Two Spaceships, .90c - Why No Faster Than Light?

[Somedude99]

You have 2 identical spaceships, each that can go .90 the speed of light relative to Earth. One spaceship tows the other, and once reaching the maximal speed (.90c relative to earth), the second ship that is currently being towed, is released, which then proceeds to accelerate toward its maximal speed (again, .90c relative to earth). Obviously the second ship, the one that is accelerating after being towed will leave the first ship behind, so why is it not possible to go faster than light?

 

[Somedude99]

Could the "light is relative to the observer" be wrong, and there is some hole in which you can travel faster? How do we know for certain that this is the truth, and that we are not mistaken?

 

[Dmitry67]

http://en.wikipedia.org/wiki/Velocity-addition_formula

You assume that 0.9c+0.9c=1.8c which is wrong.

 

[phyti]

You have 2 identical spaceships, each that can go .90 the speed of light relative to Earth. One spaceship tows the other, and once reaching the maximal speed (.90c relative to earth), the second ship that is currently being towed, is released, which then proceeds to accelerate toward its maximal speed (again, .90c relative to earth). Obviously the second ship, the one that is accelerating after being towed will leave the first ship behind, so why is it not possible to go faster than light?

You are confused, because both ships are currently moving at .9c relative to earth!
Rethink or reword your problem.

 

[Somedude99]

I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over.

What really troubles me is if someone standing still on Earth and someone in a spaceship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

I think light is an illusion of some kind.

 

[DaveC426913]

I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over.

What really troubles me is if someone standing still on Earth and someone in a spaceship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

I think light is an illusion of some kind.

The definition of velocity is a measurement of distance over time. It's not the distance that's changing, it's the time. At .999999c, the craft experiences time dilation of a factor of 700. At that factor, another ship moving relative to it, only needs to go from .999999 up to ~.99999999 in order for them to see a doubling of their relative speeds.

 

[Somedude99]

So theoretically you could go infinitely fast and never reach the speed of light? This is some fked up ****.

 

[russ_watters]

That's a contradiction. You can't go infinitely fast...you can accelerate forever and not reach the speed of light, though.

 

[neopolitan]

So theoretically you could go infinitely fast and never reach the speed of light? This is some fked up ****.

Infinitely fast relative to what?

If you don't put a "relative to" or "in relation to" in your sentence, you are going to get in all sorts of trouble. And that is not some "post-Einstein" peculiarity. We always move relative to something else - we just normally make an unstated assumption about what that something else is, the Earth's surface passing below or the air through which a plane is moving, the road we are driving or walking on, the train tracks on which a train is moving, the train body in which we are walking.

Put something into deep space and you no longer have anything obvious to move in relation to.

Before you head down that path, no, not having anything obvious to move in relation to has no effect whatsoever on relativity. Relativity works equally well for a snail as it does for a rocket traveling at 0.9c, our inability to notice the effect notwithstanding.

cheers,

neopolitan

 

[DaveC426913]

So theoretically you could go infinitely fast and never reach the speed of light? This is some fked up ****.

You could accelerate forever and never reach the speed of light.

 

[Somedude99]

Thanks

 

[phyti]

I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over..

If the ship moves at near light speed, time dilation slows all activity on the ship, which includes any rocket propulsion, thus it takes longer to accelerate. This would be observable by Earth viewers but not by the ships crew.

What really troubles me is if someone standing still on Earth and someone in a spaceship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

They do not view the same light!

It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

The train path is the same for both, they will not agree on the time it took for the trip.

Read about time dilation, if you haven't yet.

 

[Somedude99]

I just don't understand how things can look different if physical matter is taking up space. It would mean that everything we can see is just an illusion with reference to who is seeing it.

Does that mean that there could be infinite dimensions, since every speed can view its own light relative to it, which means that there are an infinite number of ways light can co-exist at the same time?

 

[russ_watters]

I just don't understand how things can look different if physical matter is taking up space. It would mean that everything we can see is just an illusion with reference to who is seeing it.

Just because things are reference frame dependent, that does not mean they are illusions.

Can you play billiards on a train moving at constant speed (on very smooth tracks!)? Of course. A billiards ball that is stationary on the table might be moving at 100 miles an hour relative to the ground, but it doesn't affect the game at all. That's frame dependent velocity, just like with Einstein's version of Relativity (well...almost). And Newton/Galileo would agree. That's the principle of relativity in action, it is just that those guys didn't know how to apply it correctly when having to do with the speed of light.

 

[gravityandlev]

You can actually view the law "nothing goes faster than the speed of light" as a consequence of the laws of electricity and magnetism: http://gravityandlevity.wordpress.c...he-speed-of-light-hints-from-electrodynamics/
Although it's probably more correct to say that the laws of electricity and magnetism are a consequence of the fact that nothing goes faster than the speed of light.

 

[scibuff]

The definition of velocity is a measurement of distance over time. It's not the distance that's changing, it's the time. At .999999c, the craft experiences time dilation of a factor of 700. At that factor, another ship moving relative to it, only needs to go from .999999 up to ~.99999999 in order for them to see a doubling of their relative speeds.

actually, time dilation and length contraction happen at the same "time", it really depends on the way you look at it (distance = velocity * time so if time is shortened, so is the distance)

 

[JesseM]

actually, time dilation and length contraction happen at the same "time", it really depends on the way you look at it (distance = velocity * time so if time is shortened, so is the distance)

The distance between two markers (say, Earth and a distant star) is only shortened in a frame moving relative to those markers. So if we consider a ship moving at 0.999999c in the frame of the Earth, in this frame there is no length contraction of the distance the ship has to travel (though the ship itself will be shortened in this frame), while there is time dilation of the ship's clock. On the other hand, in the rest frame of the ship the distance from Earth to the star is contracted relative to the distance in the Earth's rest frame, but the ship's clock is not dilated (although a clock on Earth would be in this frame).

 

[scibuff]

The distance between two markers (say, Earth and a distant star) is only shortened in a frame moving relative to those markers. So if we consider a ship moving at 0.999999c in the frame of the Earth, in this frame there is no length contraction of the distance the ship has to travel (though the ship itself will be shortened in this frame), while there is time dilation of the ship's clock. On the other hand, in the rest frame of the ship the distance from Earth to the star is contracted relative to the distance in the Earth's rest frame, but the ship's clock is not dilated (although a clock on Earth would be in this frame).

I guess i should have been clearer ... whenever two observes disagree on the time measured, they will always disagree on the length as well. That's what I meant by saying that length contraction and time dilation happen at the same time - which one of the two you see depends on your frame of reference.

 

[JesseM]

I guess i should have been clearer ... whenever two observes disagree on the time measured, they will always disagree on the length as well.

Yes, that's true, although I don't see where DaveC426913's post was contradicting this.

 

[Idjot]

I just don't understand how things can look different if physical matter is taking up space. It would mean that everything we can see is just an illusion with reference to who is seeing it.

Does that mean that there could be infinite dimensions, since every speed can view its own light relative to it, which means that there are an infinite number of ways light can co-exist at the same time?

Illusion? Consider this: The majority of today's "relativistic" scientific community is conditioned to(and obscenely comfortable with) the idea that nothing qualifies as an absolute frame of reference. This same community is also comfortable studying complex diagrams illustrating the intricacies of the various stages of imaginary twins in an imaginary experiment that proves the existence of an imaginary paradox that is in reality not a paradox at all, but a mere illusion. Although c may be constant, it takes little to distort our perception of it. If you were to take all variables into account to the smallest degree at any given time in your observation of any given event you could prove that most of what you see is indeed, an illusion. However light does not experience illusions like you do. It does exactly as it should. You just see it differently from different reference frames.

 

[JesseM]

This same community is also comfortable studying complex diagrams illustrating the intricacies of the various stages of imaginary twins in an imaginary experiment that proves the existence of an imaginary paradox that is in reality not a paradox at all, but a mere illusion.

No one in the physics community thinks it's a "paradox", it's only called the "twin paradox" because it superficially looks like a paradox if you don't relativity very well. And there is plenty of experimental evidence that clocks experience time dilation in just the way predicted by relativity--for example, the clocks of GPS satellites are designed to adjust to compensate for time dilation, if time dilation didn't occur in the predicted manner the GPS system wouldn't work.

 

[Idjot]

I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over.

What really troubles me is if someone standing still on Earth and someone in a spaceship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

I think light is an illusion of some kind.

When people imagine space ships traveling at light speed they are imagining something that is truly impossible. Period. Light is radiation not matter. That's why it can travel as fast as it does. And whether or not people want to admit it, no one truly knows why it tops out at 299,792.458 km/s. Maybe if we didn't divide our time into increments to fit our orbit, we'd find a connection somewhere by making c a whole number. Anyway, imagining a spaceship moving even remotely close to the speed of light is like hoping that water is actually made of nitrogen. It will never happen in the life of the universe. Never ever forever never. If something moves faster than light, it's certainly not visible and it certainly isn't matter, and most likely it's light itself in a specialized circumstance somewhere beyond the event horizon of a black hole, and even then if we could observe anything of it, it would probably be an illusion.

 

[Idjot]

No one in the physics community thinks it's a "paradox", it's only called the "twin paradox" because it superficially looks like a paradox if you don't relativity very well. And there is plenty of experimental evidence that clocks experience time dilation in just the way predicted by relativity--for example, the clocks of GPS satellites are designed to adjust to compensate for time dilation, if time dilation didn't occur in the predicted manner the GPS system wouldn't work.

Of course. Gravitational time dilation. No paradox. However the paradox scenario has been the subject of much discussion and debate for many years and still is and you know it.
I was goofing on the irony of so many scientific minds playing with an imaginary idea that was born from the fear of actively choosing a frame of reference, because a witty man with funny hair said they couldn't so many years ago. Sociologically speaking, sometimes Relativity reminds me of Political Correctness.

 

[JesseM]

Of course. Gravitational time dilation. No paradox.

The classic twin paradox doesn't involve any gravitational time dilation, not in inertial frames anyway.

However the paradox scenario has been the subject of much discussion and debate for many years and still is and you know it.

No debate among physicists! It's discussed only because it's an interesting pedagogical example, and because a lot of people who don't understand relativity very well think there's a genuine paradox there.

I was goofing on the irony of so many scientific minds playing with an imaginary idea that was born from the fear of actively choosing a frame of reference,

You don't need to choose a frame of reference, all inertial frames make the same prediction about how old the two twins are when they reunite.

because a witty man with funny hair said they couldn't so many years ago.

It's a basic principle of the theory that all inertial frames are on equal footing, and the theory has abundant experimental support. Nothing to do with taking Einstein's word as gospel.

Sociologically speaking, sometimes Relativity reminds me of Political Correctness.

Your comments lead me to doubt you have taken the time to understand the theory before casting these sorts of judgments on it.

 

[Idjot]

The classic twin paradox doesn't involve any gravitational time dilation, not in inertial frames anyway.

Exactly. But GPS satellites?

No debate among physicists! It's discussed only because it's an interesting pedagogical example, and because a lot of people who don't understand relativity very well think there's a genuine paradox there.

It's imaginary. People who do understand relativity should dispose of it as such. It's not logically presented. It's contradictory and confusing to people trying to learn the truth. Noone observes someone else's clock from a distance at impossible velocities. It's an imaginary scenario akin to an Escher painting. It's puzzling entertainment, nothing more.

You don't need to choose a frame of reference, all inertial frames make the same prediction about how old the two twins are when they reunite.

An inaccurate prediction based on illusion.

It's a basic principle of the theory that all inertial frames are on equal footing, and the theory has abundant experimental support. Nothing to do with taking Einstein's word as gospel.

I don't dispute that all inertial frames are on equal footing but I do think Relativity hindered the search for the absolute. Relativity convinced everyone that an absolute can not be found, when an absolute is exactly what we should be looking for. If you're going to imagine something, imagine that the absolute frame does exist out there and that Relativity is only here to get us through until we find it (and choose to accept it). Then we'll be able to figure out how much time has truly dilated for the inertial frames and how old everything truly is in the universe. But maybe that's not important to you. Maybe you're happy just knowing that clocks look different when you're passing them in space at a truly undefined velocity.

Your comments lead me to doubt you have taken the time to understand the theory before casting these sorts of judgments on it.

I do not make judgments on the theory. It's the most brilliant theory of all time and I absolutely love it. I only make judgments on the people who think it answers everything.

 

[scibuff]

Actually, the twin paradox lies in a fact that, suppose if the twin A traveled at \frac{\sqrt{3}}{2}c - so that \gamma=\frac{1}{2} for a year and then turned around and came back (at the same velocity), twin B would have aged 2 years and twin A only one year. But if you looked at the situation from twin A frame of reference, you could say that twin A was standing still and twin B was moving at \frac{\sqrt{3}}{2}c, hence twin A should think that twin B aged only a year while twin A aged 2 years ... this would be true, though, only if the situation was symmetric, which it is NOT.

 

[JesseM]

Exactly. But GPS satellites?

GPS satellites involve a combination of velocity-based and gravitational time dilation.

It's imaginary.

Virtually all physics textbook problems are highly idealized to make them simpler. In electromagnetism you might derive the magnetic field created by current traveling through a straight wire idealized to have infinite length, for example.

It's not logically presented. It's contradictory and confusing to people trying to learn the truth.

Why do you say that?

Noone observes someone else's clock from a distance at impossible velocities.

There is no need to state the twin paradox in terms of anyone observing clocks at a distance, you can state it in terms of what predictions will be made about what the two clocks read when they reunite, based on their velocities over the journey. Also, the twin paradox works just fine with small relative velocities (time dilation in a given frame occurs at all velocities, even 0.00000001c), it just makes the math a little more obvious to have them travel at a high fraction of light speed in the thought-experiment.

An inaccurate prediction based on illusion.

Are you claiming it's inaccurate that if two clocks are initially sychronized, and one of them moves inertially while the other flies away, turns around, and comes back (all in a flat region of spacetime, or close to it) that the inertial clock will have elapsed more time than the non-inertial one?

I don't dispute that all inertial frames are on equal footing but I do think Relativity hindered the search for the absolute. Relativity convinced everyone that an absolute can not be found, when an absolute is exactly what we should be looking for.

There are a number of absolute quantities in relativity such as proper time between two events on a worldline, which all frames will make the same prediction about. There is no absolute reference frame in SR, but why do you think we "should be" looking for one?

If you're going to imagine something, imagine that the absolute frame does exist out there and that Relativity is only here to get us through until we find it (and choose to accept it).

Why should we imagine this? Do you also think we need to imagine there's an absolute truth about which direction in space is "up" and which is "down"?

Then we'll be able to figure out how much time has truly dilated for the inertial frames and how old everything truly is in the universe.

The age of an object at a given point on its worldline is just the proper time from the object's beginning to that point, and proper time is frame-independent.

But maybe that's not important to you. Maybe you're happy just knowing that clocks look different when you're passing them in space at a truly undefined velocity.

Physics is basically just about using mathematical models to make predictions about the outcomes of observable events. The only objectively measurable events involving clocks are what they each read at moments when they are next to each other in space, and relativity can be used to make accurate predictions about this. Comparisons of the rate clocks are ticking when they are at different positions depend on your choice of coordinate system, and there is no physical reason at present to expect a preferred coordinate system, although it's conceivable there could be in the future.

I do not make judgments on the theory. It's the most brilliant theory of all time and I absolutely love it. I only make judgments on the people who think it answers everything.

Who says it "answers everything"? It seems that your real objection is that physicists are happy to dispense with the notion of an absolute frame as long as there is no evidence for one (and they are not dogmatic about the fact that there never could be, although there are some good arguments for judging it to be very unlikely). You on the other hand seem to have some preexisting philosophical bias for thinking there must be one regardless of whether there is any evidence to support such a notion.

 

[workmad3]

The way I've come to resolve these issues is as follows (I make no statement as to their correctness, just that it makes sense to me :
1) The speed of light is constant in any frame of reference and in every direction(given as part of Relativity and Maxwell's field equations)
2) In order for this to occur, one of two things must happen: Either the time you measure over gets longer or the distance you measure over gets shorter (According to Relativity, both occur)
3) In order for time to dilate or space to contract, the fabric of spacetime has to be distorted
4) Gravity distorts spacetime
5) Mass is the source of gravity
6) Energy is equivalent to mass
7) You need to apply energy to accelerate
8) Ergo the more energy applied to accelerate, the more mass the object gains, therefore the greater it distorts the fabric of spacetime. The limit towards this tends is the speed of light (which is composed of photons which have no mass, therefore can move at the highest possible speed with no application of energy).

 

[DaveC426913]

8) Ergo the more energy applied to accelerate, the more mass the object gains, therefore the greater it distorts the fabric of spacetime.

Hang on there. Relativistic mass does not contribute toward space-time curvature.

 

[Idjot]

GPS satellites involve a combination of velocity-based and gravitational time dilation.

Sorry. It wasn't really a question.

Virtually all physics textbook problems are highly idealized to make them simpler. In electromagnetism you might derive the magnetic field created by current traveling through a straight wire idealized to have infinite length, for example.

I can appreciate ideology by this definition. But a skewed perspective, like in the twins paradox does not indicate a paradox. It is merely an illusion that would be accounted for anyway, so there would never be a paradox to begin with. That is the paradox to me.

There is no need to state the twin paradox in terms of anyone observing clocks at a distance, you can state it in terms of what predictions will be made about what the two clocks read when they reunite, based on their velocities over the journey. Also, the twin paradox works just fine with small relative velocities (time dilation in a given frame occurs at all velocities, even 0.00000001c), it just makes the math a little more obvious to have them travel at a high fraction of light speed in the thought-experiment.

This is probably a subject for it's own thread, but do you really believe that the limits of time are dictated by electromagnetic radiation? Why not "idealize" a formula for time dilation at "idealistic" speeds above c? Seriously though, I could use such a formula in my present work. Any ideas?

Are you claiming it's inaccurate that if two clocks are initially sychronized, and one of them moves inertially while the other flies away, turns around, and comes back (all in a flat region of spacetime, or close to it) that the inertial clock will have elapsed more time than the non-inertial one?

No.

There are a number of absolute quantities in relativity such as proper time between two events on a worldline, which all frames will make the same prediction about. There is no absolute reference frame in SR, but why do you think we "should be" looking for one?

Why shouldn't we? If you accept big bang theory you might consider CBR to be a little more qualified than other frames for such a role. That's where I'm willing to leap, where you will probably say you have no logical reason to consider it special. That's understandable but even without a proven rest frame, we can still find what we determine to be the most likely or best qualified rest frame.

Why should we? It's simple. If everything is moving, everything is experiencing time dilation.
The age and velocity of an object should be measured by it's own local rate of time elapse relative to the absolute frame. That is the only way to make realistic comparisons between the properties of celestial bodies. Otherwise the results will be "skewed" as in the paradox. I just feel like we should be striving to view the universe in this way right now, even if we should choose a surrogate absolute to get us started. Does this make any sense to you or do I just sound like a crackpot? Haha.

Why should we imagine this? Do you also think we need to imagine there's an absolute truth about which direction in space is "up" and which is "down"?

Up and down are illusions caused by graviry. Time is real. Aging actually takes place. Differences in speed really cause differences in aging. Why not strive to find out what those differences really are rather than just what they are relative to each other?

Physics is basically just about using mathematical models to make predictions about the outcomes of observable events. The only objectively measurable events involving clocks are what they each read at moments when they are next to each other in space, and relativity can be used to make accurate predictions about this. Comparisons of the rate clocks are ticking when they are at different positions depend on your choice of coordinate system, and there is no physical reason at present to expect a preferred coordinate system, although it's conceivable there could be in the future.

I appreciate that you consider it conceivable.

Who says it "answers everything"? It seems that your real objection is that physicists are happy to dispense with the notion of an absolute frame as long as there is no evidence for one (and they are not dogmatic about the fact that there never could be, although there are some good arguments for judging it to be very unlikely). You on the other hand seem to have some preexisting philosophical bias for thinking there must be one regardless of whether there is any evidence to support such a notion.

Exactly. Just because we don't know which star is actually aging faster than the other does not mean that one isn't. Just because we can't tell which object is actually moving at a higher velocity than the other does not mean that one isn't. If we can accept c as an absolute, we can do the same for time somehow. We just have to find the right system. Maybe it'll end up being big bang related or maybe it'll end up being black hole related or even atom related. Whatever it is, we have to discover it eventually.