How long would it take? (Physics question, sort of. Maybe) (page 3)

DanTheMan

I think you pretty much said a lot of the same as what I did Darth.

The rod exists in physical space at the begining and end of the light year. Therefore when you push the rod at the begining of the light year the ending of the rod will be moving simultaniously although it will be a light year later. The act of moving an inch will not take an entire light year. There, will however be a light year between the recorded time of the movement between the two ends, plus the time of the whole atomic shockwave affect (if measurable).

"Simultaneity" is a tricky word when you're speaking of (relatively) vast distances such as one light year. The problem is that there is no objective, neutral point in space whereby one could say "Aha! Event A and event B happened at exactly the same time!" The concept of simultaneity is dependant on the location of the observer of the events.

In the case of the a perfectly stiff (pretend that there's no squishy atoms in this hypothetical rod to delay the transmission of the impulse) light-year-long rod, the only place in space where both ends would appear to be moving simultaneously is at the opposite end of the rod from where the impulse started off. Which, by the time you saw through your telescope a giant intergalactic AlmostAtheist pushing the rod, the light waves reaching you showing him doing this would have have been travelling for a year.

LittleToe

You're making an assumption that nothing can excede the speed of light.

What speed do quantum particles move at?

AuldSoul

I don't mean to be a monkeywrench in the works, but I think you are all making a lot of assumptions. Simultaneity is non-existent in reality. Movement, distance, time; they are each relative to something else. There is no constant by which to measure any of these, but only relative to something else which is itself inconstant.

If we assume (1) total vaccuum, (2) zero gravity except that exerted by the rod itself—which in total vaccuum would be concentrated at the center and through the core of the rod, (3) assume there is a mechanism at either end for tracking movement and marking time of movement which has no gravity—which cannot exist (but for sake of discussion), (4) assume there is a propelling force—which has no gravity—at one end capable of moving the rod one inch toward its other end, the movement would provably occur simultaneously.

In your previous explanations (except Darth Yhwh), there seems be an assumption that matter has to move inertially by atoms bumping into each other down the line until they all have moved to the new location. SteveNYC, your analogy would only apply if compression were occuring, in this case there is no resistance to movement (total vaccuum). However, even in real life that is not what actually occurs. A baseball doesn't bump its composite atoms against one another when hit with a bat until they have all flown over the wall together.

Rethink the puzzle in terms of a baseball (equal and opposite reaction), and especially think about the total vaccuum stipulation. Zero resistance to movement.

Of course, the problem presented has one glaring difficulty to overcome. Any propulsion in the construct provided would not possibly cause a move of only an inch. In total vaccuum you could not propel any object one inch. It will never stop moving once propelled until some resisting force causes it to stop.

AuldSoul

Terry

9,460,000,000,000 km

That's the distance we are talking about. How much MASS do you think that would amount to? How much inertia? Overcoming the stasis (resting inertia) of such mass is your real problem. You don't give the width, depth or other dimensions of this "object". T.

Simon

You think you have pushed it forward an inch but I reckon you just pushed yourself back an inch.

The problem with things like this is how do you measure it? How do you indicate to your friend when you have pushed the rod and they indicate back that it has moved?

kazar

Thanks for the thread and all the intelligent and funny answers! They really lifted my mood, making me laugh out loud. I have been absorbed by personal problems that have kept me away from the board for the last two weeks. This forum brightens my day.

AuldSoul

Mass in "stasis" while in total vaccuum has no resitive force in and of itself. "...Body at rest tends to stay at rest..." only applies when you have less than total vaccuum and where other mass exerts gravitational force to maintain a seeming constancy of current state. Introdution of propulsive force into a total vaccuum will alter position of matter in direction exactly opposite the propulsive force but nothing will stop the now moving object except an opposite force. Even striking another object would not stop the movement, it would only (possibly) alter its direction.

Inertia into the equation as a factor when force is applied to one end. There was zero inertia, resting or otherwise, prior to application of force to one end. Whatever inertia was generated by the initial application of force becomes the total inertia in the closed hypothetical system. Without any resistive force that initial inertia is purely applied to the entire rod simultaneously.

Of course, in hypothetical terms, I question whether matter as we know it could possibly exist in total vaccuum. I enjoyed reading Asimov's fictional tales involving total vaccuum. He hypothesized that billiards can be deadly when played across a field of total vaccuum, that any moving matter introduced into total vaccuum would immediately accellerate to the speed of light.

AuldSoul

rmt1

DanTheMan, does that book have any sort of explanation to back up the assertion? Is it saying that space-time itself cannot accomodate the transmission of an impulse, however inflexible/incompressible the medium, that moves FTL?

skinnyboy

12 Parsecs, if the rod was doing the Kessel Run IMHO!

Wouldn't the rod ends move exactly the same distance simultaneously, although to track the movement physically from one end to the other would take one light year, assuming that theres nothing faster than light. But if you put the recording device in the middle of the beam, it would take 0.5 of a light year to record the movement if you relay the push movement signal from the centre and receive and send from both ends at the same time?

FreedomFrog

(oops, I'm logged in as the soon-to-be-tattooed Gina. Oh well... -- Dave)

Interesting, interesting...

I don't think we have to worry about how would I and the other end know when such and such happened, since we'd both have digital watches synchronized and we'd log when events occurred. And of course, we'd be careful to walk quietly along the length of the object so no faster-than-light-travel-screws-up-time stuff. When the whole experiment was over, we'd jet off to a bar somewhere to compare notes and probably watch the game over beers.

Terry: As for the mass of the object, yeah, that's a problem. We'd have to manufacture it in space, since there's neither enough lift available to get it off the ground, nor enough "stuff" on the earth from which to build it. And the dimensions? I thought about using 2x4's, but they tend to flex when they get very long. We'd probably need to use 4X4's. (Treated lumber, of course) (I had in mind one of those bucky-tube things, but at the length we're talking about it's still one big bunch of carbon!)

AuldSoul: Seriously though, the "atoms would have to bump into each other to cause the motion" thing I think I have to assume is correct. Your baseball analogy is looking at things from the macro level, but even at that level the ball does compress. (Look at a high speed shot of a ball contacting a bat) I also wouldn't immediately agree that an object at rest has "zero inertia". If that were true, it wouldn't take any effort to get something big moving. Just because it isn't moving doesn't mean inertia isn't a factor, nor -- I think -- would a vaccuum have any effect on it. It isn't air that makes a car hard to get going from a stop, it's the inertia. Even in a totally frictionless environment, a body at rest wants to stay at rest.

Simon: The rocket engine was meant to overcome the "you won't push the rod forward, you'll push yourself backward" problem. The rocket would be attached to the rod, so it would push the rod.

Someone mentioned that you couldn't just push it an inch. I think that's true, you'd either get it moving, or you wouldn't. I suppose we could imagine a second rocket engine mounted in reverse (in my right buttock, no doubt) that would serve to counter the forward motion. But that needlessly clutters up an already impossible experiment.

If I am getting it at all, the point seems to be that the atoms of the rod would not actually be touching, but have some gap between them defined by forces that both hold them in proximity to each other and hold them apart. When atoms on the north end of the rod are shoved south, they attempt to close that gap. (Are we really talking about atoms, or is it molecules?) The forces won't allow that, so they push their atoms away, which causes a chain reaction resulting in the wave that Elsewhere talked about. It's the speed of that wave that we're trying to nail down. According to what DanTheMan said, that wave would travel at the speed of light. (If you were light, wouldn't you get sick of everybody copying your speed? "Get your own speed, man..")

Tell you what, why don't we just settle the question. Let's just do this thing and see what happens. I'm willing to do my part. I'll provide the digital watches, the beers, and the notepads for recording when stuff occurs. Anybody got a 9-trillion kilometer 4x4 in their garage? (Two 4.5 trillion km one's would work, if we can get one of those braces for the middle.)

Let me know if anybody wants to help get this off the ground.

Thanks, All!

Dave

How long would it take? (Physics question, sort of. Maybe)

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