posted
Ok, assuming we understand the deal with Time Dialation (it has been proven that time and space are intrinsically linked, therefor as you approach the speed of light you experience time as normal but everything external to you is moving through time at a slower pace: effectively you are travelling forward in time,)
What I don't understand in books like those of OSC and Ursula LeGuin is how come people in ships capable of travelling at these close-to-light speed can move across the galaxy from one star to another in a couple of weeks or months in their own time?
Light takes 4 years to travel to and from Alpha Centauri, the nearest star, so a NAFAL speed it would surely take about that... 4 years relative time to those IN the space craft, god knows how long to those outside!
Is there some science I am missing that someone can fill me in on as to how the NAFAL ships in these authors universe can scoot about in weeks or months? or is it just artistic liscence?
posted
It is not artistic license, for the most part. Someone else will have to explain the details, or check out "The Elegant Universe" for a very good explanation.
One serious bit of artistic license is the ansible. It allows two separated clocks to compare their times instantaneously. For various reasons, this causes problems with relativity. Again, someone else will have to explain why.
posted
It goes the other way. Traveling at 99.5% of the speed of light, the duration of a 4 lightyear voyage would seem, to someone outside the ship, to be just a hair over four years long. That much is obvious.
But to someone ON the ship, the voyage would seem to be far BRIEFER than the actual elapsed time.
The reason that all near-lightspeed voyages in sci-fi are almost as absurd as all time-travel stories is that to come close enough to lightspeed to make time-compression possible, you would have to consume such vast amounts of energy that there is no practical fuel source. We just sweep that under the rug. Plus, of course, as matter comes close to lightspeed it starts behaving like energy. The particles in your body become wavelike. I suspect this would mean that you could not sit down and eat a sandwich.
But, as with time travel, we pretend that these problems have been "solved" and just tell the stories.
In fact, though, lightspeed travel is used to reproduce the situation on Earth when crossing an ocean was a big deal that cut you off from people back home - but still didn't take SO long that history would pass you by.
If you have lightspeed travel and FTL communication (ansibles) then it's like the era of steamships with a transoceanic telegraph cable.... you can talk, but you can't GO quickly.
It's just a way of establishing degrees of isolation and raising or lowering the cost of migration.
posted
Yes, I understand the implications NAFAL travel and the introduction of the ansible have in SciFi universes... I just don't understand how traveling Nearly as fast as light gets you from A to B faster then light does...
Posts: 95 | Registered: May 2005
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posted
It doesn't. If you click a flashlight at the time your ship leaves, the light will get there first. And, the light will be going away from your ship at the speed of light, even if you are going 99.9999999% the speed of light.
Time dilation is the reason for this phenomenon (or the result of it, depending on how you look at it).
Seriously, pick up The Elegant Universe if you want a very good description. This isn't the main topic of the book, but the description in the introduction is the clearest I've read yet.
posted
ARGH! This still doesn't answer my question of how the crew about a NAFAL ship can travel a light year in what feels like days rather then close to a year!
I'll try and get my hands on the book.
There are of course other problems associated with this form of travel. At those speeds, the minutest space debris, milimeters thick, would tear a huge gaping whole through your space craft...
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posted
"ARGH! This still doesn't answer my question of how the crew about a NAFAL ship can travel a light year in what feels like days rather then close to a year!"
Because, to them, it is in fact only days. The closer you get to lightspeed, the more instantaneous your travel from your perspective. The time taken for light to travel from one location to another is perceived only by someone who is not traveling along with the light; to the light, it happens instantly.
Posts: 37449 | Registered: May 1999
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posted
I FOUND IT!! It's called "Length Contraction"
according to Albert Einstein's special theory of relativity, is the decrease in length experienced by people or objects traveling at a substantial fraction of the speed of light. It is experienced only in the direction in which the body is travelling and not transverse to this direction, which in turn is dependent on the frame of reference relative to which that motion is being measured.
At 99.999 the speed of light a light year (9.46 trillion or 9.46×10^12 km) would seem like only 4.2380800000x10^10 kilometers... A mere fraction!
I expect in the universe of OSC and the Hainish universe of Urula K. LeGuin the ships are travelling faster still.
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quote:Plus, of course, as matter comes close to lightspeed it starts behaving like energy. The particles in your body become wavelike. I suspect this would mean that you could not sit down and eat a sandwich.
I don't think this can be right. Velocities in relativity are all relative velocities. There's no such thing as the "actual speed" at which you're moving. So it's just as much the case right now that you're moving close to the speed of light as it would be if you were in what we'd call a fast-moving spacecraft. That is to say, there are now rest frames in which you are moving at .99c.
So there can't be any further wave-like effects as speed increases, because speed is not an absolute quantity.
Posts: 4600 | Registered: Mar 2000
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posted
Destineer is correct. Macroscopic objects do not behave as waves whatever their energy; in any case, with the sandwich example, your speed relative to the sandwich is zero. In fact, this would go back to the basic postulate of special relativity, namely the equivalence principle : There is no experiment you can do to tell whether you are going at lightspeed, or sitting still. (Short of opening a window, that is.) If you can't eat a sandwich at 0.995c, this is contradicted : Test whether the sandwich is edible.
I suspect comrade OSC is mixing special relativity up with quantum mechanics. As objects get smaller, they indeed behave more like waves; an electron couldn't eat a sandwich. But this applies whatever the speed of the electron is.
Antony, your explanation of length contraction is correct, but not 100% complete. It is accurate to say that the length between two stars is shorter when going at 0.995c. It is also accurate to say that the time you experience is shorter. One causes the other, or they both cause each other; it's a question of how you want to look at it. This is what is meant by saying that time becomes just another dimension.
Posts: 10645 | Registered: Jul 2004
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posted
There is one point here that everyone missed. The extreme relativistic time effects only occur when you are under CONSTANT ACCELERATION. There is a very good web site (I will try to locate the link) that provides an understandable explanation as well as some usable equations and calculated examples.
Writing science fiction with relativistic speeds is extremely difficult, and should not be attempted without either a physics degree, a good physicist friend, or significant research. It is very easy to make a mistake (this coming from someone with both a physics and math degree).
The point OSC got right is about the energy required. If you consider constant acceleration, you are looking at an asymptotic velocity function, requiring exponentially increasing energy consumption to gain that next .0000001c in velocity, but once you stop accelerating, you lose the time compression and only receive plain old vanilla time dilation. I'm not sure about the wave-like bvhavior, I'll have to research that point, but another disadvantage is that as an object approaches the speed of light, the object's effective mass approaches infinity. Hard to eat the sandwich if you can't lift it. This explains the exponential increase in energy to accelerate an object with an increasing effective mass.
I will post the link here when I dig it up. You might also look into Introduction to Special Relativity (Dover) by James H. Smith
posted
"Writing science fiction with relativistic speeds is extremely difficult, and should not be attempted without either a physics degree, a good physicist friend, or significant research."
Or a belief that the science is not itself the important thing. Posts: 37449 | Registered: May 1999
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posted
Um, no. The sandwich's effective mass only increases as measured by an outside observer, and really, that's an old-fashioned way of putting it anyway. To the eater of sandwiches, his food is not moving, so it has the quite ordinary mass we experience on Earth. Indeed, unless you are accelerating, it'll be weightless.
As for the bit about accelerating, I think you are being correct but a little misleading. Yes, you get extra relativistic effects while you are accelerating, courtesy of general relativity. But the 'plain old vanilla time dilation' from special relativity is quite powerful in its own right when travelling at 0.995c!
Posts: 10645 | Registered: Jul 2004
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posted
Tom: Refer to my comments on Science Fantasy in the thread about "Help with my Science Fiction Presentation."
I realized my mistake about the sandwich after I posted.
Correct that vanilla time dilation has some ramifications, but acceleration changes everything. Here is the link I promised. It is an effective demonstration.
As far as the particle-wave duality, there is no mention of that in any of my relativity books. I should have known, Einstein was adamently opposed to the ideas of quantum mechanics (God does not play dice). If particle-wave duality was integral to relativity, Einstein's own stubbornness would have prevented him from finishing his theories.
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posted
I think you have misunderstood your link. Certainly, accelerating at 1g is an effective way to get close to c. But the time dilation is just the same old vanilla stuff from SR. If you accelerate instantly (which admittedly is a bit more of an engineering challenge), you get the same effect - more of it, in fact, since more of your journey is spent near lightspeed. To get GR effects, you need accelerations on the order of those produced close to a black hole.
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posted
RE: Or a belief that the science is not itself the important thing. ---
Some argue that that makes your story a fantasy story set in space rather then a sci fi story... I'm not sure though! I mean, what if may of the scientific theories we believe now are proved to be wrong in the future? suddenly all that sci fi becomes fantasy because it's set in a univers with different physics Posts: 95 | Registered: May 2005
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posted
I don't. You do not need any particular acceleration to get time dilation, you need a particular speed. How you get to that speed doesn't matter. Therefore, your statement "acceleration changes everything" is inaccurate.
Posts: 10645 | Registered: Jul 2004
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posted
Once again the universe collapses in on itself as I take KoM's position (watch out for th sky!)
Time dilation is an effect of relative velocities, and the way you know that is that it was in Einstein's Special theory of relativity, which only dealt with static (non-accelerating) frames of references.
quote:Originally posted by King of Men: I don't. You do not need any particular acceleration to get time dilation, you need a particular speed. How you get to that speed doesn't matter. Therefore, your statement "acceleration changes everything" is inaccurate.
Okay, let me be explicit. There is a relationship for time dilation at constant velocity (defined as position with respect to time). Under acceleration, you have a relationship of position with respect to the square of time. To take acceleration into account, you must use the antiderivative of the time dilation equation. The physics is still special relativity as long as you are in the inertial reference frame, but the relationship is now integrated over the velocity from (say) 0 to whenever you stop accelerating. When I said everything is different, I mean that the function used to calculate the cumulative time dilation is not the same.
In response to Hobbes:
"Since this question involves acceleration, one might wonder whether special relativity supply an answer at all. The answer is that it can, so long as we ask questions about where the spaceship goes, and how long it takes as measured in an inertial system. On the other hand, we may not ask questions about things seen from the point of view of travelers on the ship, for that is not an inertial system."
Introduction to Special Relativity, Smith, James H. Dover, 1965, p83
As I said, this is very dangerous territory when writing fiction.
quote:Originally posted by Antony: I FOUND IT!! It's called "Length Contraction"
according to Albert Einstein's special theory of relativity, is the decrease in length experienced by people or objects traveling at a substantial fraction of the speed of light. It is experienced only in the direction in which the body is travelling and not transverse to this direction, which in turn is dependent on the frame of reference relative to which that motion is being measured.
At 99.999 the speed of light a light year (9.46 trillion or 9.46×10^12 km) would seem like only 4.2380800000x10^10 kilometers... A mere fraction!
I expect in the universe of OSC and the Hainish universe of Urula K. LeGuin the ships are travelling faster still.
hmmm, never heard of the length contraction before... I always just (mis)understood the whole travel at almost speed at light/relativity etc purely depending on the fact that the speed of light is always constant no matter how fast you travel yourself. Which means that the faster you travel, the slower time runs for you; therefore traveling 4 light years at 99.99999% of speed of light would seem like 4 years to anyone looking outside, but only like a few weeks to you because your perception of time has slowed.
Example: if you'd drive a car at half the speed of light, the light from your car would still travel at full speed of light away from you... that would result in light travelling at 150% of it's ordinary speed (if you shoot a gun on a moving train the bullet will move at (speed from gun)+(speed of train))... but since nothing can travel faster than the normal speed of light, the Universe had to figure out some other way to solve the problem, by making time run slower for you... so, at half the speed of light, time should run at half the normal speed too, making it appear like light is moving at it's ordinary speed for you... crank up the speed to 99.99999% of speed of light and time will be running very slow for you. Atleast that's what I got out of reading a book about Einstein's equoations I suck at explaining too so I guess none of you will have a clue what I'm talking about
Don't think too much about things like this, makes your head hurt Posts: 10 | Registered: May 2005
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quote:Originally posted by DrAbraham: ... so, at half the speed of light, time should run at half the normal speed too,
It isn't a linear relationship. At half the speed of light, relativistic effects are small (but not negligible). I recall these effects start becoming significant enough that you can't brush them off only around .85c or so.
I guessed it propably wouldn't be as easy as an linear relationship... but how in the !%!# can the speed of lite be constant, if it's not linear?
I mean if you travel at .5c, and c still appears to be the same as usual from your point of view, then time has to go at half speed too?
I believe you guys are right about this, I'm not claiming to be an expert in physics or anything, it just doesn't make sense to me if it's not linear.. but I guess that's what they mean when they said that only 12 people alive at the time Einsteins theories were written actually understood it
Guess I'll go watch Buffy instead... woman kill vampire, man dig up grave, no physics and no headache Posts: 10 | Registered: May 2005
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posted
So those physicists at the conference who told me about wavelike behavior and conversion to energy in order to reach near-lightspeed were pulling my leg. Bummer. They must have LAUGHED when I left the room.
Posts: 2005 | Registered: Jul 1999
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posted
Dr Abraham, it doesn't half to be linear, it's like a line graph that curves up the way, I'm sure you've seen many and that kind of relationship applies to many things, look:
0 % speed of light gives the ratio 1:1 10 % speed of light gives the ratio 1:1.005 50 % speed of light gives the ratio 1:1.155 90 % speed of light gives the ratio 1:2.294 99 % speed of light gives the ratio 1:7.089 99.9 % speed of light gives the ratio 1:22.366 99.999 % speed of light gives the ratio 1:224.658
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posted
Re: So those physicists at the conference who told me about wavelike behavior and conversion to energy in order to reach near-lightspeed were pulling my leg. Bummer. They must have LAUGHED when I left the room. ---
Can you expand on this please? I'm interested
As far as I know FTL IS possible, and some known particles do this, but it is not possible for matter to accelerate to light speed. I'm open to correction.
Mr. Card, if you would be so kind as to explain your scientific thinking behind the NAFAL travel in your universe and the time differentiation felt by the crews in comparison to those not moving at speed, I would be most obliged.
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quote:When I said everything is different, I mean that the function used to calculate the cumulative time dilation is not the same.
Ah, now I see what you were saying. Fair enough. Really, though, this is the sort of detail that only Heinlein could get away with going into; and his books are looking a bit dated these days.
quote:So those physicists at the conference who told me about wavelike behavior and conversion to energy in order to reach near-lightspeed were pulling my leg. Bummer. They must have LAUGHED when I left the room.
I think they were probably using baby talk, and you misunderstood them. This stuff is not actually describable in terms of words anyway, you need equations. Perhaps they were trying to tell you that you need infinite energy to reach exactly lightspeed? It's also just barely possible that you can derive that relationship from a quantum-mechanical approach, though why you'd want to is beyond me.
Antony, FTL travel is possible in principle, you just can't do it by the usual method of accelerating. However, there are no known particles that go FTL; the 'tachyon' you may be thinking of is a generic name for such - nobody has ever seen one. In fact, if I recall rightly, tachyons cannot interact with slower-than-light particles, so their existence is a bit debatable even on the philosophical level - kind of on a par with the invisible pink unicorn.
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quote:Originally posted by Orson Scott Card: So those physicists at the conference who told me about wavelike behavior and conversion to energy in order to reach near-lightspeed were pulling my leg. Bummer. They must have LAUGHED when I left the room.
I don't doubt your word. The problem is that I can't find anything to document it.
I suppose the wavelike behavior might be a very recent discovery, and therefore wouldn't be documented in any of the established relativity texts. I've done web searches on "particle-wave duality at relativistic velocities" and "wave-like behavior at relativistic velocities" and turned up nothing.
Or is it possible that the physicists were describing particles in an accelerator, and not macroscopic objects?
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posted
actually, doesn't a particle or something have to theotetically exist once you have the math figured out for it?
Posts: 1567 | Registered: Oct 2004
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posted
RE: Antony, FTL travel is possible in principle, you just can't do it by the usual method of accelerating. However, there are no known particles that go FTL; the 'tachyon' you may be thinking of is a generic name for such ------
righto, I know what a tachyon is and I've used the term in my own writing before, but I was recently informed that they now think it possible to know at one point in the universe exactly what is going on in another point at that time, er... spooky "something"... Einstein refuted this all through his career until he was really old and stated that he regretted having his mind closed to it for all those years. The conclusion of this line of thinking is that something or other must be moving FTL...
I know I sound like a bit of a novice at all this, which in a sense I am, I don't have an astrophysics degree, just a keen interest.
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posted
oh yeah, I recall reading in a swedish science magazine 3-4 months ago that some scientists are working on a theory that the speed of light hasn't always been the same; ie that the speed of light has changed over time, which would explain why some parts of the universe are too far away to have been created in the Big Bang, therefore the speed of light should've been faster than it is today billions of years ago... or something like that, I just mostly recall the complications that theory creates. For example, in the metric system 1 metre is defined as "Length traveled by light in vacuum during 1/299 792 458 of a second"... so if the speed of light would decrease suddenly we'd be left wondering is everything is suddenly growing or if the speed of light is slowing... They didn't have any proof for that theory yet though, and I hope they'll never find any... that would really mess up Einsteins equations Posts: 10 | Registered: May 2005
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posted
Antony, that's quantum entanglement unless I am vastly mistaken, and there's no particle travel invovled. (The phrase you are thinking of would be "spooky action-at-a-distance"; Einstein didn't approve.) And nothing is really moving FTL in that case.
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