What happens to a seesaw extending to another galaxy if one end moves at 100 mph?

  • Level: Graduate 
  • Thread starter Thread starter PhysicsFan11
  • Start date Start date
  • Tags Tags
    Space
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
25 replies · 3K views
PhysicsFan11
Messages
35
Reaction score
1
Einstein stated that you could actually pose a question as an image or visualize the question in thought.
My question is: If you could have an imaginary See Saw that extended from an X point here on Earth to a Y point in another galaxy, and then assuming the See Saw here on Earth started moving up at let us say, 100 miles per hour straight up into the sky, would would be happening to the Y point at exactly the same time? I have always wondered about this question and hope that someone with a background in Physics could answer it.
 
Astronomy news on Phys.org

Discussion

The question should read "what would be happening to the Y point"
 
See-Saws have a stationary pivot point.
The position of the pivot needs to be known first before anything can be said about how Y will move.

Having established that much, there are then other mechanical issues to consider, but as a purely thought experiment, the pivot is the main issue.
 
So I would think, imagine for the sake of argument that the Pivot point was stationary..and also the velocity of point X as it moved up into the sky could be not just 100 miles per hour, but up to, let us say, even the speed of light, c.
 
PhysicsFan11 said:
Einstein stated that you could actually pose a question as an image or visualize the question in thought.
My question is: If you could have an imaginary See Saw that extended from an X point here on Earth to a Y point in another galaxy, and then assuming the See Saw here on Earth started moving up at let us say, 100 miles per hour straight up into the sky, would would be happening to the Y point at exactly the same time? I have always wondered about this question and hope that someone with a background in Physics could answer it.

At exactly the same time, nothing would change at Y because the effect of what you're doing at X would propagate through the solid bar at a speed no faster than light. This is because atoms are bonded to one another by the electromagnetic force (in one form or another), and changes in the electromagnetic field propagate at the speed of light.
 
jfizzix said:
At exactly the same time, nothing would change at Y because the effect of what you're doing at X would propagate through the solid bar at a speed no faster than light. This is because atoms are bonded to one another by the electromagnetic force (in one form or another), and changes in the electromagnetic field propagate at the speed of light.
No, it would propagate at the speed of SOUND in the material, which is not just "no faster than light", it is basically standing still compared to light speed.
 
  • Like
Likes   Reactions: Chestermiller
The other point I think is that, despite any width of the See Saw, given such as long distance involved, the mass of the See Saw would be near infinite. As such it would require a near infinite force F for point X to move up into the sky, for any given velocity.
 
PhysicsFan11 said:
The other point I think is that, despite any width of the See Saw, given such as long distance involved, the mass of the See Saw would be near infinite. As such it would require a near infinite force F for point X to move up into the sky, for any given velocity.
"near infinite" isn't really meaningful. No matter how big something is, you can multiply that by any number you can write down and the result is no closer to infinity than what you started with.
 
phinds said:
No, it would propagate at the speed of SOUND in the material, which is not just "no faster than light", it is basically standing still compared to light speed.

I could've said what I did more clearly, but changes in the electromagnetic field do in fact propagate at the speed of light. This is why the speed of sound can be no faster than that of light
 
jfizzix said:
I could've said what I did more clearly, but changes in the electromagnetic field do in fact propagate at the speed of light. This is why the speed of sound can be no faster than that of light
Sure, but the way you said it, and the way you are repeating it here, you make it sound like the propagation would be at the very least in the same order of magnitude as c whereas it is, as I pointed out, basically zero compared to c.
 
jfizzix said:
I could've said what I did more clearly, but changes in the electromagnetic field do in fact propagate at the speed of light. This is why the speed of sound can be no faster than that of light

Actually it is believed that in some exotic materials the speed of sound may be greater than that of light. I don't recall what it was, though. Neutron star cores?
 
Hornbein said:
Actually it is believed that in some exotic materials the speed of sound may be greater than that of light. I don't recall what it was, though. Neutron star cores?
Passing through a neutron star core would make a bit of a mess of the see saw though.
 
Hornbein said:
Actually it is believed that in some exotic materials the speed of sound may be greater than that of light. I don't recall what it was, though. Neutron star cores?
You need a citation for this since it sounds like complete unsupportable speculation.
 
phinds said:
Sure, but the way you said it, and the way you are repeating it here, you make it sound like the propagation would be at the very least in the same order of magnitude as c whereas it is, as I pointed out, basically zero compared to c.

Fair point, but as long as we're talking about see-saws spanning thousands of light years, we may as well make it out of ultra dense neutron degenerate matter. Interestingly, there was a paper while back saying the speed of sound in such an exotic materials (being the material of neutron stars), the sp
Hornbein said:
Actually it is believed that in some exotic materials the speed of sound may be greater than that of light. I don't recall what it was, though. Neutron star cores?
The speed of sound in a neutron star:

(see) http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.031103

at least theoretically has an upper bound below the speed of light, though it can be an appreciable fraction, thereof.
 
Yeah, I guess since we're already talking about something that is utterly ridiculous in all practical senses, taking it that extra step can't make it any more ridiculous. :smile:
 
Thank you for your excellent responses. If I can just clarify one point: assuming that the See Saw is made of wood or metal, then the speed of propagation from point X to point Y would occur at the speed of sound, correct? What if the See Saw were made only of mass less Electrodynamic particles-would the speed of propagation then be the speed of light?
 
PhysicsFan11 said:
Thank you for your excellent responses. If I can just clarify one point: assuming that the See Saw is made of wood or metal, then the speed of propagation from point X to point Y would occur at the speed of sound, correct? What if the See Saw were made only of mass less Electrodynamic particles-would the speed of propagation then be the speed of light?
A see saw is a material object. You cannot make a material object out of immaterial massless objects and you cannot manipulate then the way you can manipulate material objects (you CAN manipulate them, photons at least, but not in the same way).
 
Even staying with material objects there are limits to plausibility.
You won't have much luck with a see saw made out of gas.
 
phinds said:
You need a citation for this since it sounds like complete unsupportable speculation.
Title:
Possibility of the Speed of Sound Exceeding the Speed of Light in Ultradense Matter
Authors:
Bludman, S. A.; Ruderman, M. A.
Affiliation:
AA(Physics Department, Imperial College, London SW7, England), AB(Physics Department, Imperial College, London SW7, England)
Publication:
Physical Review, vol. 170, Issue 5, pp. 1176-1184

http://adsabs.harvard.edu/abs/1968PhRv..170.1176B
 
Hornbein said:
Title:
Possibility of the Speed of Sound Exceeding the Speed of Light in Ultradense Matter
Authors:
Bludman, S. A.; Ruderman, M. A.
Affiliation:
AA(Physics Department, Imperial College, London SW7, England), AB(Physics Department, Imperial College, London SW7, England)
Publication:
Physical Review, vol. 170, Issue 5, pp. 1176-1184

http://adsabs.harvard.edu/abs/1968PhRv..170.1176B
Gads! I can't understand a word of it. In fact it sounds like gobbledygook, but that could well be just my lack of knowledge.
 
phinds said:
Gads! I can't understand a word of it. In fact it sounds like gobbledygook, but that could well be just my lack of knowledge.
Me also,
I gave up hope with "retarded neutral vector fields"
 
Hornbein said:
Title:
Possibility of the Speed of Sound Exceeding the Speed of Light in Ultradense Matter
Authors:
Bludman, S. A.; Ruderman, M. A.
Affiliation:
AA(Physics Department, Imperial College, London SW7, England), AB(Physics Department, Imperial College, London SW7, England)
Publication:
Physical Review, vol. 170, Issue 5, pp. 1176-1184

http://adsabs.harvard.edu/abs/1968PhRv..170.1176B

The models described in this paper are entirely speculative. They involve violations of the energy conditions (that's what "pressure greater than energy density" means). Stuff that violates those conditions is called "exotic matter", and many physicists don't think it can actually exist.

Also, the paper was published in 1968. To know if any of the possibilities explored in it are actually viable, it would be helpful to see if any more recent research has referenced the paper.
 
Try watching this, for anything to happen, provided your stick does not come into a net gravitational attraction, or any other particles present in between. Nothing would happen. This would help,

 
PhysicsFan11 said:
Einstein stated that you could actually pose a question as an image or visualize the question in thought.
My question is: If you could have an imaginary See Saw that extended from an X point here on Earth to a Y point in another galaxy, and then assuming the See Saw here on Earth started moving up at let us say, 100 miles per hour straight up into the sky, would would be happening to the Y point at exactly the same time? I have always wondered about this question and hope that someone with a background in Physics could answer it.
I think that you are asking about a rotational version of this:
https://www.physicsforums.com/insights/can-i-send-a-signal-faster-than-light-by-pushing-a-rigid-rod/
 
So I did read the Link as noted above about [pushing a rigid rod-faster than light]. Assuming the F force was obtainable and the rod could be pushed on one end, the signal of that force/propagation would not arrive for many years afterwards (on the opposite end of the rod). Ok, does this imply that the rod is malleable/contractible and does that then reflect the contraction of mass as in the Lorentz contraction? Or is that a separate and unrelated concept?
 
PhysicsFan11 said:
es this imply that the rod is malleable/contractible

It implies that the rod is not perfectly rigid; applied forces do not act everywhere in the rod instantaneously. Whether the result of that will be the rod contracting, expanding, or something else depends on the particular forces applied and where on the rod they are applied.

PhysicsFan11 said:
does that then reflect the contraction of mass as in the Lorentz contraction?

No.

PhysicsFan11 said:
Or is that a separate and unrelated concept?

Yes.