# Help with a thought experiment?

I stumbled into a thought experiment, consisting of:

-Two guys
-One flashlight
-One 200,000-mile-long rod.

One guy holds the flashlight in one hand, a rod end in the other; 200,000 miles away, the other guy holds the other rod end.

Guy with the flashlight simultaneously flashes the flashlight and jerks the rod forward an arm’s length.

Speed of light=186,000 miles/sec, so the other-end guy sees the flashlight flash ~ 1-second later.

But he feels the rod jerk immediately, correct? [If not, that’ll be my next Q.]

So what’s the reason this rod-jerk that says “light will arrive in one second” is not a signal sent faster than the speed of light?

Is it because, despite the 200,000-mile distance between the two guys, the shared rod establishes some “continuousness,” so in a sense, the two guys aren’t “separate,” and are in the same “object space?” So this object [rod] doesn’t travel the distance the light does? How’s this work?

No, the rod is not infinitely rigid - the movement imparted to it will travel along the length of the rod at the speed of sound in that material.

Yep; the light will arrive in about a second, but the jerking motion will take days to to be transmitted along the whole rod, if it gets there at all; I’m no materials engineer, but I half-suspect that with the amount of force needed to push a 200,000-mile rod, it would probably just compress or explode or something pretty near your end, and the motion would be dampened to nothing by the time you reach the other end.

An infinitely rigid material is impossible precisely because you can’t accelerate matter to (or past) the speed of light. Your infinitely rigid rod (I dare you to try typing that with a straight face) would by its very nature violate causality.

To put some concrete numbers on it, the speed of sound in steel is about 6000 m/s, or 1/50,000 the speed of light, and so it’d take 15 hours for a push or pull to reach the other end of your 200,000 mile rod.

No, LynnM, he cannot feel the rod jerk immediately. The problem is that nothing can accelerate faster than the speed of light. All material properties of the rod and other practical considerations regarding the rod are irrelevant for this simple discussion, including mass and the force needed to move the rod.

The rod cannot be moved at a speed faster than light.

The physicists should be along shortly to elaborate.

I asked a similar questiona while back.

Yeah, this question gets asked every so often. It’s a good idea to keep a link to Peter Morris’s thread handy to post it for the next person.

Never mind the speed of sound. What is more important, though harder to picture, is that - in your thought experiment if it worked as you hoped - somebody shooting past very fast in a space ship, headed in one particular direction, watching the whole thing and allowing for how long it takes him to receive the light from the various things he watches, would detect that the guy on the receiving end detected the jerk almost a second before the guy on the transmitting end sent it. Note - the watcher in the space ship doesn’t just see that the jerk appears to get there before it was sent, he sees that it REALLY DID get there before it was sent.

The rod will stretch.

I had a job a while back that involved doing downhole calculations for oil wells. I was totally astounded to find that the rod will flex so much that it looks like a slinky in action.

Taken to extremes, this is what makes an implosion-type Plutonium atomic bomb work. The Plutonium core is surrounded by high explosives. When they detonate, it is squeezed to around 1/3 it’s original size, and it becomes supercritical. If the metal was strong enough not to compress, this wouldn’t work.

Although the plutonium is being compressed (as is anything subjected to a pressure), its density doesn’t change a whole lot. The bulk modulus of solid materials is extremely high; even assuming a detonation wave pressure of around 1 MPSI, the density of the core material wouldn’t change by more than a few percent.

My understanding is that the core is hollow; the detonation compresses the large, hollow plutonium sphere that is subcritical into a small, (mostly) solid sphere that is supercritical. IOW, the shift to supercriticality doesn’t depend on the compression of the plutonium so much as the change in shape.

First, if the two guys are 200,00 miles apart, there is no way that the second guy will see the light of the flashlight.

Secondly, a 200,000 miles long rod, no matter how thin it is, will be too heavy to be moved, in addition, it will have to go over mountains, rivers and/or buildings and whatever else is in the way, so even if some motion could be induced at one end, no motion would be detected at the other end.

And this has nothing to do with the speed of light.

J-P L, I don’t think you grok the concept of “thought experiment”.

Could you expand on this? What kind of rod are you referring to?

Depends on how bright and well-collimated the flashlight is. For decades now they have been shining lasers at retroreflectors on the moon and detecting the returned light. The return signal is very faint, but detectable with instruments.

Who said anything about wrapping it around the earth, or even having any part of the rod anywhere near the earth? You just need about 200,000 miles of empty space; there are plenty of regions in the known universe that will meet this requirement.

Obviously we still lack the technology/resources to carry out the rod experiment, but as Blake has pointed out, this is completely irrelevant. I’d suggest reading up on the concept of a thought experiment. The important thing is this:

Nope.

From here.

The pit is hollow, but that’s so the “spark plug” (Neutron generator) can be housed in the center.

This was the original thought, but it turns out that symmetrically compressing a hollow sphere is really freaking hard, while compressing a solid sphere (allowing for the neutron generator in the center) is a bit easier.

Two guys, one flashlight, huh?

Another fun thought experiment you might enjoy. As noted, these come up here often.

I confess, I know nothing of the concept of “thought experiment”.

I read some of the Wiki page referred to by Joe Frickin Friday above and still do not understand much of it or its purpose.

I feel I’m very much planted in reality and always get a chuckle from the “what if” type of questions. My reaction is “Who Cares”

Carry On!

The people who care are pretty much limited to those with imaginations.