Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Faster-than-light experiments at home?

  1. Aug 4, 2004 #1
    Hi, I've been thinking about too quite simple ways to create what I believe to be faster-than-light speeds, and would like to know your opinion.

    First experiment:
    1. Shoot a laser pointer on the moon on a clear night sky.
    2. Move the dot through the diameter of the moon.

    If a person standing on the moon could see the "red dot" (assuming no divergence). Couldn't he "see" it go faster than c if you flicked your wrist quickly enough?

    Second experiment:
    1. Take two sheets of paper, one in each hand and hold them out in front of you, facing you, so that they overlap. Tilt one of them slightly.
    2. Now slowly seperate them.
    3. When the inside edges meet, you see an angle formed between the sheets. The vertex moves perpendicularly to the movement of the sheets.
    4. The smaller the angle, the faster the vertex will move.
    5. Since you can choose the angle to be zero, you can make the vertex go an infinite speed.

    No mass or energy is involved, so no law is broken, but its a awfully simple way to have something (a vertex) go really fast.
  2. jcsd
  3. Aug 4, 2004 #2
    About your first claim, in order to have the laser be seen moving faster than the speed of light, then you would have to flick your wrist at the speed of light. This is the maximum speed attainable in the universe, making the laser still travel at the speed of light to the observer.
    About your second claim, there is not enough info, Im not sure what you mean.

    I had the same kind of thought experiment. Look up my posts and see what the replies were. I think the post was called "speed of light broken?"
  4. Aug 4, 2004 #3


    User Avatar
    Science Advisor
    Homework Helper

    Actually, the 'dot' can move faster than the speed of light. However, the dot is most definitely not an object, but an intersection. A reasonably good occilloscope will have settings that allow a 'dot' to move faster than light as well. Of course, the intensity of the laser would have to be obscene in order to show up at that range and so on. With a reasonably fast spinning mirror, you should be able to get this type of effect on a more reasonable scale. Let's say you have a mirror spinning at 60,000 rpm and the dot is one kilometer away, you'll get a dot traveling at faster than light.

    Similarly, the vertex is an intersection, and not actually a moving object. The classic example of this is, by the way, not two sheets of paper, but a large pair of scissors.

    There's also the classic situation where an observer sees two things moving with apparent speeds of .75c in opposite directions which is also not a violation of SR.
  5. Aug 4, 2004 #4
    Why not go outside, look to the horizon at your west, pick out a Star that is many Light Years away, now turn to the east sky horizon and pick out a Star at a similar distance. The total distance between both Stars would be easy to workout, equating to several Light years distance.

    Now using a laser pointer directed at the West Star, arc your arm with laser, fast across the night sky till your laser points at the Star in the east.

    If you were to physically travel from Star to Star it would take many Light yrs, yet you have virtually located the two Stars faster than light could in real terms.

    The problem is that although you point your laser at a Star, say 10 light years away, you have not waited 10 light yrs for the light to actually arrive there before arcing your arm across the light sky!

    Imagine if the Hubble Space Telescope swivelled its direction in a similar fashion, what sort of data would it retrieve?
  6. Aug 4, 2004 #5
    Scissors are cool, but I have paper on my desk right now, and I can actually provoke an infinitely large speed using a zero angle as much as I want right now! It's so simple to go so fast, it keeps amazing me.

    I thought of this, but the background on which the "dot" travels isn't as well defined. It can't really be observed. I think the moon is a nicer surface to illustrate the concept.

    Thanks for confirming. The scissors and 60 000 rpm mirror are cool.
  7. Aug 5, 2004 #6
    We use a 30,000rpm electric motor with a tiny mirror on top to measure the speed of light in the lab. A thin beam of light travels through a glass plate (at 45 deg) then onto the mirror. It then travels 3m to another mirror and 3m back again before hitting the rotating mirror and then going back through the glass plate. In the time the beam travels 6m, the mirror has moved a tiny bit (a very tiny bit!) and by measuring the distance the beam 'moves' on the glass plate you can work out the speed of light. The motor runs both ways to give an effective speed of 60,000rpm.

    It works really well.

    (.....sorry if this is totally irrelevant!) :smile:
  8. Aug 5, 2004 #7
    Not irrelevant, thanks!
  9. Aug 5, 2004 #8
    There should be a situation in space where the illusion of FTL can happen. Say a star is moving toward the earth at an angle of a few degrees. Observations of the star would show its sidways motion to be faster than actual because light from a previous observation traveled further to reach the earth. I haven't worked the math, but there should be some speed - angle for the star that make it appear to be moving faster than light.

    Am I wrong or is it possible?

    Last edited: Aug 5, 2004
  10. Aug 5, 2004 #9
    This wouldn't work out. Instead the doppler effect would blue-shift the image and time would appear to take place faster on the surface of the star from our vantage point.
  11. Aug 5, 2004 #10


    User Avatar
    Science Advisor
    Gold Member

    Are you sure about that?

    For example:

    Code (Text):

           A'   A
           |   /
           |  /
           | /

    Imagine the observer 0, watching 'star A' moving across the sky. where A is it's inital postion and A' is it's final postion.

    Lets say that the star travels from A to A' which will be a distance of x in a straight line and with constant velcoity v. for simplicite's sake let's say that the intial sepration is sqrt(2x^2) and the final sepration is therefore x.

    The time taken for the star to move from A to A' is simply [itex]\frac{x}{v}[/itex]

    Howvere the time taken for the image o travel from A to A' will be:

    [tex]\frac{2x - \sqrt{2x^2}}{v}[/tex]


    [tex]\frac{x}{v} > \frac{(2-\sqrt{2})x}{v}[/tex]

    So the apparent velcoity of the image will be:

    [tex]\frac{v}{2 - \sqrt{2}}[/tex]

    which may be bigger than c.
  12. Aug 5, 2004 #11

    Thoughts to offer:

    First experiment... a) a pencil thickness laser beam shot from earth at the moon would reach the moon spread by at least 1-football field(uncertain about the exact spreading) so in any event a "dot" is not possible.
    b) moving the laser pointer on earth to cross the diameter of the moon in a split-second is easy enough, but, the laser light does not cross the surface of the moon in the same way. Tkae a garden hose and direct a fine stream of water at a specific point in your yard. Now, move your hand quickly to another point and look at the stream. It arcs, and is delayed. The same thing happens with light under those circumstances. No FTL.

    Second experiment... a vertex moving FTL. That has always interested me, as I have in the past considered how the vertex would move with two intersecting laser beams 10-feet apart, and then the 2 laser beams move left and right, respectively, causing the vertex to rapidly advance. Although the laser beam will arc, as described above, would the vertex move FTL? Don't know.
  13. Aug 5, 2004 #12
    Hmmm... this begs a question: Has any authoritative studies been conducted with regards to FTL vertex movement? My suspicion is that it maxes at C, regardless of what one does.
    That is, I see vertex movement to potentialize instantaneous expression, yet with the limits of propagation at C. Any thoughts?
  14. Aug 5, 2004 #13


    User Avatar
    Science Advisor
    Homework Helper

    I can't undertstand what you're saying. So let's simplify things a bit:

    Let's say I have two inclined jaws, the top jaw has a very slight up slope, and the bottom jaw has a very slight downslope, Let's say the slope is .001 m per thousand meters. So, If I lower the top beam .001 m I get a vertext traveling 1000m - thats a factor of 1,000,000. So, if I get the jaws moving 300 m/s relative to each other, the vertex is moving at faster than light.
  15. Aug 5, 2004 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The vertex is not a massive object, so it can go FTL. (Phase velocities are commonly FTL.) But if you tried to put a pebble at the vertex and push it along, you will hit a ceiling.

    The point on the moon will not travel FTL. How does flicking your wrist ensure that the dot moves correspondingly? The light will have to catch up, so the dot will lag behind the flick. In other words, the dot will reach the final position only after time, t=d/c subsequent to the flick reaches its final position.
  16. Aug 5, 2004 #15


    User Avatar
    Science Advisor
    Homework Helper

    Are you getting paid to do this? :rofl: (Reminds me of the Dilbert cartoon where they're flicking their fingers.)

    The dot moving across the moon faster than the speed of light is an illusion. The light is traveling from Earth at the speed of the light. Each photon is landing a little to left (or right) of the one before. It doesn't matter how much sooner or later the photon arrives, the speed of light is constant. So, if one were able to measure the time between the first dot appeared at one side of the Moon and the time the last dot appeared at the other side, the dot would appear to have moved faster than the speed of light. (Of course, the intensity would have been reduced in response to the spreading of the light beam due to you flicking your wrist [added on to the normal spreading of a light beam])

    Rather than trying to move a vertex really fast, just transmit a radio signal into a wave guide at an angle. If you were to measure the waves along the side of the wave guide, they would appear to be alternating between faster than light and slower than light. Once again, it's an illusion based on the point of view.
  17. Aug 5, 2004 #16


    User Avatar
    Science Advisor
    Homework Helper

    In fact, Palladin's comment about the dot spreading to the size of a football field is a perfect analogy. How much time elapsed between the light arriving at one side of the football field size dot and the other side. They're not the same photons arriving at either side - they're two different photons that happen to arrive at the same time.
  18. Aug 5, 2004 #17
    I totally agree. I am convinced because it is allowed to separate the edges with angle zero. A guillotine comes to mind. Choosing an angle 0 + d[tex]\theta[/tex] creates a vertex that can go any speed imaginable, such that [tex]lim_{\theta->0}v = \infty[/tex]. It's important to note that no information can be transmitted this way, since it must "come from the middle" , or both ends simultaneously. Pushing on one end of the blade implies information must travel to the other end through phonons, which are slower than light.

    I do not find this obvious. The dot may not move correspondingly, but I wonder if the "arrival of the next dot" (think of a machine gun) cannot move FTL. Imagine drawing a line with a water hose on the pavement... oh I think I get it... right. It comes back to the guillotine pushed on one end. I agree with you.
    Last edited by a moderator: Aug 5, 2004
  19. Aug 5, 2004 #18
    Well first, I did assume no divergence. Second, even if there was, my argument would have been about the center of each football field instead of the dots. Third, I think Gokul is right, it doesn't work. Fourth, I think two photons arriving at the same time is pretty much the same as the vertex effect, in which, ends of an edge replace photons.
    Last edited by a moderator: Aug 5, 2004
  20. Aug 9, 2004 #19
    About the scissor and overlapping paper thing:
    You said they had to be long scissors and I've heard this before but was never sure why. About a year ago I set out on finding why, did some calculations and determined that they had to be long scissors, or you just have to close regular length scissors really fast. This is because the blades don't become parallel until after the intersecting point has gone past the blade, therefore it doesn't achieve infinite velocity.? I think I found that the end velocity on typical sized scissors cutting at a typical rate, the end velocity of the intersection point wasn't really close to c. Anyway, is that how it works? I tossed out my notes about it a while ago, but just wondering if that sounds right.
  21. Aug 9, 2004 #20
    I think that for actual scissors, the vertex will never pass c. It is true that the longer they are, the "more parallel" they are at their end, as the faster the vertex will go. However, true scissors are made of atoms, and the interaction time between atoms is less than c. When you close them, their end lags the rest, they do not stay perfectly straight. For true FTL, overlaping edges such as in a guillotine work best (in which all atoms are pulled by a field simultaneously).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook