SoL from inside the spacecraft, what happens?

[kanicbird]

suppose you are in a spacecraft, not an observer on earth, you start accelerating by using, lets say, antimater engines. the type of fuel is really not important as long as you don't run out of fuel and antimater engines sounds way cool.
anyway you start accererating. now since all velocity is relitive at any given time you can say you are at 0 velocity even if you are traveling 100,000m/s faster then when you started. your acceleration rate should be the same, assuming same engine output and ignoring the loss of mass due to the propulsion system.
now what prevents you from simply accelerating to and past the 186,000 mi/sec ( = S.o.L.) past your speed you started at?

[sailor]

>> what prevents you from simply accelerating to and past the 186,000 mi/sec??

The fact that you do not have enough energy to get to the SoL. Do you know how much you would need?

[Duck Duck Goose]

AFAIK, nothing. The only limitation on space flight, science-fiction-wise, is the human life span. It might take you so long to get going faster than light that your human passengers would have died of old age long before that.

But then, I'm not a physicist, just a Trekkie.

[kanicbird]

if you could accelerate at about 1 g or 32 ft / sec, not only would you have artificial gravity but you would get to the S.o.L on about 1 yr
32 ft/s /s is appx 0.0064 mi/sec/sec so you speed would increase 0.0064 mi/sec every second, you would reach 186000 mi/sec in about 29,000,000 seconds or 8073 hrs or 336 days. so if that acceleration rate can be kept up, you would be at the S.o.L. in under 1 year.

[Derleth]

Chronos, cleanup in GQ! Until he gets here, I'll take a crack at this. As you accelerate towards c (186,252 mi/s, speed of light in a vacuum), you do two things: You gain mass and you lose length. At speeds very low compared to c, both of those are negligible. But as you get to .9c (90% of c) those effects have added up to the point where you are trying to accelerate a ship with near-infinite mass and near-zero length. It ain't gonna happen. From your reference frame, nothing has changed except your engines have begun to give a lot less acceleration per unit of fuel. Soon you have stopped accelerating and no matter how much fuel you pour in you can't accelerate. You have hit a very real wall. You either lay off the gas and coast for lightyears that fly by in months (or weeks, or even hours, depending on speed) from your frame of reference or you try to decelerate by pumping energy out the opposite direction.

[kanicbird]

[ed up to the point where you are trying to accelerate a ship with
near-infinite mass and near-zero length. It ain't gonna happen. From your reference frame, nothing
has changed except your engines have begun to give a lot less acceleration per unit of fuel]

I don't think so since velocity is relitive, your propulsion system with antimater engines is light (gamma radiation i think and a heck of a lot of it) leaving out the back of your spacecraft at the SoL, your headlights leave infrount of you at the SoL. you are effectively traveling at 0 mi/sec, there would be no reason that your mass should increase if you are on the ship

[Garfield226]

Derleth,

I don't think you'd actually STOP accelerating, I think that your rate of acceleration would decrease, approaching zero (becoming negligable), but it would only actually reach zero at the S.o.L. I think.

[Punoqllads]

Originally posted by k2dave
now what prevents you from simply accelerating to and past the 186,000 mi/sec ( = S.o.L.) past your speed you started at?


In one word, physics.

In the everyday world that we experience, we don't see things going very fast. The intuitive model that we get about how physics works is actually a simplification of how things really work.

Of note:

1) There is no absolute speed. Something can only have a velocity relative to some other thing.

2) Speeds are not additive. At very slow speeds, relative to the speed of light, the difference between the real physics, and treating speeds as additive is so small as to be almost unmeasureable.

3) Light in a vacuum moves at a constant speed, no matter what your frame of reference is. If you are moving 100,000 miles/sec. past me at the same time I shine a flashlight, and we each measure how fast the light is moving relative to ourselves, we each measure the same speed.

Because light will always be moving at the speed of light, relative to yourself, no matter how much you accellerate, you will never reach the speed of light, as sailor has said. Therefore, you will never go faster than the speed of light.

Maybe there is some way to "cheat", to go faster than the speed of light. Maybe there's something out there at the edge of special relativity that doesn't behave exactly like the theory states, as special relativity did to Newtonian mechanics. But the world has done many, many, many experiments that verify the validity of special relativity.

If you'd like to know more, I recommend that you read a good book on physics. There are many out there; one I've been perusing lately has been Clifford Pickover's "Time : A Traveler's Guide". It's been pretty good so far, and doesn't have any math more scary than square roots or fractions.

[kanicbird]

[ don't think you'd actually STOP accelerating, I think that your rate of acceleration would
decrease, approaching zero (becoming negligable), but it would only actually reach zero at the
S.o.L. I think.]
this would apply to an outside observer not for people actually on the spacecraft. the mass would also increase to an outside observer but again, not to the people on the spacecraft

[kanicbird]

[2) Speeds are not additive. At very slow speeds, relative to the speed of light, the difference
between the real physics, and treating speeds as additive is so small as to be almost
unmeasureable. ]
not sure i understand this one

but your points are valid, but I don't see how it would prevent you from going 186,000 mi/s faster then when you started. if an observer shot a laser at the spacecraft from behind, it would 'appear' to the people on the ship to come towards the ship at the SoL and if the laser missed, it would pass the ship and continue away at the Sol. the observer would 'see' the laser approach and pass the ship at the SoL.

[kanicbird]

and a little aside, why is my post count stuck at 269???

[kanicbird]

never mind that last one, I see that it is my total posts not the actual post number

[APB9999]

The simplest answer is: it takes a certain amount of energy to accelerate by a set amount, say 1m/s. But the faster you go, the more energy it takes to attain that set amount of acceleration. As you get closer and closer to c, the energy it takes to accelerate goes up and up.

So, the energy it takes to accelerate asymptotically approaches infinity as you approach the speed of light. You can never go faster than c because there isn't that much energy in the universe!

[Punoqllads]

Originally posted by k2dave
[2) Speeds are not additive. At very slow speeds, relative to the speed of light, the difference between the real physics, and treating speeds as additive is so small as to be almost unmeasureable. ]
not sure i understand this one


If you go past me at 50 mph, and throw a ball out at 75 mph, the ball will go past me at 125 mph. If you go past me at 50 mph and I throw a ball at 75 mph, it will go past you at 25 mph.

But, if you move past me at 50% of c (c = speed of light) and throw a ball out at 75% of c (pretend you're a 24th & 1/2 century baseball pitcher) it will pass me at 91% of c, not 125% of c. And, if I throw the ball out at 75% of c, it will pass you at 40% of c, not 25% of c.

but your points are valid, but I don't see how it would prevent you from going 186,000 mi/s faster then when you started. if an observer shot a laser at the spacecraft from behind, it would 'appear' to the people on the ship to come towards the ship at the SoL and if the laser missed, it would pass the ship and continue away at the Sol. the observer would 'see' the laser approach and pass the ship at the SoL.

So, the guy is shooting light blasts across the bow of the ship, which is accelerating directly away from him. If the ship was going above 186,000 mi/s, the blasts would not be able to catch up to it; if it was able to go that fast for a long enough time, it would be able to catch up to a blast that had previously passed it.

But they will, and it won't. To folks on the ship, the blasts will always be going 186,000 mi/s faster than the ship is, and will pass them going 186,000 mi/s faster than the speed it's going. No matter how quickly it accelerates, or for how long. To the observer shooting across it, the ship will not accelerate at a constant speed. The closer the ship gets to the speed of light, the less quickly it will accelerate. The ship might get within 1% of the speed of light, from the shooter's perspective, but to the occupants of the ship, they will always be 186,000 mi/s slower than light.

The math isn't that tricky, but it isn't what most people would expect. It's a part of the universe that you don't see normally, if at all, so it doesn't seem like it would be true. But it is.

[LazarusLong42]

Originally posted by k2dave
now since all velocity is relitive at any given time you can say you are at 0 velocity even if you are traveling 100,000m/s faster then when you started. your acceleration rate should be the same, assuming same engine output and ignoring the loss of mass due to the propulsion system.
now what prevents you from simply accelerating to and past the 186,000 mi/sec ( = S.o.L.) past your speed you started at?

Well, you said it right there. In order to accelerate past c, you have had to define yourself as moving at 0 m/s. Very tricky of you, to be moving both at 0 m/s and 299787 km/s at the same time.

As you said, velocity is relative. It is impossible, within the boundaries of the universe that we know, for two objects to have a relative velocity greater than the spped of light. And light _always_ has a relative velocity of c, even in the headlight beam of a car going 299000 km/s.

Because of this, if you could ride on the hood of a photon (not recommended, and illegal in 37 states), you would experience the fourth dimension as a point. All photons exist in the same time, and forward and backward in time mean nothing to them (which gives the interesting consequence that an anti-photon is exactly equivalent to a photon).

You might try an interesting book called In Search of Schrödinger's Cat by John Gribbin. Fascinating stuff.

LL

Punoqllads said;
In the everyday world that we experience, we don't see things going very fast.
Not to be picky, but...

Everything that we see is light, which, by definition, travels at the SoL, which is very fast indeed!

But seriously...

Something has always bothered me about explanations of Relativity that I've read. They all suggest that if someone went on a space trip at near the speed of light and returned, that less time would have passed for the voyager than for the people left at home. For example; ten years might have passed for you but your twin brother, left behind on Earth, would be fifty or more years older.
But, if any inertial reference is as valid as any other, you could look at this trip as if you were still, not moving in space, and that the Earth left you at speeds approaching the SoL and eventually came back. In which case they are the ones who would have experienced relativistic time-dilation, and your twin on earth would have aged a mere ten years to your fifty.

Whazzup wit dat?

[Dr. Lao]

E d'Mann said:Something has always bothered me about explanations of Relativity that I've read. They all suggest that if someone went on a space trip at near the speed of light and returned, that less time would have passed for the voyager than for the people left at home. For example; ten years might have passed for you but your twin brother, left behind on Earth, would be fifty or more years older.
But, if any inertial reference is as valid as any other, you could look at this trip as if you were still, not moving in space, and that the Earth left you at speeds approaching the SoL and eventually came back. In which case they are the ones who would have experienced relativistic time-dilation, and your twin on earth would have aged a mere ten years to your fifty.

Whazzup wit dat?You just stated the twin paradox. However it is a false paradox. The reason is that we aren't dealing with pure inertial reference frames. The twin who travels needs to accelerate to near the speed of light, turn around, then decelerate to Earth's inertial reference frame. This breaks the symmetry of the situation. You need to use general relativity to explain this fully. The twin who does the accelerating I believe will be the younger twin.

[kgriffey79]

Are you all stupid?? If you were going the speed of light, time would stop. If you were going faster than the speed of light you would travel back in time. Time is a dimension, just like height, width, and depth. Time is chronological, and cannot be penetrated.
READ EINSTEIN'S WORKS!

Dr. Lao...

I still don't really see the distinction, but since your response has at least triggered an inkling in my noggin, I will defer to your obvious qualifications.

(p.s. If you're a real Phd. or sumpin' check out my T.O.E. on my website and email me your criticisms (of which, I'm sure, there will be many.))

[matt]

E d'Mann

Suppose your two twins are sitting in a space station. One takes off in his rocket at 1g acceleration. Now, the twin in the rocket can feel an acceleration, whereas his twin in the space station can't. So the symmetry which would allow either twin to claim that they are stationary doesn't exist. If they look back at each other through telescopes, one is floating around eating paste out of plastic bags, the other is working on his juggling technique. It isn't hard to tell which one is accelerating!

I guess my real question goes back to when I first heard about the concept of inertial frames of reference;

I feel like I'm not in motion, but that's because I'm stuck at the bottom of a *gravity well*. In fact I'm moving at about a thousand miles an hour in a circular course about the Earth's center of rotation. The Earth is moving at IIRC 30,000 miles an hour in an elliptical course around the Sun. The Sun rotates along with the rest of the galaxy.
In short, I'm presently a space traveler. I'm moving.
Just as time slows with increased velocity, it speeds up as velocity decreases.
If I hopped in my handy-dandy spaceship and adjusted my angular momentum to zero and cancelled out all relative motion could I ever become truly stationary?
Perhaps in relation to the Universal average angular momentum or something like that?
How fast would time go relative to you poor saps still stuck on the moving Earth?
Could I witness the rest of the history of the Cosmos go by like a really fast-motion film?
Inquiring minds need to know.

[kanicbird]

[If I hopped in my handy-dandy spaceship and adjusted my angular momentum to zero and cancelled out all relative motion could I ever become truly stationary? ]

all you would have to do is get out of the gravity well or counteract it somehow and don't accelerate in any direction and you are at 0 velocity, no matter how fast you are moving.

Back to my OP, time dialation is the only thing that can slow you down (did some reaseach last night while sleeping). by time changing for the spacecraft passanger, it appears that he is traveling faster because he is getting somewhere in one year instaed of the 20 years as seen by his twin. the 'twin' will see the craft's mass increase as it approached the SoL and it will never get there. BUT the spacecraft itself will continue to travel faster and faster - but time will change.
Now I am not sure about this part, but it might explains why the mass increases or at least changes. as the outside observer sees it you are gaining mass, as the ship see it, everything is normal except their clocks are running slower then the observer, but time seems to continue normal for them as well. the mass of the ship to the outside observer is compressed through time and increases, basically the ship os in 2 or 3 or 1.4 or whatever places at the same time and the mass increases beacuse of that?

The above premise meens that a passenger on that ship will travel up to and past the SoL at he preceives it. and will get to his destination at that speed (mainly because of the time dialation effect). But people back on earth and mars will have to wait a very long time for him to return due to him moving .999c acording to their clocks.

The only thing I see as a problem is that the destination would appear to be moving twards them at the SoL which is a no-no. but the destination can't simply gain mass since it's not accelerating.

[Phobos]

Originally posted by E d'Mann
The Earth is moving at IIRC 30,000 miles an hour in an elliptical course around the Sun.

The Earth is revolving around the Sun at about 67,000 mph.
(whee! feel that wind in your hair!)

If I hopped in my handy-dandy spaceship and adjusted my angular momentum to zero and cancelled out all relative motion could I ever become truly stationary?
Perhaps in relation to the Universal average angular momentum or something like that?
How fast would time go relative to you poor saps still stuck on the moving Earth?
Could I witness the rest of the history of the Cosmos go by like a really fast-motion film?
Inquiring minds need to know.

There ain't no universal reference frame to adjust to. But you could throw yourself into a black hole & watch the universe behind you appear to accelerate (time wise) into the future. It would be a bit of a one-way trip though.

[ZenBeam]

From the OP:
even if you are traveling 100,000m/s faster then when you started. your acceleration rate should be the same

Your acceleration is constant in the reference frame of the ship. An observer on Earth watching you will see time slow down for you as you approach the speed of light. Since time is slowing down, even though you measure constant acceleration, the acceleration you have with respect to the observer on Earth will decrease towards zero as you approach the speed of light. Thus, you never get to the speed of light.

[Punoqllads]

Originally posted by E d'Mann
[The Earth is moving at IIRC 30,000 miles an hour in an elliptical course around the Sun. The Sun rotates along with the rest of the galaxy.
In short, I'm presently a space traveler. I'm moving.
Just as time slows with increased velocity, it speeds up as velocity decreases.


Okay, there's where the problem is. There is no velocity. There is only velocity with respect to a given reference point. The same thing goes for time and distance. For a given observer (i.e. the reference point) time will slow down for you.


If I hopped in my handy-dandy spaceship and adjusted my angular momentum to zero and cancelled out all relative motion could I ever become truly stationary?


There is no "stationary" any more than there is an "up".

Phobos;
The Earth is revolving around the Sun at about 67,000 mph.
(whee! feel that wind in your hair!)
No, but I feel the solar wind in my magnetotail!
Oops! There went another plasmoid!

[Punoqllads]

Originally posted by Punoqllads
For a given observer (i.e. the reference point) time will slow down for you.


Make that, "For a given observer (i.e. the reference point) time will slow down for you as your relative velocity increases."

[friedo]

Originally posted by kgriffey79
If you were going faster than the speed of light you would travel back in time. Time is a dimension, just like height, width, and depth. Time is chronological, and cannot be penetrated.
READ EINSTEIN'S WORKS!

Einstein never postulated what would happen at speeds FTL because an object with positive rest mass cannot possibly accelerate past C. Theories have been offered as to what would happen if an object with negative mass accelerated past C, and one of them includes backwards time travel, but I don't know if it's ever been proven. Makes great scifi, though.

It is true, however, that at relativistic speeds, time dilation does occur. If you are traveling at 90%C, time would be going slower for you than for someone at rest relative to you. If you compared clocks when you got back, you would notice a difference. The faster you go (relative to someone else) the slower time goes (relative to someone else.) This has been proven with atomic clocks, and is an important visual aide in understanding why C is always a constant. Time dilation occurs precisely because the speed of light is always the same!

[Inky-]

What happens is all the photons collect on the wall at the far end of the spaceship. Later, when you stop accelerating, they all fall at once with a humorous "Thump!" noise.

[Bad Hat]

I'm not sure if i missed it somewhere in the thread or not, but I don't think that some of the posters are appreciating the differentiation between acceleration and motion. Correct me if I'm wrong but the only waarping of the passing of time occurs during the acceleration process, once one has accelerated to .99c and sets the cruise control on said vessel, then time will relativistically be the the same as it is on a relatively slow vessel (say the earth).
of course now that I have typed this all out, it sounds kinda fishy to me, but rather than deleting it, i think I'll throw it out there and maybe get schooled, cuz I actually have managed to confuse my self about the relationship between relative motion and an acceleration in relativity...
spank me....

[friedo]

Originally posted by Bad Hat
I'm not sure if i missed it somewhere in the thread or not, but I don't think that some of the posters are appreciating the differentiation between acceleration and motion.

It's an important distinction. An object travelling at a contant velocity (no acceleraton) is exactly the same as an object not moving at all. Why? Well, suppose you're in space travelling at 30mph. Another spaceship beside you is travelling 30mph. Since there is nothing else to act as a reference point, you might as well be standing perfectly still. Indeed, there is no such thing as "perfectly still" since there is no universal reference point. Everything standing still is simply moving with the exact same velocity vector as the thing you are comparing it to.

Acceleration, on the other hand, means your velocity is increading (or decreasing) with respect to the velocity of another object.

Acceleraion is the same thing as gravitation. If you accerate at 9.8m/s^2 in outer space, you have "artificial" gravity. This is where gravity and time get all mixed up together and your brain explodes.

[Dr. Lao]

E d'Mann:(p.s. If you're a real Phd. or sumpin' check out my T.O.E. on my website and email me your criticisms (of which, I'm sure, there will be many.))I'm not a real PHd. I hope to be some day, but it won't be in physics it will be in chemistry. I find relativity very interesting, and I am familiar with some of the common issues people have with it but I'm no expert. My post handle is a second hand reference to a cult-popular cable television puppet show.

kgriffey79:Are you all stupid?? If you were going the speed of light, time would stop. If you were going faster than the speed of light you would travel back in time. Time is a dimension, just like height, width, and depth. Time is chronological, and cannot be penetrated.
READ EINSTEIN'S WORKS!I find that one should always be careful about calling others stupid before making a supposedly grand pronouncement on a particular topic. It can be very difficult to retract if you are wrong. First of all, Einstein never said this. If you can provide citations showing otherwise, I'll gladly look a them. The equations for time dialation describe an asymptotically slowing of time of the traveler from the rest obsever's reference frame. It makes no sense to describe a situation at or beyond the S.o.L. using these equations. Such descriptions lie beyond the scope of relativity.

k2dave:The only thing I see as a problem is that the destination would appear to be moving twards them at the SoL which is a no-no. but the destination can't simply gain mass since it's not accelerating.You forget that it is all relative. From the spaceship's reference frame his destination is traveling toward him at 0.999c. It will appear to the traveler that his desitination is experiencing time dialtion, Lorentz contraction, and increases in mass. These observations are due to the relative velocities of the reference frames. If the space craft decelerates to his destinations reference frame all the relativistic effects he observed will disappear.

[Whack-a-Mole]

Let's assume two observers.

Observer A is on the spaceship

Observer B is ouside watching the spaceship

Let's also assume the spaceship has an infinite amount of massless fuel (just go with me on this).

Also, through the magic of hypotheticals assume Observer B can clearly see the spaceship and Observer A throughout this whole process without violating any physics or 'moving' himself.


Ok...

Observer A, throughout his entire trip (even if it is forever), will never notice anthing change. His watch ticks like it always has, he's still 6 feet tall, the ship is accelerating at a constant 0.0064 mi/sec/sec and he measures the speed of light at 186,000 miles per second. He will observe all of this to be true from day 1 of the trip till the end of his life 70 years from now. To help you buy this imagine Observer A has no windows in his spaceship. Indeed, if we want to say he was born there then as far as he knows the inside of the ship is the entire universe. As far as he is concerned he is standing still in a gravity well (acceleration and the effects of gravity are indistinguishable from each other...this is called the Equivalence Principle).

Now, Observer B knows different. He put the baby on the ship and has watched it race away. Observer A gets closer and closer to light speed but never quite reaches it. Observer B also notices that while the spaceship is spewing out the same amount of fuel it always has it is accelerating slower and slower. Instead of 0.0064 mi/sec/sec we'll say it's now down to 0.000064 mi/sec/sec. Again through the magic of examples Observer A measures the spaceship and Observer A's mass as increasing. No wonder it's slowing down.

Question: Why does Observer A notice no difference? Why doesn't he notice a slowing of acceleration?

Because time is slower AND distance is smaller...his rulers are shortening compared to Observer B's rulers.

So, he is now only accelerating at 0.000064 mi/sec/sec but since his ruler has shortened one mile to Observer A seems like 1/1000th (or whatever it really is...I'm not up to the math) of a mile to Observer B. Since time also slows down when Observer A starts his stopwatch it will run slower than Observer B's stopwatch. When measuring the speed of light Observer A's photon has less distance to travel from Observer B's perspective and Observer A's watch will slow (compared to Observer B) in a combined manner to give him exactly the same measurment for the speed of light as Observer B.

Finally, another way to look at what would happen to you if you reached the speed of light --

An infinite mass may be hard to swallow so take it from another angle. Distance shortens for you as you approach light speed. This will have the effect of making the universe appear smaller (to you). At light speed the universe will be the size of a point to you. You are now effectively (from your perspective) everywhere in the universe at once. Neat huh! Unfortunately, from your perspective, if you decided to hit light speed for just the briefest moment, say as fast as you could hit the button to start and then stop (i.e. hitting a button twice), the universe would come to an end before you could turn your engines off. Not so neat...

[kanicbird]

thats the missing piece of the puzzle
[Lorentz contraction]

as the traveler approaches c, the perceived distance would increase. you could accelerate more but doing so would only 'push' your destination further away. So much for my 3 year trip to Alpha Centuri.
so due to Lonentz contraction is it worth accelerating any more or just cruse at 0.99c.

Here's more of my oversimplification that I don't know if it is true or not, but it seem to fit:
I have read that for photons, anything is infinite distance from them and they are not moving through time, the Lorentz contraction and time dialtion fits here. for a photon everything is 'pushed' further away - very very far away, but since time doesn't exist, it can travel through space.

[Punoqllads]

Originally posted by k2dave
as the traveler approaches c, the perceived distance would increase. you could accelerate more but doing so would only 'push' your destination further away. So much for my 3 year trip to Alpha Centuri.
so due to Lonentz contraction is it worth accelerating any more or just cruse at 0.99c.


Actually, the opposite happens. To an observer watching the ship, it takes the ship four years to get there going 0.99c. In the traveler's eyes, the distance to Alpha Centauri contracts as they accelerate, and the trip takes about 15 weeks.


Here's more of my oversimplification that I don't know if it is true or not, but it seem to fit:
I have read that for photons, anything is infinite distance from them and they are not moving through time, the Lorentz contraction and time dialtion fits here. for a photon everything is 'pushed' further away - very very far away, but since time doesn't exist, it can travel through space.

I have no idea what you are talking about. As far as photons and time, by our equations, anything moving at the speed of light does not experience time as we know it; no time elapses for them from the beginning of their journey to its end.

[kanicbird]

[Actually, the opposite happens. To an observer watching the ship, it takes the ship four years to get there going 0.99c. In the traveler's eyes, the distance to Alpha Centauri contracts as they accelerate, and the trip takes about 15 weeks.
]

so how fast can one get from point A to point B using distance from an observer not on a ship and using time as observed on the ship?
According to the above post it is possible to effectivally travel FTL IF you are willing to also travel into the future.

[Punoqllads]

Originally posted by k2dave
[Actually, the opposite happens. To an observer watching the ship, it takes the ship four years to get there going 0.99c. In the traveler's eyes, the distance to Alpha Centauri contracts as they accelerate, and the trip takes about 15 weeks.
]

so how fast can one get from point A to point B using distance from an observer not on a ship and using time as observed on the ship?
According to the above post it is possible to effectivally travel FTL IF you are willing to also travel into the future.

We are always traveling into the future. Moving between two positions cannot be done without moving through time as well. Space and time are inextricably linked. Asking to measure the distance in one frame of reference and the time in a different one makes as much sense as measuring the x & y distance between point A and B in one frame of reference, and the z distance between them in a different one.

You are never going FTL. By accelerating, you alter your frame of reference to one where the distance is shorter, and then by decelerating, you return to your previous frame of reference. The journey still took 4 years, even though you experienced less time.

[Chronos]

If I have a very good (but not theoretically impossible) drive that lets me travel arbitrarily close to the speed of light, then I can get anywhere I want in an arbitrary short amount of time. If I go .99999999 times the speed of light, I can cross the observable Universe in my lifetime (ok, I fudged the math... Fridays are my day off. The point is, it's possible to cross he Universe.) From anyone else's viewpoint, time has almost stopped for me, so it just takes me a heckuva long time to die of old age. From my viewpoint, the Universe has flattened out to the extent that my destination is now just a hop, skip, and jump away. Regardless of whose viewpoint, I still reach there before dying.

It's not clear if it's possible to actually "move" faster than the speed of light (it'd take imaginary mass, not just negative... Even weirder), but there may (or may not) be a few ways to get the same effect, using wormholes or warp bubbles or whatever. To produce one of these, you do need negative mass. The thing is, that any method which can give you effective FTL travel (meaning you arrive there before light could), can also be used to give you time travel into the past, even if you do use one of those funky backdoor methods like wormholes.

[kanicbird]

[You are never going FTL. By accelerating, you alter your frame of reference to one where the distance is shorter, and then by decelerating, you return to your previous frame of reference. ]

ok let me rephrase, it is possible to travel 4.5 light years (here to alpha centuri) in lets say 3 year as measured by time on the ship? I'm not concerned about how old or dead people are on earth.

[Punoqllads]

Originally posted by k2dave
ok let me rephrase, it is possible to travel 4.5 light years (here to alpha centuri) in lets say 3 year as measured by time on the ship? I'm not concerned about how old or dead people are on earth.
No, you won't travel 4.5 light years in 3 years. I tried to express why in my previous post:
Space and time are inextricably linked. Asking to measure the distance in one frame of reference and the time in a different one makes as much sense as measuring the x & y distance between point A and B in one frame of reference, and the z distance between them in a different one.
Maybe this will help show why. Start out with two points, a & b:
a



b
Next, map two frames of reference on top of them, Q & R, each of which have their x & y directions swapped with the other.
Q R
^ ^
| a | a
| |
x| y|
| |
| b | b
+- - - -> +- - - ->
y x
Now, the distance of two points from each other, in two dimensions, x & y, is sqrt(x2 + y2). Let's take the x distance from frame R, and the y distance from frame Q.

Well, the x distance in R happens to be zero, as does the y distance in Q, and sqrt(02 + 02) = 0, so the total distance is zero. Therefore, a & b are the same point!

Except they're not. You can't use measurements from different frames of reference without transforming them into a single frame of reference. Otherwise, the calculations you get are gibberish.

[kanicbird]

i understand about the differing frame of refrence Punoqllad. But Earth and Alpha Centuri are real points (a and b) seperated by a distance 4.5 lightyears. it would take light 4.5 years to get from one system to another. If I took one of those slow ships traveling at lets say 0.1c it would take me about 45 years. now we know from an observers point of view nothing can exceed the SoL, and only light itself can travel at the SoL. fine
As the ship is aproaching c, time slows down for the ship's crew and space itself contracts in the direction of flight. time and space are vastly diffrent from 2 frames of ref. but time still goes on on the ship. when the ship decelerates at Alpha Centuri back down to its velocity it was at when it started the trip (at real point a) it would be at the real point b. The time for the trip as seen by the observer on earth would have taken many years (would that be 4.5+ yrs at 0.999c?), but how long has it taken for the crew, in other words how many years have they aged since they left? now this is open ended, it depends on the speed relitive to the start or destination point, but it seem like the crew could make the trip aging only 3 yrs

[Chronos]

Yeah, k2dave, you could reach Alpha Cen in three years, as measured by a person on the ship. Of course, we don't currently have any hope of a spacecraft that fast, but as long as it stays below the speed of light, it's a problem of engineering, not physics.

[Lemur866]

k2dave:

Yes, you could travel to Alpha Centauri in a month of shipboard time, if you traveled at a high enough fraction of c. But it would seem to the people back at earth that it took you 4 years. You can lower your ship-board time to whatever you like, as long as you can increase your velocity to an arbitrarily close fraction of c...the closer to c, the shorter the trip. You can't travel faster than c, but from the perspective of an observer on a ship, it seems as if you can...you can travel 4.5 light years in what seems to be 3 months, say.

So, if you set out for the the Zaxulbrtz galaxy in your arbitrary ship, and set engines for .999999c, you could cross millions of light years in what seems like years, weeks, days, or seconds, depending. Of course, when you get back to Earth, everyone will have green tentacles and you'll be put in a zoo as a sub-human life form.

Heinlein's classic novel, "Time for the Stars" explores this. Tom sets out on a relativistic ship to explore the local stars, in telepathic contact with his twin Pat. He crosses dozens of light years, but he only ages a few years himself, while his twin gets older and older. Of course, we have the problem that Heinlein's torch ships wouldn't be able to carry enough reaction mass to continuously boost at 1.25 g for several years, and of course telepathy doesn't happen, and if it did, it wouldn't be FTL. But everything *else* makes sense.

Or Ursula K. LeGuin's novels, postulating Nearly As Fast As Light travel...you can get to other planets in a few seconds, but everyone back home will have aged as many years as the distance you traveled. This is fine if you don't want to come back.

[kanicbird]

[Yeah, k2dave, you could reach Alpha Cen in three years, as measured by a person on the ship]
that's what i wanted to know, now the next step is calculating the mass of fuel required to do this
Thanks