Traveling at Light Speed Through Space: A Thought Experiment

[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.