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Hard questions

  1. Dec 14, 2006 #1
    My school is getting a visit from a scientist that is told to be really good on special and general relativity. My teacher of course wants us to ask really tricky and hard questions and try to put him in place. Now I asking you guys for really hard questions :smile:
    Do you know something really hard, that can help me?
  2. jcsd
  3. Dec 14, 2006 #2
    Asking tricky questions? Put him in place? What is this teacher thinking? :confused:

    My advice is not to waste the time of this person but instead ask questions that would help you and others get a better understanding of SR and GR.
  4. Dec 14, 2006 #3
    Well thats my teacher :grumpy:
    Well one tricky question would be really fun to ask. I will probarly ask question that will help my understadning to.
  5. Dec 14, 2006 #4
    Dude. There's an expert on relativity theory coming to speak to you. Isn't there anything you want to know?
  6. Dec 14, 2006 #5


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    Does your teacher expect you to be able to catch him out? Because I doubt that that will happen.

    No, I think you've probably misinterpreted your teacher; in him asking you to think up questions, I suspect he hopes that you'll put some time thinking about relativity, and that you'll use this opportunity to get the answer to some questions that you may be unsure about.
    Last edited: Dec 14, 2006
  7. Dec 14, 2006 #6


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    Yeah, lose the "put him in his place" stuff. Just be happy for the opportunity to hear from a person working in such an interesting field. I'd brush up on relativity some, like spend some time at wikipedia.org. And then I'd consider asking questions about some of the more interesting stuff, like gravity waves and black holes and such.
  8. Dec 14, 2006 #7


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    What school are you at? and who is this expert? and who is your teacher?
  9. Dec 14, 2006 #8


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    Ask him this:

    Suppose I had two identical watches, set to the same time. I throw one up in the air and catch it. The other I hold in my hand. Which one shows the later time.

    There's a good chance that he'll answer that the one thrown in the air went on a journey, and therefore was the "traveling twin", and therefore the one in your hand aged more, and so the one in your hand shows the later time.

    If he does answer this, he's dead wrong, so ask him this question:

    Suppose you were on a steadily accelerating spaceship and you had two identical watches. You kept one in your hand and you threw the other up ahead of you, so you the spaceship would eventually accelerate you up to it and you would catch it. Now which one shows the later time.

    Since gravity is equivalent to an acceleration, the answer must be the same in both cases, but physicists have a strong tendency to give different answers. The reason is that they think of the man on earth as being an unaccelerated reference frame when it actually is accelerated by g.

  10. Dec 14, 2006 #9


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    The scientist might not appreciate being asked questions that make it obvious that the person asking the question already knows the answer and is trying to "test" them; presumably the point of this Q&A is so students can learn something?
    Last edited: Dec 14, 2006
  11. Dec 15, 2006 #10
    I looked up your original posting at https://www.physicsforums.com/showthread.php?t=78666" and I have a question:

    Do you think that in the gravitational field the maximum distance between the two watches as measured from the watch in the accelerated frame is exactly the same as the distance measured from the thrown watch in the inertial frame?

    I am just curious how you define distance in the accelerated frame, since obviously there is no such thing as an accelerated frame.
    Last edited by a moderator: Apr 22, 2017
  12. Dec 15, 2006 #11
    Why not YouJennifer?
    You can define it.
    Poetically speaking we all live in an accelerated frame!
  13. Dec 16, 2006 #12
    Ask the Expert:

    1) Whether General Relativity is necessary to explain the Twin paradox of Special Relativity

    2) Is there an experiment that measures the gravitational constant by itself - one that is not dependent uponsome quantity of mass such that that the experiment gives a constant value for the MG product, rather than a constant value for G. (Example, Long term radar ranging experminents conducted on the moons of Mars reveal the Mass of a Moon times G is temporally invariant).

    3) Does the total mass of the universe determine the locally measured inertial reaction of masses subjected to acceleration (aka Mach's Principle), and if so, how does it couple instantaneously, and if distant matter does not cause local inertia, what does?

    4) Why does mass curve space (what is the physical mechanism that brings about curvature)

    5) Whether General Relativity is necessary to explain the Sagnac effect - and if not, why

    6) What does the expert have to say about the Silvertooth Experiments, and those of Roland Dewitt

    Some of these will be sure to evoke some interesting discussion
    Last edited: Dec 16, 2006
  14. Dec 18, 2006 #13


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    1. Does relativity implies that nothing can move faster than light?
    If he answers "yes", then ask him about tachyons.

    2. Gravity is a curved spacetime. But gravity is also a spin-2 field in a flat spacetime. How both can be true?

    3. Can gravity change the topology of spacetime?
  15. Dec 18, 2006 #14


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    Ask him this: If he had to pick only one of quantum mechanics or general relativity as being correct, which would he choose?
  16. Dec 18, 2006 #15
    Ask him about the explicit and implicit hypothesis underlying the theory of general relativity.
    Ask him then to tell you if he believes some of these hypothesis could sometimes appear as the Achilles’ heel of General Relativity.
  17. Dec 18, 2006 #16


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    Since nobody said that he is an expert for quantum mechanics, he would probably choose GR. :wink:
  18. Dec 18, 2006 #17
    Unfortunatelly he would be probably forced to pick QM as bloody correct.
  19. Dec 18, 2006 #18


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    I agree with techno. More particularly, to the extent that QM and GR cannot be reconciled in a single theory, they impeach each other. And of the two, the one that is more deeply entrenched in experiment is QM.
  20. Dec 18, 2006 #19

    Chris Hillman

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    Ambush questions about relativistic physics?

    Hi all,

    I agree with those who protested that trying to "put [a visiting scientist] in his place" seems a very unlikely goal for the (science?) teacher who (presumably?) invited him. But I seem to be the only forum member cynical enough (or with enough experience at the junior high level?) to suspect that Hymne has played us... although I don't doubt that someone somewhere wants to humiliate some alleged knowitall. Be this as it may, I have some comments about some of the proposed "ambush questions", beyond the obvious one already made by several others: ambushes are not nice.

    I suspect that MeJennifer might have been thinking of my own comments here in the past month or so about the existence of multiple competing notions of distance for accelerating observers (even in flat spacetime), when she asked CarlB:

    Carl's thought experiment isn't affected by the points I raised; his mention of distance in his June 2005 post is a red herring, since the truly relevant question in this scenario is: "which watch is accelerating?"

    Indeed you can, but I hope you (tehno) will read the post I just mentioned, since my point was that there are multiple notions which give multiple distinct operationally significant definitions of distance, even in flat spacetimes. That is, procedures real physicists could carry out in real spaceships which would give different values for the same pair of particles (spaceships, whatever) in a real situation. These notions of distance do agree "very nearly" for very small regions, as you would expect from the Equivalence Principle, and I am not refering to Lorentz transformations (which are defined at the level of a single tangent space) but to effects which become apparent only in "local neighborhoods". Here's the link:

    yogi suggested these questions:

    These are perennial "trick questions" having no correct answers--- or at least no correct short answers. I would not regard them as particularly suitable for a junior high school classroom discussion! yogi, you might want to consider starting new threads on some of these "challenge questions", if you have sufficient patience to spend some weeks continually reformulating your questions as you learn enough to appreciate various relevant conceptual issues, but do I have a few quick comments:

    1. Depends upon what you mean by "twin paradox" and "neccessary" (see recent "pedagogical" arxiv eprints presenting slight variations on ideas which were extensively discussed many years ago in sci.physics.relativity, of all places, in posts by Nathan Urban, Tom Roberts, and Steve Carlip).

    2. Speaking of Steve Carlip, he tends to give excellent concise answers to questions like these. You can try to invoke him at sci.physics.research!

    3. Depends upon what you mean by "Mach's principle", "distance" (aha!---MeJennifer, take note!), "inertia", and so on. See http://www.arxiv.org/abs/gr-qc/9607009 for the multiplicity of Mach principles. Incidently, I guess that this question may have been prompted by the book Gravitation, by Ciufolini and Wheeler. If so, be aware that, unfortunately, the authors of this monograph entirely omitted to define the Mach principle they have in mind, or to point out that the one they appear to describe in the first chapter is not well-defined in gtr (think "hyperslice" and mull over my discussion of distance). This unfortunately limits the value of this otherwise engaging and thought-provoking book unless one has already mastered enough gtr from other sources to spot (and to understand the significance of) these omissions.

    4. Demanding exhibition of a "physical mechanism" runs counter to the history of physics since Newton. But a less confrontional question could indeed lead to a good discussion: "what do theorists try to accomplish by formulating physical theories, and how any why do experimental physicists try to test them?"

    5, 6. DWR!!

    Similarly, Demystifier suggested:

    Again, I doubt that any of these are very suitable topics for a junior high school classroom discussion, but I note in passing:

    1. Both str and gtr (but particularly str) are among the best tested of all scientific theories. In contrast, the notion of a tachyon is hypothetical and theoretically dubious. In fact, this idea is generally regarded as "unphysical" (one of the more deprecatory terms in theoretical physics!), although explaining why quickly becomes difficult, unless all parties have a solid appreciation of what a "physical theory" is and how it interacts with experiment. See http://www.math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#19

    2. Gtr does not say "gravity is a curved spacetime", but it does model the "gravitational field" (and in particular, its "universal character") in terms of spacetime curvature. It is true that from the example of QED one would expect gtr to arise as an effective field theory from a quantum theory in which a hypothetical particle called the graviton would play a role analogous to that played by photons in QED, and that the graviton would be a spin-two particle, but explaining why this naive viewpoint has turned out to be seriously flawed is not easily done in a few sentences. See for starters http://www.arxiv.org/abs/gr-qc/0311082

    3. Depends upon what you mean by "gravity", "change", and "cause".

    In my opinion, this is a silly question, since it has not been established that one cannot have both. Rather, a huge amount of research in recent decades has established that formulating a workable quantum theory of gravitation turns out to be much harder than anyone could have anticipated in 1950.

    One problem in trying to discuss gtr in forums like this is that it would be hard to name a theory dogged by a greater variety of well-entrenched popular misconceptions. The real strengths and limitations of gtr (and its competitors) are very different from what one might have anticipated from studying earlier theories, but this only becomes clear once one has unlearned most of what one "learned" from misleading pop sci accounts.

    But here too a less confrontational formulation could lead to a good classroom discussion (at least, at the college physics level): "What are the most characteristic hypotheses of the general theory of relativity (gtr)? What are some possible theoretical weaknesses of gtr? In what situations do physicists currently guess that it might one day fail (to produce accurate predictions)?"

    By the way, returning to the issue of good discussion questions, a student looking forward to a class visit by an expert on relativistic physics could profitably prepare by looking at some popular science sites to see what relativity-related questions seem interesting. Try:

    http://www.sciam.com/index.cfm (search "relativity")

    But the most valuable advice would probably be simply to listen closely and to go ahead and ask about anything which pops into your mind as you listen!
    Last edited by a moderator: Apr 22, 2017
  21. Dec 18, 2006 #20
    now that ^^^^ is a detailed response
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