Let say there's a particle that is travelling very near the speed of light. Lets say I have a camera capable of filming this high velocity particle and I film the particle on my camera then I fast forward it so it would be faster than the speed of light. What would be shown by the camera? and why?
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1$\begingroup$ It's just a video. If you go into your favorite animation program and animate the impossible, that won't make the impossible possible either. $\endgroup$– user2357112Commented May 19, 2014 at 9:25
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$\begingroup$ What did you mean by "film the particle"? Did you mean to make a video of it? $\endgroup$– SensebeCommented May 19, 2014 at 13:01
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2$\begingroup$ If I film myself smashing a glass, does playing the movie backwards break the second law of thermodynamics? $\endgroup$– boyfarrellCommented May 19, 2014 at 13:15
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$\begingroup$ You might be interested to read about apparent superluminal motion in jets from active galaxies. $\endgroup$– rob ♦Commented May 19, 2014 at 13:47
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4 Answers
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A film is just a series of still images and playing the film is just looking at the still images in sequence. Playing the film at a different speed just means you look at each of the still images at a different rate. What's in those images doesn't change depending on how fast you look at them.
answered May 19, 2014 at 7:40
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$\begingroup$ Lets consider the video of a simple situation, consider BigBohrs running at the speed of Bolt. Doesn't BigBohrs appear to run faster than before, if we fast forward the video? I think BigBohrs would beat the record of Bolt in the fast forwarded video. $\endgroup$– SensebeCommented May 19, 2014 at 13:15
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$\begingroup$ I thought of this as well. Conceptually yes, as long as you can add the velocities... The problem with the light particle on the screen, you will have to make something - say the beam of the beamer - move faster than light. Which is where the dog bites its tail (if you know what I mean...) $\endgroup$– JanCommented May 20, 2014 at 9:17
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As John Rennie pointed out, a film is just a series of still images. So when you play the film, you actually move these still images but not the things on those images. You do not have a particle to move.
Think of this another way: when you watch a movie you don't make the actors move. Yes, you will see a particle moving faster than the speed of light but you will not make it move faster than the speed of light. When you see Superman fly though space it does not mean that the actor (human) flies though space.
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I believe, due to relativity, bodies moving near light speed appear to slow down to a standstill to observers.
If you somehow could stand next to something moving near enough to light speed, it might not appear to be moving at all.
Think about it this way: the light from the particle approaches you at the speed of light, but the particle is moving near the speed of light.
Now let's draw an analogy: Imagine you're on top of a train that is moving left at 5 units velocity. Imagine now that you have a baseball in your hand which you throw to the right at 4.9 velocity units (relative to your position on the train, of course).
From your perspective on the train, the ball moves away at the speed you threw it.
From the perspective of someone not standing on the train, the net motion of the ball is
(5 - 4.9) = 0.1 speed units in the left direction.
Now imagine this near-light-speed particle - when you observe this particle, you're observing the light that comes from it. But the particle is moving near the speed of light! So the result is that the light which comes from the object (the light you observe that tells you where the object is spatially localized) gets kind of draaagged out behind the object. You can think of this as the object having almost "outrun" its own light.
From the perspective of the object, it races away at the speed of light. But from your perspective, the light from the object at each position takes extremely long to reach you because it approaches you at a velocity that is the equivalent of the sum of the vector velocity of the object and the vector velocity of the light which allows you to physically observe that object... very confusing, but the basic result is that you will see the object come closer and closer to a standstill the nearer it comes to the speed of light.
If you're still confused, imagine what would happen if an object raced away from you AT the speed of light. In this case, the light from the object moving away from you will be moving towards you at the same speed that the object is moving away from you, and the result is that the object will appear to be permanently frozen in space.
If you filmed the object, you'd only be filming its light at the speed that the light reaches you; so I don't see any reason why you wouldn't be able to speed the tape up. You'd just be speeding up the recording of the light (not the physical motion of anything).
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$\begingroup$ I'm going to have to give this a -1 as it stands. The answer in your last paragraph is correct, but you seem to have taken a pretty strange and convoluted route to get there. $\endgroup$ Commented May 19, 2014 at 13:57
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$\begingroup$ Comes as no surprise, I was in the middle of an all-nighter for my thermo class. But come on, I was being really detailed! And that took a long time to type! $\endgroup$ Commented May 19, 2014 at 21:32
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If a particle is travelling faster than the speed of light, it would radiate energy in the form of Karchov radiation. Such has been observed.
You would probably observe a doppler effect, as well.
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1$\begingroup$ Do you mean Cherenkov radiation? Playing a video at a non-standard speed would not produce this physical effect. $\endgroup$– rob ♦Commented May 19, 2014 at 13:45