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I can't slow down!

  1. Sep 24, 2013 #1
    In real life when we accelerate we cover every possible speed we can right? Einstein said that it would take an infinite amount of energy to go the speed of light, but going faster than the speed of light would take almost no energy(forgive me if I'm wrong).

    Now lets pretend we are in a space ship in space, and we could some how skip over a few "speeds", using this, we could skip over the speed of light, but we can only do this once on our space ship. So we skip over the speed of light and are cruising above the speed of light. Then we want to stop so we slow down again, but as we decelerate toward the speed of light, we can't slow down because you can't go at the speed of light.

    So what would happen? What if you hit a wall? Would you naturally skip over it again and radically decelerate?
  2. jcsd
  3. Sep 24, 2013 #2


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    No, you have misunderstood something. He never said that.

    Anything that starts off slower than light will always be slower than light, no matter how much it accelerates.
  4. Sep 24, 2013 #3


    Staff: Mentor

    This is not correct. You must be misunderstanding something. Perhaps you could post the source which made you think this.
  5. Sep 24, 2013 #4
    If something starts out faster than the speed of light, can it ever go slower than the speed of light?
    And does any thing ever start out faster than the speed of light?
  6. Sep 24, 2013 #5


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    ...... No
  7. Sep 24, 2013 #6
    This might be an overgeneralization but I hope its not just flat out wrong (in which case I hope someone will jump in and correct it):

    Anything that has mass can never travel at the speed of light. Anything that has no mass will always travel at the speed of light.

    Anything that travels at the speed of light does not experience the passing of time. Anything traveling slower than the speed of light will experience the passing of time (but time will slow as you approach the speed of light)

    I believe those are fairly safe rules (again correct me if Im wrong). Assuming those are the case, going through this trans-light speed would cause many more issues than simply not having your breaks work properly :)
  8. Sep 25, 2013 #7
    The part in bold face isn't quite correct because speed is relative so it doesn't really make sense to simply say that you are moving close to the speed of light. You must say that you're moving close to the speed of light from the point of some observer and that (s)he perceives your time to slow down, but of course you perceive your own time to flow normally and you perceive the other observer's time to slow down since from your point of view (s)he is the one that is moving.
  9. Sep 25, 2013 #8


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    If anything could start off faster than light, it would always be faster than light, no matter how much it decelerated. However nobody has ever detected anything faster than light and there are many reasons to suggest such things (called tachyons) shouldn't exist.
  10. Sep 25, 2013 #9
    xcourrier: If you want another general rule for special relativity, try something like this :

    "A local clock always ticks at a steady rate." This is like one you carry with you.

    As was posted above, only another inertial observer in relative motion to the clock will see it tick more slowly.

    Even more universe, to include general relativity, you can say:

    "Only two things affect clock rates: relative gravitational potential and relative speed."

    edit: Note that no clock [mass] can move at the speed of light.
  11. Sep 25, 2013 #10
    Yeah, I realize now that it was probably unclear what I meant by "time slows down." In attempts to keep it simple I was trying to minimize the use of terms like "reference frame" and things like "person A and person B and what A thinks of B" kind of thing, but I suppose thats necessary to keep things clear.

    In terms of your last post I almost jumped in and said light clock, but quickly realized how wrong that was and kept myself from sticking my foot in my mouth :)
  12. Sep 26, 2013 #11
    I have inserted 'foot in mouth' here a number of times...... not to worry....somebody will hopefully correct such mis statements.....that's how we learn.

    Regarding "reference frame" and light......

    Note that in prior discussions in these forums the impossibility of a reference frame for photons [light] has been clarified: there is none.

    An easy way to remember this is that if clocks and rulers can't travel at 'c' then we can't define an inertial reference frame. Or that the Lorentz transform doesn't work at v = c.
  13. Sep 26, 2013 #12


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    An (IMO) better way of thinking about it is to consider that the phrase "<something>'s reference frame" is a just a convenient shorthand way of saying "a reference frame in which <something> is at rest" - but since light moves at c in all frames, there are no frames in which light is at rest.
  14. Sep 27, 2013 #13
    The Lorentz factor is:


    where [itex]\beta[/itex] is you relative speed as a fraction of the speed of light.

    So we want [itex]\beta > 1[/itex]. Let's say [itex]\beta = 2[/itex].

    That would make [itex]\gamma=\frac{1}{\sqrt{1-\beta^{2}}} = ±\frac{-i}{\sqrt{3}}[/itex].

    Relativistic mass is related to rest mass as follows: [itex]m_{rel}=\gamma m_{0} = \frac{-i}{\sqrt{3}}m_{0}[/itex].

    Since we are looking to move you from rest to twice the speed of light, you relativistic mass will go from [itex]m_{0}[/itex] to [itex]\frac{-i}{\sqrt{3}}m_{0}[/itex], a change of [itex](\frac{-i}{\sqrt{3}}-1)m_{0}[/itex].

    So the amount on energy we will need to apply to you to get you going at that speed will be"

    [itex]e = mc^{2} = (\frac{-i}{\sqrt{3}}-1)m_{0}c^{2}[/itex]

    That will be the same amount of energy that your brakes must absorb to bring you back to rest. I'll let you calculate how hot your brakes will get in the process.

    I wouldn't count colliding with anything in this subluminal world. Since you're traveling faster than light, any interaction or information exchange with normal matter would cause serious causality issues. So if you want to get back, carry your own braking system.
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