Speed of Light problem?

Question

This riddle popped in my head one night and cant figure it out. Lets say you have a secure standing stick that is one light year tall and you cut the very bottom of it. Obviously the bottom will fall to the ground imediatley, but will it take one year for the top to start falling since information travels at the speed of light. If that indeed happens, then is the stick actually getting tallerer when the bottom drops to the ground. Where is this extra mass coming from. If this is so, then any standing and secure object that is cut at the bottom acually increases in height (though immersurable) while following. If u have objects that are millions of light years tall than the object will increase in height dramatically? Please help me with this

Answer 1

what's height gotta do with the top falling a year later,there's no change in height whatsoever

Answer 2

In my opinion tgypoi is on the right track.
As there is no such thing as a completely rigid body, the stick, before being cut, should be compressed in the part where earth's gravity is still influent (say the first 1000km) and so its overall length sound be slightly shorter than the same stick in space. When the stick is cut ALL of it will start falling (but not instanteneously) because the earth's pull is "felt" by the part of the stick which is in the earth's gravitational field and this pull is transmitted along the stick at the speed of light (depends on the material it is made of). So you might think the stick is longer (because the increasingly distant parts fall "after") but it's just that you don't know that the far away parts have started to fall before that information reaches you! The problem here is relating what happens here (where I cut the stick) and now (the instant the stick is cut) to what happens there (the other terminal part of the stick) and then. Separated events in space have to be judged simultaneous or not with care. As usual one thing is to measure things (1 light yr stick), another thing is looking at things (1 light yr stick) and still another is to calculate things (1 light yr stick)!!

Answer 3

Nice thought. I think it will get longer, but not for any relativistic reasons. The stick was under compression which caused the molecules to fit closer together than they would if the stick were say in free fall. After the cut, the expansion wave will start at the bottom and propagate upwards at the speed of sound thru the stick material.

FYI, one of the early clues astrophysicists had that strange things could happen to stars was a calc indicating that beyond a certain density, the speed of sound thru the collapsing material would exceed the speed of light. Knew that was a no no.


Edit: Promethius, how can you have a physics degree and ever use the word instant?

For the stick to not change length as you suggest, it would have to be perfectly rigid to have avoided contraction under the compression forces. No such thing as perfectly rigid. The speed of sound thru it would exceed the speed of light. Not allowed. NO information can propogate faster than c.

Taking the case of a constant gravitational field, the stick is in static equilibrium with the downwards force of gravity from the overlying stick being felt by each atom balanced by the upwards force on the bottom end (sans the weight of the underlying stick) conveyed upwards. When that bottom force is removed, the effect is not and absolutely cannot be instantly felt at the top, but only felt by each atom when the atoms below begin to move downwards, but they first have to wait for the atoms below them, etc. This wave of expansion will travel upwards from the bottom end at the rate a compression/expansion wave travels in the stick, ie the speed of sound.

Answer 4

Since when does information travel with the speed of light?
And falling to the ground is a question of gravity, so where on Earth are you going to have this huge object. You can't have anything that huge in the Earth's gravity, so it has to be in space, in freefall, so it won't fall to any ground at all. Get some sleep. What are you smoking?

Answer 5

sorry you missed a quite important piece of the problem the stick would not fall except upwards once it had been disconnected from its base sorry check the problem more thoroughly next time also if the object was to fall to earth it would do so with the acceleration of 32 fps sq and the stick would take longer for response figuring a length of approx6 trillion miles your stick should be moving at nearly .6 of light this is a rough guess not a firm figure ask NASA they might enjoy working the figures and who knows something could come of the idea !

Answer 6

Holy craptacular answers.

The stick does not change lengths at all. "information" does not propegate at the speed of light: interactions of forces are immediate. So let's play with your example:

A flat plane above which is a universally constant gravitational field. perpendicular to the plane is a rigid pole 1 light year in length. We cut one foot off the bottom of the pole. All the particles at the bottom begin free-fall. Gravity AT THE SAME TIME pulls on the particles above those free falling... Since the previously supporting particles are now in free fall, they offer no resistance to the gravitational force, ergo all particles in the stick begin to fall at the same rate and at the same time.

In the case of the non uniform gravitational field (assuming gravity was still enough to pull the entire pole down) the electromagnetic force between particles would instantly pull the other particles down, preventing dilation.

Answer 7

It's an interesting question, but not about the mass. There is no reason for the mass to change - if it stretches (as I believe it would) the mass would remain the same - you might think of the molecules as getting a little farther apart.

I would think the stresses would likely break the stick elsewhere, but if they don't then it seems as if, as you said, it would need to stretch, since the information cannot progogate faster than the speed of light.

Answer 8

Wrong way around, methinks. If you were 2 light-minutes away from the Bank's clock and your watch was still 2 minutes fast then your watch would show 10:04 when you saw the Bank's clock flick to 10:00

Answer 9

That would look so cool!
The tower doesn't actually get taller, it just looks taller to you. When we look through telescopes at stars that are millions of lightyears away, many of them have actually exploded since then, but we won't see that for a few more million years. The fact that we can still see the star doesn't mean that it's still there.

Think of it this way - your friend breaks his arm, then sends you an email about it. Until you read the email, you don't know that his arm is broken, but he does! Same principle. &)

Answer 10

Read about length contraction and time dilation, Einsteins relativity theory, the hieigh will not increase, rather the distance the stick falls, using the acceleration you can determine how long it will take.

Answer 11

Hey you cant think of space like that man. Space is relative, that means if you drop the pole from here, it will drop from the other end at same time, however the timeframes are relative. so do not oversimplify, theory of relativity is not a very simple subject.