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.

 

[Ryan_m_b]

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

Don't worry about it.

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.

Theories in science have accumulated vast wealths of independent evidence all strongly indicating the theory to be true. You shouldn't complain that Einstein's theories are not the "ultimate truth", they are not meant to be. What does even "ultimate truth" mean? Einstein's theories are brilliant at describing what they are supposed to describe, nothing more.

Just because we can never say with 100% certainty that SR and GR are certain does not mean that we cannot say they are true. All evidence points towards their veracity. Look at it this way, over the centuries our understanding of the shape of the Earth has improved. When we could only measure the curvature of the horizon as 0 we believed it to be flat. Eventually better tools allowed us to measure a curvature and people thought the Earth was round. Eventually far better tools showed the Earth to be an oblate spheroid. In the future better tools may be available but is it sensible to suggest that they may show the Earth to be something entirely different than what we measure now?

 

[ZapperZ]

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.

Then by the same token, why pick on Relativity alone? One could say that ALL science is like that! So what is this discussion about? Relativity, or Science? Is this a philosophical discussion, or a science discussion with an actual physics content (the latter is required for this thread to remain open, or to be discussed in the physics forums of PF)?

The issue I have here is that people who don't quite understand SR or even how it has worked, are producing stuff that they THINK can violate SR. This thread can't even find the proper physics forum relevant to the topic!

And for your information, there are plenty of theoretical proposals beyond just handwaving arguments or simplistic "thought experiments", of ideas that could possibly violate Lorentz invariance, etc. We continue to make such measurements to detect such violations (physicists, by nature, LOVE to find things that violates or break beyond current understanding, believe it or not).

Zz.

 

[Physicist1231]

Unfortunately, nature cares not what you "feel". There is a heck of a lot against traveling >= C.

Thank you for that. That was awesome. There is a lot that is supporting not being able to exceed C however, even the growing scientific community is talking about faster than light objects or events. IE Quantam Physics and two particles being linked together and it does not matter how far apart they are. If one particle moves the other does the same thing at the same time. So what I present is not new.

Rather most scientific breakthroughs have been achieved by thinking outside the box. And viewing something as simple as speed as a limitation is a little too "in the box".

Here is a good example. If the speed of light is the max any object (we will say a photon) can reach and that is relative to any point or object in space then what if you have two cars with cool little engines under the hood. They are X distance apart and traveling directly twoard each other at 50mph each.

Cumulitively they are approaching at 100mph.

Speed it up to 300mph each... you get 600mph closure. (Car A will see Car B approaching at 600mph)

Keep going to .25C... cumulative of .5c (no one has exceeded C yet...)

Now get to .5C each... You have a total of 1C for closure. According to Relativity this would be the limitation.

But neither one actually exceeded the speed of light. Bump the speeds up to just over .5C (which is still possible according to either Newtons physics or relativity) and now you have a combined closure speed of >1C. Photons of light do this all the time say from one star to the next or even photons reflecting from the Earth back in the direction of the sun. Or even simpler... two candles 5 feet from each other are emitting photons with a closure rate of the photons of 2C (excluding things like gravity, reflection, and refraction that may slow it down a little).

So >C is possible even with relativity.

You can counter this argument with Lenth and time contractions. To exclude those you have observer C that is standing equidistant from each object and measures each objects (A and B) approach and sees them both coming in at 1C apiece. He can logicly conclude that the rate of approach is 2C... Fun stuff to think about.

 

[Ryan_m_b]

Thank you for that. That was awesome. There is a lot that is supporting not being able to exceed C however, even the growing scientific community is talking about faster than light objects or events. IE Quantam Physics and two particles being linked together and it does not matter how far apart they are. If one particle moves the other does the same thing at the same time. So what I present is not new.

Rather most scientific breakthroughs have been achieved by thinking outside the box. And viewing something as simple as speed as a limitation is a little too "in the box".

Here is a good example. If the speed of light is the max any object (we will say a photon) can reach and that is relative to any point or object in space then what if you have two cars with cool little engines under the hood. They are X distance apart and traveling directly twoard each other at 50mph each.

Cumulitively they are approaching at 100mph.

Speed it up to 300mph each... you get 600mph closure. (Car A will see Car B approaching at 600mph)

Keep going to .25C... cumulative of .5c (no one has exceeded C yet...)

Now get to .5C each... You have a total of 1C for closure. According to Relativity this would be the limitation.

But neither one actually exceeded the speed of light. Bump the speeds up to just over .5C (which is still possible according to either Newtons physics or relativity) and now you have a combined closure speed of >1C. Photons of light do this all the time say from one star to the next or even photons reflecting from the Earth back in the direction of the sun. Or even simpler... two candles 5 feet from each other are emitting photons with a closure rate of the photons of 2C (excluding things like gravity, reflection, and refraction that may slow it down a little).

So >C is possible even with relativity.

You can counter this argument with Lenth and time contractions. To exclude those you have observer C that is standing equidistant from each object and measures each objects (A and B) approach and sees them both coming in at 1C apiece. He can logicly conclude that the rate of approach is 2C... Fun stuff to think about.

Entanglement offers no way for information to travel FTL (indeed nothing is traveling FTL there). As for the closing speeds two objects traveling towards each other at velocities >.5c but <1c would seem to close the distance between each other faster than if one were stationary and the other traveling at 1c but only to an observer who isn't moving relative to them. From the point of view from one of the objects the closing speed is still less than 1c http://en.wikipedia.org/wiki/Faster-than-light#Closing_speeds

Just because objects can have closing speeds in excess of that of the speed of light does not mean faster than light speeds can be reached

 

[DaveC426913]

IE Quantam Physics and two particles being linked together and it does not matter how far apart they are. If one particle moves the other does the same thing at the same time.

What exactly are you talking about?

They are X distance apart and traveling directly twoard each other at 50mph each.

Cumulitively they are approaching at 100mph.

No they are not. They are traveling toward each other at a hair's breadth less than 100mph. Really.

True, at anything less than relativistic velocities this is usually ignored, but since you want to get picky and scale it up, velocities do not add the way you think they do. You must use the proper velocity addition formula.


Likewise, two objects approaching each other at .9c are not traveling wrt each other at 1.98c, they are traveling wrt each other at approx .0.994c.

If they were approaching each other at .999c then there combined relative v is 0.9999994994997501.

 

[mishrashubham]

Here is a good example. If the speed of light is the max any object (we will say a photon) can reach and that is relative to any point or object in space then what if you have two cars with cool little engines under the hood. They are X distance apart and traveling directly twoard each other at 50mph each.

Cumulitively they are approaching at 100mph.

Speed it up to 300mph each... you get 600mph closure. (Car A will see Car B approaching at 600mph)

Keep going to .25C... cumulative of .5c (no one has exceeded C yet...)

Now get to .5C each... You have a total of 1C for closure. According to Relativity this would be the limitation.

But neither one actually exceeded the speed of light. Bump the speeds up to just over .5C (which is still possible according to either Newtons physics or relativity) and now you have a combined closure speed of >1C. Photons of light do this all the time say from one star to the next or even photons reflecting from the Earth back in the direction of the sun. Or even simpler... two candles 5 feet from each other are emitting photons with a closure rate of the photons of 2C (excluding things like gravity, reflection, and refraction that may slow it down a little).

So >C is possible even with relativity.

You can counter this argument with Lenth and time contractions. To exclude those you have observer C that is standing equidistant from each object and measures each objects (A and B) approach and sees them both coming in at 1C apiece. He can logicly conclude that the rate of approach is 2C... Fun stuff to think about.

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).

Just to echo what Dave has already said in this thread.

 

[Physicist1231]

Entanglement offers no way for information to travel FTL (indeed nothing is traveling FTL there). As for the closing speeds two objects traveling towards each other at velocities >.5c but <1c would seem to close the distance between each other faster than if one were stationary and the other traveling at 1c but only to an observer who isn't moving relative to them. From the point of view from one of the objects the closing speed is still less than 1c http://en.wikipedia.org/wiki/Faster-than-light#Closing_speeds

Just because objects can have closing speeds in excess of that of the speed of light does not mean faster than light speeds can be reached

You may want to read your links. The link you posted supports your thoughts on Closing speeds but just a couple paragraphs down you see your initial statement about Quantam Mechanics is shown wrong by the same site.

Quantum mechanics - Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light.

There were other examples too for FTL thoughts on that same site.

There are several people saying that "nothing" can travel faster than light. but by the very theories they support FTL travel is possible for certian things... So... which is it. Is C an unachieveable speed by "anything" or not?

 

[podd]

Theories in science have accumulated vast wealths of independent evidence all strongly indicating the theory to be true. You shouldn't complain that Einstein's theories are not the "ultimate truth", they are not meant to be. What does even "ultimate truth" mean? Einstein's theories are brilliant at describing what they are supposed to describe, nothing more.

I'm not complaining, just saying 300 years ago scientists thought Newton's law of gravitation was true, and it turns out his equation was only an approximation of the effects of gravity. What prevents the same from happening to Einstein's equations?
You could think of "ultimate truth" as infinite precision, can you be 100% certain this applies to Einstein's equations? (not holding a grudge against Einstein it's just we happen to be speaking of him).

I was making the point that all theories are only models that try to explain our universe and that keep being remodeled and superseded by other theories which adopt a different point of view. As such you can't claim a theory is absolutely certain, it's like claiming a theory will remain the one widely accepted 10 years or 10000 years from now. We comprehend so little of the universe that there is still a lot of room for groundbreaking discoveries to be made and for change.

Otherwise I'm pretty confident that eminent scientists know their stuff better than I do, I was just reacting on the "xxx's theory is absolutely certain" really

 

[Ryan_m_b]

You may want to read your links. The link you posted supports your thoughts on Closing speeds but just a couple paragraphs down you see your initial statement about Quantam Mechanics is shown wrong by the same site.

Quantum mechanics - Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light.

Clearly you haven't read the other things posted here. The closing speed is faster than light when observed by an observer who is at rest relative to the moving objects. The moving objects will never measure their closing speeds as >c

Quantum mechanics does not transmit information. You cannot transmit information using two entangled particles. A good analogy is to thing that Alice and Bob both have a spinning coin in a box. To open to box they can press a button on the top, the box will open 1 second later. When a button is pressed the coin immediately falls over (showing either Heads or Tails). The other coin falls onto the other face. If Bob opens his box and sees Heads he does not know if it is Heads because he pressed his button or if Alice pressed hers at an earlier time. Do you see?

 

[JaredJames]

I was just reacting on the "xxx's theory is absolutely certain" really

Nobody (nor science) has claimed that without the caveat (or implication) of "based on current data".

 

[Ryan_m_b]

Otherwise I'm pretty confident that eminent scientists know their stuff better than I do, I was just reacting on the "xxx's theory is absolutely certain" really

Which I never said. We can discover more detailed theories and can expand our knowledge. But suggesting "well it's not 100% true so you can't say that this isn't possible" is hugely fallacious and unhelpful.

 

[DaveC426913]

...you see your initial statement about Quantam Mechanics is shown wrong by the same site.

Quantum mechanics - Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light.

No it doesn't.

Read Ryan's post above for an analogy.

 

[DrGreg]

You may want to read your links. The link you posted supports your thoughts on Closing speeds but just a couple paragraphs down you see your initial statement about Quantam Mechanics is shown wrong by the same site.

Quantum mechanics - Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light.

You may want to read your links. The link Physicist1231 posted goes on to say that no information actually goes faster than light.

If relativity is correct then faster-than-light travel would inevitably imply backwards-in-time travel.

Nothing traveling slower than light can be accelerated to light speed or faster, as that would require infinite energy. Photons always travel at the speed of light and cannot go faster or slower. It is postulated that particles called "tachyons" might exist, which always go faster than light and never slower, but there are lots of reasons for doubting their existence.

 

[podd]

Which I never said. We can discover more detailed theories and can expand our knowledge. But suggesting "well it's not 100% true so you can't say that this isn't possible" is hugely fallacious and unhelpful.

What is fallacious is claiming something is absolutely certain when it's not.
One should avoid to make false claims or generalized assertions when explaining a subject to someone less knowledgeable, because it more than often can lead to misconceptions. We are very confident the theory is true, no need to claim more.

But I still agree with your post.

Nobody (nor science) has claimed that without the caveat (or implication) of "based on current data".

Indeed but that's not the impression I had from reading some of the posts here. Anyway, sorry for the off topic.

 

[Physicist1231]

If relativity is correct then faster-than-light travel would inevitably imply backwards-in-time travel.

Not necessarily being able to go back in time (thus having the ability to change it) but yes seeing things in a reverse order. And what would be wrong with that if we could?

We already know that if you put a camera in space and look at the Earth (or other distant object) that what you are seeing is not up to date with what is actually happening on that object. It t would be the equivelant to watching a movie. If you sit still you are watching the event at normal speeds.

Should you approach the event you are speeding it up (even though the even already took place. There will be a limit as you can never see what will happen to the object in the future be cause it has not happened yet. The closest to the present that you can get by approaching is at the object itself.

Receeding away would be the equivelant of pressing the slow button. Once you hit the speed of light away from the object you have effectively hit the stop button... go faster to chase the photons that you have already seen and you will see them again... welcome to the rewind button! Nothing wrong with that as you cannot actually change the past... just view it.

 

[DaveC426913]

Not necessarily being able to go back in time (thus having the ability to change it) but yes seeing things in a reverse order. And what would be wrong with that if we could?

If one sets aside the laws of physics and then asks to see what might happen, one might as well expect faeries and unicorns.

Relativity allows faeries and unicorns at least as freely as it allows massive objects to exceed c.

It is true, there are hypothetical ways of traveling through time that are not forbidden by relativity (rotating cylinders, toroidal black holes, stable wormholes) but exceeding c isn't one of them.

There's more real science than you can study in a lifetime, why get caught in an eddy of fantasy?

 

[ZapperZ]

Rather most scientific breakthroughs have been achieved by thinking outside the box. And viewing something as simple as speed as a limitation is a little too "in the box".

Actually, this is NOT true.

Most breakthroughs and new ideas are based on the impetus of either a logical inconsistencies of an established understanding (eg: non-covariant of Maxwell equation under galilean transformation), or based on empirical observation that simply violates current theories or has no explanation (eg: high Tc superconductors). Only THEN would one needs to come up with new physics to describe these things. One just doesn't start with thinking "outside the box" for no apparent reason!

Here is a good example. If the speed of light is the max any object (we will say a photon) can reach and that is relative to any point or object in space then what if you have two cars with cool little engines under the hood. They are X distance apart and traveling directly twoard each other at 50mph each.

Cumulitively they are approaching at 100mph.

Speed it up to 300mph each... you get 600mph closure. (Car A will see Car B approaching at 600mph)

Keep going to .25C... cumulative of .5c (no one has exceeded C yet...)

Now get to .5C each... You have a total of 1C for closure. According to Relativity this would be the limitation.

But neither one actually exceeded the speed of light. Bump the speeds up to just over .5C (which is still possible according to either Newtons physics or relativity) and now you have a combined closure speed of >1C. Photons of light do this all the time say from one star to the next or even photons reflecting from the Earth back in the direction of the sun. Or even simpler... two candles 5 feet from each other are emitting photons with a closure rate of the photons of 2C (excluding things like gravity, reflection, and refraction that may slow it down a little).

So >C is possible even with relativity.

You can counter this argument with Lenth and time contractions. To exclude those you have observer C that is standing equidistant from each object and measures each objects (A and B) approach and sees them both coming in at 1C apiece. He can logicly conclude that the rate of approach is 2C... Fun stuff to think about.

This is a clear example of the faulty application of velocity addition in Special Relativity. Before you proceed any further, please do some homework on why the galilean velocity addition that you've used isn't valid for speeds approaching c. This is basic Special Relativity 101. This is not "thinking outside the box". This is not knowing what's in the box.

Zz.

 

[cosmik debris]

Quantum mechanics - Certain phenomena in quantum mechanics, such as quantum entanglement, appear to transmit information faster than light.

There were other examples too for FTL thoughts on that same site.

I think this is referring to the EPR paradox and the key word is "appears". If you read some of the posts on Bell's theorem you will get some idea of the problem.

There are several people saying that "nothing" can travel faster than light. but by the very theories they support FTL travel is possible for certian things... So... which is it. Is C an unachieveable speed by "anything" or not?

No information can be transmitted FTL.