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Is a bullet at the speed of light lethal?

  1. Jan 23, 2010 #1
    I was wondering what a bullet would look like to an outside observer traveling at near (.9999999 or so) the speed of light. Ignoring everything else and all the problems in it, if someone fired a bullet at you going almost the speed of light, would it actually appear to be moving quickly enough to kill you? Or would time slow down relative to the outside observer that it would be harmless?

    I don't know if I'm just spouting nonsense or not (I'm not the kind of person that can wrap my head around this stuff). Is it a clear answer or blurred?

    Last edited: Jan 23, 2010
  2. jcsd
  3. Jan 23, 2010 #2
    A bullet at 0.00000001 times the speed of light can kill you, so surely a bullet at the speed of 0.99... c will be able to kill you.

    You're messing up a couple of things... let me make it clear. According to the observer, the time in the bullet's inertial frame is slower than his own. So if the bullet would have an internal clock, it would run slower than the watch of the victim.

    This, however, does not mean that the bullet itself slows down. If the gun was fired from the sun and the bullet would travel at a constant speed of 0.999c, it would almost hit you as soon as a sunray emitted at the same time. That's pretty darn fast for a distance that big.
  4. Jan 23, 2010 #3
    Time is only dilated for an observer in the frame of reference of the bullet. To the person who is about to get hit the bullet is still traveling .9999c. So yes it's gonna kill you pretty well lol.
  5. Jan 23, 2010 #4


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    I'm not so sure.... the original question proposed a velocity of 0.9999999c. Assuming a 10 gram bullet that's got an energy of about 2*1018 J; which is equivalent to a 480 megaton thermonuclear weapon.

    Sounds bad. But this bullet is going to drill a nice clean hole and carry on, taking nearly all that energy with it. As long as the bullet gets a fair way away from you before dissipating all that energy, you may be a lot better off than if you were hit by a 1000 m/s bullet. The slower bullet carries some 5000 J of energy that gets used to tear up your insides.

    Cheers -- sylas
    Last edited: Jan 23, 2010
  6. Jan 23, 2010 #5
    I'm going to counter by saying the friction between the bullet and your body as it passes through you might generate enough heat to instantly vaporize you. lol. Of course that is completely unsubstantiated, please don't ban me for overly speculative posts.
  7. Jan 24, 2010 #6
    With a powerful gun, just getting hit in the arm can kill you because of the shockwave generated inside your body.

    I also imagine a bullet hitting you at that speed, even if made out of adamantium, will vaporize instantly, and not just pass cleanly through.

    I'm pretty sure you'd be dead, as will most anybody around you. In fact, even if it didn't hit anybody directly I'm pretty sure it could kill you, just from the heat and horror generated by something like that tearing through the air, if not space-time itself.
  8. Jan 24, 2010 #7
    It doesn't matter what it's made of, its state won't really make a difference. At such a high velocity, the kinetic energy of the various atoms and subatomic particles exceeds that needed to break them apart by quite a margin, and they will interact with the target as individual particles...the bullet will have the impact ballistics of a brief but extraordinarily intense burst of particle radiation. In its own frame, the bullet sees you similarly. The impact may vaporize the bullet, but the gases won't have time to expand by any meaningful amount before they pass through the target, any spread within the target will be due to scattering of individual particles. (there's a lot of stuff in the way for them to scatter off of though)

    I would expect that while the initial removal of material might be a fairly clean-edged cone, there would be enough heat deposited in the surrounding tissues for quite a large bang to follow. Even a near miss might scatter enough ionizing radiation your way from passing through the air to kill you.

    And yes, a bullet moving at 0.9999999c as measured by the target is measured as moving at 0.9999999c. It doesn't slow down. A clock on that bullet will appear to tick slower to the outside observer, and a bullet-camera that compensates for doppler effect will see the target as moving in slow motion. Each will also see the other as being shortened in the direction of relative motion, the bullet being pancake shaped and the target looking like something from a shooting range, they will disagree about distances as well as times. They both agree on the impact velocity, however...0.9999999c.
  9. Jan 25, 2010 #8
    I didn't even think of the fact that the beam bullet wouldn't have time to "vaporize." I'm pretty much a layman on this subject.

    However, as a layman I took a glance at the LHC Wikipedia entry. Apparantely one beam (weighing as much as a grain of sand) fired by this thing has the energy of 87 kilograms of TNT. And, from what I understand, half the reason the detectors are so large is because they have to withstand this energy.

    Needless to say, a bullet traveling that fast would have significantly mroe energy than a grain of sand. Also, a human is significantly smaller, and weaker, than an LHC detector.

    I imagine whatever would happend if a bullet like that hit a human, the result would be quite spectacular.
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