Traveling at Light Speed Through Space: A Thought Experiment

[michonamona]

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

 

[BruceW]

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 traveling 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]

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.

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).

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]

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]

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]

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]

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]

\displaystyle{P = \frac{mv}{\sqrt{1-\frac{v^2}{c^2}}}}

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]

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]

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]

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 traveling >= C.

 

[BruceW]

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]

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]

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]

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]

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]

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]

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.

 

[JaredJames]

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.

Just take a look at the LHC. Should tell you all you need to know.

 

[mishrashubham]

Not arguing, didn't the Spitfire cross it?

(Or some old aircraft that done it in a nose dive?)

Nah just rumours. It was allegedly the P-47 but it definitely could not cross the sound barrier.

 

[Ryan_m_b]

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.

The theory has been confirmed far beyond reasonable doubt. As JaredJames rightly says you should check out the LHC. All that power to accelerate particles as close as pos to the speed of light.

IF in the future some way is discovered to transmit mass faster than light (and I am not holding my breath) it would have to be by changing the parameters of what you are trying to do. If you could magically turn the mass of the object in question into tachyonic mass then you may have made it travel faster than light but it is no longer ordinary matter is it?

The above paragraph is to illustrate the point (FTL is not a question of breaking the theory, it would be a question of circumventing it). I in no way endorse it as a real scientific statement, don't take from it "ah then we should just find a way of doing that".

 

[BruceW]

I agree that circumventing the theory or changing the parameters of what you're trying to do is the most likely way that speeds greater than c would be achieved.

But I'm also saying: imagine Einstein's laws were very slightly wrong (so slightly that current particle accelerators, which give roughly 10^{-8} joules of energy to the particle, wouldn't be able to detect the inconsistency). Then if someone built a particle accelerator that gave the particles a much higher energy, then it might be possible for speeds greater than c to be achieved.

Einstein's relativity is almost certainly correct. But it's not completely certain. Which is why it is good to have debate on what would happen if it were wrong.

 

[JaredJames]

Which is why it is good to have debate on what would happen if it were wrong.

Well I'm sure it's a lovely debate to have, but not here. That's about as far from mainstream as you can get.

 

[Ryan_m_b]

I agree that circumventing the theory or changing the parameters of what you're trying to do is the most likely way that speeds greater than c would be achieved.

But I'm also saying: imagine Einstein's laws were very slightly wrong (so slightly that current particle accelerators, which give roughly 10^{-8} joules of energy to the particle, wouldn't be able to detect the inconsistency). Then if someone built a particle accelerator that gave the particles a much higher energy, then it might be possible for speeds greater than c to be achieved.

Einstein's relativity is almost certainly correct. But it's not completely certain. Which is why it is good to have debate on what would happen if it were wrong.

That's a bit like saying "the ATM say's I have £0.05 but imagine if there was a slight calculation and it was actually £500! Let's debate that..." to a debt collector.

Imagining that what we know to the best of our knowledge is wrong and then trying to debate that can be fun but here is no place for it.

 

[BruceW]

OK, I guess I was trying to start up a conversation on the meaning of science.
BTW, is there a section for that on physicsforums?

 

[mishrashubham]

OK, I guess I was trying to start up a conversation on the meaning of science.
BTW, is there a section for that on physicsforums?

I don't think so. PF is very strict about sticking to mainstream science.

 

[JaredJames]

OK, I guess I was trying to start up a conversation on the meaning of science.
BTW, is there a section for that on physicsforums?

You can try philosophy, but you need to follow the new rules for posting there.

 

[Ryan_m_b]

OK, I guess I was trying to start up a conversation on the meaning of science.
BTW, is there a section for that on physicsforums?

If it's a question related to the philosophy of science (i.e. what are the advantages of deductive empiricism?) then that could work in the philosophy section. If you just wanted to ask the question "what would the world be like if we could travel FTL?" then perhaps the general discussion forum would allow it but you'd have to read the posting rules.

 

[BruceW]

Thanks for the advice, hopefully I'll be able to contribute to PF better in the future

 

[podd]

Einstein's relativity works very well, but it leaves many phenomena unexplained and is based on a materialistic view of the universe which may not hold forever. Einstein's theory isn't the ultimate truth, so without getting into a "what if" debate it's still fair to say that the theory isn't certain.