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Faster than light Information Possible?

  1. Oct 4, 2009 #1
    Ok my question has to do with a thought experiment which seems to allow faster than light transfer of information. I'm sure there is a problem with it....I just cant seem to figure out what that problem is...so I thought you guys could help.

    First you build a sort of telegraph system that uses a laser beam which is constantly on. One station shines a laser at the other, and by rocking the beam back and forth it either hits the tiny receiver or does not....in other words imagine a laser pointer hooked onto a sort of pivot. It is adjusted so that the beam hits a recieving unit, but the recieving unit is very small. When the laser hits the unit then that makes a tone that the person monitoring the receiver can hear. Now by turning the laster slightly on its pivot it can be made to miss the unit and the tone will go off. If you wanted to you could put a tiny spring onto the pivot so that it holds the laser in a position where the beam misses the reveiver....then by pushing down on the laser pointer it will pivot and the beam will hit the receiver and make a tone...then by letting up the spring makes it pivot back out of range of the reciever...(In this way it is basically just like a telegraph system capable of sending little dots and dashes)
    Ok sorry for all that background....I just wanted to make the operation of the device as clear as possible....now on to my thought experiement.
    Imaging the reciever is on the moon and the laser is on the earth.....basically the same set up as before, except of course in this set up the laser Pointer is VERY powerful and can make a beam hit the receiver on the moon with no problem. Ok so the long beam of light is just a straight beam and when you move it it instantly moves the straight beam as well....therefore the longer the beam, the more distance the information will have to travel from sender to receiver.....and therefore the faster the information will be traveling.
    Now Light takes something like 1.5 seconds (thats approximate...I dont remember exactly but I think Im close) to go from the moon to the earth, or vice versa. Since the long beam of light is just a straight beam like a stick and it moves at the same rate on the moon that it does on earth then the telegraph information would travel basically instantly every time you wobble the beam would it not? Does this not allow for faster than light transfer of information? I am fairly certain that I am correct in assuming that the laser beam would not bend....but if I am wrong about this ....what if you were to substitute some kind of exceptionaly long stick that was perfectly rigid that went from the earth to the moon....(same set up as before...just using a long stick instead of a beam of light.....I didnt use this Idea before because it is hard to visualize a stick which could be this long and still be completely rigid...so it is basically hypothetical.....none the less it too would seem to transfer information faster than light....you could even get rid of the reveiver and just have the guy watching the stick on the moon to see when it wiggles)
    Anyway....could someone please help me see where it is that I am wrong...(assuming I am of course and that transfer of info faster than light speed is impossible :devil:)
     
  2. jcsd
  3. Oct 4, 2009 #2

    Pengwuino

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    This is a common misconception. Lets say your device is being received as "on", as in the laser is hitting it as the person on the moon sees it. Now, the instant the person at Earth moves his laser, you still have 1.5 seconds (if that is the correct time) worth of photons moving towards the receiver that will still hit the reciever. Only when 1.5s has passed, will light that was moving towards a spot not on the receiver arrive.

    Think of the Sun and the Earth. If the sun were to mysteriously disappear at some instant, light would still be received by the Earth for 7 or so minutes since photons that were emitted from the sun still are still in transit despite the sun disappearing. So 7 minutes later, all of a sudden on Earth, light would stop arriving and we'd see the "signal" that the sun disappeared.
     
  4. Oct 4, 2009 #3

    Janus

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    Once the light leaves the laser it is no longer effected by anything the laser does after that. When you move the laser, the light that left it before that moment continues along in the same direction as it was before you moved the laser. The whole length of the beam will not move, just that part consisting of light that leaves the laser after it started to move. In other words, it will take 1.5 sec between the time that you move the laser and the detection of that movement by the receiver.
     
  5. Oct 4, 2009 #4
    Because the light that leaves the laser emitter takes 1.5 seconds to reach the moon, it would take 1.5 seconds for info to be transported, assuming that any info can be transfered using the emitter. any light that leaves the laser will no longer be affected by ht eactions of the emitter so there would still be a delay.

    Hope this helped,
    BOB
     
  6. Oct 4, 2009 #5
    Lol, I pondered this exact scenario with a stick..... anyone think instantaneous information transfer is possible with an exceptionally long stick? I.e. Moving it on one side will cause instantaneous moving on the other; theoretically moving information this way would be faster than moving information by something like RF or wire which is constrained by the speed of light...

    Funny how im not the only one thinking of this weird question..
     
  7. Oct 4, 2009 #6
    I just discovered there are many threads about this here in physics forums; we are like minded, hah.
     
  8. Oct 4, 2009 #7

    russ_watters

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    Sticks are not rigid. The information will travel at the speed of sound in the stick.

    For your laser idea, you can demonstrate the effect with a garden hose, sweeping it rapidly back and forth and seeing how it curves.
     
  9. Oct 5, 2009 #8
    Even if we assume there is an ideal stick that is perfectly rigid if any part of the stick approached the speed of light it would gain mass as all things non light do. Thus making it impossible to go faster without more energy which in turn adds more mass. Group velocities can travel faster than light but that is more of a mathematical construct than physical..sort of.
     
  10. Oct 5, 2009 #9
    Right I noticed that as well, of course after I posted my question! I guess everything lies on whether or not the laser beam bends when you move it.....which according to what Ive read here it would.....I remember reading this scenerio where a physicist claimed that it was a special exception to information traveling faster than light.....I was a bit skeptical, and now Im starting to think that the guy who suggested this must have been a poor physicist LOL! It all seems to lie upon the absolute rigidness of the medium....whether it be a ray of light or a stick....and it seems that absolute rigidness cannot truley exist (as in the stick it would travel in waves at the speed of sound according to what other posters here have said). I will try to find the "physicist" who claimed that info would travel faster than the speed of light, just in case I missed something in my description of his idea, but I think I represented it as he stated it.....I was stuck on his assertion that a laser beam would not bend, but after reading peoples posts and giving it some thought, I would have to concurr with what everyone else has said....Light from a laser probably would bend(By bending I am meaning what it would look like as veiwed from the side and also of course assuming some kind of medium like smoke or dust for it to shine through....which is not likely considering the distance from here to the moon is mainly made up of the vaccume of space LOL....but it is just a thought experiement, and thinking of it this way helps me visualize it!) So since this thougt experiement relies on something absolutaly rigid, and nothing like that could theoretically exist..... unless someone can suggest something that long that COULD remain rigid...then I would guess this one is pretty much case closed.

    PS I found one more example of the "rigid stick" Idea that at first seems to side step the whole rigidness aspect....and that is using stick....but instead of wiggling it....pulling on it.....You would hold one end....and your buddy on the moon would have a hold of the other....when you yank your side...he feels it instantly, and therefore by a combination of yanks you could send a message.....I admit at first this seems pretty good, but the source I read was pretty convincing that although it might seem instantanious as we imagine it in our head...in reality at such a distance it would not be as instant as we might imagine.....the movement would in practice not move any faster than would sound waves through the medium.
     
  11. Oct 5, 2009 #10
    Yeah I am starting to agree with this....I wrote a longer thing about my thoughts on what people had said in another reply here on this page.....I just had to reply to you as well 'cause I wanted to say KUDOS on those very nice qoutes at the bottom of your reply!!! I really liked 'em. Im gonna copy them and save them in my notebook. I like to collect cool phraises and qoutes. Thanks!
     
  12. Oct 5, 2009 #11
    No, the beam of light would not "bend" or even appear to because you moved the source. It can do so in a gravitational field but that is another topic. The suggestion that if you tug that it sidesteps the rigidness is also incorrect. The modulus of elasticity dose not change for a given material. You are only changing the direction of the force to align parallel with the greatest section of material. You are right that it would transfer at around the speed of sound. As I said previously even if we assume there is an ideal stick that is perfectly rigid if any part of the stick approached the speed of light it would gain mass this has been experimentally verified and you can not get around it. If your physicist was talking about "information" traveling faster than light he could have been talking about the aforementioned group velocities. If i remember correctly this is demonstrated in plasmas. But don't hold me to that last statement.
     
  13. Oct 5, 2009 #12
    Whoah....now wait just a second....this whole scenerio is based on whether or not a beam of light 'bends' when you move it....if it does not then it would indeed be the same as having a perfectly rigid stick. How can you claim that it does not bend (or appear to....lets just assume every time I say 'bend' I add "or appear to" right after it so that doesnt become a source of contention:smile: ) If light is made of photons and you change the angle of the emiter of photons, then at that moment the light beam will bend and that bend will travel down the length at the speed of light.......and the speed of light will take time (approx 1.5 sec) to reach the moon!!!! How can that not be a "bend"? I suppose U could call it a wave....does my confusion simply lie in us defining the term "bend" differently? Perhaps I should call it a "break" instead of a bend? If my terminology was confusing then I appologise....however if you did understand my meaning before then you are attesting that the entire beam of light changes angles instantly and there is no bend...then it would indeed have to be able to send information instantly....therefore faster than light! It would be no different than having a perfectly rigid stick! I do hope we are only differing in termanology.....if not then youve lost me! LOL!
     
  14. Oct 5, 2009 #13
    Ive got one side question for you aside from the main one. You state that as something approaching the speed of light it gains mass, which I know is based on Einsteins theories....this I am not doubting....however you state that this fact has been proven by experimentation. Id like to know about the experiment(s) in which this was done....it sounds interesting. Could you let me know the name of the experiment(s) so I can look it up online...Thanks!
     
  15. Oct 5, 2009 #14

    russ_watters

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    I think physalis misunderstood what was happening. By "move" we really mean "rotate" the laser beam. The beam does, in fact, curve in that case.
     
  16. Oct 5, 2009 #15

    russ_watters

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    The functioning of particle accelerators is dependent on SR.
     
  17. Oct 5, 2009 #16
    I don't know alot about this stuff, but i think i understand why bhornbuckle75 is confused with physalis statement.

    It seems as is if you are picturing light to be one long "thing" like a stick; however, it is better to think of it in packets or photons. Think of it like shooting a bunch of BB's. As you fire one BB, or one photon, it will start to move toward the moon, and is not longer effected by how you move the gun or laser.

    So a laser in the scenario is more like a machine gun, then a stick.
     
  18. Oct 6, 2009 #17
    Sure I understand that Satalite Positioning systems we use to navigate here on earth also use SR theory in order to operate properly.....I am not asking for experimential proof of SR in and of itself....but I am asking what experiment was there that was done which proved a particular concept of it...namely that as objects speed up to light speed they start to gain mass. I was not aware of any experiment done which showed this particular Idea of SR. Simply because one part of SR can be proven does not mean that every thing about it has been proven experimentaly. Not that I am expecting it to be disproven or anything....I was simply asking as to what experimental evidence has been done to prove this one particular aspect of SR.
     
  19. Oct 6, 2009 #18
    Right this is how I understand it now...I was originaly going by what some "physicist" said who may not have been all that knowledgeable about what he was talking about (I wont say for certain until I can find the original link and share it with you guys) Another way to think of it bending would be to think of one of those sprinkler hoses that shoot out water in a 'rat-a-tat-tat' sort of way(for lack of a better way to say it LOL) Well when you rotate (another poster pointed out this is a more correct way to suggest the movement I was talking about with the laser, and I agree) the sprinkler head...you can still see the 'stream' of water 'appear' to bend.....even though it is made of of individual little 'packets' of water....Actually I guess I could say the same thing about any 'stream' of water since it is made up of individual little drops...which themselves are made of individual atoms....etc. this is why I tried to say the light 'appeared' to bend in some of my posts....to avoid confusion.
     
  20. Oct 6, 2009 #19

    russ_watters

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    You missed my point: particle accelerators accelerate particles up to relativistic velocities with energy inputs consistent with SR.
     
  21. Oct 6, 2009 #20
    I am too lazy to read this whole thread, but has anyone mentioned quantum entanglement yet?
     
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