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So you are driving a car at (almost) the speed of light

  1. Mar 31, 2013 #1
    You are driving down the road at almost the speed of light. And you fire a powerful laser that is attached to the front of your car.

    The laser beam hits a guy standing at the side of the road and goes through his hand.

    But to the guy at the road, your laser beam can't hit him, because to him, your laser beam can't go any faster that your car, So it is impossible for this laser beam to hit him.

    So does the guy have a hole in his hand or not?
  2. jcsd
  3. Mar 31, 2013 #2


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    He does. (You said he does.)

    You also said I'm driving at almost the speed of light. That's good because if you had said "at the speed of light", like your title, you'd be in deep trouble. So at almost the speed of light, the laser is going faster than me. Where do you see a problem?
  4. Mar 31, 2013 #3
    Well your laser would be going faster than your car , but not by much, his hand would have moved before the laser got there
  5. Mar 31, 2013 #4


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    The speed of light is the same for all observers, regardless of the speed of the source. If I can hit the hand from a stationary (relative to the hand! Never speak of a speed without being clear about what the speed is relative to!) car, I can hit from a moving (again, relative to the hand) car at the same point.
  6. Apr 1, 2013 #5

    Doc Al

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    Thread reopened.
  7. Apr 1, 2013 #6
    Yes and no he obtain a hole in his hand but the event would happen after you and your had passed him. The hand if we are talking a normal human and centre of the palm, would take around 3 seconds for the laser to burn through without carbonisation but 2.5 nanoseconds for the pain to be processed by the spine. Give these times the car and laser would of passed by the the hole finished forming. He'd feel the pain but not what caused. He'd also see the hole form.
  8. Apr 1, 2013 #7


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    I'm really confused as to where your figures came from. THe original poster didn't say anything at all about the laser taking three seconds to burn through the hand. Also, you didn't specify whose seconds they were (it matters). The OP didn't either, because he didn't mention seconds at all.

    It's possible to imagine that the laser wasn't powerful enough to burn through the hand i the alloted time. But that wasn't what the original poster assumed/asked. Changing the question midstream is just going to generate confusion.

    Also, light can move about 2.5 feet in 2.5 nanoseconds, so I think your figure for the amount of time it takes to process the pain is way off. (And I don't understand it's relevance).
  9. Apr 1, 2013 #8
    I was taking the measurement from aproximating the length of my arm with central part of the hand where the is no bone I assumed the heat of the laser to be around 2000 degrees c as I knew this numbers. He also didn't add the length of the road either so the question he put towards was unanswerable. Though as a side note I took into account of gunshot survivors who never hear the gunshot until they see the muzzle flash and feel the pain. Used this as a guide to but changed the speeds appropriatelly.
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