# 5 Light-Year long stick question

RCulling
My friends and I are having an arguement 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.

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RCulling

- Thank you very much

BL4CKCR4Y0NS

I understood all of it until
at the speed of sound

Why is it the speed of sound?

Staff Emeritus
Gold Member

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.

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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 lets 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?

Staff Emeritus
2021 Award

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.

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical lets 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

Homework Helper

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 thats 5 light years long, but have trouble with the physics of a stick 5 metres long.

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Mentor

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 thats 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 thats 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 realise 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 realise 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?

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

Mentor
2021 Award

well i thought that was a given since people have responded to the OP's OP. its asking about a 5 LY stick.
I realise 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! :rofl:

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! :rofl:
I realise what I'm hypothesising probably sounds a bit out there and maybe theres 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 realise what I'm hypothesising probably sounds a bit out there and maybe theres 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 realise my argument could have been thought out better, but as i said I don't like your putdown.

Frame Dragger

i realise 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.

i realise 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

Mentor

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 thats 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 realise 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" [Broken]" 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

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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" [Broken]" 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

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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

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Homework Helper

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 thats 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.

Staff Emeritus
Gold Member

oh for chrissake - you're taking this too literally - the very question is a postulate - more of a philosophical lets 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 thats 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... :tongue2: