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' Travel

  1. Aug 19, 2008 #1
    While SR states one can never measure a velocity greater than C, for all practical purposes one can travel 'faster than light'. Here is what I mean. . .

    Lets assume I have a Spaceship with the means of constantly accelerating at 1G. The fact that this is technologically unfeasible is irrelevant to the principles involved.

    We choose a destination and for our purposes assume linear travel.

    I calculate that one is able to travel many light years of distance within a human lifetime.
    We will base this though experiment on SR and traveling at constant acceleration of 1G.

    We will acclerate for the first half of the trip and decelerate for the second half. Obviously would could just keep accelerating forever if we wanted to get there faster, but this would leave us going to fast to enjoy the destination.

    I calculate the time it takes using:
    Time(years) = SQRT(Distance/2*2/accel)*2

    I attached a Excel file showing various times for various distances you want to travel at 1G or 2G in your POV and Earth's POV.

    In summary from Earth's POV you will asymptotically accelerate toward C so for long distances it would take you roughly 1 year per light year.

    But from your POV, for practical purposes, you would be able to travel:

    4 light years in 3.94 years using 3.7E20 J of energy
    100 light years in 19.7 years using 9.3E21 J of energy
    1000 light years in 62.31 years using 9.3E22 J of energy

    Given 10^22 J of energy you could feasibly travel 1000 light-years in your lifetime!(what is really happening is that the energy is shrinking space to allow you to travel that far, but for all practical purposes of travel this works)

    to put the energy in perspective, a large nuclear exploxive can give 8.4E13 J of energy so we would need 10 million times this much energy to travel 4 light years in 3.94 years which is entirely possible but highly improbable.
     

    Attached Files:

  2. jcsd
  3. Aug 19, 2008 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Yes, you are right that due to time dilation and length contraction, you can travel seemingly long distances (measured from your pre-launch stationary frame) in a short time, without ever measuring your speed to be greater than C. If we ever get the technology to accelerate a spacecraft to a high fraction of C, we could send spaceships to distant stars and back within the lifespan of the crew. The catch, of course, is everyone who helped build the craft will be long dead when they get back.
     
  4. Aug 19, 2008 #3

    Dale

    Staff: Mentor

    Hi nahkaimurrao,

    This is also quite well described on the relativistic rocket page. I didn't look at your excel spreadsheet, but you can check your calculations with theirs.
     
  5. Aug 20, 2008 #4
    Thanks
    It appears my calculations are different than those, but I think I understand the concept at least.
    Mostly I wanted to point out that it is possible to travel to distant star/galaxies within a reasonable time (from the traveler's POV). This doesn't seem to be understood by the mainstream public who tend to think that it would take at least 1 yr to go 1 lightyear etc.

    I feel that saying the speed of light is a barrier is misleading. It is more of another dimension. When we accelerate, out frame of reference seems to disassociate with Earth's frame of reference. We are literally 'warping' space-time. The speed of light only appears to be a barrier because as you warp space you are creating a new space with it's own properties distinct from that of other spaces and within this space the speed of light is still observed.
     
    Last edited: Aug 20, 2008
  6. Aug 20, 2008 #5
    Actually I think the term 'warping' is misleading here since your example is obviously using flat spacetime.
     
  7. Aug 22, 2008 #6
    Instead of discussing speed of light, talk about speed of information. Gravitation also travels at the speed c. Physical objects consist of information. Information cannot travel faster than itself.
     
  8. Aug 22, 2008 #7

    Dale

    Staff: Mentor

    If you want to avoid the term "speed of light" the usual term is "invariant speed".
     
  9. Aug 22, 2008 #8
    It does take a year to travel one light year. For the external observer. How about him returning? Twin paradox.
     
  10. Aug 25, 2008 #9
    What I mean by 'warping' is that as one person accelerates, his reference frame (both space and time) lose continuity with that of his initial reference frame (perhaps his twin who stayed at home).

    It is almost like you are becoming 'distant' but by a dimension distinct from those we normally experience. It is analogous to communication over long distance, say between earth and mars. If both people talk at the same time (sending radio waves to the other's planet) the other person appears to be delayed. But which person spoke first? Each person observes that they spoke first because the other's message was delayed by distance.

    When two beings are traveling at greatly differing velocities, each appears to the other to have slowed down. But who is it that truly slowed down? It is unclear becomes space has become less linear, more 'warped' so that different observers measure different time/dimensions. Time runs differently for each observer, and space measures differently for each observe.
     
  11. Aug 25, 2008 #10
    Again that has nothing to do with warping of space.
     
  12. Aug 26, 2008 #11
    I'm afraid your rebuttal leaves something to be desired. Would you please enlighten me with your understanding of the 'warping of space' and how it differs from my illustration thus far?
     
  13. Aug 26, 2008 #12
    Once we discuss acceleration, we must discuss general relativity, not simply special relativity. The basic equation of GR is G=T. G is geometry, "warping of space". That is, with acceleration, we cannot have "flat space".
     
  14. Aug 26, 2008 #13

    JesseM

    User Avatar
    Science Advisor

    Not true, you can certainly have accelerating test particles (with negligible mass so they don't curve spacetime) in flat spacetime. See for example this page from the Usenet Physics FAQ:
     
  15. Aug 26, 2008 #14

    I dont think length contraction is relevant. only the time dilation matters.

    or rather, you get the benefit of one or the other but not both at the same time.
     
    Last edited: Aug 26, 2008
  16. Aug 26, 2008 #15

    Dale

    Staff: Mentor

    I think you two are just having a miscommunication. Spacetime "warping" as it is usually used refers specifically to spacetime having a non-zero curvature which can cause inertially moving observers to accelerate wrt each other (which doesn't happen in flat spacetime even with accelerating observers). You are refering to "warping" as any change in time or distance measurements. An imprecise analogy would be MeJennifer refering to bending a metal bar and you refering to stretching it both using the same word.
     
  17. Aug 26, 2008 #16

    russ_watters

    User Avatar

    Staff: Mentor

    They are two parts of the same thing. The reason you can travel to, say, Alpha Centuari in less than 4.5 years (at close to the speed of light) even though it is 4.5 years away is you measure the distance to be much smaller than 4.5 LY.

    Heck, it may actually be better to say that from your point of view (the traveler), it is length contraction alone that allows you to get there that fast.
     
  18. Aug 27, 2008 #17
    Thanks for all the replies. I admit I have not had any formal education regarding GR and only undergrad classes regarding SR. Other than that I was basically going off intuition.

    Is it generally understood according to SR that a photon simultaneously exists everywhere along its path i.e. that in its POV there is no distance from beginning to end?
     
  19. Aug 27, 2008 #18
    yes. from the point of view of the stationary observer only time dilation matters. from the point of view of the traveler, only the length contraction matters.
     
  20. Aug 27, 2008 #19

    JesseM

    User Avatar
    Science Advisor

    A photon doesn't actually have its own POV (i.e. its own inertial rest frame) in SR. All inertial frames are ones that are the rest frames of sublight objects. This has to do with the assumption that the laws of physics should work the same in all inertial frames (which wouldn't work if there was a frame where photons can be at rest), and also to do with the fact that the coordinates of each frame are supposed to be defined in terms of measurements on rulers and clocks at rest in that frame, and one can't construct rulers or clocks that move at the speed of light.
     
  21. Aug 27, 2008 #20
    This may be more philosophy than physics but what you said makes sense assuming what I said is true, that light exists simultaneously everywhere along its path as if there is no such thing as distance, space or time. In other words in the photons POV there is no POV cuz spacetime hardly exists at all.

    Two distant electrons that communicate via photon are connected (via the photon's POV) by zero distance or time as if they were the same electron, this seems to support the "one electron" universe model.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: 'Faster than Light' Travel
Loading...