Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 21/09/2020 15:38:02
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Yu-Feng asks:
Came across the idea that if you travelled at the speed of light (or slightly faster) you couldn't possibly see anything as lights reflected from objects would never reach your eyes. My question is if that was true, wouldn't it go against the idea that light will always travel at the speed of light relative to me regardless how fast I'm traveling?
What do you think?
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Since this could only happen in a movie, it depends what the script-writer says.
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This question falls under the category of " What would physics predict if you ignored the laws of physics"
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The question doesn't necessarily violate relativity, as we can, for the sake of argument, say that the observer is made of tachyons. Such particles have never been detected, but at least they avoid the "more than infinite energy" problem when going faster than light. That being said, tachyons travel back in time and length contraction makes distances negative. That's a bit difficult for me to wrap my head around when it comes to what a tachyonic observer should see.
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The question doesn't necessarily violate relativity, as we can, for the sake of argument, say that the observer is made of tachyons. Such particles have never been detected, but at least they avoid the "more than infinite energy" problem when going faster than light. That being said, tachyons travel back in time and length contraction makes distances negative. That's a bit difficult for me to wrap my head around when it comes to what a tachyonic observer should see.
The problem with a "tachyonic observer" is that tachyons can't travel at anything less than c, And that includes as measured with respect to each other. Also, with tachyons, it takes a increase of energy to slow them down towards c, and if they lose energy, they speed up.
And you can't say that time goes backwards for them or that length contraction is negative. The time dilation formula results in answers which are a square-root of a negative number when v>c, which is undefined. So does the length contraction equation.
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we can, for the sake of argument, say that the observer is made of tachyons.
...it depends what the script-writer says.
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A contradiction in terms that one. Nothing can go faster (propagate) than light that we have found. So to get to that 'FTL speed' you're discussing you just left the definition of lights speed in a vacuum. You want to eat the cake and keep it, right?
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Take the sun and the earth (distance 8 light minutes). Let's say the time is 1:30 on earth, we see the sun at 1:22, moving at the speed of light, we just hit the sun at 1:30 (8min.), Moving 8 times faster (for indicative and), we overcome this distance is only 1 minute and we get into the sun at 1:23, and since the distance between us and the earth is 8 holy minutes, we (if we could) see the earth at 1:15. If we go back to earth at the same speed, we will get home at 1:16, that is, in our past.
But such a movement is not possible, because the speed of light is the maximum possible speed.
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Take the sun and the earth (distance 8 light minutes). Let's say the time is 1:30 on earth, we see the sun at 1:22, moving at the speed of light, we just hit the sun at 1:30 (8min.),
No. If it is 1:30 on the Earth, and we "see" 1:22 at the Sun, we know that it is 1:30 at the Sun, since the light we are now seeing from the Sun left it when both the Sun's and Earth's clock's read 1:22. If we Now travel to the Sun at c ( thus ignoring Relativity), it will take 8 min ( Earth and Sun time) to get there and we will arrive at the Sun at 1:38, while seeing 1:30 at the Earth (But knowing that it is actually 1:38 at the Earth. Moving 8 times faster (for indicative and), we overcome this distance is only 1 minute and we get into the sun at 1:23, and since the distance between us and the earth is 8 holy minutes, we (if we could) see the earth at 1:15. If we go back to earth at the same speed, we will get home at 1:16, that is, in our past.
You'd arrive at the Sun at 1:31, see the Earth at 1:23 (During your trip you would see the Earth clock tick backwards), But you would still say that it is 1:31 at the Earth. If you return to the Earth, you will arrive when it is 1:32 (two minutes after you left), and see the Sun read 1:24 ( have seen the Sun clock run backwards during your trip)
All this would have to ignore Relativity, which would not allow you to travel at even c, let alone faster.
[/quote]But such a movement is not possible, because the speed of light is the maximum possible speed.
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Now there is a scenario which applies FTL speeds using Relativity, that would allow you to get a response to a message before you sent it, but this involves two inertial frames moving with respect to each other, each with the ability to send FTL signals between observers at rest with respect to each other. It involves the fact that the two inertial frames would have different determinations of simultaneity between clocks.
Here's how this would work using the Sun and Earth example. For simplicity, we'll assume messages can be sent instantaneously.
You are traveling from Earth to the Sun at 0.8 c. For you, the Distance between Earth and Sun is length contracted to 4.8 light min. You will see the Sun's clock read 1:22 as you pass the Earth. However, due to relativity of simultaneity, you will not say that the the Time at the Sun id 1:30, but already 1:36:24. It will take, by your clock 6 min to reach the Sun. During which time, both the Sun's and Earth clocks advance 3 min 36 sec. So that the clock at the Sun reads 1:40 when you arrive, and the clock on the Earth reads 1:33:36 ( not the image you see but the actual time on Earth according to you.)
Now you had a brother trailing 4.8 light min behind you( by your measure). This means that according to you, he is just passing the Earth as you reach the Sun. You send an instantaneous message to him, which he relays to the Earth.
Now at the Earth, it is 1:33:36, and in the Earth-Sun rest frame, it is also 1:33:36 at the Sun. If the Earth then relays this message instantaneously back to the Sun, it gets to the Sun at 1:33:36 Sun time. This is before you arrive at 1:40 Sun time.
The answer to the message you sent upon arriving at the Sun will be already waiting for you when you arrive. The answer arrived before you sent the message.
This is just one of the truly paradoxical scenarios that arise if you apply FTL speeds under Relativity.