I have been curious about this one for a while,

if you push one end of a metal rod, doesn't the other end move at the same time?
cause if you have a light-year long rod don't you just need to push it to send signals that are faster than light.....

is there some kind of light speed wave that runs through the rod when you push on an end? :confused:




I got another question too for ya: you've got a rocket ship that can travel as fast as light, you take off near a huge clock, you also have the greatest telescope in the world pointed out of the back of you r ship,

you blast off at the speed of light, you look back at the clock and time has stopped,

a book I read said this is a good example of time traveling, but if you go back to the clock at a regular pace don't you just come back to the time you were at before and not time travel at all???

Hi!
I think when you are pushing the rod, the other end doesn't respond instantaneously. It is just that ordinary rods are too short for the time to be easily noticed. When you push the rod, the molecules transmit the force from one to the other. Is that right?



But you haven't move back in time, so I think that's not time travel.:(

The classical gedanken experiments to the rescue...

:D

Superluminal Scissors:

http://www.public.iastate.edu/~physics/sci.physics/faq/scissors.html

Twins paradox:

http://zebu.uoregon.edu/~js/glossary/twins_paradox.html

When you push on a rod, your push travels down the rod at the speed of sound in the material the rod's made of. That is much slower than the speed of light, so this method could not be used for faster than light communication.

Our everyday life experience mislead us easily!;)

:)

thanks guys, I just needed to get those off my mind, it's been buggin me for a while

Hi!

Many people wonders about time travel and faster than light things, one of them is me.:) :) :)
________________________________________________
Is it the general or special theory of relativity that allow the existence of particles travelling faster than light?
;)

Keep wondering ... it is a pleasant way to waste an afternoon, or evening,
or morning.

Take care ;)

Even wasting my time in lessons!
:rolleyes:

When you push on a rod, your push travels down the rod at the speed of sound in the material the rod's made of.

I don't want to nitpick here, but are you sure it's the speed of sound?

Connect on one end of a 2 mile rod an explosive device. On the other end, connect a loud horn that can be heard at least 2 miles away. Place yourself near enough to the explosive device so that it blows you to bits, but make sure you're between the explosive device and the sound of the horn.

Apply a force on one end of the rod that will ignite the explosive device and at the same time sound the horn. According to your statement, you'll hear the sound of the horn before being blown to bits.

According to the "Superluminal Scissors Gedanken Thought Experiment" in a link provided above:

The electromagnetic force which binds the atoms of the scissors together propagates at the speeds of light. So if you displace some set of atoms in the scissor (such as the entire handles), the force will not propagate down the scissor instantaneously, This means that a scissor this big *must* cease to act as a rigid body. You can move parts of it without other parts moving at the same time. It takes some finite time for the changing forces on the scissor to propagate from atom to atom, letting the far tip of the blades "know" that the scissors have been closed.

I don't think you'll hear the horn, nest pas? :D

<i>I don't want to nitpick here, but are you sure it's the speed of sound?</i>

Fairly sure. But remember, it's not the speed of sound in <i>air</i>, but the speed of sound in the material the rod is made of. If the rod's made of steel, the speed of sound in steel is considerably faster than the speed of sound in air. So, the impulse which triggers the explosion will reach you before the sound of your horn can travel to you through the air.

Essentially the same thing applies to the superluminal scissors - they can only deform at the speed of sound in the material of the scissors.

Does that make sense?

Essentially the same thing applies to the superluminal scissors - they can only deform at the speed of sound in the material of the scissors.

IF the energy propagating through the iron is in the form of sound, you are correct. Sound waves move through iron by bumping one molecule into the next. Sound waves depend on the weight of molecules as their medium of exchange. At similar temperatures, sound waves will travel through iron at 13,330 mph as opposed to 742 through dry air.

Would you say the sound wave is the same force required in our thought experiments?

The force used in the "Superluminal Scissors" propagates at near the speed of light. ;)

Would you say the sound wave is the same force required in our thought experiments?

I think no matter in the thought experiment or the rod, the force are the same. Both are the electromagnetic force between the atoms. Even in the rod, the sound wave is due to the vibration of the atoms. The interaction between the atoms is just the electromagnetic interaction with speed near speed of light.
When I push the rod, it is just setting a compression in the rod, i.e. the compression of thge sound wave, therefore the signal should be travelling at the speed of sound. ;)
Though I don't quite know how we get the speed of sound by considering the force in the rod.
:)

As I see it (and I'm still a beginner at this stuff)...

Speed of sound has nothing to do with the speed of movement of something like this. A jet fighter pushed by a constant explosion is not necessarily moving at the speed of sound. Whatever the speed of sound is in a perfectly rigid rod running along a lightyear for example, just push it at half or twice that speed; there's nothing stopping you. Except the mass of the rod, the forces acting on it, et cetera...

However, if you could make a perfectly rigid rod a lightyear long, and you could manage to push it, the speed of the rod will still only be the total distance moved divided by the time it took to move that distance. So if you push it one metre, and it takes two seconds, well, that's hardly lightspeed. Look at each end individually, or the rod as a whole, and it's not moving that fast.

However, the effect alone could move faster than light I think. If the rod is one hundred lightyears long, and the rod is perfectly rigid, and you were able to move that mass as easily as you pushed open a door, the effect or relationship between one end and the other would be moving heaps faster than light.

Now, the question is "What do you define as a signal?" As far as I'm concerned, any change in state at either end constitutes a signal. You make it binary if you want. If the far end moves, that's a 1. If it doesn't, that's a 0. Wow, transfer of information. The only way you would have to worry about the speed of vibration (sound) through the rod is if you were trying to transmit a signal by vibration along the thing. Which you're not in this case.

I agree with Dreamsa.

Adam:

I'm afraid you're wrong. The speed of sound limits the rate at which you can deform or move the rod. When you push on one end, the other end cannot "know" instantaneously about your push (how could it?). What must happen is that a signal of some kind must travel from one end to the other. No signal can travel faster than the speed of light, so we can actually rule out faster-than-light communication <i>a priori</i>. But if you want a reason for this particular case, the reason is that the push is transmitted by electromagnetic forces between atoms. The push is an impulse at one end of the rod, which creates <i>phonons</i> (sound waves) in the rod which propagate at the speed of sound in the material.

You said: <i>If the rod is one hundred lightyears long, and the rod is perfectly rigid, and you were able to move that mass as easily as you pushed open a door, the effect or relationship between one end and the other would be moving heaps faster than light.</i>

That would be true apart from the "if"s in your sentence. In fact, no rod can be perfectly rigid. If you put a bending force on one end, the other end does not react instantly. The bottom of the rod bends before the top end, and the top end cannot move faster than the speed of light under any circumstances.

No rods can be perfectly rigid, I think!;)

The motion of a solid object is not dependent on EM interaction, although can be influenced by such. That's why I said "rigid'. And "if". If you push the end of a solid, the thing that moves the rest of it is the bonds between atoms. Yes, those bonds are EM in nature. But those bonds and atoms will still only be moving at a speed of distance-travelled/time-of-journey. Much less than light speed. If the rod is perfectly rigid, then that same speed and distance is applied to all atoms in the rod along its length. But that's not the point either. If you move that rod one metre in two seconds, and the rod is one hundred lightyears long, then the effect of motion is then registered at the other end of one hundred lightyears over two seconds. The only signal through an object which is dependent on speed of sound is vibration.

But, as I said, I am still a beginner, and I do realise I could be completely mistaken. If so, please run through those points I just mentioned so I know where my knowledge is lacking.

The motion of a solid object is not dependent on EM interaction, although can be influenced by such.
What I think about this is that why can we apply a force to a object is because of EM interaction. There are only 4 fundamental forces in nature: gravitaionla, EM, strong and weak. Therefore in fact all the forces we deal with when pushing an object is EM interaction. It is the repulsion of the molecules which make us able to push things, is that right? I just think so!;)


The only signal through an object which is dependent on speed of sound is vibration.
When you are pushing a rod, you're just seetting up the compression of a vibration.
The distance/time is valid, but when you move the rod at one end for 1m in 2s, this doesn't mean that the whole rod has move forward 1m in 2s.;)

Let me stress again that no rod can be considered as perfectly rigid.;)

That is what I think, open for queries!:rolleyes:

Adam,

Once again, when you push one end of the rod the other end doesn't move simultaneously. It doesn't "know" about the push on one end until some signal has propagated to the other end. That signal, in the case of a push, is a compressional wave travelling along the rod. In other words, a sound wave.

In atomic terms, when you push one end of the rod, the atoms there push against the neighbouring atoms (via the electromagnetic force), which then push against their neighbours and so on. All of that takes time. When an atom pushes against a neighbour it also feels a restoring force pushing it back towards its initial position. The end result is a compression (sound) wave in the rod.

I also doubt you could have a perfectly rigid rod for this thing. But if you had one, would there be any compression moving along it?

A rigid body is one that does not deform during its motion. The distance between any two points in the rigid body remains fixed.
But for that large rod it is inevitable that it will deform on pushing.
;)

Talking about a rigid rod is not applicable here! I think!;)

Maybe I am wrong!;)

Solids involve crystalline lattice, which is bound at the atomic level. If you were trying to move gas, then it would be the speed of sound.

Atoms contacting atoms = sound waves, whether in solids, liquids, gas, or plasma. Same diff.

Here's an old poem for your amusement:

There was a young lady named Bright,
Who could travel much faster than light.
She took off one day
In a relative way
And came back on the previous night!

Solids involve crystalline lattice
The vibrations in the crystal lattice are just sound waves!;)

no, they are bound atomicly. There is no space inbetween them, and sound involves two unconnected atoms hitting each other.

"Bound atomicly" is peculiar grammar and spelling, and irrelevant physics. There is no such thing as rigid contact at the atomic level, in the sense we are used to thinking of it at the macro (usual, human, visible) scale. Atoms are in constant motion with regard to each other, in elastic bonds if a solid, temporary loose bonds in a liquid, in independent motion in gas, and complex electrostatic and magnetic patterns in a plasma.

The "surfaces" of atoms are electron clouds which are shared to greater or lesser degree with nearby atoms, and they stretch, vibrate, change and dissolve gazillions of times a second. The only atomic structures bound as units are the newly created Bose-Einstein condensates, at microscopically low temperatures, which are in the same quantum state; they are sometimes called "superatoms."

Hey guys!
Don't you think it's not too interesting arguing which one is the speed of light or sound in the rod.
Our physics doesn't know any signals that can travel faster that light.
So, our problem is to think about existence of such signals.
I guess the clue is in our measuring instruments, that just can't register any super-light speed.
It's the same as we can't see objects smaller than light wavelength...

Maybe anyone has any suggestions about super-lightspeed, or super-timepropagating stuff. With our technology we can only imagine such things, but using science.
I guess neutrino is very suspicious for this topic, what do you think? And btw, gravity is a cornerstone in this problem, for sure.

Their travel time from known supernovas etc. has been measured. They travel at lightspeed.

There are many measurements of such effects as slowing of time and mass increase which have in fact been made with large objects and small particles which verify the increase of mass and other predicted changes at near lightspeed.

At this time, any and all FTL technologies are fairy tales, with no backing in science, other than tachyons. Unfortunately, they are theoretical particles which travel only faster than light, and go backwards in time...so by the time you decide to use them, it's too late (if you do, you would already have seen the results in the past).

I love sci-fi, but see zero prospect for FTL in any current scientific models. The posts on this and other threads and topics which talk about it are obvious babble by hopeful but uneducated writers. Sorry 'bout that.

I thought the light from stars and other such things was travelling at C (300 million m/s or whatever) divided by the refractive index of space? Coz it isn't a void up there. It's chockas with dust, gas, all sorts of crap. Gravitational forces, EM fields, all sorts of things which affect the passage of EM signals.

Was Lijun Wangs experiment at NEC not evidence of a signal moving faster than light? As far as I know, there has not yet been any disproof of his experiment. Also, the former head of Australia's CSIRO, who lives around the corner from me, told me of another experiment involving the transmission of the old "Mary had a little lamb" signal via FTL microwaves; that, he said, had also not yet been disproven.


http://www.neci.nec.com/homepages/lwan/

http://physicsweb.org/article/news/4/7/8/1

Ever heard of Popper? He's a prominent philosopher of science. I like his attitude. He says that any scientific assertion ("law") is a testable, disprovable statement which has not yet been disproven or shown to have exemptions despite thorough, competent attempts to do so.

It is important to compare every scientific statement to this standard. Where most pseudoscientific statements fall down is in not being testable or not having been subject to competent attempts to disprove them.

None of the FTL or time reversal assertions you make qualify under any of these standards.

The experiment described in Adam's post actually do not vilate the special relativity as said in the article. Does anyone know the explanation?
:)

What is the difference between 'phase velocity' and 'group velocity'?:confused:

Was Lijun Wangs experiment at NEC not evidence of a signal moving faster than light?

No. It was simply a re-shaping of a light pulse. No FTL occured and no information transfered. The entire process is predicted from Relativity. NEC was terribly embarrassed by the misunderstanding.

http://www.salon.com/people/feature/2000/08/03/light/

What is the difference between 'phase velocity' and 'group velocity'?

http://www.phys.virginia.edu/classes/109N/more_stuff/Applets/sines/GroupVelocity.html

If the electrons are moving at C, and somehow the effect is moving faster than C, whay is that not a signal? Any change in state at the far end constitutes a signal. If something registers at all, regardless of what it is, that's a 1. If nothing registers, it's a 0. So even if you can't send a tune through as PART of the signal, the binary changing of state at the other end should still exist. You don't NEED to send the tune as part of the signal if there is any change of state at the afr end which can be interpreted AT THAT END. Or not? Can someone explain the difference?

Adam,

Imagine you're transmitting a continuous Zero signal, by any method you like. Then, you decide to switch to a One signal. The time it takes for the receiver to know that Zero has gone to One is limited by the distance between source and receiver and the speed of light. There is no existing experiment and theoretically no way to make the change from Zero to One propagate faster than the speed of light.

So what does the experiment by Wang mean?:confused:
That fuzzy thing with speed greater than speed of light!:confused:

Anyone got anything interesting to say about these things I have seen on various websites (some of you will probably know what I'm talking about):
- Superluminal scissors idea.
- Spin around on the spot at night, and relative to you the moon moves around faster than light.

And other such things mentioned on many websites. This is all new stuff for me still. I must say, from what I already know and what the teachers tell me, there seem to be many very large holes in our understanding of physics. I have all these questions in class, and the teacher always says "Nobody knows yet."

Dreamsa,

<i>So what does the experiment by Wang mean?</i>

There a pulse made up of many different frequencies of light travelled faster than the "normal" speed of light. The explanation is to do with a phenomenon called <i>anomalous refraction</i> in a specially prepared atomic medium. The individual waves making up the pulse all travel at different speeds, giving the impression of the pulse moving faster than light. This is possible because the component waves themselves <i>can</i> move faster than light. That is quite normal and not contrary to relativity since they carry no signal.

It's a little complicated to explain in detail, but I might try a little later on.


Adam:

<i>Spin around on the spot at night, and relative to you the moon moves around faster than light.</i>

Yes, but the important thing is that no information moves from the moon to you faster than the speed of light.

James R....
Exactly. And no information can be transferred between two points on the moon or, say, between two stars by having an illuminated spot swing between them.

I don't know how fast the scan point on a CRT screen can move, but regardless of such "apparent" motion, there is no transfer of matter, energy, or information between separated points of the screen.

Wow you guys made this topic complicated, but I was thinking somemore and I thought when mankind discovers the smallest building block of them all (the higgs or whatever it will be) couldn't you theoretically line them up, touching each side to the next, therefore creating a rigid structure that doesn't flex because there are no atoms or smaller pieces inside of each particle to compress and hit the next..........

No.

:D

Hi all,

Nanok, I think I may assume that you will not feel satisfied with Brain0's "No" ;). Allow me to elaborate a bit on his (correct) answer.

You cannot just "align" atoms or building blocks of matter: first of all you have a quantum uncertainty at that level, and this implies that you cannot hold a particle perfectly still. You need to subject every building block to a force - or in more common quantummechanical language, a potential - that more or less keeps the building block at its place (or pulls it back into its place in case of a distortion). Because this potential is responsible for keeping those building blocks where they belong, you can only move the particles by changing that potential. Hence the theoretically maximum possible speed at which a configuration of building blocks can change is the propagation speed of the force (already assuming that the building blocks react instantly to a change of the potential). This maximum speed is the speed of light.

Since in all practical cases you cannot change the potential that keeps the building blocks into place, this maximum speed is never reached. The best you can do is partially move some particles (most of the time a lot less than the speed of light for macroscopic objects) and watch the effect propagate along the rod you constructed. I'm not entirely certain that the deformation will propagate at the speed of sound in the material - as some people mentioned above - since there are a lot of effects to consider for different kind of building blocks. In either case: it's not faster than the speed of light.

Bye!

Crisp

Nanok & Crisp..

It gets even worse than that. There are no ultimate "particles". In QM all particles are instances of field effects; at base, only fields exist. To be crude, only twitches of space-time+ which persist and interact.

So there are no ultimate billiard balls. The effort to reconcile physics with intuitive images of hard little things is wasted effort. It cain't be done 'cause it ain't so.