Simple Setup Answers Complex Question On The Physics Of Solids

Speed of motion test setup

Thought experiments can be extremely powerful; after all, pretty much everything that [Einstein] came up with was based on thought experiments. But when a thought experiment turns into a real experiment, that’s when things can get really interesting, and where unexpected insights crop up.

Take [AlphaPhoenix]’s simple question: “Are solid objects really solid?” On the face of it, this seems like a silly and trivial question, but the thought experiment he presents reveals more. He posits that pushing on one end of a solid metal rod a meter or so in length will result in motion at the other end of the rod pretty much instantly. But what if we scale that rod up considerably — say, to one light-second in length. Is a displacement at one end of the rob instantly apparent at the other end? It’s a bit of a mind-boggler.

To answer the question, [AlphaPhoneix] set up a simple experiment with the aforementioned steel rod — the shorter one, of course. The test setup was pretty clever: a piezoelectric sensor at one end of the bar, and a hammer wired to a battery at the other end, to sense when the hammer made contact with the bar. Both sensors were connected to an oscilloscope to set up to capture the pulses and measure the time. It turned out that the test setup was quite a challenge to get right, and troubleshooting the rig took him down a rabbit hole that was just as interesting as answering the original question. We won’t spoil the ending, but suffice it to say we were pleased that our first instinct turned out to be correct, even if for the wrong reasons.

If you haven’t checked out [AlphaPhoenix] yet, you really should. With a doctorate in material science, he’s got an interesting outlook on things, like calculating pi using raindrops or keeping the “ultra” in ultra-high vacuum.

17 thoughts on “Simple Setup Answers Complex Question On The Physics Of Solids

  1. This physics is the basis of a high strain-rate testing device called the “Hopkinson bar”
    (Which might well be mentioned in the video, I haven’t had chance to watch it yet)
    At one point I looked in to building a Hopkinson bar for some research I was employed to do, and found myself consulting with a man working in a bunker in Derbyshire using explosives to launch his pulse in the bar.
    The same testing site had a vertical U-shaped pair of railway tracks and trucks arranged to not entirely miss each other for testing high energy impacts too.

  2. Solids are tightly packed atoms/molecules that are held together by atomic forces. The neat part is that which forces are keeping them as a solid can vary. Everyone knows atomically bonded solids but then there are weird solids that are held together by static friction alone.

    1. Really the idea of “are solid objects solid” boils down to how much energy you are applying to said object. Too much energy and it will undergo change, from deformation to explosion. It’s not wise to apply energy to solids that explode.

  3. The problem is that there’s a conflation between the mass effect of the entire unit (rod), and the sonic velocity of the material. You can carry this to ad absurdium; “what if the light-second long rod were traveling just below the speed of light and it was struck on one end – would the impulse now propigate faster than the speed of light?”

    1. I think there is a rule that allows you to travel up to the speed of light within your own space-time reference. If the impulse was in the rod and the rod was travelling just below the speed of light, it could in fact also travel at / near the speed of light within the rod. To the apparent external viewer’s frame of reference it would be faster than light speed but not locally. Same reason that we can see light from galaxies travelling away from us at near light speed. The reason is that space itself is expanding between us.

  4. This is good stuff. Nonetheless, the phrase “thought experiment” strikes me as an oxymoron. A lot like the phrase “adult children” which had me going a while back. Maybe “adult offspring” would be better.

    As long as it ends up a real experiment (and it did) we are on safe ground. Note that this whole business of establishing whether two events at different locations are simultaneous is what led to the theory of relativity.

    And there is a great quote, supposedly by Einstein: “in theory, theory and practice are the same. In practice they are not”.

    This difference between thought experiments and real experiments is what differentiates philosophy and science.

    1. I don’t agree. To me a thought experiment may be able to be proven mathematically or based on other assumed rules of physics. Relativity was an example of this until it was experimentally confirmed years later. Same with the Higgs Boson which was a part of the standard model way before it was confirmed to exist experimentally.

      Theoretical science is not any less science than experimental. One kind of requires the other to make real progress.

      Philosophy is almost entirely a set of values that are a matter of opinion in which absolute truth is not a goal. You and I can have completely different philosophies without one of us being “wrong”. We cannot have two mutually exclusive scientific theories without one of us being wrong.

  5. It wouldn’t appear faster than light speed to an external viewer. It’ll appear as if it’s traveling at the speed of light in the outside observers reference frame so it will look like it’s travelling down the pole at lightspeed – pole speed. The reason it also looks like it’s travelling at the speed of light to someone in the poles frame of reference is because time is moving slower for them (with reference to the outside observer).

  6. This phenomenon is an everyday challenge for the control of ‘roped elevators’, where it can take a large fraction of a second for the tension exerted at the sheave, by the motor at the top of the hoistway, to reach the cab and counterweight. As the time is dependent on where in the hoistway the two cars are, the control system has to actively adjust control parameters to account for this.

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