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:
MeJennifer said:
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.
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?"
tehno said:
Why not YouJennifer?
You can define it.
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 referring 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:
Chris Hillman said:
Way back in February 2003, franznietsche asked:...
yogi suggested these questions:
yogi said:
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
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:
Demystifier said:
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?
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".
CarlB said:
Ask him this: If he had to pick only one of quantum mechanics or general relativity as being correct, which would he choose?
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.
lalbatros said:
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.
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")
http://www.aip.org/history/einstein/
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!
