# The Naked Scientists Forum

### Author Topic: How "fast" does force "travel"  (Read 12799 times)

#### f6

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##### How "fast" does force "travel"
« on: 11/02/2008 22:32:28 »
is the transmission of force instantaneous? if yes does this mean information travels faster than light?

if I have a long stick, say 10 light years long, OK so its a really long stick. If I push on the near end of the stick, how long until the force can be measured at the far end of the stick? does it take 10 years?
« Last Edit: 09/03/2008 14:46:16 by ukmicky »

#### JP

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##### how "fast" does force "travel"
« Reply #1 on: 11/02/2008 23:04:43 »
Force travels at light speed or slower.

For the example of your stick, nearly all the forces involved will be electromagnetic.  Electromagnetic forces are "carried" on a basic level by photons (i.e. little packets of the electromagnetic field).  In order for one atom of your stick to push on another, it has to send these photons to its neighbor.  Photons are light, so the fastest this force can be sent from neighbor to neighbor is the speed of light.  Therefore, the fastest the force can travel down the stick is at the speed of light.  (If the atoms aren't absorbing and emitting the photons perfectly and immediately, there might be delays in the force, so it might actually go slower than the speed of light.)

#### another_someone

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##### how "fast" does force "travel"
« Reply #2 on: 11/02/2008 23:36:42 »
I would have thought it would go much slower than the speed of light - the speed of sound would be what I would expect in that particular instance.

#### f6

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##### how "fast" does force "travel"
« Reply #3 on: 12/02/2008 00:23:03 »
rats, there is no getting around this speed of light thing is there. -

hey, here's a thought: could the value of c change and we not be able to detect it? I am guessing here, but c is a velocity, ie a ratio of distance and time, so if that ratio changed then time and distance must change, but seeing as these are the things we use to measure velocity, how would we know they had changed? or is there some strange mathematics that measures v (or c) not using distance and time?

#### DoctorBeaver

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##### how "fast" does force "travel"
« Reply #4 on: 12/02/2008 07:44:33 »
But don't forget that as you travel faster, time does funny things - like stretch.

#### syhprum

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##### how "fast" does force "travel"
« Reply #5 on: 12/02/2008 09:01:28 »
It is quite easy to notice the delay in the transmission of force on the old fashioned railway signaling system where the movement of a lever pulled a long metal bar that in turn moved the signal up.
The force is of course propergated at the speed of sound in the medium.

#### f6

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##### how "fast" does force "travel"
« Reply #6 on: 12/02/2008 09:15:36 »
are we confusing the mass of the object reacting to the force with transmission of the force. I would imagine the long railway signal tie would resist movement due to its own mass, probably even causing some stretching or compressing of the member and this would retard the movement of the far end, but would it retard the effect of the force over the entire length of the member? For the member to stretch say, the near end must pull away from the far one, the mass at the far end of the tie must therefore be supplying equal and opposite reactions to the force at the near (at the first instant at least, until the entire member begins to move).

#### f6

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##### how "fast" does force "travel"
« Reply #7 on: 12/02/2008 09:18:50 »
It is quite easy to notice the delay in the transmission of force on the old fashioned railway signaling system where the movement of a lever pulled a long metal bar that in turn moved the signal up.
The force is of course propergated at the speed of sound in the medium.

Does this mean a supersonic aircraft or a high power bullet could reach the signal before it changed?

edit: oops, realised my own stupidity, the speed of sound in steel is of course much higher than in air.

#### another_someone

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##### how "fast" does force "travel"
« Reply #8 on: 12/02/2008 13:29:58 »
are we confusing the mass of the object reacting to the force with transmission of the force. I would imagine the long railway signal tie would resist movement due to its own mass, probably even causing some stretching or compressing of the member and this would retard the movement of the far end, but would it retard the effect of the force over the entire length of the member? For the member to stretch say, the near end must pull away from the far one, the mass at the far end of the tie must therefore be supplying equal and opposite reactions to the force at the near (at the first instant at least, until the entire member begins to move).

Yes and no.

The problem is that the mass at the far end of a beam is not actually reacting to a force at the near end of the beam, it is reacting to a force applied to it by its nearest atoms, which is then reacting to the atoms nearest to them, and all the way up the beam.  On an atom by atom basis, you could argue that each atom has a delay in reacting to a force, but a force that is transmitted to it by the speed of light; but the next atom down the chain cannot receive the force from atoms upstream until those atoms upstream have reacted.

So, on an atom by atom basis, you are correct; but looking at the entire system, the mass at the far end will actually have to wait for that time (the time determined by the speed of sound in the material) before it even receives the force.

#### lyner

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##### how "fast" does force "travel"
« Reply #9 on: 12/02/2008 14:13:03 »
Why do people instantly reach for the 'big' explanation before clearing up the conventional bits?

There is a very good 'intermediate level' answer to the question and that is that force is transmitted along a bar or wire as a shock wave. The density and stiffness of the material govern the speed of the wave.

The stiffness is a function of the electric fields between atoms and, if you absolutely have to, you can talk about the interactions between adjacent atoms in terms of photon interaction. But, do you really have to?
It's clear that the 'speed limitation' is largely due to the enormous masses of the atoms which have to be got moving and vibrating in space. There will be a delay during the photon interactions and a minuscule delay in the photon traveling time.

When you can understand all the levels, you can see the bigger picture but not everyone can. What is needed by so many of the readers is the simple explanation. Physics is not just about posh-end stuff. So many of the misconceptions we read are about the real basics - which are also great fun, don't forget.

#### lightarrow

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##### how "fast" does force "travel"
« Reply #10 on: 12/02/2008 14:31:07 »
is the transmission of force instantaneous? if yes does this mean information travels faster than light?

if I have a long stick, say 10 light years long, OK so its a really long stick. If I push on the near end of the stick, how long until the force can be measured at the far end of the stick? does it take 10 years?
One of the main differences between newtonian mechanics and special relativity, well understood even before Einstein's theory, is the fact that in the first theory interactions can propagate at infinite speeds, in the second they have a finite limit: c.
That said, in the case of a stick, as others wrote, interaction's speed is sound speed in that medium (one of the highest is in diamond: 12,000 m/s).

Edit. In the metal Berillium sound's speed is even higher: 12,870 m/s.
« Last Edit: 15/02/2008 17:38:03 by lightarrow »

#### JP

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##### how "fast" does force "travel"
« Reply #11 on: 12/02/2008 16:03:25 »
Why do people instantly reach for the 'big' explanation before clearing up the conventional bits?

I think that most people who aren't scientists/engineers want to know the most fundamental explanation that science can currently give expressed in the simplest terms possible.  Scientists and engineers are much more forgiving, since they have the training to understand that all these theories are just models that are accurate in certain limits.
is the transmission of force instantaneous? if yes does this mean information travels faster than light?

if I have a long stick, say 10 light years long, OK so its a really long stick. If I push on the near end of the stick, how long until the force can be measured at the far end of the stick? does it take 10 years?
One of the main differences between newtonian mechanics and special relativity, well understood even before Einstein's theory, is the fact that in the first theory interactions can propagate at infinite speeds, in the second they have a finite limit: c.
That said, in the case of a stick, as others wrote, interaction's speed is sound speed in that medium (one of the highest is in diamond: 12,000 m/s).

The speed of sound is going to be right for the bulk of the force.  This makes me wonder, though, if it's like light being transmitted through a medium: the bulk of the light travels at speed v=c/n where n is the refractive index, but a tiny portion of the light travels through exactly at speed c.

#### another_someone

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##### how "fast" does force "travel"
« Reply #12 on: 12/02/2008 16:32:14 »
The speed of sound is going to be right for the bulk of the force.  This makes me wonder, though, if it's like light being transmitted through a medium: the bulk of the light travels at speed v=c/n where n is the refractive index, but a tiny portion of the light travels through exactly at speed c.

I would guess so, but only a very tiny proportion.

Since the force is fundamentally a coulomb force, so the force (even from the farthest atom) has an infinite range (even if dispersed so thinly that at any macroscopic range it is not perceptible).  Clearly, the force from the nearest atom (or even before that, the force felt by the nearest atom) will have a slight residual impact even at the furthest end of a long rod; but the bulk of the force will travel atom to atom, and so be very much slower.

#### lightarrow

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##### how "fast" does force "travel"
« Reply #13 on: 12/02/2008 19:00:27 »
The speed of sound is going to be right for the bulk of the force.  This makes me wonder, though, if it's like light being transmitted through a medium: the bulk of the light travels at speed v=c/n where n is the refractive index, but a tiny portion of the light travels through exactly at speed c.
"A tiny portion of the light travels through exactly at speed c". Don't understand the reason.

#### JP

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##### how "fast" does force "travel"
« Reply #14 on: 12/02/2008 19:17:36 »
The speed of sound is going to be right for the bulk of the force.  This makes me wonder, though, if it's like light being transmitted through a medium: the bulk of the light travels at speed v=c/n where n is the refractive index, but a tiny portion of the light travels through exactly at speed c.
"A tiny portion of the light travels through exactly at speed c". Don't understand the reason.

Physically, there's always a tiny bit of light traveling through a material that "misses" interacting with the matter and passes through at the speed of light.  (It goes through before the atoms get excited and start interacting to slow down the light).  It's so tiny that in most situations it would be swamped out by noise in any detector you have to detect it.  This is relevant in defining the speed that information can travel if carried by electromagnetic fields, since the information-carrying bit of a pulse seems to pass through media at the speed of light.  Of course, in a real detector, you have to have a high enough signal-to-noise ratio, that you usually end up waiting until the slowed-down part of the wave arrives.

But I agree with the speed of sound limit.  You might get a microscopic "tap" at the speed of light, and then the force would arrive at the speed of sound.

#### f6

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##### how "fast" does force "travel"
« Reply #15 on: 12/02/2008 20:14:52 »
Why do we say force travels at the speed of sound in the material? It must be more than coincidence...

I think that most people who aren't scientists/engineers want to know the most fundamental explanation that science can currently give expressed in the simplest terms possible.
.

Please enlighten a person who only got as far as 7th form (year 12) physics
« Last Edit: 12/02/2008 20:17:34 by f6 »

#### lyner

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##### how "fast" does force "travel"
« Reply #16 on: 12/02/2008 22:03:06 »
Quote
Why do we say force travels at the speed of sound in the material? It must be more than coincidence...
They are one and the same thing. The sound travels through the material as compressional forces, propagating as waves. The actual speed may vary with the frequency but it's the same thing, in principle; watch a loudspeaker cone vibrating - a force is making it do that. You can hear a train coming down the line (don't try this at home) because the forces are propagating along the steel in the track.

#### f6

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##### how "fast" does force "travel"
« Reply #17 on: 12/02/2008 22:07:29 »
ahhh, ding! now it the gears in my head mesh and it makes sense. Thanks.

#### lyner

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##### how "fast" does force "travel"
« Reply #18 on: 12/02/2008 22:08:39 »
But I agree with the speed of sound limit.  You might get a microscopic "tap" at the speed of light, and then the force would arrive at the speed of sound.
I  guess this is true in principle, but I feel it's the same order of magnitude as the likelihood of a pea tunneling through a table and falling onto the floor. The energy associated with the photons involved in mechanical / sound waves is very small. What sort of signal to noise ration would you need in order to detect the 'tap'?

#### JP

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##### how "fast" does force "travel"
« Reply #19 on: 13/02/2008 17:30:27 »
But I agree with the speed of sound limit.  You might get a microscopic "tap" at the speed of light, and then the force would arrive at the speed of sound.
I  guess this is true in principle, but I feel it's the same order of magnitude as the likelihood of a pea tunneling through a table and falling onto the floor. The energy associated with the photons involved in mechanical / sound waves is very small. What sort of signal to noise ration would you need in order to detect the 'tap'?

You'd need zero noise, I believe, or very close to it--so it's a theoretical limit and not practical in most cases.
It's probably akin to trying to detect the pea by looking for pea atoms that had tunneled through the table.

#### lyner

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##### how "fast" does force "travel"
« Reply #20 on: 13/02/2008 22:57:23 »
Yup!

#### thinkoutsidethe

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##### Re: How "fast" does force "travel"
« Reply #21 on: 19/06/2012 10:26:06 »
Force can travel faster than the speed of light as far as i'm concerned. In fact how could the universe maintain its stability and form if that was not the case? its impossible otherwise. All these interconnections are somehow unconnected as a whole?

Let me put it a different way. You have a steel tube which is longer than meters c. At one end of the tube you have a plunger and push with a massive force. The other end is blocked. Whilst maintaining the pressure on the plunger you release the blocked end. The pressure will instantaneously burst.

Sure the observer won't be able to see this event because he can only use light as an observing factor. But having two timed clocks at either end will prove the effect.

I feel like this is a fundamental floor in the train sequence of relativity. Einstein was using light as a reference. Just because something is not sighted does not mean it does not exist. In fact the whole night sky as we see it is not the universe as it is but rather how our eyes perceive it. It is totally warped by time because each point is travelling to our eyes from different distances and hence have a different time continuum. One photon has traveled 6  years and another 7  years. What you are seeing is different points in time in space.

#### imatfaal

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##### Re: How "fast" does force "travel"
« Reply #22 on: 20/06/2012 10:16:43 »
Force can travel faster than the speed of light as far as i'm concerned. In fact how could the universe maintain its stability and form if that was not the case? its impossible otherwise. All these interconnections are somehow unconnected as a whole?
Intuition often fails in astronomical calculations - and those involving einstein's theories.  Our theories currently do no have force transferred instantaneously - or if you are using a formulation without forces, then the interaction is still limited to SoL.

Quote
Let me put it a different way. You have a steel tube which is longer than meters c. At one end of the tube you have a plunger and push with a massive force. The other end is blocked. Whilst maintaining the pressure on the plunger you release the blocked end. The pressure will instantaneously burst.
Nope - this is testable and false.  A pressure wave will travel from one end to the other at about the speed of SOUND within the material.
Quote
Sure the observer won't be able to see this event because he can only use light as an observing factor. But having two timed clocks at either end will prove the effect.
Whilst using light to observe can introduce interesting extra factors - this form of experiment is not a problem as we can observe that the effect is transferred at far lower than SoL

Quote
I feel like this is a fundamental floor in the train sequence of relativity. Einstein was using light as a reference. Just because something is not sighted does not mean it does not exist. In fact the whole night sky as we see it is not the universe as it is but rather how our eyes perceive it. It is totally warped by time because each point is travelling to our eyes from different distances and hence have a different time continuum. One photon has traveled 6  years and another 7  years. What you are seeing is different points in time in space.
Basically Special Relativity is as well proven as any theory is gonna get - it could still be wrong, but the only thing that will show it is wrong is experimental data

#### syhprum

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##### Re: How "fast" does force "travel"
« Reply #23 on: 20/06/2012 13:00:05 »
c

#### LetoII

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##### Re: How "fast" does force "travel"
« Reply #24 on: 28/06/2012 04:30:58 »
in case of entanglement, instantly....

#### The Naked Scientists Forum

##### Re: How "fast" does force "travel"
« Reply #24 on: 28/06/2012 04:30:58 »