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Traveling at Light Speed Through Space: A Thought Experiment

by michonamona
Tags: experiment, light, space, speed, traveling
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michonamona
#1
Apr29-11, 09:27 PM
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My brother-in-law proposed the following thought experiment: Suppose a person was sitting on the nose of a spaceship traveling at the speed of light through outer space. Now suppose that person pushed off against the spaceship launching himself ahead of it. Is it the case that that person will maintain his speed at the speed of light, since both the spaceship and the person is traveling through a vacuum and neither experience wind resistance? Also, is it true that the person will forever drift in space since nothing can travel faster than the speed of light?

Thank you for sharing your insight,

M
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BruceW
#2
Apr29-11, 11:17 PM
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Imagine Tim sitting on earth watching this spaceship with Tom on it. The spaceship could accelerate to get close to, but not exactly the speed of light. As viewed by Tim, Tom could increase his speed slightly by pushing off from his spaceship, but no matter how hard Tom pushed off, he would still be travelling less than the speed of light as viewed by Tim. If Tom then floats through space at close to the speed of light (as viewed by Tim), then it would take a lot of work, but it is always possible for Tim to catch up to Tom.
DaveC426913
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Apr29-11, 11:26 PM
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Quote Quote by michonamona View Post
My brother-in-law proposed the following thought experiment: Suppose a person was sitting on the nose of a spaceship traveling at the speed of light through outer space.
Can't happen. Spaceship can't reach the speed of light.

But let's say for the sake of argument it's going .999c.

Quote Quote by michonamona View Post
Now suppose that person pushed off against the spaceship launching himself ahead of it. Is it the case that that person will maintain his speed at the speed of light,
No.

Relativistic velocities don't add this way. They add using this formula:

v(final) = (v1 + v2) / (1+ (v1+v2/c^2))

You will find that, when you add the ship's v (v1) and the person's jump (v2), it will always result in a number less than c.

Even if you had a 2nd stage rocket blast off from the first stage at .999c, the final v of the second stage will still be less than c (0.9999994994997501c in fact).



Quote Quote by michonamona View Post
since both the spaceship and the person is traveling through a vacuum and neither experience wind resistance? Also, is it true that the person will forever drift in space since nothing can travel faster than the speed of light?

Thank you for sharing your insight,
Wind resistance has nothing to do with it.

Ryan_m_b
#4
Apr30-11, 04:43 AM
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Traveling at Light Speed Through Space: A Thought Experiment

Quote Quote by michonamona View Post
My brother-in-law proposed the following thought experiment: Suppose a person was sitting on the nose of a spaceship traveling at the speed of light through outer space. Now suppose that person pushed off against the spaceship launching himself ahead of it. Is it the case that that person will maintain his speed at the speed of light, since both the spaceship and the person is traveling through a vacuum and neither experience wind resistance? Also, is it true that the person will forever drift in space since nothing can travel faster than the speed of light?

Thank you for sharing your insight,

M
As has been stated it's not possible to get mass to travel at the speed of light. However if the person pushes off the edge of the rocket at .9999c they will drift through the vacuum at that speed. There is no wind in space BUT there will be resistance.

Vacuum isn't perfect, IIRC there are approximately 2 atoms per square metre in space (this of course changes). Over time this resistance will slow the person. Though I doubt they could ever be slowed to standing still in the lifetime of the universe
michonamona
#5
Apr30-11, 01:51 PM
P: 122
Thanks everyone for your response. I'm an amateur in physics so could someone explain why it is not theoretically possible for a spaceship to travel at the speed of light? Does it have something to do with Einstein's E=mc^2?

Also, does ".99c" stand for ".99 percent of the speed of light"?

Thank you,
M
jtbell
#6
Apr30-11, 02:15 PM
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Quote Quote by michonamona View Post
Also, does ".99c" stand for ".99 percent of the speed of light"?
"1.00c" is 100% of c. "0.99c" is 99% of c (not .99%, watch the decimal point!).
Ryan_m_b
#7
Apr30-11, 02:54 PM
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Quote Quote by michonamona View Post
Thanks everyone for your response. I'm an amateur in physics so could someone explain why it is not theoretically possible for a spaceship to travel at the speed of light? Does it have something to do with Einstein's E=mc^2?

Also, does ".99c" stand for ".99 percent of the speed of light"?

Thank you,
M
Mass cannot travel at the speed of light. I'm not a physicist myself but as an objects velocity increases it's mass increases. Therefore the energy required to accelerate it increases.

I.e. at .9c the energy required to accelerate to .91c is more than to accelerate from .1c to .11c

It requires infinite energy to accelerate matter with mass to light speed.

p.s If you could travel at faster than light velocities then you would have invented time travel!
BruceW
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Apr30-11, 03:30 PM
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[tex] \displaystyle{P = \frac{mv}{\sqrt{1-\frac{v^2}{c^2}}}} [/tex]

This is the momentum of an object in special relativity, where m is the rest mass of the object.
When the velocity tends to the speed of light, the momentum tends to infinity. To give an object infinite momentum requires infinite energy, which is why the object can't ever get to the speed of light.
Also interesting, is that when the velocity is much less than the speed of light, the bit in the square root is almost equal to one, so the equation becomes P=mv, which is just the classical equation for momentum
Physicist1231
#9
May1-11, 01:51 PM
P: 103
it seems it us the opinion of most that a mass cannot exceed the speed of light. we also thought the speed of sound wash a limitation. i feel the speed of light can be achieved and exceeded. if true then a lot of funny stuff would be noted. for instancence if money head where ahead of their feet in the path of travel (at C) then he would see his feet frozen in time. but if accelerated past C, then he would see his feet moving as if someone hit the rwind button... if C could be exceeded.
khemist
#10
May1-11, 01:59 PM
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Quote Quote by Physicist1231 View Post
it seems it us the opinion of most that a mass cannot exceed the speed of light. we also thought the speed of sound wash a limitation. i feel the speed of light can be achieved and exceeded. if true then a lot of funny stuff would be noted. for instancence if money head where ahead of their feet in the path of travel (at C) then he would see his feet frozen in time. but if accelerated past C, then he would see his feet moving as if someone hit the rwind button... if C could be exceeded.
I would not call it opinion. The fact it is impossible to reach C is experimentally and theoretically grounded.
JaredJames
#11
May1-11, 02:05 PM
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Quote Quote by Physicist1231 View Post
it seems it us the opinion of most that a mass cannot exceed the speed of light. we also thought the speed of sound wash a limitation. i feel the speed of light can be achieved and exceeded. if true then a lot of funny stuff would be noted. for instancence if money head where ahead of their feet in the path of travel (at C) then he would see his feet frozen in time. but if accelerated past C, then he would see his feet moving as if someone hit the rwind button... if C could be exceeded.
Unfortunately, nature cares not what you "feel". There is a heck of a lot against travelling >= C.
BruceW
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May1-11, 04:04 PM
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If someone could accelerate past c, then all of Einstein's relativity would be wrong. Einstein's relativity is probably the most well established theory that has ever existed.
If someday, someone was accelerated past c, then I would have to concede that Einstein's relativity is incorrect. Until that day, we can assume Einstein's relativity is correct.
DaveC426913
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May1-11, 10:48 PM
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Quote Quote by BruceW View Post
If someone could accelerate past c, then all of Einstein's relativity would be wrong. Einstein's relativity is probably the most well established theory that has ever existed.
If someday, someone was accelerated past c, then I would have to concede that Einstein's relativity is incorrect. Until that day, we can assume Einstein's relativity is correct.
No, that is misleading. It is not simply a 'we'll wait and see' idea. That some day won't come. We can tell today that mass cannot each c.

It is not the same as the 'sound barrier' myth.
BruceW
#14
May2-11, 04:11 AM
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The point that I'm trying to make is that no theory is absolutely certain.
Theory must fit experimental observations, and the results of future experiments cannot be assumed.
This is the foundation of all science, in my opinion.
mishrashubham
#15
May2-11, 04:19 AM
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Quote Quote by DaveC426913 View Post
It is not the same as the 'sound barrier' myth.
Exactly! The "sound barrier" was not a physical limit but a technological one. Scientists and engineers did not believe that objects could not travel faster than sound. They believed that it was immensely difficult to develop the technology required to cross the speed. Planes at that time could not survive the shock-waves produced by supersonic travel nor did the people think they could build one which could.
JaredJames
#16
May2-11, 04:52 AM
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Quote Quote by mishrashubham View Post
Planes at that time could not survive the shock-waves produced by supersonic travel nor did the people think they could build one which could.
Not arguing, didn't the Spitfire cross it?

(Or some old aircraft that done it in a nose dive?)
Ryan_m_b
#17
May2-11, 05:11 AM
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Quote Quote by BruceW View Post
The point that I'm trying to make is that no theory is absolutely certain.
Theory must fit experimental observations, and the results of future experiments cannot be assumed.
This is the foundation of all science, in my opinion.
Whilst no theory is absolutely certain all evidence points to requiring infinite energy to accelerate a mass to light speed. Often during the further developing of theories, filling out the details we discover a practical way of doing something deemed impractical. But there is not a time when a theory that shows something to be true is completely negated!

It's not that we have a "pretty good idea" that mass cannot travel at light speed, we have evidence to show it is not possible. This is not a case of "we don't know but perhaps we will find a way in the future", it is a case of "evidence shows us it is not possible"/.
BruceW
#18
May2-11, 05:39 AM
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If Einstein's relativity is incorrect, then 'perhaps we will find a way in the future'.
Just because Einstein's relativity agrees with all experimental evidence so far, doesn't mean its impossible that someday an experiment is done that disproves the theory.


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