Test Your Knowledge: Visit from a Scientist on Relativity

[Hymne]

Hi.
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
Do you know something really hard, that can help me?

 

[MeJennifer]

Hi.
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
Do you know something really hard, that can help me?

Asking tricky questions? Put him in place? What is this teacher thinking?

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.

 

[Hymne]

Asking tricky questions? Put him in place? What is this teacher thinking?

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.

Well that's my teacher
Well one tricky question would be really fun to ask. I will probarly ask question that will help my understadning to.

 

[cesiumfrog]

Dude. There's an expert on relativity theory coming to speak to you. Isn't there anything you want to know?

 

[cristo]

Hi.
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
Do you know something really hard, that can help me?

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.

 

[berkeman]

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.

 

[robphy]

What school are you at? and who is this expert? and who is your teacher?

 

[CarlB]

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.

Carl

 

[JesseM]

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.

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?

 

[MeJennifer]

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.

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.

 

[tehno]

.. since obviously there is no such thing as an accelerated frame.

Why not YouJennifer?
You can define it.
Poetically speaking we all live in an accelerated frame!

 

[yogi]

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

 

[Demystifier]

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?

 

[CarlB]

Ask him this: If he had to pick only one of quantum mechanics or general relativity as being correct, which would he choose?

 

[lalbatros]

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.

 

[Demystifier]

Ask him this: If he had to pick only one of quantum mechanics or general relativity as being correct, which would he choose?

Since nobody said that he is an expert for quantum mechanics, he would probably choose GR.

 

[tehno]

Unfortunatelly he would be probably forced to pick QM as bloody correct.

 

[CarlB]

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.

 

[Chris Hillman]

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:

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?"

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:

Way back in February 2003, franznietsche asked:...

yogi suggested these questions:

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:

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".

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.

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!

 

[3dsmax]

now that ^^^^ is a detailed response

 

[tehno]

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.

I don't believe in Mach principle in his original form.Matter of fact
I think that inertia could be local effect arising from disturbing quantuum vacuum fields.Sort of speaking the massive object acts upon itself.
But that's just my opinion.
Did you know that Einstein changed his mind about Mach principle several times in his life?
IIRC,he said something like it was useful during first years of constructing GR but that's not a way one should interpret it .


As of tachyons.Physically :Tachyons=rubbish.They arise from math ,not from physics.

 

[Chris Hillman]

Math produces rubbish? No.

Hi, tehno,

I don't believe in Mach principle in his original form.Matter of fact
I think that inertia could be local effect arising from disturbing quantuum vacuum fields.Sort of speaking the massive object acts upon itself.

You might be interested in some old speculations of Sakharov; see MTW.

Did you know that Einstein changed his mind about Mach principle several times in his life?

Sure, he changed his mind several times about many topics. The subject of Mach principles is impossible to discuss, however, unless all parties clearly recognize the existence of and are familiar with precise formulations of some specific examples of such principles, however.

IIRC,he said something like it was useful during first years of constructing GR but that's not a way one should interpret it .

He would have to explain what "it" is. Unfortunately, he's dead, so if you could find the passage you recall, you would have to try to infer from context which Mach principle he had in mind.

Physically :Tachyons=rubbish.They arise from math ,not from physics.

Well, most physicists would say "right conclusion (probably); wrong reason". That is, whether or not an idea touted in physics arose from a mathematical consideration or a physical consideration does not determine its validity in physics! Rather, one must use both mathematical criteria (such as internal consistency of a proposed theory) and physical criteria (such as conformity with accepted physical principles and agreement with experimental evidence) in assessing the value (in physics) of a given idea.

 

[Demystifier]

As of tachyons.Physically :Tachyons=rubbish.They arise from math ,not from physics.

It could be true. But the point is that even if there are theoretical reasons why they are not viable, the principle of relativity is NOT such a reason. Therefore, the theory of relativity by itself does NOT exclude superluminal velocities, unless this theory is combined with some OTHER physical assumptions.

 

[Chris Hillman]

Just to clarify what demystifier said, in the more general context of physics formulated on spacetime models (Lorentzian manifolds): spacelike world lines are geometric conceivable in Lorentzian manifolds; just draw any spacelike curve and declare that this is a world line. So the theoretical objections to tachyons (never mind the obvious experimental objection, that no tachyons have been observed) must draw on physical principles, and perhaps also on mathematical reasoning which goes beyond the elementary geometry of Lorentzian manifolds.

 

[Gib Z]

Umm as to the two watches ones, where we are being accelerated at g, in our frame of reference arent the g-forces and electromagnetic repulsion from the electrons in our body to the ones in the ground, cancel out each other, with no resultant force? I'm sorry its been a while since I've done relativity.

 

[tehno]

It could be true. But the point is that even if there are theoretical reasons why they are not viable, the principle of relativity is NOT such a reason. Therefore, the theory of relativity by itself does NOT exclude superluminal velocities, unless this theory is combined with some OTHER physical assumptions.

Everything can be true:)

And if one doesn't have even the slightest chance ( theoreticaly) to detect "tachyons" in our "subluminal world" what's the point in saying they could exists?
Example:Please note this not the same as the questions about "virtual particles" existence.Effects of their interactions with our world can be detected ,and they are also important in the theory.

 

[CarlB]

And if one doesn't have even the slightest chance ( theoreticaly) to detect "tachyons" in our "subluminal world" what's the point in saying they could exists?

At the present time, we have absolutely no experimental evidence for the characteristics of particles with energies on the order of the Planck mass. To assume that such particles follow the rules of the very low energy particles that we observe, requires that we assume that we can exptrapolate over many, many, many orders of magnitude in energy.

Previous extrapolations have failed miserably, as the higher energy particles get weirder and weirder. For example, extrapolating from the low energy situations observed in the early 20th century, we would expect that all particles have integer charges. And these are still very small energies compared to Planck mass.

Carl

 

[tehno]

Well I think a big accelerator we have to build around globe would answer many of the questions of that type one day.
But,still I don't expect tachyons to be detected by it.

 

[Demystifier]

And if one doesn't have even the slightest chance ( theoreticaly) to detect "tachyons" in our "subluminal world" what's the point in saying they could exists?

I do not see why an interaction between tachyons and subluminal particles could not exist.
Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

 

[dextercioby]

As of tachyons.Physically :Tachyons=rubbish.They arise from math ,not from physics.

(Theoretical) physics is simply math for the sake of description of nature, the nature we observe and the nature that someday we could observe. It relies on the fundamental concept of "model" and this "model" is supposed to make (hopefully someday) experimentally (dis)provable predictions. A model that makes no predictions is useless. One of the most important features of nature is symmetry or sometimes lack of symmetry. So far the breaking of the Poincare' symmetry within experimantally testable theories has not been encountered (yet) and therefore anyone claiming the Poincare' symmetry is broken is raising some eyebrows.

So if you're willing to admit on purely physical/experimental grounds that the Poincare' symmetry is an exact symmetry of Nature, then it's simply a result of physics (surely gotten by means of sophisticated maths) that both continuous spin and the existence of tachyons are plausible physical realities.

Daniel.

 

[tehno]

I do not see why an interaction between tachyons and subluminal particles could not exist.
Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

OK,let me put my question in this way:
What kind of experiment do you propose which could (in principle) detect the tachyons?

 

[CarlB]

Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

Tachyons could also be of extremely large energy and consequently we would be unable to detect them easily.

 

[tehno]

What math says (and crackpot unphysical theory agrees with) is the higher their velocity- less energy they have (and closer their velocity to c more energy they have).But that was not my question.

 

[vanesch]

I do not see why an interaction between tachyons and subluminal particles could not exist.
Such an interaction could be weak, which would explain why they have not been detected yet, but we would still have chances to detect them in the future.

In the past, you mean

I thought the main objection against interactions between tachyons and subluminal particles was that it is then possible to build a paradoxical machine: a machine that sends itself a message to send another message.

True, spacelike worldlines are geometrical entities. However, with the right combination of spacelike worldlines and timelike worldlines, we can make loops in a flat metric. The combination is exactly the interaction. Along such a loop, a contradiction can be sent.

 

[JesseM]

What math says (and crackpot unphysical theory agrees with) is the higher their velocity- less energy they have (and closer their velocity to c more energy they have).But that was not my question.

The entry on tachyons from the Wolfram science pages says:

It has been proposed that tachyons could be produced from high-energy particle collisions, and tachyon searches have been undertaken in cosmic rays. Cosmic rays hit the Earth's atmosphere with high energy (some of them with speed almost 99.99% of the speed of light) making several collisions with the molecules in the atmosphere. The particles made by this collision interact with the air, creating even more particles in a phenomenon known as a cosmic ray shower. In 1973, using a large collection of particle detectors, Philip Crough and Roger Clay identified a putative superluminal particle in an air shower, although this result has never been reproduced.

There's an abstract for a similar experiment http://www.iop.org/EJ/abstract/0305-4616/5/1/003. And http://physics.gmu.edu/~e-physics/bob/tachyons.htm by Robert Ehrlich of George Mason University has a little more on tachyon-detection experiments, especially in http://physics.gmu.edu/~e-physics/bob/h.htm . But I'm not sure how all this squares with the statement at the bottom of this page from John Baez's site that even if tachyons existed, quantum field theory indicates it would be impossible to use them to transmit information faster than light.