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

Rotional Motion Time Traveling

  1. Oct 3, 2004 #1
    I have a first clock tied to a string. At the other end of the string is the second clock. I swing the second clock around the first clock. I bring the second clock near the speed of light, while the first clock is at near rest compared to me.

    A constant force must be applied to the first clock to keep the second clock spinning around it. Does this constant force that produces a near light speed for the second clock mean that the static energy is the same for both clocks?

    If not, then does energy become denser in one of the clocks?

    If the energy becomes denser in one clock, it must be transfered to this denser area during acceleration. Since both clocks have the same quantity of mass, the internal motion of one clock's mass must then be greater for energy to be denser. Where is this energy transfered from but the other clock? It is transwered during acceleration. During constant rotational motion, the energy gradiant from the first clock to the second clock stays constant. During deceleration, the energy transfered to the clock with greater energy is transfered back to the clock from which the energy came. Therefore, no time change can occur, because energy transfer was equal and opposite in direction during the periods of speed up and slow down.

    If the energy doesn't become denser, then there just isn't a density effect and again time couldn't be different in each clock, even during acceleration.

    If things at high speeds do appear to have time traveled, then some other force must interact according to the previous logic.

    Or what am I missing or mixing up here?
     
    Last edited: Oct 3, 2004
  2. jcsd
  3. Oct 3, 2004 #2
    Is the first clock spinning such that if they were cubes, the clocks would constantly be face to face during rotation?
     
  4. Oct 4, 2004 #3
    The second clock would behave much like the moon does, only showing the other clock one face while it rotates around the other clock, because the string extend perpendicular from it's center of mass. The string extends tangent from the top of the second clock.

    If you want to isolate how these clocks are attached to make the discussion simpler, please do.
     
  5. Oct 4, 2004 #4
    Well ok, before this goes too far, I'll first reinstate that within SR, time dilatation deals primarily with frames of reference irrespective of energy transfer, forces, or directions. Two statements from observationnal facts, and an thought experiment involving two mirrors, a flashlight and a traveling wagon are sufficient to demonstrate it quite irrefutably.

    Now I must ask, how would you intend to decelerate the second clock? There are many ways, and energy transfer (kinetic energy of the 2nd to whatever form afterwards) will happen differently depending on how you choose to do so.
     
  6. Oct 5, 2004 #5
    What do you mean by time dilation?

    Please define, frame of reference too. I don't know what you mean.
     
    Last edited: Oct 5, 2004
  7. Oct 6, 2004 #6
    I'll start with reference frames and we'll work our way to time dilatation.

    If I stand in a room, I can view the room as my reference frame. I am at position (x', y') within it. (0,0) being the corner of a room, (x',y') = (2ft, 5ft) my position. Mathematically, it is simply an xy position graph (in 2 dimensions, it could be 1D or 3D).

    The room itself however is at position (x,y) relative to another reference frame, that might be the city. The room is at position (x, y) = (1 mile, 3 miles) from city hall. Just about anything that is rigid (room, city, ruler, automobile ...) can be a reference frame.

    In the case of the room and the city, two frames are not needed, one would suffice, because they are fixed to one another. But if the room was in a train wagon and the wagon was moving, it is useful to have both frames, and we note them (x,y) for the city frame and (x',y') for the wagon frame.

    Roughly stated, special relativity's 2 postulates are :

    1. A complete physical experiment will give the same results in whichever reference frame you do it (in a moving wagon or in city hall).
    2. The speed of light c is constant.

    1. is common sense, and 2 is based on both experiment (Michelson-Morley) and theory (Maxwell's equations, which are themselves based on experiments, and involve all of classical electric and magnetic phenomena).

    I gotta go, time dilatation in my next post. Think about 2, it is deeper than it sounds.
     
  8. Oct 6, 2004 #7
    I understand up until this part:

    I'll make an assuption, so you can see what it seems to imply to me.

    When I learned about the concept inertia, I imagined an object in motion that had no forces acting upon it. No touching interaction of objects we see, internal interactions changing its velocity or some kind of force field exerting upon it. This object maintains a constant velocity because it has no force acting upon it. (Although this never occurs, it's a necessary theory to isolate a property that can be discussed in relaiton to the others that have been isolated outside of intertia framework.) When you state a room is inside a wagon moving in relation to the rest of the world, is this intertia concept what you are using in the example? Is the wagon and room isolated?

    If so or not, is there such a thing as the concept instantaneous in this explanation, like in classical physics? If there is, it is only usefull to use an instantaneous frame if things are moving in relation to one another. Since the wagon moves, then it would be usefull. And, any value of say three frames alows us to deduce or induce certain things about changes in instantaneous frames. Acceleration.
     
  9. Oct 7, 2004 #8
    "When you state a room is inside a wagon moving in relation to the rest of the world, is this intertia concept what you are using in the example? Is the wagon and room isolated?"

    Yes, since you seem to understand this pretty well, the 1st postulate would indeed be stated more completely if I had said "inertial frame". The wagon will be moving at constant velocity. Introducting acceleration isn't necessary to show time dilatation, so inertia is implied. We can add acceleration and non-inertial frames later and see what happens.

    I'm not sure what you mean by "instantaneous frame". Or which event you would qualify to be "instantanous". We don't have to touch the word, nor the concept.
     
    Last edited by a moderator: Oct 7, 2004
  10. Oct 7, 2004 #9
    Instantaneous is principally based upon the concept inertia. A moment, frame, or state, which implies constancy. If you choose to eliminate it, we can just use inertial frames though for clarity.

    So, intertial frames. The the city frame and room frame are at intertial rest. The room-wagon frame is in an interial motion frame compared to the city. The reason why the room and the city rest intertial frame are not needed because we are interested in intertial motion frames, which is only the wagon room compared to the city?

    I want to make sure we are clear on something else. When things are in inertial motion frames, would you agree it only takes two individual consecutive frames to test the velocity of each frame? (Considering the person measuring is in a third inertial frame.)
     
  11. Oct 8, 2004 #10
    I don't see what relation you are seeing between "instantaneous" and "inertia", but we simply don't need either concept, so I wouldn't bother with this.

    Why would you introduce a 3rd frame to measure the velocity? Whatever velocity the wagoneer sees the city go by is the same as what the citizen sees the wagon go by, and this number is constant in value and opposite in direction.
     
    Last edited by a moderator: Oct 8, 2004
  12. Oct 8, 2004 #11
    I introduce it, because we need a velocity of each from the viewer, or there would only be one velocity for both. Do we only need one velocity for all?
     
    Last edited: Oct 8, 2004
  13. Oct 9, 2004 #12

    rcgldr

    User Avatar
    Homework Helper

    All this special relativity stuff is based on the concept that the speed of light appears to be constant regardless of the observers frame of reference (regardless of the observers speed relative to other observers).

    If two observers pass by at high speed, it will appear to each obsever that the other guys clock is running slow. At least in the case that the clock is based on the speed of light. Say the clock shoots out a beam of light against a mirror and uses the time it takes the light to travel between 2 parallel mirrors as it's reference for time. Each observer sees the beam of light from his own clock bouncing back and forth between 2 mirrors that move as the same speed the observer moves at. However when looking at the other guys' clock, the beam moves at an angle while it bounces between the 2 mirrors of the clock moving by at high speed. Since the observed speed of light is constant, it appears that the moving clock is running slower, since the beam of light has to travel farther to bounce between the two mirrors moving by at high speed.

    Another similar dilema is distance compression at high speeds. If you observe a long object approaching you at very high speed, the time it takes for light at the back of the object to get to you will be less that the time it took for the front of the object to get to you, because the back of the object will have moved closer to you by the time you see the front of the object, and the object will appear to be shorter. However in special relativiy, it's not just appearance, the object has actually shrunk relative to your frame of reference. At the same time your depth is much shorter relative to the any observer in the approaching object. As the object passes by and is departing it appears to get longer (I think).

    Back to the original question. It's the acceleration caused by a force that counts here. If force is used to accelerate a vehicle up to high speed, then then used to slow it back down again to return it to it's original frame of reference (like a quick trip to the moon and back), then the time for the acceleratees will have passed by slower than those that didn't get accelerated. When the vehicle returns, it's clocks will be a bit behind those that weren't accelerated. I'll post a thought experiment for this in a bit.

    General relativity also addresses accelerations within gravitiation fields, but I'm not aware of what the effect is supposed to be. The statement is made that from within a closed system, you can't tell the difference between acceleration and gravitation. Again using the beam of light bouncing between two mirrors example, both grativity and acceleration (in the "upwards" direction) should cause the beam to move downwards over time. However this ignores the obvious case of a very tall closed system. If it's acceleration that's bending the beam, the beam acclerates downward at the top of the closed system the same as it does at the bottom of the closed system. If it's gravity, the beams rate of downwards acceleration will increase as it approaches the bottom of the closed system, because the pull of gravity is stronger at the bottom of the closed system than it is at the top.
     
    Last edited: Oct 9, 2004
  14. Oct 9, 2004 #13

    Alkatran

    User Avatar
    Science Advisor
    Homework Helper

    It's true for ALL clocks.

    When something is moving relative to you it IS shorter than it's length in its own frame. Always.

    Just go with the twin paradox. Because you accelerated you moved further through time.

    ... You assume the gravity is as strong at the top as it is at the bottom if you're comparing it to acceleration downwards.
     
  15. Oct 9, 2004 #14

    russ_watters

    User Avatar

    Staff: Mentor

    That constant force keeps the 2nd clock from flying off into space, but it does not "produce" the "near light speed" for the second clock. That must come from somewhere else.

    The second problem with your thought experiment is that there is another force on the 1st clock that you are not considering - it must be anchored to something otherwise the force between the two clocks will send both flying off in a straight line (while revolving around their common center of mass).

    Consider the case of a GPS satellite (its easire to use than the moon). The foce that accelerated the satellite up to orbital velocity was provided by a rocket, not by the earth. But the force that keeps the satellite in orbit is provided by the earth. This force makes no difference for SR time dilation (but it does for GR time dilation). Also, since the earth is so much larger than the GPS satellite, the force they exchange doesn't move the earth much at all.

    Are you looking for a flaw in SR here?
    I think there is a language issue here: the word "frame" is not referring to frames in a film. A "reference frame" is simply a set of coordinates in space. So the phrase "individual consecutive frames" has no meaning.
     
  16. Oct 10, 2004 #15

    rcgldr

    User Avatar
    Homework Helper

    Until they make extremely large flat planets, I'm going to stick with the idea that gravition fields are not constant in strength.

    After thinking about it, it doesn't matter if the objects are approaching or departing, both will "see" the other as shorter. So I agree here.
     
  17. Oct 10, 2004 #16
    Do you think energy source is absolutely neccessary? If so, please explain why we miss something crucial. Otherwise, lets not digress from the main point of high speeds supposedly creating time travel.

    That sounds principly to be the same argument, just different perspectives. Expanding force(exerting) on the first clock, and compressing force (anchor). The anchoring force must be strong enough to conduct the exerted force though of course.

    This sounds interesting, but how is it relevant to a high speed creating time travel?

    I would really like to stick to the example I've stated at the begining of the thread, which isn't a gravity based example, like the one you have stated.

    If you look at the language in this thread, I'm just being carefull so I understand what people mean. I'm not trying to prove anything wrong. That will reveal itself unless those who do comment decide not to be patient and thorough with the vocabulary and principles of the dicussion.

    For example, the badly written argument:

    A "reference frame" is simply a set of coordinates in space. So the phrase "individual consecutive frames" has no meaning.

    Now, we can be more picyunne about the principles and or we can be more picyunne about vocabulary words used. I'm voting for a principly in-depth discussion vs. superficialalities of chosen words. I agree we need to make sure what words are equivalent. Do not assume different words and phaseologies mean meaninglessness or you'll error on the side of vocabularous dogma.

    A reference frame is a set of coordinates. Cartesian and geometry concepts give those coordinates numbers. From this we can see length, width and depth of frames. When we combine this with the simple speed concept, we get frames of reference that express displacement compared to other frames. When a frame is checked for displacement compared to another frame, and it has displacement, the two smallest increments noticeable by a human are instantaneous frames. When one instant passes, you have two consecuative frames. Yes, like a film. This geometry is in many physics books and so is displacement. Speed is usually described with a partical type model. Partical displacement seems to be one dimensional displacement. Add two more dimensions and we have frame of reference displacement.

    If frames of reference and velocity are not seen like this, then how are they supposed to be seen? If so, I'm on the right track, but didn't explain it real good, then do better.
     
    Last edited: Oct 10, 2004
  18. Oct 10, 2004 #17

    rcgldr

    User Avatar
    Homework Helper

    It's not the high speeds, it's the acceleration to high speed that causes the change in observed time.
     
  19. Oct 11, 2004 #18
    That's new to me. I heard that time travel and special relativity was described only with constant velocity, not acceleration. Would you explain then why I acceleration creates time travel?
     
  20. Oct 11, 2004 #19

    rcgldr

    User Avatar
    Homework Helper

    It's not time travel, it's just changing the rate of time. As posted in another thread, a spacecraft can accelerate to high speed, take a trip to the moon and back, decelerate and end up back on earth. Although both the spacecraft and the earth observe each other at high speed, only the spacecraft (and its occupants) are affected by the slowing down of time. The clocks on the spacecraft will be a bit behind the clocks on earth.
     
  21. Oct 11, 2004 #20
    Acceleration doesn't change time.

    Time never changes. What does change is the speed of entities.

    If you say something accelerates at a high rate of speed, it only means it moved faster. We know it moved faster because we used a constant to know the rate of acceleration. We wouldn't know the acceleration if we didn't have a constant to base the acceleration upon.

    Quantifiable time is does not exist in human consciousness without a known constant in which to compare it to. A quantified constant is needed to test for quantified acceleration. Without a quantified constant, acceleration cannot be quantified. Therefore time travel could not be detected.

    To say, oh this thing traveled this far in time, is a quantified acceleration and cannot known without a constant. Time didn't accelerate, a thing did.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Rotional Motion Time Traveling
  1. Time Travel (Replies: 1)

  2. Time travel (Replies: 3)

  3. Time travel (Replies: 3)

  4. Time Travel (Replies: 4)

  5. Time travel? (Replies: 3)

Loading...