Interesting physics question

[reese4221]

Lets say i have a bar that is one light year long now lets say i move this bar forwards and back and theres someone at the other side observing this motion would there be a delay or would it be instant?

[Nightside]

11 minutes ago, reese4221 said:

Lets say i have a bar that is one light year long now lets say i move this bar forwards and back and theres someone at the other side observing this motion would there be a delay or would it be instant?

Assuming this rod doesn’t collapse in on itself due to its own gravity. 
Applying a force at one end of a solid object would compress the object like a spring, causing it to shorten slightly over its length. If it has a decent modulus of elasticity, it will then rebound to its original size. This strain will travel through the material at some speed consistent with its material properties, but much slower than the speed of light.

[Kerwood Floyd]

In other words, your original question had the hidden assumption that the bar is perfectly rigid, while in reality there is no such thing as a perfectly rigid bar.

[Entropian]

It would have a delay.  The forwards and backwards movement creates a wave moving through the bar, which travels at a speed dependent on the material the bar is made of.  Information can't travel faster than the speed of light.

[SOXBLOX]

Ah. You have tried to cheat the universe. But it does not work.

Seriously, though, the self-consistency of physics never ceases to amaze me.

[cubinator]

The wave travels through the bar at the speed of sound (a few kilometers per second in steel, for instance)

The wave will have to be very long if you want to compress it much, but it won't go faster than sound. There's a similar problem where you have a very long bar like that, but twist it instead. The 'twist' again moves at the speed of sound. Any bar that long will appear to be very flimsy and wiggly, even if you assume it has structural integrity.

Edited September 1, 2020 by cubinator

[YNM]

There's no such thing as a fully rigid bar - look at any physical structure around you, they all have to give way (deformation) in order to transmit force or even displacement.

The compression would travel at the speed of sound within the material (as sound is simply waves of compression - there is also shear wave in solids though). Even diamond is reported to only have a speed of sound of 12 km/s; this is barely above the orbital escape velocity from Earth from sea level, and definitely a microscopic amount compared to the speed of light. You'd have a better chance at getting there faster by beaming a laser, or even just fling a much shorter rod really fast.

What does intrigue me, however, is the fact that P-waves and S-waves have different speed in any material; would this mean that, if you both push and bend the rod a bit, will it arrive as two different motions in sequence ? Or is it like earthquakes, where only the S-wave matters (P-waves causes little deflection after all)

Edited September 1, 2020 by YNM

[Shpaget]

 

But what if you swing the rod, you may ask yourself now.

[kerbiloid]

The bar is a "material object", it consists of interacting parts.

The interaction speed is finite.

99 bottles of beer on the bar, 99 bottles of beer.
Take one down, push it along, 98 bottles of beer on the bar to move...

Edited September 1, 2020 by kerbiloid

[Hannu2]

5 hours ago, YNM said:

There's no such thing as a fully rigid bar - look at any physical structure around you, they all have to give way (deformation) in order to transmit force or even displacement.

This is true. If there is a natural law which set maximum speed of information it makes impossible to have perfectly rigid material in such world.

5 hours ago, YNM said:

What does intrigue me, however, is the fact that P-waves and S-waves have different speed in any material; would this mean that, if you both push and bend the rod a bit, will it arrive as two different motions in sequence ? Or is it like earthquakes, where only the S-wave matters (P-waves causes little deflection after all)

Yes, you get pushing and twisting signals at different times. They are called longitudinal and transversal waves in material physics and they have generally different speeds.

[YNM]

1 hour ago, Hannu2 said:

Yes, you get pushing and twisting signals at different times. They are called longitudinal and transversal waves in material physics and they have generally different speeds.

Have anyone tested what is it like though ? I mean, in earthquakes, the waves that we have to worry about, the ones that gives the most of the ground acceleration and most of the deflections, displacements and destructive power, are the S-waves (and surface waves) only; it's the reason earthquake early warning works by detecting P-waves, since they arrive a while before all the bad stuff. True that our rod would be barely confined at all, unlike tectonic plates, and it'd be of little importance... But if anyone want to (or have had) test it out in real, I'd be grateful.

[magnemoe]

16 minutes ago, YNM said:

Have anyone tested what is it like though ? I mean, in earthquakes, the waves that we have to worry about, the ones that gives the most of the ground acceleration and most of the deflections, displacements and destructive power, are the S-waves (and surface waves) only; it's the reason earthquake early warning works by detecting P-waves, since they arrive a while before all the bad stuff. True that our rod would be barely confined at all, unlike tectonic plates, and it'd be of little importance... But if anyone want to (or have had) test it out in real, I'd be grateful.

It should be pretty easy to test on something like an railway rail and lab equipment. 

[Cattette]

Another interesting thing to ponder is how much energy you'd need to move a 1 ly bar  in units of Saturn V's 

Edited September 1, 2020 by Cattette

[sevenperforce]

16 minutes ago, Cattette said:

Another interesting thing to ponder is how much energy you'd need to move a 1 ly bar  in units of Saturn V's 

Well technically you can "move" a 1 ly bar with any impulse at all. It just won't move very fast.

As far as the minimum...assume an impossibly rigid steel "bar" one single atom thick (that's the minimum) and one lightyear long. Metal-metal bonds are on the order of 1.6 angstroms, so we're looking at 5.84e25 atoms, or a "bar" weighing about 5.42 kilograms.

14 hours ago, reese4221 said:

Lets say i have a bar that is one light year long now lets say i move this bar forwards and back and theres someone at the other side observing this motion would there be a delay or would it be instant?

Lots of good answers have been given already, but I just have to say that I really like this question and I'm glad you asked it! Keep it up.

[Gargamel]

5 minutes ago, sevenperforce said:

or a "bar" weighing about 5.42 kilograms.

You mean wire.      And one that will slice you into tiny pieces when you try to push it.   (Looks at his machinist hands and sighs). 

[sevenperforce]

3 minutes ago, Gargamel said:

You mean wire.      And one that will slice you into tiny pieces when you try to push it.   (Looks at his machinist hands and sighs). 

If it was in fact impossibly rigid it would probably go right through you without noticeably interacting.

If it was not impossibly rigid it would shred at the slightest touch.

[SOXBLOX]

It would also oxidize kinda fast...oh, wait, this is space. Nvrmnd.

[Shpaget]

That's one hell of a splinter.

[magnemoe]

5 hours ago, Gargamel said:

You mean wire.      And one that will slice you into tiny pieces when you try to push it.   (Looks at his machinist hands and sighs). 

You might not note notice and yes the wire will buckle crazy within mm. 
Yes an single atom with blade is an old scifi consent. Very nice for cutting but probably more useful for structure or armor. 

[mikegarrison]

42 minutes ago, magnemoe said:

You might not note notice and yes the wire will buckle crazy within mm. 
Yes an single atom with blade is an old scifi consent. Very nice for cutting but probably more useful for structure or armor. 

A single atom structure wouldn't work. It would be kind of like stacking spheres on top of each other. Even atoms like carbon that can form chains don't form them in straight lines.

Thin tubes or columns would work, depending on the atom. Something like one hexagon clocked-stacked on another hexagon clocked-stacked on another hexagon, etc.

Edited September 1, 2020 by mikegarrison

[JoeSchmuckatelli]

On 8/31/2020 at 7:32 PM, reese4221 said:

Lets say i have a bar that is one light year long now lets say i move this bar forwards and back and theres someone at the other side observing this motion would there be a delay or would it be instant?

 

On 8/31/2020 at 7:55 PM, Nightside said:

Assuming this rod doesn’t collapse in on itself due to its own gravity. 
Applying a force at one end of a solid object would compress the object like a spring, causing it to shorten slightly over its length. If it has a decent modulus of elasticity, it will then rebound to its original size. This strain will travel through the material at some speed consistent with its material properties, but much slower than the speed of light.

 

On 8/31/2020 at 10:04 PM, Entropian said:

It would have a delay.  The forwards and backwards movement creates a wave moving through the bar, which travels at a speed dependent on the material the bar is made of.  Information can't travel faster than the speed of light.

 

On 8/31/2020 at 10:29 PM, SOXBLOX said:

Ah. You have tried to cheat the universe. But it does not work.

Seriously, though, the self-consistency of physics never ceases to amaze me.

 

On 9/1/2020 at 12:36 AM, cubinator said:

The wave travels through the bar at the speed of sound (a few kilometers per second in steel, for instance)

The wave will have to be very long if you want to compress it much, but it won't go faster than sound. There's a similar problem where you have a very long bar like that, but twist it instead. The 'twist' again moves at the speed of sound. Any bar that long will appear to be very flimsy and wiggly, even if you assume it has structural integrity.

I've enjoyed reading the question and subsequent answers - but I have a follow on question: presuming a rigid 1ly bar that doesn't collapse under its own gravity... 

Would the inertia be such that both ends could move simultaneously - or does 'suppose-itional' physics require compression at one end followed by a speed of sound transmission to the far end before the movement can occur? 

[cubinator]

1 hour ago, JoeSchmuckatelli said:

I've enjoyed reading the question and subsequent answers - but I have a follow on question: presuming a rigid 1ly bar that doesn't collapse under its own gravity... 

Would the inertia be such that both ends could move simultaneously - or does 'suppose-itional' physics require compression at one end followed by a speed of sound transmission to the far end before the movement can occur? 

The best you could probably do is have a material so rigid that its speed of sound is equal to the speed of light, or close to it. Then it would only take 1 year for the compression to reach the other side. Otherwise, you need to ignore relativity and the speed of light and such if you want the speed of sound to be infinite. The action of compression is reactionary: Each atom has to move a little bit to create a force on the next, so each atom's inertia limits how fast the wave can go.

Of course, both ends could be moving simultaneously if you kept wiggling the bar for the whole year, and it could have resonances just like any other bar, tube, or piece of twine with a wave traveling through it. Your first push wouldn't reach the other end for some time, though.

Edited September 2, 2020 by cubinator

[sevenperforce]

44 minutes ago, JoeSchmuckatelli said:

 

I've enjoyed reading the question and subsequent answers - but I have a follow on question: presuming a rigid 1ly bar that doesn't collapse under its own gravity... 

Would the inertia be such that both ends could move simultaneously - or does 'suppose-itional' physics require compression at one end followed by a speed of sound transmission to the far end before the movement can occur? 

The latter. The two ends would not move simultaneously. 

[Bill Phil]

59 minutes ago, JoeSchmuckatelli said:

I've enjoyed reading the question and subsequent answers - but I have a follow on question: presuming a rigid 1ly bar that doesn't collapse under its own gravity... 

Would the inertia be such that both ends could move simultaneously - or does 'suppose-itional' physics require compression at one end followed by a speed of sound transmission to the far end before the movement can occur? 

Information can't travel faster than light.

As for the speed, I'm not sure if it would be the speed of sound or not. However, the application of a force would create a stress locally, and that stress would have to propagate. Stress waves propagate differently depending on whether or not the stress is normal stress or shear stress. I believe normal stress propagates at the material's speed of sound, provided that it's isotropic and of constant density. However, shear stress propagates at much lower speeds generally since it's based on the shear modulus of the material which is usually lower than its elastic modulus. 

Basically, the impulse is only applied locally. The force applied creates local stress which then propagates like a wave.

12 minutes ago, cubinator said:

Otherwise, you need to ignore relativity and the speed of light and such if you want the speed of sound to be infinite.

Interestingly, this would also mean that the material would have an infinite elastic modulus - infinite tensile/compression strength.

[K^2]

43 minutes ago, Bill Phil said:

Information can't travel faster than light.

This statement should always come with the words "locally" or "in flat space-time".