Physics Question - Speed of Light

Maximus Alexander

Hi guys,

I have no formal physics background, I just tend to do a lot of reading on the subject in my spare time, mainly in relation to astronomy and cosmology.

There has been one question in the back of my mind for a while that I haven't quite been able to find an answer for. It probably sounds a bit silly actually, and the answer might be quite simple, but I'll put it out there anyway.

If information cannot be transmitted faster than the speed of light, does that mean that different parts of a rigid object respond to movement at different times?

For example, let's imagine I have meter long ruler on my desk and I push one end of it. The other end appears to move instantly, but is there actually a speed of light delay here, perhaps due to the elasticity of the material, or something else I haven't thought of?

I assume that if I could somehow create a ruler 1 light year long, I could not push and pull on one end of it to send faster than light "morse code" messages. It must take a year or more for the other end to respond to the movement, but I'm not sure how or why.

LeighH

LeighH wrote: »

................

I assume that if I could somehow create a ruler 1 light year long, I could not push and pull on one end of it to send faster than light "morse code" messages. It must take a year or more for the other end to respond to the movement, but I'm not sure how or why.

Imagine the energy required to move that ruler

clearz

Actually the fastest you can transmit a push through an object it at the speed of sound in that object. Think about it, if you shout at the end of a ruler the sound wave from your voice would hit the end then continue through the material as a wave until it reaches the other end. It doesnt matter if it's a sound wave hitting it or your hand. We don't notice this wave in small things like a ruler because of their relative size compared to the speed of sound but if you had a hypothetical ruler a light year long and had the means to push it, what you would see is it to bend by the amount you push it and then this bend would shoot along it at the speed of sound.

sink

From the perspective of an observer at either end of the ruler it would appear straight but time would pass at a different rate for each observer. Rather than the ruler distorting it's relative position in space (bend) it would distort it's relative potions in time.

TBH I don't know what i'm talking about.

im invisible

clearz wrote: »

Actually the fastest you can transmit a push through an object it at the speed of sound in that object.

That cant be right?

krd

RoverJames wrote: »

Imagine the energy required to move that ruler

What if you had a very big hand?

krd

krd wrote: »

What if you had a very big hand?

Then you'd need very big gloves if it got cold.

clearz

im invisible wrote: »

clearz wrote: »

Actually the fastest you can transmit a push through an object it at the speed of sound in that object.

That cant be right?

Check my revised answer. I'm typing on an iPad on a bus so I'm a bit slow and my grammar sucks.

tricky D

There's a major issue with using the term rigid (or solid) which is misleading. The ruler is not rigid as like just about everything else it has elastic properties so it's more like a stiff spring. For ease of analysis, at our regular scale, it's fine to call a ruler rigid as the elasticity is for intents and purposes negligible, so it appears that both ends move instantly, when actually they don't. However, at a molecular scale it's different.

Gurgle

I'm likely to be corrected by the real experts, but I think what you're looking for is Lorentz contraction.
Ladder Paradox

seamus

As tricky D says, the problem is thinking about a solid object as rigid. In the theoretical example of pushing one end of a ruler which is 1 light year long, it's more appropriate to think about the energy transfer moving as a wave along the ruler.

I'm sure there's a way to calculate this. I'm not sure if the "speed of sound" is correct, because it's not a sound wave.

Conceptually I think seems as a ruler a light year long was suggested surely we are to presume it remains rigid much like a foot long ruler appears to

seamus

RoverJames wrote: »

Conceptually I think seems as a ruler a light year long was suggested surely we are to presume it remains rigid much like a foot long ruler appears to

Ah, but we can't, because there's no matter in the universe which would display such a level of compactness and be 1 light year across.

Visually the ruler would remain rigid, though the energy would still transfer in a "wave" action.

But you can conceptually imagine a ruler a light year long ............ :pac:

Maximus Alexander

Thanks for all the responses guys. Special thanks go to:

Gurgle wrote: »

I'm likely to be corrected by the real experts, but I think what you're looking for is Lorentz contraction.
Ladder Paradox

While the article doesn't specifically answer this question, it does contain the following two paragraphs:

Wikipedia wrote:

The difficulty arises mostly from the assumption that the ladder is rigid (i.e. maintains the same shape). Ladders seem pretty rigid in everyday life. But being rigid requires that it can transfer force at infinite speed (i.e. when you push one end the other end must react immediately, otherwise the ladder will deform). This contradicts special relativity, which states that information can only travel at most the speed of light (which is pretty fast for us to notice in real life, but is significant in the ladder scenario). So objects cannot be perfectly rigid under special relativity.

In this case, by the time the front of the ladder collides with the back door, the back of the ladder does not know it yet, so it keeps moving forwards (and the ladder kind of "compresses"). In both the frame of the garage and the inertial frame of the ladder, the back end keeps moving at the time of the collision, until at least the point where the back of the ladder comes into the light cone of the collision (i.e. a point where force moving backwards at the speed of light from the point of the collision will reach it). At this point the ladder is actually shorter than the original contracted length, so the back end is well inside the garage. Calculations in both frames of reference will show this to be the case.

The bolder parts touch on the answer, which appears to be in line with the sort of "movement transmitted as a wave" idea which another poster supplied. I'm comfortable with that for the moment until such a time as I feel like getting involved with the mathematics of GR.

krd

RoverJames wrote: »

Then you'd need very big gloves if it got cold.

What if your hands were very hairy - like a sasquatch? An interstellar sasquatch?


It's people like you, who ruin thought experiments.

krd

seamus wrote: »

Ah, but we can't, because there's no matter in the universe which would display such a level of compactness and be 1 light year across.

No, but as an abstract problem. Let's just say you could.

Visually the ruler would remain rigid, though the energy would still transfer in a "wave" action.

You know something. If you've ever played around with one of those metal tape measures, you can see the wave action as you move it.

You say, for the sake of argument, it's perfectly rigid. In reality it couldn't be.

As long as you discount many parts of reality, the information - change in velocity of the far end of the ruler would be faster than the speed of light. In reality, it will be nowhere near the speed of light. The compression will have to travel the length of the ruler. Which will probably be the speed of sound through that medium - so it would take years and years to work it's way through.

Sound isn't just what you hear. When a bomb goes off, the heat and light may not get you, but the sound wave may blow out all the windows - and mash up you and your bones.

FISMA

clearz wrote: »

Actually the fastest you can transmit a push through an object it at the speed of sound in that object.

You may have been confused by what is going on internally, rather, than external.

Consider some solid crystalline structure: regular repeating lattice.

The atoms in the lattice are not locked into place, rather, they vibrate.

However, the atoms are bonded together, so they are not able to vibrate independently.

The vibrations will be weakly damped and longed lived - giving a coherent motion.

These vibrations propagate through the material and can be considered sound waves, which propagate through the material at the speed of sound, for that material.

But that's not to say that a Force applied to one end of a diamond rod (for example) will not travel, very fast.

Anyone remember phonons? Wouldn't they be worth discussing here?

krd

FISMA wrote: »

Consider some solid crystalline structure: regular repeating lattice.

The atoms in the lattice are not locked into place, rather, they vibrate.

However, the atoms are bonded together, so they are not able to vibrate independently.

That's a confusing statement.........What's the difference between being bonded and locked.

FISMA

krd wrote: »

That's a confusing statement.........What's the difference between being bonded and locked.

I agree and thought the same when I was typing. It's been a while since Solid State Physics - can't say I aced it the first time around!:o So I won't pretend to be the resident expert - just throwing in my $0.02.

I used the term "locked" colloquially. A lot of people believe that atoms are locked in a position in a solid. They don't realize that the atoms vibrate and have a fundamental frequency.

So atoms vibrate (move) a bit, but not a lot, because they are bonded. There's a limited range of motion.

Does that make sense?

Anyone who did ace Solid State, or who can tell us more about those wonderful phonons than I can remember - please advise.

krd

FISMA wrote: »

I used the term "locked" colloquially. A lot of people believe that atoms are locked in a position in a solid. They don't realize that the atoms vibrate and have a fundamental frequency.

The first time I heard of phonons is when you mentioned them. I thought they existed before - or thought that would be the explanation for few things.

Looking at the wikipedia entry, the maths are a little tricky for me, but I think they explain a few things from thermodynamics to sound generation.

So atoms vibrate (move) a bit, but not a lot, because they are bonded. There's a limited range of motion.

How are they bonded? I imagine the electrons of different atoms repel each other - but something happens in the condensed state where they spontaneously find the least repulsive position in relation to each other, and this position is repeated to build the crystal structure. And that all condensed matter is crystalline.

And what explains triboluminescence? Or how does that fit in.

Anyone who did ace Solid State, or who can tell us more about those wonderful phonons than I can remember - please advise.

I looks like interesting stuff....I never knew a SASER was possible.....I would have thought it was impossible.