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Speed of light what if question.

  1. Apr 22, 2005 #1
    Here is something I have thought about and would like to hear some other peoples opinions on what would happen if.
    You had a huge disc in space. Made from an indestructable material that wouldn't bend or twist. The disc has a circumference of 599,584,916 meters. The disc would be spun by a shaft with a diameter of say 19 meters. Then you space clocks at regular intervals in line with each other from the center of the disc to the outer edge. With another clock not attached, to be a control clock. Then you spin the shaft at 60 RPMs. Or one turn per second. So saying that nothing would fall apart. A clock placed at 9.5 meters out from the center of the disc. Would be traveling approximately 60 meters per second. (That is equal to the circumference of the disc at that point.) As you kept moving out each clock would be traveling faster and faster than the ones before it. Then at about 4,771,351 meters out from the disc center the clock there would be spinning at the speed of light. (Traveling a circumference of 299,792,458 meters) The clock at the outer edge of the disc would be traveling 2x the speed of light.
    Some things I know would happen are. Each clock although inline with each other would be in different space times. The disc itself would be in different space times from itself. From 4,771,351 out from the center and past that it would be going back in time. Right? Also the mass of the disc would increase from the center out. At 4,771,351 meters out from center and past that it would reach infinite mass? Would that mean that even though the shaft that spins the disc isnt moving that fast. 60 meters a second at its fastest point. It couldn't go that fast cause it would then be attached to a disc that after a certain point has an infinate mass? So the shaft would then have a speed limit of just under 30 rpms? (Which would mean that the outer edge of the disc would be traveling at just under the speed of light)
    So even with the shaft having a max speed of just under 30 RPMs. The clock and the atoms at the outer edge of the disc would be barely moving through time, compared to the control clock (cause the would be moving just under the speed of light). Or to a clock and the atoms at the center of the disc. If the disc were to keep spinning. Say 100 yrs on the control clock. The atoms and the clock on the outer edge of the disc would be almost 100 years younger than the clock and the atoms at the center of the disc. So the same disc would exist both in the present (in relation to the static control clock) and nearly 100 yrs in the past?
  2. jcsd
  3. Apr 22, 2005 #2
    I have another question to ask: does the retard time affect the speed of light?
  4. Apr 22, 2005 #3
    Wrong assumption from the beginning... Do you see why?

    The inner part of the disk would have to break from the outer edge, otherwise you would need an infinite force (impossible) to pull the string.
  5. Apr 22, 2005 #4
    I was thinking more of it accelarating up to speed and turned by a motor. Also the whole thing being indestructable. Yes though it would take an impossible infinite force to turn it at 60 RPMs. What about the second part though. It could theoretically reach just under 30 RPMs. Which means none of it would pass the speed of light (just under at the outer edge) which means it wouldnt need an infinte force to spin it so theoretically possible. So at just under 30 RPMs what about the time thing with the outer clock barely moving in time compared to the clock in the center, or the control clock. The disc could theoretically exist in the present (control clock) and 100 yrs in the past (clock at outer edge)?
  6. Apr 22, 2005 #5
    You seem to have made a lot of inacurate assumptions, so we can "guess"the result but we cannot rely on our current equations to calculate something they are not meant to.

    1. None of the clocks would be traveling at c or 2c. Even if you are traveling past me at .9c and I am traveling past you at .9c in the opposite, it does not mean that our velocities are 1.8c. Spacetime does not allow this.

    2. Infinite force :surprised Well if we assumed that, we could assume a lot of things which relativity forbids.

    Im just pointing out a few things, I like getting hypothetical over things to hehe
  7. Apr 22, 2005 #6
    I think you misunderstood what I am saying. The clocks are all in line with each other. Each going further out on the disc, but they stay in line. Example: 1 Clock in the center of the disc 1 clock at 4meters out from center another at 100meters out etc.. With clocks spaced out all the way to the edge. With the disc spinning at just under 30 RPMs. The circumference of the disc is 599,584,916 meters so with almost 1 rotation every 2 seconds The farthest point on the edge of the disc where the last clock is would be moving nearly the speed of light. The clock at 4 meters out would be moving approx. 25 meters per second. (The circumference of the circle at 4 meters out from center.) So even though the clocks never pass each other and stay perfectly in line. They will be moving faster as you go out from the center cause they travel a greater distance with each rotation. Then cause the faster something moves the slower time moves. (relative to a stationary object) Compared to the stationary control clock. The clock at the outer edge would be barely moving through time. If you started all the clocks at say 12:00 on 1/1/2006. Then let the disc spin for 100 yrs ( on the stationary clock) The stationary clock would read 12:00 on 1/1/2106. The clock at the very edge cause it has been moving nearly the speed of light may only read 12:05 on 1/1/2006. Each clock from the center on out would show a lower date and time as the one before it. All the clocks although in line with each other and traveling the same RPMs. Would be moving at different speeds. Faster as you went out from the center. So wouldnt that mean that the disc would be 100 yrs old or so at the center and only 5 mins old at the edge? The same disc would exist in the present and 100 yrs in the past?

    The infinite force would be the reason that the disc could only spin at just under 30 RPMS. That way no part of the disc would be spinning at the speed of light. Which causes infinite mass. It would need infinite force to move an infinit mass. My 1st question breaks laws of physics. The 2nd part though with the fastest point of the disc spinning just under the speed of light doesnt break any laws.
  8. Apr 22, 2005 #7

    how about my question...../.\
  9. Apr 22, 2005 #8


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    The disk would be of various ages as you move outward, that much is correct (the center of the disc experiences 100yrs while the edge experinces only 5 min), But that does not mean that mean that the outer parts of the disc would "be in the Past".
  10. Apr 22, 2005 #9


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    The speed of light is the same for all inertial frames.
  11. Apr 23, 2005 #10
    What would that mean then for the RPMs and speed of the disc? The motor turning it is set to 29 RPMs. If you used the control clock or a clock at the center. In 100 years at 29 RPMs the disc would have made 1,524,240,000 rotations. According to the outer clock though it would have made that many rotations in 5 mins. Which would mean that it would have to be turning at alot more RPMS so turning faster, but it is really turning at 29 RPMs. What would it be? Would that mean only a stationary clock is accurate?
  12. Apr 23, 2005 #11
    well i mean, if a light is near a gravity field, then the time will retard. Then we see the light travelling slower...
  13. Apr 23, 2005 #12
    do you mean time delation here? retarded time in relativity means the other thing....
    Well, the answer is NO , However, in fact, the time delation affects the light in the other way. i.e. frequency.... Have you heard about red shift? Can you use time delation in gravity field to explain it?
  14. Apr 24, 2005 #13
    If the above is true. Would that mean that there IS a constant time, but when something is moving it is avoiding the full passage of the constant time? There is a "constant time" it is just that different objects experience its passage at different rates?

    Also would that mean if someone were able to stand on the outer clock. They would see 100 yrs of events happen in what would be to them 5 mins?

    Or would it also mean that you could think of a "constant time" like a wave that moves at the speed of light. Then when an object moves the "constant time" waves dont pass the object at the same rate as a stationary object. Then if an object moves at the speed of light. It is like hoping on and riding a crest of the "constant time" wave. Therefore it it doesn't experience its passage at all?

    Would it also mean that traveling into the future in a way IS possible. By moving at or near the speed of light or just moving at all. Cause by moving at the speed of light you are staying in the present time riding a "constant time" wave. Then say you do that for a "constant time" of 100 years. Then when you stop because some "constant time" has been steady moving at a constant rate. (Like the spinning disc has always been turning at 29 RPMs and turned 1,524,240,000 times in the 100 yrs that passed on the control clock. Only 5 mins passed on the outer edge clock, but the disc still turned 1,524,240,000 times at 29 RPMs, and the outer clock experienced it all in 5 mins ) You stop 100 yrs in the future, to you though it feels as no time has past?
    Last edited: Apr 24, 2005
  15. Apr 26, 2005 #14
    While the hypothesis that started this thread is intriguing the effects that occur are so localized as to be meaningless. From center to edge there is a light wave length shift and that is about it. To an observer 10 million miles away relative to their point in space the disk is nothing of particular interest. The reflective light emanating from all points on the disk is a constant wavelength.

    Furthermore you have not established two isolated points in the continuum. Since the disk is considered a single point theoretical applications do not apply. Again the post made one think but it has no applicable significance.
  16. Apr 27, 2005 #15

    Wouldn't it be pretty much the same way they tested the theory that time does slow down when an object is moving. They used 2 atomic clocks one on the earth the other in an airplane. After flying the plane for awhile the clock on the plane was slightly behind the clock on the ground.
    The clock on the ground was moving with the earth rotating. So was the plane just faster.

    It is the same with the disc right? The outer clock is moving just under the speed of light. So time would have to slow down for it, just like the clock on the plane. Time would barely move on the outer clock compaired to the center clock or an outside control clock. The motor would have made that many rotations in 100 yrs. It couldn't at 29 RPMs have done it in the time the outer clock would show. So the outer clock is inaccurate. The non- attached control clock, also the least moving would show the most accurate time. Time kept ticking (the disc kept turning) but the outer clock missed most of it.

    Maybe in some ways it is like time is a river, and clocks (any object) measure time by how much water flows past them. If the clock is not moving, the maximum amount of water (time) flows past. Time passes for it at the fastest rate. Then the faster the clock moves with the current the less water passes it. So it counts time slower than when stationary. If the clock is moving at light speed (speed of the current) no water passes it therefore no time. The river though always flows at the same rate. The same way the disc kept spinning at 29 RPMs. So time would be constant? Objects just experience it at different rates?

    If that were all true. Would it mean that if there were 2 twin objects created at the beginning of the universe. One of the objects never moved. The other forever moved at near light speed.

    The stationary object would have experienced as much time as is possible in the universe. It would be as old and would have aged as long as the universe. If it were a clock it would show the true constant time.

    The other, moving object even though created at the same time. (And would really be the same age. Around since the beginning.) It would have only experienced and aged a small amount of time. Basically a smaller amount of time would have passed it by or even happened for it. So just because it doesnt know it doesnt mean that time didnt keep moving. Time just didnt move past it as fast as possible.
    Last edited: Apr 27, 2005
  17. Apr 27, 2005 #16


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    Something rather similar exists in nature ... young pulsars such as that in the Crab Nebula. Particles trapped in the (intense) magnetic field of the pulsar move around it with the same period as the pulsar itself, ~30 ms in this case. What happens where the electrons (etc) are moving relativistically?
  18. Apr 27, 2005 #17


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    The thought experiment is flawed. It assumes material properties that cannot and do not exist. A neutron star is the densest known state of matter in the universe, and it does not exhibit any such properties.
  19. Apr 27, 2005 #18
    Some pulsars (neutron stars) rotate with a period that would give a surface velocity of something like 0.7c if it was a rigid body. It is clearly possible to imagine something to rotate fast enough as to experience measurable relativistic effects.

    The thought experiment is perfectly valid, that the disc edge is spinning at close to the speed of light is just a way to make things clear. The effects would arise at any speed and for any size of disc. A CD in a CD player would work just as fine.
  20. Apr 27, 2005 #19
    UglyEd all you say is correct but once again it is a closed system. So it is only relative to that particular environment. You are applying Macro Theories to the micro. If all that were true I could use a plate spinning on top of a stick and claim interdivisional displacement.
  21. Apr 27, 2005 #20


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    It's not hard to do the calculation; some neutron stars will have a surface that is moving relativistically (say, >0.1c).

    However, what I was referring to is electrons trapped in intense magnetic fields, above the neutron star's surface - they will co-rotate with the star. Here is a simple account of neutron stars (electrons trapped in the magnetic field is mentioned near the bottom).
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