5 Light-Year long stick question

[RCulling]

My friends and I are having an argument over this question:

"Someone 5 LY away on a planet is getting "poked" by a 5 LY long stick from here on earth"

Does it take five years (or more) from the time one end of the stick is pushed until the person right next to the other end is poked by it? Or does it happen in next to no time at all?
- I personally believe that it will take alteast five years.. since if the person being poked had a telescope and watched the person push the stick, it would take the light ("information") 5 years to reach him.. and he can't get poked by the stick if it hasn't been pushed?

Is that right to say?

*Ignoring the fact of obvious problems with the situation, like requiring a massive force to move the stick.

 

[JesseM]

You're right, it takes more than five years, if you push one end, this creates a pressure wave that travels at the speed of sound in the material the stick is made of, the other end doesn't move until the pressure wave reaches it. There's a good discussion here:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4

 

[RCulling]

- Thank you very much

 

[BL4CKCR4Y0NS]

I understood all of it until

at the speed of sound

Why is it the speed of sound?

 

[Fredrik]

Because it's a longitudinal wave in the material, and that makes it "sound" by definition. Sound has a specific speed in each material, so that's the speed this wave will have.

You could of course hit that stick with a hammer or something that moves faster than the speed of sound in the material, and then the first layer of atoms will move faster than the speed of sound. But the end of the stick that you hit will shatter, and the wave you caused will propagate faster than sound for a while, tearing the material apart at first and losing lots of energy, only to turn into a regular sound wave after a while.

The wave that starts out going faster than the speed of sound is definitely slower than the speed of light, since atoms are massive and the interaction between atoms is electromagnetic. Massive particles move at speeds <c and waves in the electromagnetic field propagate at speed c.

 

[BL4CKCR4Y0NS]

Okay I get that... but what if instead of poking,prodding and hitting, we use an electric shock? How would that be calculated? (ie. how long would it take until Person B felt the shock sent by Person A)

Am I right to say that some factors are... the material of the stick (resistance), how strong the power supply is (volts) and how fast the current is moving (amps)?

And with those factors, am I then right to say that there can be no answer given unless I provide the specifics?

 

[hamster143]

Then you would generate an electromagnetic wave that travels down the stick at approximately the speed of light and still takes 5 years to reach the other end.

 

[q_interested]

lets say that the theoretical stick/rod has zero mass and is perfectly rigid (it doesn't deform), so wherever you go along its length, it moves the same distance in the same manner as it does at the point of transmission..what then?

 

[hamster143]

lets say that the theoretical stick/rod has zero mass and is perfectly rigid (it doesn't deform), so wherever you go along its length, it moves the same distance in the same manner as it does at the point of transmission..what then?

It is impossible to make a perfectly rigid stick.

 

[RCulling]

Zero mass? Why Zero mass? Kind of defeats the purpose then...

 

[q_interested]

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical let's suppose..apparently its impossible to travel at the speed of light, but it didn't stop Einstein theorising.
I actually think the Original question is a bloody good one.
okay what if the stick is 5 metres long - and I poke you with it...you will feel the movement instantaneously - forgetting relativism, and internal factors for a second - so notionally, the resultant poke isn't governed by speed - time goes out of the equation...agreed?

 

[Vanadium 50]

Why stop with perfectly rigid rods? Why not imagine invisible pink pixies?

Once you start with something which violates the laws of physics, you're not going to be able to draw a physical conclusion.

 

[sylas]

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical let's suppose..apparently its impossible to travel at the speed of light, but it didn't stop Einstein theorising.
I actually think the Original question is a bloody good one.
okay what if the stick is 5 metres long - and I poke you with it...you will feel the movement instantaneously - forgetting relativism, and internal factors for a second - so notionally, the resultant poke isn't governed by speed - time goes out of the equation...agreed?

The answer to the original question is that such a thing is physically impossible. You can ask "what if" all our physics is incorrect, but what then will you assume to give an answer?

On the other hand, one thing you can do without breaking the laws of physics is this.

Instead of just pushing one end of the stick, have the whole stick mounted on little wheels, attached to clocks. Make sure all the clocks are synchronized with each other. Then, at a given time, ALL the wheels rotate, and the whole stick moves forward one inch.

So at least the movement of this stick is going to look like what you are proposing, right?

OK. Now here's the surprising thing. Whether the "front" or "back" of the stick moves first, or both at the same time, depends on how fast you are moving past the stick when the clocks tick over. This is not merely what "seems" to occur. The times really are different depending on the observer.

Weird, heh! But that's consequence of well tested and completely uncontroversial physics.

Does this help?

Cheers -- sylas

 

[HallsofIvy]

lets say that the theoretical stick/rod has zero mass and is perfectly rigid (it doesn't deform), so wherever you go along its length, it moves the same distance in the same manner as it does at the point of transmission..what then?

It is precisely this thought experiment that shows that, even theoretically, there cannot be a "perfectly rigid" object.

 

[q_interested]

It is precisely this thought experiment that shows that, even theoretically, there cannot be a "perfectly rigid" object.

perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

 

[Doc Al]

where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation?

Why go to a physics site if you're not interested in what physics has to say?

it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

Why do you say that?

 

[Frame Dragger]

perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

Your original question has been answered in every way possible, so all that is left was to talk about the nature of the thought experiment you posed. Would you rather be ignorant? Here's a question... imagine a NON-ideal "stick" which follows a "normal" (curved) path who's total length = 5 Light Years (ly). Does that change what you're asking, AT ALL, which is really just a way of asking if Information (term of art) can exceed "c". The answer is still no, so don't blame a room full of experts (and duffers such as myself) and expect that to hold our interest long.

Why not learn about WHY you can't construct a striaght, rigid... ok... um... am I the only one who has been laughing inside ever since hamster said "It is impossible to make a perfectly rigid stick."? Just me? Hmmm, ok, I'm the only one who never matured... fair enough.

Anyway, why not learn about why a perfectly straight and rigid body can't exist? You might learn about geodesics, and all sorts of other fascinating material. Sylas gave you a fantastic answer to your (third, or fourth... I lost track) question, which you seem to have ignored in favour of telling a bunch of people on PF why they should ignore Physics in the Relativity sub-forum. Doesn't that strike you as a combination of odd, and deeply ungrateful?

 

[q_interested]

Why go to a physics site if you're not interested in what physics has to say?
I found the topic via a search engine - not thinking that I would find anything.
Why do you say that?

well i thought that was a given since people have responded to the OP's OP. its asking about a 5 LY stick.
I realize it sounds airy-fairy but I'm beginning to consider in terms of there there being no such thing as time, I guess I'm an absolutist...I believe that if two things don't happen "simultaneously", its merely that they happen one after the other - "after" not being related to a temporal concept, but merely non-coincidental - the temporal is a manmade idea IMO., albeit a deeply rooted one.
this is why i think this whole thing about lightspeed is a moot point. there is no speed, only travel in a direction.
another idea i had along these lines is that if you had an escalator of people as one got on, one would get off - there was no "give" - so what is happening there?

 

[Frame Dragger]

well i thought that was a given since people have responded to the OP's OP. its asking about a 5 LY stick.
I realize it sounds airy-fairy but I'm beginning to consider in terms of there there being no such thing as time, I guess I'm an absolutist...I believe that if two things don't happen "simultaneously", its merely that they happen one after the other - "after" not being related to a temporal concept, but merely non-coincidental - the temporal is a manmade idea IMO., albeit a deeply rooted one.
this is why i think this whole thing about lightspeed is a moot point. there is no speed, only travel in a direction.
another idea i had along these lines is that if you had an escalator of people as one got on, one would get off - there was no "give" - so what is happening there?

Ok... here is the answer to your question, and a hand-grenade to feed to your pet theory.

http://en.wikipedia.org/wiki/Relativity_of_simultaneity

 

[Dale]

well i thought that was a given since people have responded to the OP's OP. its asking about a 5 LY stick.
I realize it sounds airy-fairy but I'm beginning to consider in terms of there there being no such thing as time, I guess I'm an absolutist...I believe that if two things don't happen "simultaneously", its merely that they happen one after the other - "after" not being related to a temporal concept, but merely non-coincidental - the temporal is a manmade idea IMO., albeit a deeply rooted one.
this is why i think this whole thing about lightspeed is a moot point. there is no speed, only travel in a direction.
another idea i had along these lines is that if you had an escalator of people as one got on, one would get off - there was no "give" - so what is happening there?

What is it about the anti-time crowd that insists on hijacking other people's threads rather than starting their own?

If you wish to discuss your anti-time musings you can do so in the philosophy section or on many other internet sites (e.g. SciForums). This forum is for discussing mainstream science where an idea is judged not by how pretty it looks on paper but by how well it agrees with experimental evidence. Judged in that light, the idea of time passes with flying colors having several centuries worth of experimental support and the idea of no time fails miserably.

 

[Frame Dragger]

What is it about the anti-time crowd that insists on hijacking other people's threads rather than starting their own?

If you wish to discuss your anti-time musings you can do so in the philosophy section or on many other internet sites (e.g. SciForums). This forum is for discussing mainstream science where an idea is judged not by how pretty it looks on paper but by how well it agrees with experimental evidence. Judged in that light, the idea of time passes with flying colors having several centuries worth of experimental support and the idea of no time fails miserably.

Maybe believing in a timeless universe turns you into an *******? All kidding aside, really there are respected notions of imaginary time as a CONSTRUCT, but outside of that I'm really tired of this. No time = the realm of pseudoscience or a science advanced beyond our current ability to test it meanignfully... i.e. NOT SCIENCE.

I wonder if "no time" is somehow a step on the way to being a crank? Perhaps "no time" is the "hearing voices" of kooks!

 

[q_interested]

Maybe believing in a timeless universe turns you into an *******? All kidding aside, really there are respected notions of imaginary time as a CONSTRUCT, but outside of that I'm really tired of this. No time = the realm of pseudoscience or a science advanced beyond our current ability to test it meanignfully... i.e. NOT SCIENCE.

I wonder if "no time" is somehow a step on the way to being a crank? Perhaps "no time" is the "hearing voices" of kooks!

I realize what I'm hypothesising probably sounds a bit out there and maybe there's a better place to talk about this, but I'm interested in this kind of thing and I don't like your putdown.

 

[Frame Dragger]

I realize what I'm hypothesising probably sounds a bit out there and maybe there's a better place to talk about this, but I'm interested in this kind of thing and I don't like your putdown.

I'm intersted in the Proto-Saxon language... shall I recite the Dream of The Rood here? No? Oh, because it's PHYSICS forums... not "whatever the hell floats through your head" forums. Got it. For the record, you're not "hypothesizing" anything, you're flailing without fully stating your case. I'm guessing that's because you don't HAVE a case to make with even the rigor requires to formulate a hypothesis.

P.S. "Feala ic on þam beorge gebiden hæbbe wraðra wyrda..."

 

[q_interested]

I'm intersted in the Proto-Saxon language... shall I recite the Dream of The Rood here? No? Oh, because it's PHYSICS forums... not "whatever the hell floats through your head" forums. Got it. For the record, you're not "hypothesizing" anything, you're flailing without fully stating your case. I'm guessing that's because you don't HAVE a case to make with even the rigor requires to formulate a hypothesis.

P.S. "Feala ic on þam beorge gebiden hæbbe wraðra wyrda..."

i realize my argument could have been thought out better, but as i said I don't like your putdown.

 

[Frame Dragger]

i realize my argument could have been thought out better, but as i said I don't like your putdown.

You don't HAVE an argument that you've presented, just the first hints of crackpottery. If you feel that having the absurdity of your own actions thrown in your face is a "putdown"... tough luck.

 

[sylas]

i realize my argument could have been thought out better, but as i said I don't like your putdown.

Welcome to physicsforums, by the way. We have been remiss not to say that earlier.

It's an unfortunate aspect of the internet that somehow when we are not talking face to face its easy to forget the person on the other side. Meaning no offense, but this applies to you as much as to anyone else.

So relax, everyone.

q_interested, if you would like to talk about the physics relating to your question (this is physicsforums, after all, and the question is in the relativity forum) then let's move on to that. You are interacting here with a group of people who are well placed to answer questions on physics, and the answers are not likely to be any different even if packaged differently.

The trick is understanding the answers. This isn't easy. Understanding physics is generally a lifelong journey. You have to actually like that journey to stick with it, and that will apply for most folks here.

In [post=2653065]msg #13[/post] I did my best to put your question into a slightly different form where it no longer has the problem of being inconsistent with physics... which is what you really need if you want to use physics to get an answer.

The answer I gave is one that rocks the world of most people when they first understand it. It took me a while to get to grips with it, when I was first learning about relativity.

If you would like to comment on that post, or even ask what on Earth I am talking about (a common reaction, don't sweat it, I won't take offense), then I think this is the most likely way to get at the implications of the question. By developing it a bit further, we can also get to an understanding of why the original question phrasing is physically impossible... which is a perfectly sensible answer to the question.

Cheers -- sylas

 

[russ_watters]

perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

Yes, if I have a perfectly rigid and massless rod, I could send and impulse faster than the speed of light.

And yes, if the Santa Claus myth is true, it is entirely possible for a 400lb man to land on my roof with flying reindeer and squeeze down a 1' square chimey to delever presents.

So...now how are either of these answers at all helpful? They are both completely meaningless. It seems your real purpose here, though is the assumption that if you can imagine something to be true that it must be true. Sorry, but the universe doesn't work that way. It is not required to conform itself to your desires.

However, if you are interested in in a free-form idle-speculation discussion that won't help you learn physics, there are a lot of places on the net where you can have that discussion till your heart's content. This isn't one of them. We don't humor people's crackpot musings here.

 

[utesfan100]

Yes, if I have a perfectly rigid and massless rod, I could send and impulse faster than the speed of light.

And yes, if the Santa Claus myth is true, it is entirely possible for a 400lb man to land on my roof with flying reindeer and squeeze down a 1' square chimey to delever presents.

So...now how are either of these answers at all helpful? They are both completely meaningless. It seems your real purpose here, though is the assumption that if you can imagine something to be true that it must be true. Sorry, but the universe doesn't work that way. It is not required to conform itself to your desires.

However, if you are interested in in a free-form idle-speculation discussion that won't help you learn physics, there are a lot of places on the net where you can have that discussion till your heart's content. This isn't one of them. We don't humor people's crackpot musings here.

Interesting post...

The speed of sound in a material is the square root of the ratio of the modulus of elasticity to the density.

How stiff of a rod would you need for the classical speed of sound to be the speed of light?

If you got close to this stiffness I think you would need to look here.
http://en.wikipedia.org/wiki/Relativistic_Euler_equations

 

[Altabeh]

i realize my argument could have been thought out better.

No it couldn't! All people here shared their feelings over the "philosophical" musing of yours and I think all were quite right about what they suggested as someone who can actually think the way a physicist does not a pragmatic philosopher. I believe you are a perfectionist even in the type of philosophy you are into but physics does not accept people of your type just because it's against perfectionism but realism. We don't care what the theory is in case it is not in agreement with experiment. A friend of mine has the same approach to physics as yours but is always unfortunate to proceed in the understanding of most simple things in physics because of the narrow ideas and solutions he got in facing the poblems, all being related to his own version of philosophy and don't get me wrong, no offense, he can easily be referred to as being ignorant by this crackpot way of looking at physics.

Would you be able to name some of those people who probably made history in this zone through their own version of "Newton's second law", I don't know, in terms of crackpot theoretical musings\languages? As an alive example, you can find the "non-mainstream" unified field theories including http://en.wikipedia.org/wiki/Heim_theory" " which have been almost put aside by the physics community due to being so much less strong than the usual approaches and mainly "out the zone" because of offering a more limiting framework to work within it! Furthermore, these theories are sometimes so inaccurate in their predictions and this part contributes more to them being rejected by physicists than the other reasons menstioned above! I don't know if you really belong to this way that you think it's right in physics' perspective but the history has showed us this won't take you anywhere good and the fate is this that you see how people react to your ideas!

AB

 

[sylas]

No it couldn't! All people here shared their feelings over the "philosophical" musing of yours and I think all were quite right about what they suggested as someone who can actually think the way a physicist does not a pragmatic philosopher. I believe you are a perfectionist even in the type of philosophy you are into but physics does not accept people of your type just because it's against perfectionism but realism. We don't care what the theory is in case it is not in agreement with experiment. A friend of mine has the same approach to physics as yours but is always unfortunate to proceed in the understanding of most simple things in physics because of the narrow ideas and solutions he got in facing the poblems, all being related to his own version of philosophy and don't get me wrong, no offense, he can easily be referred to as being ignorant by this crackpot way of looking at physics.

Would you be able to name some of those people who probably made history in this zone through their own version of "Newton's second law", I don't know, in terms of crackpot theoretical musings\languages? As an alive example, you can find the "non-mainstream" unified field theories including http://en.wikipedia.org/wiki/Heim_theory" " which have been almost put aside by the physics community due to being so much less strong than the usual approaches and mainly "out the zone" because of offering a more limiting framework to work within it! Furthermore, these theories are sometimes so inaccurate in their predictions and this part contributes more to them being rejected by physicists than the other reasons menstioned above! I don't know if you really belong to this way that you think it's right in physics' perspective but the history has showed us this won't take you anywhere good and the fate is this that you see how people react to your ideas!

AB

This is getting into philosophy of science now. I'm not entirely sure of what you are saying here. Science does indeed manage fine with mavericks, but I am not aware of ANY useful discovery in physics made by someone who hadn't taken the time to learn about the theories they sought to replace -- unless it was by accident and recognized by someone else who had the knowledge to see the significance.

Lisi is a physicist in the full sense of the word. He got his PhD in physics in 1999, and he publishes in science journals and goes to all the normal conferences. He's different because he doesn't have an academic position and spends a lot of time in adventure sports rather than being physics only. But that's no matter. He himself recognizes that his "exceptionally simple theory of everything" is a bit of a long shot, and that's okay. He has done the hard yards, for many years, of learning about the field which makes his proposals more credible than someone who hasn't learned about the science they want to change. His theory hasn't worked out, but that's normal for new ideas.

I don't know so much about Heim, but I believe it is a similar case.

Now where physicsforums comes into the picture is that we are an education site. The main aim here is to learn about physics as it is currently used by working physicists. No doubt that will change in the future, and perhaps even someone who started out learning physics here might bring about those changes.

That person will need to be able to think outside the box. But before you can think outside the box, you have to know where the box actually is! And that's what we are about here.

There's quite definitely no prospect whatsoever of the questions in this thread leading to some new insight to contribute to physics. But that's fine, because if you check the guidelines, the idea is to get insights to help contribute to the knowledge of members about physics. And that is definitely possible.

If someone objects to being told emphatically that a certain thought experiment is physically impossible, then they are probably in the wrong place. They should go to some other site where people try to invent new theories without troubling to learn the old ones first... and the rest of us can ignore them.

But if they want to know WHY a certain though experiment is physically impossible, then they are on the road to learning about physics, which is ultimately the only way they are ever going to redefine notions of what is and is not physically possible. And they won't do it from this thought experiment. The value in this thought experiment is in what it can tell you about physics, and nothing else.

I think you may be saying much the same thing... but I'm not entirely sure. So here's my take on it.

Cheers -- sylas

 

[Altabeh]

The whole idea of my last post was that even making a new theory that can describe something seeked out theoretically, which means has been proven to exist in reality, requires strict knowledge of all other related sciences to not be possibly violating them. In the Lisi's theory, there are many problems, the biggest of them being the fact that two generations of fermions do not have the correct quantum numbers in his model leaving much doubt about the capacity of the theory to be adequate in the long run to answer the fundamental questions. This kind of deficiency is structural and leaves no room for so many of physicists to believe it can do something. The other theory is suspected to have wrong predictions partially and that it is said to be a Higgs-less theory. These are from non-mainstream physics and it is clear that they could be largely subsided in comparison to a theory like standard model and this is normal unless one sees the theory from a unique angle where the outcome is not as important as the process or let's say the result can be anything in case you seem to be satisfied enough by the procedure itself to ignore all facts together!

In this case, since the outcome cannot be accepted by the ordinary kind of physics you and everybody else around here deals with on a regular basis even if the reasoning might make sense somehow, one the other hand taking for granted the fact that one can't call things such as a "timeless universe" or "no speed exists at all" pictures physics, there's no such thing as middle ground at least between me and the idea holder on the subject as long as I find myself in a situation attached to the observational aspects of the problem in question. I tend to not pass the red lines of "reality" and "imagination".

AB

 

[HallsofIvy]

It is precisely this thought experiment that shows that, even theoretically, there cannot be a "perfectly rigid" object.

perhaps I'm on the wrong forum, but again, this seems to be breaking down into minutaie before its even begun.
maybe i haven't got the mathematical skills or am not as great as some of you, but often one's imagined greatness is an obstacle to enquiry. does physics ever progress?
where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation? -I'm not getting at you Ivy, its a more general attack.
it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

I said nothing about a five light year long stick. I said that this shows why, in relativity, objects cannot be perfectly rigid.

 

[Fredrik]

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical let's suppose..apparently its impossible to travel at the speed of light, but it didn't stop Einstein theorising.
I actually think the Original question is a bloody good one.
okay what if the stick is 5 metres long - and I poke you with it...you will feel the movement instantaneously - forgetting relativism, and internal factors for a second - so notionally, the resultant poke isn't governed by speed - time goes out of the equation...agreed?

You're overlooking something very important here. First of all, this is the relativity forum, so when someone asks about a perfectly rigid object, the natural interpretation of the question is "what does relativity say about these things?". Second, a perfectly rigid rod (of any length) in special relativity is a logical inconsistency. We don't mind questions that start with things that are impossible in practice ("If I eat a million hamburgers..."), but we understand that questions about things that are impossible in principle ("If I eat myself...") don't have any meaningful answers. (And yes, that example works better in a language where "eat" can't be interpreted as a sexual act, but it's still the best way I know to explain the difference between impossible in practice and impossible in principle).

where would i go to discuss things like this 5 year long stick in a more free environment. where i am not told what cannot happen, but am offered constructive conversation?

There is no such place. There's also no place where you'll get better answers than here.

it seems everyone has somehow accepted that there can exist a stick that's 5 light years long, but have trouble with the physics of a stick 5 metres long.

As I said, we don't mind unrealistic assumptions, but we can't answer questions that assume that the theory we're supposed to use to answer the question is logically inconsistent.

 

[Gatchaman]

I know where we can put this rod!

 

[Frame Dragger]

I know where we can put this rod!

...

...

Heh...

 

[Azeroffs]

So I came across this discussion via stumbleupon, and I'm astounded that no one spotted where q_interested was coming from. Everyone just decided to get all bent out of shape about creating the idea of a perfectly rigid stick.

q_interested states that when you poke someone 5 meters away from you the poke happens instantaneously so he's thinking that logically you could assume that the 5LY stick would also poke instantaneously.

The problem is that the 5 meter stick isn't poking instantaneously. The 5 meter stick actually does compress slightly as you push it forward. So the push-poke isn't happening simultaneously. There is a very tiny delay even in the 5 meter stick, but the delay is so negligible that it seems to be instantaneous. Multiply that delay by the proportion of 5 meters to 5 LY and you'll get a delay of >5 years.

Also, it's perfectly within the laws of physics to conceive of a perfectly rigid stick. Just as you can conceive of a frictionless surface and create thought experiments based on that using physics.

If there was a perfectly rigid substance, then it would be instantaneous no matter how long the stick, but as so many people so adamantly stated, such a thing does not exist and cannot exist in reality.

 

[Fredrik]

So I came across this discussion via stumbleupon,

Never heard of it, but now I have to check it out. Welcome to Physics Forums.

and I'm astounded that no one spotted where q_interested was coming from. Everyone just decided to get all bent out of shape about creating the idea of a perfectly rigid stick.

q_interested states that when you poke someone 5 meters away from you the poke happens instantaneously so he's thinking that logically you could assume that the 5LY stick would also poke instantaneously.

The problem is that the 5 meter stick isn't poking instantaneously. The 5 meter stick actually does compress slightly as you push it forward.

I don't know why you think no one understood that. The finite propagation speed was brought up in the very first reply the OP got. (See post #2 by JesseM).

Also, it's perfectly within the laws of physics to conceive of a perfectly rigid stick. Just as you can conceive of a frictionless surface and create thought experiments based on that using physics.

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

In principle, you could still get the different parts of the stick to start moving at the same time in a specific inertial frame by attaching a rocket to each component part and set a timer in every one of them, so that they all turn on their engines at the same time.

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

 

[Azeroffs]

Never heard of it, but now I have to check it out. Welcome to Physics Forums.

Thanks

I should probably add a disclaimer.. Stumbleupon is like procrastinator crack. You've been warned :P

I don't know why you think no one understood that. The finite propagation speed was brought up in the very first reply the OP got. (See post #2 by JesseM).

well its clear that q_interested wasn't aware.

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

In principle, you could still get the different parts of the stick to start moving at the same time in a specific inertial frame by attaching a rocket to each component part and set a timer in every one of them, so that they all turn on their engines at the same time.

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

You'll have to bear with me because I'm not entirely familiar with these concepts, but I think the idea of a completely rigid stick would be equivalent to one very large inertial frame.

Tbh, I don't understand a lot of that post, so I'll just take your word for it. Correct me if I'm wrong, but I'd think there are logical inconsistencies with frictionless surfaces and instantaneous acceleration as well.

 

[JesseM]

This is actually not true. The existence of perfectly rigid sticks would make special relativity logically inconsistent. That's why rigid sticks are unacceptable, and things like frictionless surfaces and instantaneous acceleration are OK.

It depends what you mean by "rigid" I think. As you say:

First problem: If the component parts start moving at the same time in one inertial frame, they don't start moving at the same time in others. So if it's "rigid" in one inertial frame, it's not in others.

This does show you'd get a logical inconsistency if you assumed that pushing a particular stick at a particular time would result in the other end moving "instantaneously" when the same set of events was analyzed using different inertial frames--according to the relativity of simultaneity, if two events are simultaneous in one frame (like the event of pushing one one end and the event of the other end accelerating) then they are non-simultaneous in other frames (in some frames the far end will accelerate after the near end was pushed but before there's been time for a light signal to get from the event of the near end being pushed to the far end, in other frames the far end will actually accelerate before the near end was pushed). However, this doesn't necessarily show a problem with the idea of a stick made out of some kind of tachyonic material which always accelerates instantaneously in its own rest frame, which you discuss below:

The much more serious problem: Suppose that a stick made of pure "unobtainium" has the property that a push at one end will make all the component parts start moving at the same time in the inertial frame in which the rod started out at rest. Then we can derive a logical inconsistency, by the methods used here. (The post contains a typo. See #138 for the correction).

You correctly point out that FTL signalling (including 'instantaneous' signalling) would lead to causality violations (sending information backwards in time) in SR, but I don't think that's the same as a "logical inconsistency". As I'm sure you're aware, causality violations appear in solutions of general relativity which contain closed timelike curves but most physicists don't think that means we can rule out such solutions a priori on purely logical grounds, instead they usually invoke something like the Novikov self-consistency principle to show how such solutions need not lead to any logical paradoxes. This seems to be the type of resolution that Demystifier was talking about in post #137 on that other thread.

 

[sfan]

I've found a good site here that actually backs up the OP's theory and possibly prove some of the doubters wrong, it's backed up with technical drawings as well, so if you could get a pole long enough, it would be possible.

http://bit.ly/9SjS2u

 

[Dale]

Hi sfan, welcome to PF

That website is pretty clearly intended to be a joke making fun of the OP and other similarly wacky ideas. I especially enjoyed the 9-down-6-up generator idea. Very funny.

 

[sfan]

Yeah i know, i just came across this thread from stumbleupon and it instantly reminded me of those troll science cartoons

 

[JesseM]

I've found a good site here that actually backs up the OP's theory and possibly prove some of the doubters wrong, it's backed up with technical drawings as well, so if you could get a pole long enough, it would be possible.

http://bit.ly/9SjS2u

Hahaha, that page is hilarious, thanks for the link!

 

[Fredrik]

...something like the Novikov self-consistency principle to show how such solutions need not lead to any logical paradoxes.

It sounds like that principle is just dismissing the solutions that contain actual inconsistencies, while permitting the ones that don't. I agree that there are consistent solutions that contain a CTC. (I'm thinking specifically about the scenario described in the article that involves a billiard ball coming out of a wormhole in a way that knocks a younger version of itself into the other end of the wormhole). This idea makes sense in GR where spacetime containing the CTC and the motion of matter in it are different aspects of the same solution of Einstein's equation. But in SR, matter is added "manually" to a background spacetime, so the principle would at least have to be modified to deal with that. The only modification I can think of that avoids paradoxes is to simply say that we're not allowed to add matter to spacetime in a way that leads to paradoxes. So the principle doesn't really "resolve" the paradox I described in the other thread; it just says that I'm not allowed to construct it because it contains a contradiction. It doesn't say which of the ingredients are disallowed, it just says that I'm not allowed to include all of them.

I don't think a "resolution" like that has any value. It's essentially just saying that "OK, you have a scenario where tachyons cause a contradiction, but that doesn't mean that tachyons don't exist. It could also mean that if you try to set up that experiment, a ninja turtle will appear out of nowhere and chop your head off". Demystifier argued that I would simply be unable to choose to complete the setup, but I don't find that any more plausible than ninja turtles, since it violates the illusion of free will. This was discussed in the other thread:

If the reply you get is "the message you sent hit your daughter in the head and killed her", and you're still unable to stop yourself from sending the message, you don't even have the illusion of free will.

There are ways to make sure that the reply can be trusted, at least to such a degree that you would feel that sending the message would be to gamble with your daughters life with nothing substantial to gain. You can e.g. use encryption and digital signatures, and put someone you trust at the other end. That someone doesn't even have to be a person. It could be a computer that you programmed yourself, and rigged to explode if tampered with.

 

[JesseM]

It sounds like that principle is just dismissing the solutions that contain actual inconsistencies, while permitting the ones that don't.

How could a "solution" contain an inconsistency? A solution is a spacetime manifold where the Einstein field equations hold at every point (and if you consider other laws of physics like electromagnetism, they should hold locally at every point too), I don't see how you can imagine a spacetime that contains an "inconsistency" but where this is still true. For example, if you try to imagine an inconsistent scenario where a billiard ball goes into a wormhole, then emerges in the past and knocks its younger self away so it doesn't go into the wormhole, then either you'd need to imagine two parallel versions of the region of spacetime where the billiard ball is headed for the wormhole (or at least parallel truths about whether the billiard ball's worldline passes through certain points in spacetime, like points near the wormhole mouth where the ball's trajectory would take it if it wasn't hit by its future self), or you'd have to imagine a discontinuity in the worldline of the billiard ball such that after being knocked out of the way, it suddenly jumps in position so it's back on course to go into the wormhole and go back in time and knock its younger self out of the way.

I agree that there are consistent solutions that contain a CTC. (I'm thinking specifically about the scenario described in the article that involves a billiard ball coming out of a wormhole in a way that knocks a younger version of itself into the other end of the wormhole). This idea makes sense in GR where spacetime containing the CTC and the motion of matter in it are different aspects of the same solution of Einstein's equation. But in SR, matter is added "manually" to a background spacetime, so the principle would at least have to be modified to deal with that. The only modification I can think of that avoids paradoxes is to simply say that we're not allowed to add matter to spacetime in a way that leads to paradoxes.

When you state it that way it sounds like a circular argument, but I would again state it in terms of the idea that all the laws of physics, including the ones governing the behavior of matter, work the same way in each local neighborhood of a point, and we're looking for a single 4D manifold where this is true in the neighborhood of every point on the manifold. The absence of "paradoxes" should follow directly from this, it isn't an additional requirement.

I don't think a "resolution" like that has any value. It's essentially just saying that "OK, you have a scenario where tachyons cause a contradiction, but that doesn't mean that tachyons don't exist. It could also mean that if you try to set up that experiment, a ninja turtle will appear out of nowhere and chop your head off".

But the sudden appearance of some weird phenomenon like a ninja turtle would actually be a violation of the principle because it would mean the same local laws don't apply in each local region, instead you need new laws that only get invoked when the danger of paradoxes looms. Pay particular attention to this section of the wikipedia article:

The Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes either that there is only one timeline, or that any alternative timelines (such as those postulated by the many-worlds interpretation of quantum mechanics) are not accessible.

Given these assumptions, the constraint that time travel must not lead to inconsistent outcomes could be seen merely as a tautology, a self-evident truth that cannot possibly be false, because if you make the assumption that it is false this would lead to a logical paradox. However, the Novikov self-consistency principle is intended to go beyond just the statement that history must be consistent, making the additional nontrivial assumption that the universe obeys the same local laws of physics in situations involving time travel that it does in regions of spacetime that lack closed timelike curves. This is made clear in the above-mentioned Cauchy problem in spacetimes with closed timelike curves, where the authors write:

"That the principle of self-consistency is not totally tautological becomes clear when one considers the following alternative: The laws of physics might permit CTC's; and when CTC's occur, they might trigger new kinds of local physics which we have not previously met. ... The principle of self-consistency is intended to rule out such behavior. It insists that local physics is governed by the same types of physical laws as we deal with in the absence of CTC's: the laws that entail self-consistent single valuedness for the fields. In essence, the principle of self-consistency is a principle of no new physics. If one is inclined from the outset to ignore or discount the possibility of new physics, then one will regard self-consistency as a trivial principle."

 

[Fredrik]

How could a "solution" contain an inconsistency?

OK, that's a good point. It can't, of course. But that has to mean that the principle doesn't actually say anything about general relativity. The text you're quoting (from the Wikipedia article) suggests that all it does is to rule out spontaneously appearing ninja turtles and similar stuff that aren't part of the theory anyway.

When you state it that way it sounds like a circular argument, but I would again state it in terms of the idea that all the laws of physics, including the ones governing the behavior of matter, work the same way in each local neighborhood of a point, and we're looking for a single 4D manifold where this is true in the neighborhood of every point on the manifold.

So how you would you apply this principle to SR, where matter is added manually to a fixed spacetime? (This is true both for classical SR and special relativistic QM). How does it invalidate the argument that instant messages makes SR inconsistent?

 

[JesseM]

So how you would you apply this principle to SR, where matter is added manually to a fixed spacetime? (This is true both for classical SR and special relativistic QM). How does it invalidate the argument that instant messages makes SR inconsistent?

Whatever particles/fields/etc. are added, it's assumed there are some known Lorentz-invariant laws governing their behavior, laws which can be stated in local form like the differential form of Maxwell's equations. And if tachyons exist and don't have a preferred frame, one should be able to write down some local Lorentz-invariant laws governing their behavior too. So, can't you just impose the condition that the global collection of local facts about particles/fields at each point in spacetime must satisfy these same local laws at every point? This global condition on allowable "histories" of particles/fields throughout the entire 4D spacetime should guarantee that the Novikov self-consistency principle would hold.

 

[PAllen]

It always struck me that tachyons observe the math of SR, but not the physics; more that the derivation of SR would break down, so you would need something different if tachyons existed.

Simultaneity could be objectively defined a frame independent way, for example. That would seem to require a major modification of SR.

 

[JesseM]

Simultaneity could be objectively defined a frame independent way, for example. That would seem to require a major modification of SR.

Only if you assume causality must hold would tachyons necessarily pick out a frame-invariant definition of simultaneity. If you allow tachyons to travel backwards in time in every inertial frame, which is the idea I and Fredrik have been discussing, then they can still behave in a frame-invariant way and satisfy the postulates of SR, at the cost of violating causality and allowing information about a given event E to be transmitted into E's own past light cone.

 

[PAllen]

Only if you assume causality must hold would tachyons necessarily pick out a frame-invariant definition of simultaneity. If you allow tachyons to travel backwards in time in every inertial frame, which is the idea I and Fredrik have been discussing, then they can still behave in a frame-invariant way and satisfy the postulates of SR, at the cost of violating causality and allowing information about a given event E to be transmitted into E's own past light cone.

I am not understanding this. Can you exlain more how causality fits in? I'm just thinking I can send signals as fast as I want, so I can set up absolute simultaneity between any two regions of space to any desired precision. Why would either party think they are getting messages from the future, each believing the other's messages were from the future? One would instead believe a common 'now' has been established to arbitrary precison over arbitrary distance.