Relativity and space travel - what's the problem?

[billy_boy_999]

Well sure, as long as you don't keep falling into a very similar misconception about how the concept of speed is defined, we're fine. We weren't in disagreement about that point about the effects of time dilation - but that isn't what you were saying before.

russ - this is what i have been saying all along, maybe you have misunderstood what i was trying to say or haven't been following along or maybe i haven't been clear enough...if you go back and read the posts, as well as the title of this thread, you will find that my point has always been that relativity does not bind us to obeying a relative velocity of light, only a local one...in other words, the misconception that you cannot cover a distance of 10 light years in one year is a misreading of relativity...

Do you want to use physics or not? If you use physics, the "speed" you are calculating isn't a speed.

what is "speed"? speed is simply a measure of distance over time...what i was saying is that we can take the distance as measured from Earth and the time as measured from a moving spaceship and get a speed that is much faster than light...this is not some pedantic trick of logic - this is how you would go about measuring speed if you were undertaking such a space flight...you would measure the distance before you took off and measure how long it took you to get there in local time...

relativistic increases in mass due to high speeds do not apply and relativity does not prohibit this sort of space travel...

 

[Doc Al]

... my point has always been that relativity does not bind us to obeying a relative velocity of light, only a local one...in other words, the misconception that you cannot cover a distance of 10 light years in one year is a misreading of relativity...

If all you are saying is that if a ship is moving fast enough (some high fraction of c) with respect to the earth, then that ship can cover 10 earth-measured light years of distance in one ship-measured year, then sure! But don't think for a minute that its speed relative to Earth is now somehow faster than light. (But it doesn't have to be, in order to get the effect you are talking about.)

what is "speed"? speed is simply a measure of distance over time...what i was saying is that we can take the distance as measured from Earth and the time as measured from a moving spaceship and get a speed that is much faster than light...this is not some pedantic trick of logic - this is how you would go about measuring speed if you were undertaking such a space flight...you would measure the distance before you took off and measure how long it took you to get there in local time...

"Speed" is well-defined. It's the distance traveled divided by the time it takes---but both distance and time must be measured in the same frame. If you just mix and match measurements made in different frames, you'll get nonsense.

But, nonetheless, I believe your basic point is correct. If, somehow, you had the energy to get a ship moving fast enough (never greater than the speed of light, mind you!) then the occupants of that ship could traverse galactic distances within their lifetimes. (From the ship's point of view, these galactic distances are now quite small.)

Of course, as has been pointed out here, practical energy considerations make this scenario unlikely.

 

[billy_boy_999]

thanks for conceding the point Doc...

"Speed" is well-defined. It's the distance traveled divided by the time it takes---but both distance and time must be measured in the same frame. If you just mix and match measurements made in different frames, you'll get nonsense.

your definition of "speed" is very specific and of course this is the tacit conventional definition...but really, why do you have to keep insisting this is nonsense? the practical consequence is that you can travel 10 light years in 1 year...

as far as that not being 'faster than light' in a local sense...of course! in a local sense your speed as measured against light is always ZERO!

why are we so married to this pedantic language in insisting that 'nothing can travel faster than light'? it seems like a meaningless mantra to me, and i think it contributes to a popular misapprehension of relativity...(and popular enough to be evidenced on this board as well, it seems)...

your point about practical energy considerations making the scenario unlikely is absolutely valid...THESE considerations are the limits of space travel, NOT relativistic 'speed limits'...

 

[selfAdjoint]

You seem to think you know more about relativity than the others responding to you on this thread. But all I have seen from you is wild assertions and language tricks.

Relativity (the special kind) is derived logically from two postulates:

1. All inertial frames experience the same physics within themselves.
2. All inertial frames experience the same value of the speed of light.

From these one can deduce the Lorentz transformations, and the Lorentz transformations say that it would take an infinite amount of energy to accelerate any mass up to the speed of light, and that the relative speed of two inertial frames cannot exceed the speed of light.

If you have a theory in which these things are false then it isn't relativity; it's something else. And as something else it has to be justified the same way relativity has been, by experiment.

 

[Doc Al]

thanks for conceding the point Doc..

What point did I concede? This is all relativity 101.

your definition of "speed" is very specific and of course this is the tacit conventional definition...but really, why do you have to keep insisting this is nonsense? the practical consequence is that you can travel 10 light years in 1 year...

"My" definition of speed is the only one used in physics. Your definition is a quaint earth-bound one! Imagine you were born in Galaxy X, which just happens to be moving at 0.9c with respect to earth. You would get a kick out of those old-fashioned "earth light years" that they insist on using. Would you insist that they really should be using "Galaxy X" light years instead--the one true unit!

as far as that not being 'faster than light' in a local sense...of course! in a local sense your speed as measured against light is always ZERO!

Huh?

why are we so married to this pedantic language in insisting that 'nothing can travel faster than light'? it seems like a meaningless mantra to me, and i think it contributes to a popular misapprehension of relativity...(and popular enough to be evidenced on this board as well, it seems)...

If you wish to talk about speeds and times and distances then you must define things carefully. If you wish to talk about when the ship will arrive at its destination as measured by Earth clocks, then you'd better measure the speed of that ship properly. It will still take Time = Distance/speed to reach the destination. No one on Earth will say that the ship is traveling faster than c. (Well... apparently some will. )

 

[russ_watters]

c is not constant and atomic clock rates aren’t either.

If you want, start a thread in TD and prove it. Don't attemt to hijack other people's threads.

your definition of "speed" is very specific and of course this is the tacit conventional definition...but really, why do you have to keep insisting this is nonsense? the practical consequence is that you can travel 10 light years in 1 year...

Yes, our definition of speed is very specific: and it has to be. That's the reason we mentors are such sticklers for accuracy in definitions. If you use your definition and plug it into the equations of physics, it will return nonsensical answers.

Further, who'se clock and who'se meterstick would you use in your definition? Any one you want? Let's say you travel to a star that by Earth's measurement is 10 LY away. Due to your speed, you think its 5 LY away. Another ship passes you on the way and according to them, its 3 LY away. Couldn't you reasonably use whatever distance you choose according to your definition? Doesn't that therefore make such a definition useless?

The same problem (and solution) exists in Galilean relativity. A man is walking forward at 1m/s on a train which is moving at 10m/s. He sees himself to be moving at 1m/s. A woman at the station sees him and measures his speed to be 11m/s. Different speeds, but one very important thing in common: both measure the distance and time in their own frame of reference.

 

[billy_boy_999]

DocAl -

billy_boy_999said: as far as that not being 'faster than light' in a local sense...of course! in a local sense your speed as measured against light is always ZERO!

DocAl said: Huh?

The speed of light is the same for all observers. Again, relativity 101.

russ_watters said: Further, who'se clock and who'se meterstick would you use in your definition? Any one you want? Let's say you travel to a star that by Earth's measurement is 10 LY away. Due to your speed, you think its 5 LY away. Another ship passes you on the way and according to them, its 3 LY away. Couldn't you reasonably use whatever distance you choose according to your definition? Doesn't that therefore make such a definition useless?

Look it's perfectly simple...I'm taking the distance measurement from the Earth's frame and the time measurement from the spaceship's frame...the other spaceship can do likewise...or it can take whatever measurements from whatever frames it wants, it will end up measuring a speed relative to someone else's frames...

you call this a useless definition or nonsense or 'not physics'...try telling that to someone planning a hypothetical interstellar space voyage...the distances they measured from Earth and the time they measure en route - these are certainly very real quantitites, even if in real 'physics' we aren't supposed to consider them together...

anyways, i'll shut up now as i think the point has been conceded...

 

[russ_watters]

Look it's perfectly simple...

Yes, we get it. Time in one frame and distance from another.

the distances they measured from Earth and the time they measure en route - these are certainly very real quantitites, even if in real 'physics' we aren't supposed to consider them together...

That's certainly also true.

Look, all we're telling you here is that to use equations like Newton's laws of motion or Einstein's Relativity, you have to use the definition of speed they use otherwise the answers you get to your calculations won't jive with what you observe. If you want to use a different definition of "speed" for whatever reason, you can - you just have to change all the relevant equations to make it work. You can certainly add an extra set of transformations to make the speedometer readout on the spaceship say whatever you want it to, but I would think most physicists would just as soon use the equations and definitions they have (because they work).

Not that I necessarily want to bring David back into the conversation, but this is essentially what he's talking about with C+v and C-v calculations he brings up all the time. By using time and distance from different frames, you can get a variable speed of light if you want to.

 

[billy_boy_999]

okay, so all i was trying to say was that there is no speed barrier because there is no mass increase when you measure it in your own inertial reference frame...i thought this was an important clarification...

thus, these figures...

Janus: Even with a fusion drive you would have to burn a mass of approx. 1/5 that of the Moon for every kilogram you would want to deliver at another star system , traveling at only .25c!

...are completely inapplicable...they have nothing to do with our inertial reference frame, they only come into play when a stationary observer on the Earth becomes involved...

You can certainly add an extra set of transformations to make the speedometer readout on the spaceship say whatever you want it to, but I would think most physicists would just as soon use the equations and definitions they have (because they work).

okay, i thought i had made this clear...i'm not talking about selecting an arbitrary set of reference observations to manufacture an arbitrary speed...i'm talking about measuring how long it will take a spaceship to go a certain (non-arbitrary, actual, physical) distance...i think physicists will be willing to use any equations that represent a real, physical reality...

a spaceship can travel 10 light years as measured from Earth in only a year's traveling time, at least relativity doesn't prohibit it...that was my entire point...

and now i'll shut up...

 

[Doc Al]

okay, so all i was trying to say was that there is no speed barrier because there is no mass increase when you measure it in your own inertial reference frame...i thought this was an important clarification...

There is very much a speed barrier! The fact that there is no "mass increase" as measured in the rest frame of the ship is irrelevant. If you wish to move with respect to something, then that "mass increase" matters a great deal. It is what prevents you from achieving the speed of light with respect to anything else-- since that requires infinite energy. No can do!

Now to get that relativistic space travel working, that ship's got to move with respect to the earth. And that takes energy. That's the point of Janus's figures.

thus, these figures...
...are completely inapplicable...they have nothing to do with our inertial reference frame, they only come into play when a stationary observer on the Earth becomes involved...

Those figures are critical in estimating what it will take to attain a certain speed. Remember that the ship must move with respect to the earth.

okay, i thought i had made this clear...i'm not talking about selecting an arbitrary set of reference observations to manufacture an arbitrary speed...i'm talking about measuring how long it will take a spaceship to go a certain (non-arbitrary, actual, physical) distance...i think physicists will be willing to use any equations that represent a real, physical reality...

If you want to travel 10 Earth light years in one ship year, then you'd better haul ass--attain a real speed with respect to the earth! The relative speed is not arbitrary.

a spaceship can travel 10 light years as measured from Earth in only a year's traveling time, at least relativity doesn't prohibit it...that was my entire point...

If the relative speed of the ship and the Earth is high enough, no problem! But if you are the one who needs to accelerate that ship from rest to that high speed---there is a huge energy cost for doing that.

 

[David]

c is not constant and atomic clock rates aren’t either.

If you want, start a thread in TD and prove it.

It has already been proven. Read this, posted in an article on the excellent website “Bernhardt Media Physics Post”:

“Since the segment closest to the star is aging slower, the speed of light there is slow by the same proportion according to Einstein's view described here. The light pulse moves with a velocity less than "c" along the curve relative to reference observers far away, but it still travels with the velocity "c" to the slower aging observers on the local metric since their clocks run slower and they are aging slower. I can't draw speed into the picture, so you'll need to imagine that in figure 25, the light pulse slows down as the wavelength decreases.”

http://www.physicspost.com/articles.php?articleId=101&page=12


Shapiro also proved the part about “c” not being “constant” and many other scientists have already proved that the tick rates of atomic clocks are not “constant”.

 

[jcsd]

It has already been proven. Read this, posted in an article on the excellent website “Bernhardt Media Physics Post”:

“Since the segment closest to the star is aging slower, the speed of light there is slow by the same proportion according to Einstein's view described here. The light pulse moves with a velocity less than "c" along the curve relative to reference observers far away, but it still travels with the velocity "c" to the slower aging observers on the local metric since their clocks run slower and they are aging slower. I can't draw speed into the picture, so you'll need to imagine that in figure 25, the light pulse slows down as the wavelength decreases.”

http://www.physicspost.com/articles.php?articleId=101&page=12


Shapiro also proved the part about “c” not being “constant” and many other scientists have already proved that the tick rates of atomic clocks are not “constant”.

Beware! what Eric really should of said: is that the remote coordinate speed of light is less than c, which is different from the local coordinate speed of light which is always c. As we're talking only about special relativity that's irrelevant anyway as in special relativity the coordinate speed of light is always c no matter where you are.

Of course atomi cclock rates aren't constant they're always going to be peturbed by slight variations in their local environs, but they are very, very close to constant.

 

[David]

Beware! what Eric really should of said: is that...

Eric said exactly what he should have said, and I know what he is talking about. The overall real physics situation is more complicated than what you just said. See the Max Born quote I posted on the other thread regarding the redshift and blueshift of the light from stars that are fixed relative to the sun, the shifts that we observe because the Earth is moving either away from or toward the stars in its annual motion around the sun.

 

[jcsd]

Eric said exactly what he should have said, and I know what he is talking about. The overall real physics situation is more complicated than what you just said. See the Max Born quote I posted on the other thread regarding the redshift and blueshift of the light from stars that are fixed relative to the sun, the shifts that we observe because the Earth is moving either away from or toward the stars in its annual motion around the sun.

No, you should always be precise inn your terminology when your talking about the remote coordinate speed as it leads to erroneous conclusions such as c isn't constant.

In regards to redshift blueshift, what's the point in talking about relativity when your not even using the non-relativistic equations, which are only the limits of the actual relativistic equations.

 

[billy_boy_999]

okay, sorry, i promised i'd shut up but Doc Al, you really have hugely misinterpreted relativity here...

There is very much a speed barrier! The fact that there is no "mass increase" as measured in the rest frame of the ship is irrelevant.

it is absolutely relevant, in fact it's the whole point. this is why there is no speed barrier! no increased mass from the ship's point of view means no additional (proportionally) energy is needed to accelerate it!

Now to get that relativistic space travel working, that ship's got to move with respect to the earth.

what?!? to travel you need to move with respect to the earth? the Earth is outside the ship's inertial reference frame! it doesn't matter what the Earth does, the Earth can stay in its orbit (now who's moving from whom?) or it can bob up and down or it can disappear - it doesn't affect the speed of the ship!

Those figures [of Janus's] are critical in estimating what it will take to attain a certain speed. Remember that the ship must move with respect to the earth.

that is absolutely wrong in every possible way...the Earth is immaterial to the space ship...think about what you're saying...now every body's speed has to be accelerated against the earth! do you not understand the basis of the principle of relativity? there is no absolute motion or rest...you're talking about apparent relative effects like they are physically applicable to the spaceship's reference frame! if that were the case then in respect to some faraway galaxy the Earth would have to be accelerating at very near the speed of light - which means the Earth's mass is close to infinity...[and it is, of course, but only in the faraway galaxy's reference frame - a very important caveat, otherwise the infinite mass of the Earth would turn us all to cooked spaghetti!]...

If you want to travel 10 Earth light years in one ship year, then you'd better haul ass--attain a real speed with respect to the earth! The relative speed is not arbitrary.

haul ass, yes - the relative speed is not arbitrary? that sentence in itself makes no sense..."relative speed" means exactly that...there is no absolute speed, the Earth's measurements are inconsequential to a spaceship that is moving under its own power...

If the relative speed of the ship and the Earth is high enough, no problem! But if you are the one who needs to accelerate that ship from rest to that high speed---there is a huge energy cost for doing that.

yes, this is absolutely true, but very different from having an infinite mass due to relativistic effects...that infinite mass only applies when someone else who is stationary in relation to the moving body accelerates it (such as is the case with particles in accelerators)...a ship moving under its own power feels none of these effects, a closed inertial reference frame feels no effects - this is the whole point of the theory of relativity!

and now, definitely, i will shut up...i promise...

p.s. - sorry about all the exclamation points...

 

[Janus]

yes, this is absolutely true, but very different from having an infinite mass due to relativistic effects...that infinite mass only applies when someone else who is stationary in relation to the moving body accelerates it (such as is the case with particles in accelerators)...a ship moving under its own power feels none of these effects, a closed inertial reference frame feels no effects - this is the whole point of the theory of relativity!

The numbers I gave came directly from using the relativistic rocket equation:

v = ctanh \left(\frac{V_{e}}{c} ln(MR)\right) <br />



MR is the mass ratio of reaction mass to payload and V_e is the exhaust velocity. For a fusion drive you won't get an effective exhaust velocity of more than a few percent of c.

How much reaction mass you need to reach any given velocity will be the same from the view of the Earth or the Space ship.(It has to be, or you would run into paradoxes.)

Another point is that at .25c the Relativistic effects amounts to very little.

If you use the classical Rocket equation:

v = V_{e}ln(MR) <br />

You would end up using about 1/3 the reaction mass, but 1/15 the moon's mass per kilogram is still quite a bit.

Changing your frame of reference does not allow you to "exceed the speed of light" The rocket is an accelerated frame, and as such will see relativistic effects different than those seen from an unaccelerated frame. The combined results of these effects will be such that the rocket will never measure the universe's relative motion to itself as greater than c. So, yes, even from the rocket's frame there is a c limit. It is just that from the rocket's frame, the distance traveled is less.

 

[billy_boy_999]

janus - yes, yes, you can throw out all the rocket equations you like...the point i was trying to make by starting this thread was this. relativity does not prohibit traveling distances of 10 ly in only 1 year - if you measure the distance before you take off and the time it takes you to get there, which is exactly how you would do it...

So, yes, even from the rocket's frame there is a c limit. It is just that from the rocket's frame, the distance traveled is less.

yes, this is exactly what happens, the result is that you can travel "faster than light" by which i mean you can travel 10 ly away from Earth in a one year space flight...is this a nonsensical, non-relative equation?...if so, it certainly is a fairly practical one...i must say i find these objections pedantic at this point...you can look at it any way you want, you can correct for time dilation, relativistic mass and space accomodations, but the effect is the same: you can make a journey of 10 ly in one year, or at least the theory of relativity does not (as is popularly assumed) prohibit these speeds...other physical laws might...

i have been labouring for four pages of posts to get you to concede this very simple point...you and others have responded with non-applicable equations relating a relativistic increase in mass that does not have the slightest effect on our inertial frame's discrete acceleration...

will you concede this: there is a misconception that relativity prohibits a spaceship reaching a certain distance in a certain amount of time because of the constancy of the speed of light. rather, measuring the distance you will travel before you take off and measuring the time it takes you to get there on board your ship, there is no relativistic speed limit in your own ship's inertial reference frame. in fact, this would absolutely contradict the principle of relativity.

if you will concede this point, then i will shut up...

 

[Doc Al]

i have been labouring for four pages of posts to get you to concede this very simple point...you and others have responded with non-applicable equations relating a relativistic increase in mass that does not have the slightest effect on our inertial frame's discrete acceleration...

Your "labors" have been a mishmash of misconceptions about relativity. You make statements that just aren't true and you expect agreement? Your wacky personal definition of "speed" has not helped you. In fact, it will just prevent you from understanding how the world really works according to relativity.

will you concede this: there is a misconception that relativity prohibits a spaceship reaching a certain distance in a certain amount of time because of the constancy of the speed of light. rather, measuring the distance you will travel before you take off and measuring the time it takes you to get there on board your ship, there is no relativistic speed limit in your own ship's inertial reference frame. in fact, this would absolutely contradict the principle of relativity.

Again, your non-standard (and plain wrong) definition of speed is blocking you from understanding basic relativity concepts. Saying things like "speed limit in your own inertial frame" is gibberish. In its own frame, everything is at rest. The relevant speed is always a relative speed between one thing and another, defined in the usual physics manner.

If you made the following simple statement, all relativists would agree: If a ship were moving fast enough (with respect to the earth), then it could travel large Earth distances (say 10 Earth light years) in small amounts of ship time (say 1 ship year).

If that's all you wish to say, then we're done! (And everyone is happy.) But no, you go on to try to "explain" that effect, and, in so doing, garble up what relativity says.

Relativity really does impose a universal speed limit. And even to get to a small fraction of that limit, there is a huge energy cost, again due to relativity. You are welcome to disagree, but don't expect agreement from those of us who understand relativity. It's just not going to happen, not in the Relativity forum.

 

[billy_boy_999]

Doc Al - yes, thanks for pointing out that I'm speaking "gibberish"...

as it happens you have just admitted my point, is it "gibberish"? yes, an object is always at rest in its own frame of reference...if you had read my posts you would have found that this was exactly my point...as such there is no limit to how far a spacecraft can travel in a set amount of time <--a large admission!

my statement: will you concede this: there is a misconception that relativity prohibits a spaceship reaching a certain distance in a certain amount of time because of the constancy of the speed of light. rather, measuring the distance you will travel before you take off and measuring the time it takes you to get there on board your ship, there is no relativistic speed limit in your own ship's inertial reference frame. in fact, this would absolutely contradict the principle of relativity

your counterstatement: If you made the following simple statement, all relativists would agree: If a ship were moving fast enough (with respect to the earth), then it could travel large Earth distances (say 10 Earth light years) in small amounts of ship time (say 1 ship year).

is there a qualitative difference between these two statements? do you feel the need to paraphrase my statement in order to satisfy your semantic requirements?

i think my 'misunderstanding of relativity' is really owing to my refusal to listen to other misinterpretations...the relativistic mass increases do not affect the ship's inertial reference frame when it is being accelerated under its own power...

it seems like getting you say this is like pulling teeth...anyway, yes, thank you for the concession, and yes i think the point of this thread has now been thoroughly exhausted...and i shall shut up...

 

[heretofore]

It seems that both "sides" of the argument here are correct. In billy_boy's point of view, his hypothetical scenario effectively allows the spaceship to travel 10 LY with only 1 year passing for the craft's occupants. Meaning that while the craft is traveling, relativity "shortens" the 10 LY as observed in the craft, so that no FTL is achieved. But when the craft lands, a measurement of Earth shows it is 10 LY away (which the occupants left 1 year ago according to their clocks).

The other point of view is presented, which is more scientifically accurate. The spacecraft never exceeded c because of the time elapsed on Earth is far longer than 1 year (and longer than 10 years). Thus, no FTL travel has happened.

But that isn't the point. It's a thought experiment: IF no onboard fuel is required (other than enough to run your ecosystem for 1 year), and IF you can constantly accelerate (then decelerate) using whatever wonderful technology, you could travel to a planet 10 LY from Earth and personally experience 1 year's passage.

 

[jtbell]

Um... did you happen to notice that the last previous posting in this thread was over three years ago?

 

[heretofore]

Yes.

 

[Doc Al]

Oy! Let's not start this one up again...

 

[heretofore]

My apologies. I found the topic interesting.