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Faster than lightspeed

  1. Nov 15, 2014 #1
    Just a random thought or question.
    Why can't something go faster than the speed of light?
    It almost seems like some laws of energy or physics do not agree with light speed.

    Consider this; imagine earth pays to construct a light speed ship, and sends me up with a baseball.
    Once at lightspeed, I go outside, standing in my space suit, and throw the baseball ahead.
    Now the baseball is travelling at lightspeed + 40 mph.

    Every piece of science seems to deny that my ball could travel over the speed of light.

    So then what happens to the ball? We're at 0 G's, I throw it, and it just stays in space where I let go of it? That doesn't make sense either.
    I exert a force on it, so it should react. It should move where I throw it, unless some other force disables it from doing so, like a wall, or water.

    So what would happen to the baseball thrown ahead of a spaceship traveling at light speed?
  2. jcsd
  3. Nov 15, 2014 #2


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  4. Nov 15, 2014 #3


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    You've skipped a step here.

    Here's the short answer:

    1] You, on the spaceship see the baseball moving at 40 mph. No violation there.
    2] People on Earth see the spaceship travelling at .999c with a baseball moving at .9999c. Still no violation.

    The key is that, as you and your spaceship approach the speed of light, Earth will see time for you slow. So, in slow motion, they see you throw a baseball at merely 0.1 mph. They never see a baseball exceed c.

    (The numbers are wild approximations, merely for the sake of clarity.)
    Last edited: Nov 16, 2014
  5. Nov 15, 2014 #4
  6. Nov 16, 2014 #5
    You're displaying a common misunderstanding that everyone's motion is relative to a universal, "common" frame of reference. First of all, you can't get to lightspeed, so your example fails right there, but let's say you are going very close to lightspeed, say 0.99c. But that would be a speed, say, that was seen by me watching you move through space. According to you, though, you are not moving at 0.99c, according to you, you are not moving at all, you are at rest. So when you throw your ball, you're going to see it travel not at 0.99c + 40 mph, you're going to see it travel at 40 mph relative to your velocity, which is zero. So, you will see it travel at 40 mph. I, on the other hand, looking at you throw the ball, will see you basically in slow motion throwing the ball, and I will see it leave your hand and move forward from you at a velocity faster than what you are traveling at from my perspective, but it will move forward very slow compared to you and won't ever reach what I perceive to be the speed of light.
  7. Nov 21, 2014 #6
    Wow cool, I love all the thought in here!
    I am trying to grasp and comprehend a few things here still.
    This is a very tough concept for me to grasp.

    So here is where I'm at.

    If I am traveling at .999 light speed, towards a slow object, say an earth orbit satellite, which is near 0 speed relative to me on my space ship, and I fire a laser beam at it, 299 792 458 meters from it, does my laser light strike the satellite before my ship does?

    If I understand correctly, if I was sitting on the satellite, I would see the laser light leave the laser transmitter in slow motion, as the ship hit's me directly behind that.
    However, if sitting on the space ship firing the laser, I would see the laser light emit from the transmitter for an entire second before impact.

    To make it even weirder, what if someone had a series of hypothetical telescopes from a few light years away, and they could view this event, what would it look like to them?
  8. Nov 21, 2014 #7


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    You always have to be clear about what a speed is relative to. Are you trying to say that the you are moving at a speed of .999c relative to the satellite? If so we can describe that situation as either the satellite at rest while you approach it at .999c, or as you at rest while the satellite is moving towards you at .999c (and a third observer not at rest relative to either of you would say that you're both moving towards one another, possibly at different speeds). If that's not what you meant, then the problem is still underspecified - you have to tell us what the satellite is "moving slowly" relative to.

    Assuming that problem is as I described above:

    You in the ship will fire your laser beam for one second, starting when the satellite is one light-second 299 792 458 meters) away and ending when the satellite is .001 light-seconds away. About a millisecond after that, the satellite will impact your ship.

    Satellite guy will describe the situation as you approaching him at .999c; the leading of the laser pulse won't even reach him until you are a mere .001 light-seconds away and about one millisecond before impact. All the energy of the entire laser pulse will arrive during that millisecond. Thanks to the doppler effect (google for "doppler effect" if you are not familiar with that term) the incoming radiation will be enormously blue-shifted relative to satellite guy, so delivers much more energy per unit time and ends up delivering the same total amount of energy.
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