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

Can Light Travel Faster Than Speed of Light

  1. Mar 25, 2010 #1

    If you fire a gun standing still the bullet will travel at a certain velocity.
    If you fire a gun sitting in an airplane, the bullet's velocity will be speed of plane plus the the velocity due to the gun.

    If you fire a bullet with a light being emitted from it: will the velocity of the light be:
    The light plus the velocity of the bullet?
  2. jcsd
  3. Mar 25, 2010 #2
    No, the speed of light will be the same, whatever frame of reference :)
    The reason : when you say that “If you fire a gun sitting in an airplane, the bullet's velocity will be speed of plane plus the the velocity due to the gun.” you simply add the two velocities (the one from the plane plus the one from the bullet). This law (adding the two velocities) is only an approximation when the velocities are small wrt the speed of light; it is incorrect when you have to deal with near-speed-of-light velocities.
    When you use the correct composition of velocities law, you see that the speed of light stays the same whatever frame of reference :)
  4. Mar 25, 2010 #3
    No, that could never happen, since photons are massless particles and they will always have that same speed, as for:

    [tex]c = \nu\lambda[/tex]

    And you could check the ranges of both the wavelength and the frequency in a electromagnetic radiation table and see how they concur on maintaining the [tex]c[/tex] constant. Which, however, must not to be confused with the energy of the photon:

    [tex] E = \frac{hc}{\lambda} = h\nu[/tex]
  5. Mar 25, 2010 #4
    the bullet's velocity will travel with the velocity of light, but it wont attain its velocity, will it ? !

    Obviously, the speed of light is far too much greater to make the velocity of gun compatible with that of light
  6. Mar 25, 2010 #5
    I don't think I understand your question, but an object such as a bullet to reach the speed of light, infinite energy would be needed. No matter how fast is the bullet traveling, the speed of light c will be the same for any inertial frames of reference moving with the relative velocity [tex]V[/tex]. According to the velocity transformations of Lorentz:

    [tex]v' = \frac{v - V}{1 - vV/c^2}[/tex]

    So, again you can see from here that if you set [tex]v = c[/tex], one can only say that the speed of light is the largest, since for [tex]V > c[/tex] would lead you to imaginary numbers as a result, if we regard the other lorentz transformations of the velocity.
    Last edited: Mar 25, 2010
  7. Mar 25, 2010 #6
    I realize this isn't "physics", but the title of this thread is a tautology; light can't travel faster than light, because then light would be traveling that fast.

    So, maybe thinking about the issue long enough to formulate a thread title would be a good idea?

    As for the basic question you're posing, NO. This is the what SR was about, with light flashes on trains and such. I suggest basic research into first SR, and then GR. Your question is the premise upon which relativity is BASED; that invariant of "bullet", the light ALWAYS moves at local light-speed (c in a vacuum). You can't "add" velocity to light, or subtract it, except insofar as the velocity changes in a given medium, which has nothing to do with your question.

    EDIT: This is also a question about Relativity, not Quantum Physics. May I suggest you ask a staff member to move this thread to the appropriate forum? In your case, I would suggest the Acadmic Guidence sub-forum, because you really need to be starting at first principles here. (at least, in terms of Relativity and QM)
  8. Mar 27, 2010 #7
  9. Mar 27, 2010 #8
    The difference are virtual photons. I've not read the link, but i know a fair bit to realize i think this is the type of superluminosity of particles of light. For a short time virtual particles can travel faster than light under the uncertainty principle.
  10. Mar 27, 2010 #9
    Moreover, whilst it may not lead to causal paradoxes, the Casimir Force does, because it violates the most well-known conservation law of energy; This is the fine line between which is classical, and which obeys the uncertainty principle.
  11. Mar 27, 2010 #10
    I am not aware of the fact that the Casimir force violates “the most well-known conservation law of energy” :confused: Could you clarify this point ?
  12. Mar 27, 2010 #11

    The Casimir Force is a direct experimental result of which is believed to be a background energy of virtual particles; also known as the Zero-Point Field. It is from here, energy is released spontaneously and created even in a lab. The conservation in which states energy can only change states and cannot be created is therefore violated, since energy apparently can be created from the virtual vacuum which is a resavior of negative energy - an infinite amount presumably.
  13. Mar 27, 2010 #12
    I know what the Casimir force is since it is my research field :biggrin:
    I can assure you that there is no violation of any energy conservation principle :smile:
  14. Mar 27, 2010 #13
    I can assure you there is. It's a well-known fact.

    The classical theory states no energy can be created. But we can create energy from the vacuum, from virtual to real.

    It stands to reason this is a violation of the classical sense.
  15. Mar 27, 2010 #14
    There is no energy created, don't trust those QED mumbo jumbo :biggrin:
    Rather, consider Casimir interaction as usual van der Waals interaction (which it is) :smile:
  16. Mar 27, 2010 #15
    Au contrare;

    In a recent discussion on this forum, i repeatedly told a person that there is no general consensus whether van der Waals interaction was literally the case of reality. The ZPF is still highly respected, and one must remember that the ZPF was proposed to exist according to the rules of quantum mechanics long before there was any confirmation of its theoretical evidence in the form of the attractive and repulsive Casimir Force.

    One can choose either theory - but the ZPF is still i would estimate, the leader of the pack over this new interpretation, which i am not fond of.
  17. Mar 27, 2010 #16

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    No, it's not. (And it takes quite some chutzpah to argue this point with someone who does this for a living!)

    There is no more a violation of conservation of energy than finding a rock on a hilltop and rolling it down.
  18. Mar 27, 2010 #17

    Yes there is. Taking for granted that the total energy of the universe is still zero, sporadic appearances of energy from the vacuum which is a creation process goes directly against the idea that energy cannot be created by the classical law.

    I'm not going to argue about it. You can agree to disagree if this is what it will consist of. I've studied physics for many years now, and i am positive i have a clear grasp on the subject of the zero-point field.
  19. Mar 27, 2010 #18


    User Avatar
    Homework Helper
    Gold Member

    There is no violation of conservation of energy in quantum mechanics, and quantum mechanics fully accounts for the Casimir force.
  20. Mar 27, 2010 #19
    Holy ****, what have you been reading? That sounds like a nasty pastiche of pop-sci notions and your own suppostions, but it has nothing to do with phyics.

    And now you're taking on Vanadium. Wow. I am... wow.

    EDIT: Wow, I'm really going to miss ManyNames... he knew how to mix blazing idiocy with semi-science and arrogant disregard so well. There should be a little cage on the forum for "The Banned", where we can all go to blow off steam. :smile: Oh, and I love PF staff. Nothing like waking up and having a burr magically vanish from one's paw...
    Last edited: Mar 28, 2010
  21. Mar 30, 2010 #20
    An example of why some people might get confused with regards to the speed of projectiles fired in near lightspeed vehicles:

    Suppose you're in a very very long spaceship, traveling at 99.99999% C. You're at the back end of the spaceship and you fire a laser gun, which travels to the front end and kills the pilot just as it was reaching the destination. According to you the laser reached the pilot at the same time the front end reached the destination, but it took 1 year more for the last end of the ship, from where it was fired to reach the destination. So the gun that was at the same distance as the laser took one year longer to reach the destination than the laser, aka, the laser arrived one year ahead.

    The key is that it took one year measured by the clocks inside the ship, but from the outside this is not the time difference that is measured.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook