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Faster than light

  1. Apr 29, 2009 #1
    As I understand it (I don't study physics) any object with mass gets heavier and heavier as it accelerates, so it never reaches c.

    'Light' has no mass, but can be regarded as essentially a fluctuation in the electric/magnetic field? From the definition of c^2=1/epsilon0/mu0, I interpreted that c is the fastest speed that a perturbation at one location can spread through the E/B fields, in a vacuum.

    But, if it was possible to avoid interacting with these fields, and had no mass, what would stop this thing from travelling faster than c? Am I missing something to do with time dilation?

    Also, would it be impossible to detect?
  2. jcsd
  3. Apr 30, 2009 #2
    There are several theoretical particals known as exotic particles, that are faster than light.

    The tachyon and supersymmetry are examples of these exotic particles.

    Remember that the study of any type of particles that are faster than the speed of light, is still out of our league of understanding and even measuring.
  4. Apr 30, 2009 #3
    My current understanding is that particles that have mass as we understand it have to move in a 'time-like' way and therefore at sublight speeds. Particles with no mass move on the edge of the 'light-cones' of Relativity and move at exactly the speed of light. Particles at superluminal velocities are theorised as 'tachyons' and potentially move in a 'space-like' way. The theory also goes (from my understanding) that if such particles existed, it would be as impossible for them to drop to sub-luminal speeds as it seems to be for us to accelerate to super-luminal speeds.
  5. Apr 30, 2009 #4
    i dont study physics either but i would be bold to say that you observe faster than light already. The problem is how much faster than a universal "at rest" constant. The earth is moving and because it moves and we are on it, in any and all directions light is the same speed. If it were faster (from the point observed), you would not see it because it would never have the time to be seen from earth. Much like a black hole, making sure light never reaches us to observe it.

    Just my theory, i dont study physics books... just common sense. Im sure some mathematical physics junky will figure a way to throw my pseudo-science out the window. (which i welcome)
  6. Apr 30, 2009 #5


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    That's pretty much correct, but most of us prefer to let "mass" mean "rest mass", which doesn't change. Search the forum for "relativistic mass" if you want to know more about that. I prefer to say that the work required to accelerate a massive particle from speed 0 to v goes to infinity as v goes to c.

    Maxwell's equations have some solutions that describe waves. These waves all move at c.

    The fact that there's a speed that's the same in all inertial frames doesn't really have anything to do with electromagnetism. It's a property of spacetime. What I said about work above is a consequence of that property.

    Anything that interacts with matter can be detected, at least in principle. It can however be difficult, if the interaction is very weak.

    Particles that move faster than light are called tachyons. The supersymmetric partners of massive particles aren't tachyons. There's no evidence for tachyons or supersymmetry, unless you count the existence of gravity as evidence of supersymmetry. (Gravity is a prediction of (super)string theory).

    I was going to, but I couldn't make sense of what you said. :smile:
  7. Apr 30, 2009 #6
    in other words....

    if something went faster than the speed of light it would technically go back in time. Light is relative anyway... we can slow it down and speed it up beyond 300,000,000 m/s
  8. Apr 30, 2009 #7


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    I've read this three times and I don't follow this at all.

    Nothing in our subluminal universe can go faster than light. Tachyons - hypothetcial particles only - do, and yes, they are equivalent to going backwards in time. But there's no way to get from here to there. Or there to here.
    Not true.
  9. Apr 30, 2009 #8


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    If something moves faster than light from event A to event B in one inertial frame, it would be going back in time in some other inertial frame. So it's not "going back in time, period". It's just that different observers disagree about which of the two events is the emission event and which is the detection event, and also about which direction the particle is going. For example, you fire your tachyon gun, which emits a tachyon going to the left. It hits a target and destroys it. Some moving observers would say that this is what happens: The target is destroyed and emits a tachyon going to the right. Then you pull the trigger just before the tachyon reaches your gun, and finally the gun absorbs the tachyon.

    It's possible to show that this leads to paradoxes, unless tachyons has some very peculiar properties. The only way I know to avoid the paradoxes is if the the time it takes to emit or detect the signal grows at least linearly with the distance the signal travels between emission and detection.
  10. Apr 30, 2009 #9
    Light is not realitive, motion and time are. Light has no acceleration, when light is emmitted it always and will always travel at the speed of light, no matter how fast the object the light is being emitted from is traveling.

    Light is the universal speed limit, with our current understanding at least.
  11. May 5, 2009 #10
    Thanks for all your replies. A couple were really enlightening. I've been through the forum now and looked at Tachyons, and a few more of the faster-than-light topics. Interesting stuff, but they don't seem to address my issue.

    Ok, particles with non-zero mass (real or imaginary) can't accelerate or decelerate through c without infinite energy. Is it always the case that a thing (particle, if you like) with zero mass will move at c? What's stopping them from going slower/faster? In the case of light I think Maxwell's Equations explain it... are there any other massless things?

    Also, isn't there circular reasoning in using the speed of light as our definition of a metre? If so, doesn't that imply from the outset that c is a constant?
  12. May 5, 2009 #11
    There is a circular definition, but no circular reasoning. You can change the units of the speed of light to anything you want and it all still works out the same. c is constant due to it being a constant of proportionality in the equations (and with appropriate units to make c = 1, then energy = mass). The rest is just to keep in with the conventions we like to use for units :)
  13. May 5, 2009 #12


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    We didn't start to use the speed of light to define the meter until after many experiments had been done to test the constancy of the speed of light using the old definition of the meter. Eventually, measurements of the speed of light became precise enough that the limited precision of the old definition of the meter prevented further improvement.
  14. May 5, 2009 #13


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    Yes, c is defined to be an exact constant under todays definition of the meter, but that was not true with former definitions of the meter. If today you were to improve the accuracy of an experiment which previously would have been used to measure the speed of light, then the result will be a more accurate measurement of the length of the meter.
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