    One of the funniest oneliners I've seen in a technical paper:

    " It is therefore impossible to have a Laser deliver more than about a GigaJoule of coherent light. The implied limitations to military systems, such as those inspired by SDI, are so tragic they shouldn’t even be contemplated. "

    http://arxiv.org/gr-qc/0603073

    Bee Hossenfelder was one of the people thanked in the acknowledgments of this paper, so maybe she can give us some extra insight.

    Magueijo has come up with something that could be TESTED and which is derivable from some forms of (Loop) quantum gravity that require DSR. It is a limit on the amount of energy you can stuff into a coherent laser pulse.

    This could presumably be used to FALSIFY those forms of DSR-LQG which imply this energy limit. That seems like it might be helpful. And also quite intriguing if they PASSED the test----meaning that this limit exists.

    The oneliner I quoted is on page 10, one paragraph from the end of the article, in the conclusions.
     
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  3. Mar 31, 2006 #2

    Chronos

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    It looks too simplistic to me I think you could phase pulse a coherent light beam nearly to GR limits.
     
  4. Apr 1, 2006 #3

    marcus

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    Hi Chronos, I am glad to see that you have taken an interest in this. Time goes on I admire Joao more and more, I think this is a great paper.

    I would urge EVERYBODY to watch the PI seminar talk he gave on TESTING MOND IN THE SOLAR SYSTEM

    the all out brilliant thing about Joao is his sophisticated focus on testing. He is keen on discovering ways that far-out ideas can be FALSIFIED.

    You can tell from he seminar talk that he is very balanced about whether MOND is right or wrong, he clearly sees goods and bads in a kind of cold way. What is creative about the talk is that he points out that you could measure MOND effects at a saddlepoint between here and the earth/sun L1 point. this is a point that the spacecraft Lisa Pathfinder may be crossing and Lisa has an unprecedent accurate gravitometer.

    So the guy is smart and dispassionate and likes risk. I like him very much.
    ===============

    Here he is pushing DSR to the brink of a precipice and he is saying coldly that DSR looks like it might (at least in some versions) predict that you cant have a coherent laser pulse with more than Planck energy!
    That is fantastic.

    He is not arguing for that. he is not trying to impose this idea on you Chronos (you probably have enough puzzles on your mind already:smile: ) He doesnt BELIEVE this thing about lasers. What he is saying is this could be dangerous to DSR. So let's TEST it!

    Of course IF it were true (if DSR passed that test) and you actually COULDNT have a coherent batch of energy bigger than Planck---then that might make some StarWars people look silly.

    that is Magueijo's joke, but that is not really the message. the StarWars people (assuming you gave them lots of money) would figure a way around that one, like sending lots and lots of separate pulses

    and after all a Gigajoule is a lot of energy and could do quite some damage----so that about StarWars is just a joke.

    but as a solution to the Soccerball puzzle this paper is lovely, I think.

    you don't agree?
     
  5. Apr 1, 2006 #4

    marcus

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    I should post the abstract just for explicitness

    http://arxiv.org/abs/gr-qc/0603073
    Could quantum gravity be tested with high intensity Lasers?
    Joao Magueijo

    "In quantum gravity theories Planckian behavior is triggered by the energy of elementary particles approaching the Planck energy, E_P, but it's also possible that anomalous behavior strikes systems of particles with total energy near E_P. This is usually perceived to be pathological and has been labelled "the soccer ball problem''. We point out that there is no obvious contradiction with experiment if coherent collections of particles with bulk energy of order E_P do indeed display Planckian behavior, a possibility that would open a new experimental window. Unfortunately field theory realizations of deformed special relativity never exhibit a "soccer ball problem''; we present several formulations where this is undeniably true. Upon closer scrutiny we discover that the only chance for Planckian behavior to be triggered by large coherent energies involves the details of second quantization. We find a formulation where the quanta have their energy-momentum (mass-shell) relations deformed as a function of the bulk energy of the coherent packet to which they belong, rather than the frequency. Given ongoing developments in Laser technology, such a possibility would be of great experimental interest."

    My comment to clarify for people who react like Chronos to this: SCIENCE IS NOT ABOUT HAVING THINGS TO BELIEVE IN it is about having things to TEST. And maybe this coherent pulse thing he says is wrong and maybe it is right. Either way it is a fascinating possibility and one definitely wants to know!
     
    Last edited: Apr 1, 2006
  6. Apr 3, 2006 #5
    Hi Marcus,

    I can tell you the same I told Joao and others who have asked me for my opinion on that. It's an interesting work and it's great that he investigates different possible realizations of DSR as to it's outcome for experiments.

    I like the idea of the paper but I don't specifically like the approach to start from the modified dispersion relation - if you indeed start from a DSR, there is less ambiguity in the quantization procedure. I find it very confusing to think that I have a choice how to write down the Lagrangian and therefore can have different theories. You don't have that choice if you start from DSR in the first place. But I guess that's a matter of taste.

    Maybe most importantly, I still don't get the point with the soccer ball problem. It's a classical field description. Naturally, there is no soccer ball problem, because there are no particles that build the soccer ball. I don't see how the analysis solves the question how the transformation behavior of one single particle's momentum (DSR) turns into the standard one for bound multi-particle states. The question is not even addressed.

    What is really good about the paper is that it is a step towards the important question which quantum field theories with DSR are possible and reasonable, and how we can formulate them.

    Best,

    B.
     
  7. Apr 3, 2006 #6

    marcus

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    hello Bee, thanks for replying. BTW i am almost certain that you know Etera Livine, or have met him while at Perimeter. Etera has written about the soccerball problem. Sometimes he comes here to PF (his member name is simply etera.) This is someone who could define the issues very quickly and clearly for us, if he was around.
     
  8. Apr 4, 2006 #7

    Chronos

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    Your chastening is appropriate, marcus. And indeed, I have many puzzles in mind. However, I welcome good arguments. What I find soft in Magueijo is the DSR connection. I'm not sold on DSR, and without that 'a priori', I think the argument is weak. So basically, I have a dumbed down version of hossi's argument [hossi is pretty bright IMO].
     
    Last edited: Apr 4, 2006
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