Does Pushing Objects Relativistically Create Pressure Waves?

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Discussion Overview

The discussion revolves around the concept of whether pushing an object at relativistic speeds generates pressure waves within that object, specifically focusing on the implications of such waves in solids and the resulting motion of different parts of the object. The scope includes theoretical considerations and conceptual clarifications related to relativistic effects and wave propagation in materials.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the impulse from a push travels through an object as a longitudinal pressure wave at the speed of sound, suggesting that this could lead to one end of the object moving before the other.
  • Another participant challenges the initial discomfort with the idea of pressure waves in solids, asking for clarification on the reasoning behind it.
  • A different viewpoint supports the notion of pressure waves by referencing the behavior of a steel rod when struck, indicating that a shock wave travels through the rod, causing the far end to move later than the point of impact.
  • Some participants assert that solids are not fundamentally different in terms of compressibility and that they behave elastically under small displacements.
  • One participant expresses discomfort with the idea of one end of an object moving before the other, comparing it to concepts from quantum mechanics and Zeno's paradoxes.
  • Another participant argues that if both ends of an object moved simultaneously, it could imply the possibility of faster-than-light signaling, which is not permitted in current physics.
  • A further perspective questions the implications of sound propagation in solids and the mechanics of earthquakes, suggesting that understanding pressure waves can clarify these phenomena.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the implications of pressure waves in solids. While some support the idea that pressure waves exist and can lead to differential motion, others remain uncomfortable with the conclusions drawn from this concept, indicating that the discussion is unresolved.

Contextual Notes

Participants highlight limitations in understanding the implications of pressure waves, including assumptions about the behavior of solids under relativistic conditions and the potential for misinterpretation of simultaneous motion.

Jonathan
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When you push on an object and accelerate it to some relativistically (so we can really, really ignore lorentz contraction) small velocity (say a nice 1 m/s), does the impluse from your push travel through that object as a longitudinal pressure wave at the speed of sound (what ever that may be for that object)? It seems like that should be true, but that also implies that the object should contract slightly and that one end of the object would attain the 1 m/s velocity before the other end does, (assuming that the object is long enough that one can reach this velocity before the pressure wave meets the other end) and for some reason I don't like that conclusion, even though it makes sense.
 
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Perhaps your post would make sense if you explained WHY you don't like the idea of pressure waves in solids.
 
Learn to like that conclusion. This falls in line with something I just posted in the thread entitled 'energy absorption'. Consider a steel rod that is about 5 to 10 feet long. Hit it with a hammer from one end. The 'shock wave' will arrive at the other end a short time later. The 'shock wave' is basically the far end being moved by the hammer only at a later time. So yes, there is a speed of push so to speak.
 
There isn't anything about solids that special and makes them not compressible. Metals are actually very close to perfectly elastic for small displacements.

Your conclusion is correct.
 
That's what I thought; HallsofIvy: What I don't like is that one end will move before the other. It feels to me like macroscopic quantum superpostion, ie two places, one time, one object. (I knows that that's unrelated, it just feels as weird as that would.) It's like one of Zeno's paradoxes or something.
 
Originally posted by Jonathan
That's what I thought; HallsofIvy: What I don't like is that one end will move before the other. It feels to me like macroscopic quantum superpostion, ie two places, one time, one object. (I knows that that's unrelated, it just feels as weird as that would.) It's like one of Zeno's paradoxes or something.

Read my post in 'energy absorption'. If both ends moved at the same time then an object could be used for faster than light signaling. It's not permitted.
 
Another way to consider your objection, what would happen to sound in a solid if there weren't pressure waves moving around in there? Heck, how would an earthquake work (would the whole Earth shake?)?

Some things in science don't make sense when you look at them from one direction, but if you look at them from another direction, they make a lot more sense.
 

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