# Physics question

#### Maj

##### Emperor
This thought has been gnawing away at me for the past couple of years and I would really like for someone to explain this to me:

Suppose you had a steel rod, about 1 cm in diameter and 1 light-minute in length. On one side of the rod is an astronaut ready to pull the rod towards him: Side A. On the other side (1 light-minute away) is nothing but, well, the other end of the rod: Side B. The relative time at both ends of the rod is 0 (seconds). Were the astronaut to pull the rod one metre towards him at 0 , would the end of the rod at Side B not move until 60 seconds had passed or would it move at the instant the astronaut pulled it? If the latter is true, would that mean "information"(for lack of a better word) travels faster than the speed of light?

I'm certain there's some obvious or obscure law(s) of physics I haven't accounted for that would expalin this all. The only explanation I've been able to conjure up is that the atoms that compose the rod would undergo a freigh-train effect where there would be an extremely tiny lapse in time between the bonds of each atom a bit like when a freight train engine starts moving, the cars further back don't begin to move until the engine has travelled a short distance. But I'm not sure if the attachment of freight cars can be compared to the bonds between atoms.

- Maj

I know almost nothing about physics, but maybe I can help

Have you considered that a light- minute has nothing to do with light, it is how far light can go in a minute. So, I believe that the rod would move the instant the Astonaut pulled it.

I am probably very wrong. Actually, this could not be proved at all. I think I should have been a Physisist. Sound svery interesitng.

I'd also say that it would move at the instant.

But there's another question quite similar to this:
Imagine a spaceship that can travel FASTER than lightspeed (that fast that it can pass the distance of a lightyear in less than a year). Now that ship is passing at some planet, star-system or just some point in space a lightyear away. Now imagine someone on Earth is looking through a telescope at that place (therefore he is watching the place's situation a year ago) almost a yeah after that. A day after that ship was passing there it started on a flight to Earth at it's faster-than-light speed. Would it arrive before he could see it?

Think about it, the ship would be faster than it's image (the light that it reflects).

Now imagine it's a huge asteroid and say a prayer

God I love that question, Maj.

I have no freakin' idea, but its one of those mindbenders to think about. My gut instinct would be to say that the movement would be instantaneous....but I'd bet that Einstein probably would have something to say about that.

Maybe when the astronaut pulls on the rod it actually does take times for the atoms within to react causing a delay....kind of like the moving hump in a long rope or hose when you stand and move your hand up and then down real fast where the hump moves from you hand toward the end of the rope......just like you were saying Maj. Einstein would have a reason why because supposedly the absolute speed limit is the spead of light.

Sheesh.....

Seems like what I always think would make sense turns out to be false.

But I think I can say confidently that the answer to Hitro's question is that you would see yourself at the the other star system. When you look at a star that's 4,000 lightyears away, you're seeing that star as it was 4,000 years ago, period.

So if the point is 50 lightyears away and you were there 50 years ago, you'd be seeing yourself.

What's cool about that (and makes me fear that maybe i'm wrong here ) is that you would also see yourself leave and, i guess, eventually catch up to you.....

I think it will need a while to start moving. Think about it as a bunch of particles - when you pull one he quickly pulls the next. Quickly, but it still takes a very small amount of time.

Anyway what is most likely to happen is that the Austronaut will be pulled away. An item as large as the rope you described has a very large mass so moving it will require a strong base and a strong force.

Imagine a spaceship that can travel FASTER than lightspeed (that fast that it can pass the distance of a lightyear in less than a year). Now that ship is passing at some planet, star-system or just some point in space a lightyear away. Now imagine someone on Earth is looking through a telescope at that place (therefore he is watching the place's situation a year ago) almost a yeah after that. A day after that ship was passing there it started on a flight to Earth at it's faster-than-light speed. Would it arrive before he could see it?

Well, time slows down as you approach the speed of light. So if the FTL ship traveled to that point at a faster-than-light speed. If that happened, you could see the ship at the the star BEFORE it even left Earth, since it would have travelled back in time.

Suppose you had a steel rod, about 1 cm in diameter and 1 light-minute in length. On one side of the rod is an astronaut ready to pull the rod towards him: Side A. On the other side (1 light-minute away) is nothing but, well, the other end of the rod: Side B. The relative time at both ends of the rod is 0 (seconds). Were the astronaut to pull the rod one metre towards him at 0 , would the end of the rod at Side B not move until 60 seconds had passed or would it move at the instant the astronaut pulled it? If the latter is true, would that mean "information"(for lack of a better word) travels faster than the speed of light?

*thinks back to lessons*

IIRC, the rod would move, but you would not see the astronaut pulling the rod for 60 seconds.

As long as the rod was a typical maths rod, which is inextensible then it should move straight away.

Originally posted by Sixchan
Well, time slows down as you approach the speed of light. So if the FTL ship traveled to that point at a faster-than-light speed. If that happened, you could see the ship at the the star BEFORE it even left Earth, since it would have travelled back in time.

As far as I know the theory of time slowing down is not proven. Please correct me if I'm wrong. But what I mean is that is starts its FTL engine AFTER passing that point. Therefore it would arrive before it's image would, don't you agree with that? It would "overtake" its own images from the points in space behind it.

This is a classic question from relativistic physics. The thought experiment of using a long, rigid rod for FTL communications was first explored by Paul Dirac, and thus you may hear such a method of communication referred to as a "Dirac Rod".

The short answer: it won't work.

When you are dealing with the real world, the concept of a "perfectly rigid, solid rod" is a fiction. Solid objects are made of atoms, and held together by a matrix of electron shells.

When you pull (or push) on a solid object, you are imparting kinetic energy to the nearest atoms, which transfer it to the atoms adjacent to them, and so forth. On a human scale, this happens so fast that the whole object appears to move as a single rigid body. But what is really taking place is that a "wave front" of atoms is moving, and pushing or pulling the adjacent atoms along with them. This compression/rarefaction wave cannot travel faster than the speed of light.

Bottom line: you can't make a "perfectly rigid rod" out of real world materials. If you push it, it will compress. If you pull it, it will stretch. The change of position will be transmitted no faster than the speed of light.

(For further reference, take a year or two of physics and chemistry courses at a good technical college.)

I'd imagine that real-world materials would actually transmit significantly slower than the speed of light. A light-minute is a really long rod, so there's plenty of chance for it to slow down.

Even from a relativistic point of view the rod as a whole will not move instantaniously, even if it is the mathematical infinite stiff one. It all comes down to the basic 4 forces (strong, weak, electromagnetic, gravity) where all other forces are derived from. All 4 have to deal with the speed of light to get their forces working over distance.

So it will always take at least 60 seconds before the other end of the rod starts moving!

of course it moves.
But the dude doesn't see it until 60 seconds later.

No, it fundamentally would not move instantaneously. The velocity of movement of the kinetic energy through the medium would be directly related to the magnitude of the force applied, and also to the nature of the material it is made from.

Additionally, the mass of the rod would be such that an incredibly large force would be needed to move it. It would also generate large gravity-fields which would influence the space-time around it.

Such an article cannot be considered to be 'an object' as such. If it was, hypothetically, infinitely rigid and infinitely strong, and sufficient force could be mobilised to cause it to move, then there are going to be all sorts of permeations of the space-time continuum caused by such a convergence of material rigidity and infinite energy. Einstein's theory of relativity would simply have the thing bending in on itself, such that its dimensions change to restore the properties of space-time.

The point is, it HAS to compress or stretch, even if the material itself does not do so, because space-time will compress or stretch it anyway. Thus, there is no capacity for it to move instantaneously unless it is travelling infinitely fast, in which case it does not actually remain at one-light-minute's length, but will compress into an infinitely small and dense particle whose mass prevents it from accelerating beyond the speed of light.

Every motion of every thing - a coin say, is not instantaneous either, but appears that way due to our limited perception.

"There is [absolutely] no simultaneity of events".

Interesting question though. Have you ever considered studying physics?

If a spcaeship travels at a speed faster then the speed of light it's mass will have to be smaller then nothing -> There's no way you can see it.

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