Do you think it's possible?
If you have a Delorean
and some plutonium
Yeah and then you just need to accelerate to 88 MPH :D
Ok, I love that movie, in fact Back to the Future is one of my favories.
But my question is somewhat serious.
Imho time travel maybe possible but only for traveling to the past. I think future isn't determined yet so you cannot go there. On the other hand, going to the past and changing only a minor thing can lead to huge effects on the present. Just is you go to the past, you've already changed it.
You know the chaos theory. :)
Anyway wether it is possible or not I think time travel remains in the world of sf for a couple more centuries.
I watched a whole show on this on either Discovery, Learning Channel, or oddly enough, History Channel. Maybe I watched it on Nova back when I was still watching Nova.
Anyway, they talked about Einsteinian physics and said that as far as we understand the universe time travel is inherently possible. Science has accepted that. But, what does that mean? Is it possible to go back in time to kill your grandfather before your father is born? That's the paradox question that comes up every time you get into time travel.
The rest is left up to science fiction because science has no answers.
Once we invent the Flux Capacitor...
>>6 Nah, the flux capacitor is most likely a simple wave generator.
I'm more worried about the 1.21 gigawatts of power needed...
We don't have portable plutonium-powered nuclear reactors currently.
>>8 But Mr. Fusion is in the future... TIME PARADOX!
Back to the topic (oh dear god no!! I didn't mean to)
I don't believe so. I'm too tired to explain it, but I thought about it enough to right a 3 copy page essay on it last year.
Good times ^_^
At least thirty years back, there were some papers published discussing the possiblity. I mean scholarly papers, from physicists, who were looking at the mathematical models in Einstein's theory of relativity and examining them under unnatural, possibly impossible conditions.
Google for "Tipler machine," "Kerr metric space warp," or "closed timelike curve" and you will see some very brilliant stuff.
I don't know whether any of it was ever found to be mathematically inconsistent with relativity, or proven to be mathematically impossible, though the reduction-to-practice aspects are daunting.
A Kerr metric space warp, for instance, would require the mass of a star crammed into a superdense black whole whose core was not in the natural spherical shape that gravity would push it into, but, rather, a torus--a ring, in other words--that was spinning at almost the speed of light. Pass through the center, go into the past. Or something like that. It's a fascinating idea but you can't exactly build one in the garage.
>>10 Time travel is a reality. I'm travelling in time right now. And so do you. At 1x speed.
Your humble opinion is completely wrong. Time travel into the future is very easy. Besides the fact that you're travelling there right now, simple special relativity tells you that if you're in a hurry, you just need to go move very fast and you'll get there quicker. Once you're there, though, you're stuck.
Unless the esoteric relativistic solutions like >>11 mentions ever turn out to be feasible, but that's very uncertain. Those are, however, not exactly "time machines", but something much, much stranger.
Using what I've heard and read of time travel and parallel universes, most likely what would be experienced in time travel is just going to a universe that is however far back as you think you've traveled (say 2006 to 1920s, just 86 years behind ours). Because of this, coming back here will show no know changes because it affected another world entirely.
I'm guessing of course.
>>13 Yeah. There is also gravitational time dilation: for example the GPS satellites we have in orbit have their clocks running behind. I guess one could say that the satellites are time-travelling into the past, by a minute amount.
GPS satellites, as they are orbiting or just falling through the vaccum of space, aren't affected (or very little) of gravity. Somehow matter moves at a slower pace when not under gravity. So clocks on satellites tend to stay behind earth-time and need to become updated in a periodic basis.
I think that beings tend to live a little longer when in weightlesness.
I don't know wether time-travel will ever be possible or not. But I have been experimenting a little bit with myself as of how time shifts according to situations. Whenever I sleep with my music on, I can unconciously hear it (sometimes) and notice a faster beat rate, meaning that the music goes faster than what you are accustomed to hear. Therefore, I can say that my body is moving at a slower rate, because it is in sleep mode, and time moves faster. I remember that I once was listening to music when suddenly the phone rang. And because it is located at the other end of my home, I had to run. Once I hung up I continued to listen to my music, but noticed it was moving slower. It seemed that, under that desperate situation, my body modified its biological clock and made time go slower, hence making the music become slower and have a lower pitch.
Maybe different situations make our body change its biological clock to respond correctly and more efficiently to them.
What do you think?
I think biological clocks have as much to do with time travel as sunglasses with have with nuclear fusion.
>>17 Lol. Good analogy.
Yes, time travel is possible under Einsteinian physics. Unfortunately we have no clue how it works nor how to move backwards through time. Since to move backwards in time forces us to break the speed of light, which also under Einsteinian physics is impossible. Tests have shown however that time travel is possible. We have tested and measured small instances of slowing time, most notably through the use of atomic clocks placed in orbit.
You are technically correct that we have "tested and measured small instances of slowing time", but this really is a bit misleading. Not only are experiments confirming this (by necessity) done every day, but many everyday occurances depend on it - notably, GPS navigation.
Adding to my comment >>15, "time travel into the past", I'm still not sure of my assertion. Is it the GPS clock that moves backward in time, or is it the clock on the ground that moves (fast)forward in time?
I think you're misunderstanding. Both are moving in time at the same speed according to the effects of both gravity and motion. Since according to Einsteinian physics the closer you are to a gravitational center the slower you move in time. Also the faster you travel the slower you move in time.
By moving an object away from the center of gravity (Earth) you are minutely moving it faster through time. But! The effects of being away from the center of the earth is negligable compared to the effects of traveling at high speed that slows it down.
Once one atomic clock is seperated from the other and placed in orbit it begins to move at a different rate through time. It's not much but it is measurable because atomic clocks are amazingly accurate.
So to answer you, neither atomic clock (you call them GPS clocks but I imagine you meant atomic clocks) is moving forward or backward in time. They are both moving forward in time at different rates, the one in orbit moves slightly slower through time then the one on the ground because it's slowing time as it approaches the speed of light.
People... "time" doesnt exist...
its invented by humans to make life a little easier, like: "hey lets meet at 7 ok?"
the so called "ageing" is just a procces off cells breaking down
so how can time traveling be possible is there isnt even time to travel with?
sigh but anyway its getting late and its time to go, see you next time!
>>21 Okay, but which time should a time traveller sync his clock to? I suppose that with the Moon orbiting Earth, they are variations in the atomic clocks on the ground as well. Also planets/the Sun moving closer or away. Don't those ground clocks get out of sync with each other at times? Or not?
Yes, a clock set on the moon will move at a different rate then one on Earth, or the Sun, or Jupiter, or wherever it's placed. It's unavoidable. We do the best we can with what we've got though and right now since Greenich standard time is universal on Earth you can assume that it's universal in the Galaxy (assuming of course that we're all there is in intelligent life). Any clock which gets out of sync presumably would be reset to GMT whenever possible.
Since interstellar travel is still a dream we don't know exactly what happens when people begin traveling through space and colonizing other worlds. Right now it's left to science fiction how to handle clocks in that future.
The basic principle of relativity is that there is no preferred reference frame. That is to say, there is no clock to sync to. All clocks run different, and there's nothing you can do about it, except choose an arbitary one to call your standard time. Even if you do, it won't have anything to do with any deeper aspect of reality, it will just be bookkeeping for yourself.
You should also realize that this goes further than that. The concept of "simultaneity" doesn't exist, either. The concept of two things happening in different place simultaneously is meaningless, because this is also dependent on your reference frame (that is, the speed you're moving at, gravity, and such). Things can only happen at the same time if they also happen in the same place.
This is a pain in the ass to wrap your head around, but unfortunately it's how the universe works.
There is another way of timetravelling into the future that hasn't been mentioned above yet, and that's temperature.
For example, food in the fridge decays slower that in the heat.
So this is yet another factor that could influence an atomic clock, isn't it?
Are those forward-timetravel factors (gravity + speed + cold) cumulative?
> But! The effects of being away from the center of the earth is negligable compared to the effects of traveling at high speed that slows it down.
But! all movement is relative, so it isn't actually moving at all.
Cold only slows down the atoms. I don't believe cold has anything to do with slowing time to any degree. It's used to preserve foods because it keeps bacteria and molds inactive.
No. Lay off the acid.
>>28 Wait, wait... if cold is slowing down the atoms, why has it no influence on an atomic clock?
>>30
No. Lay off the acid.
Just because both the statements "slowing down the atoms" and "atomic clock" have the word "atom" in them, does not mean they are directly related.
You don't age more slowly by walking slower.
>>32 Well, okay. I need to look up how an atomic clock works.
Rephrasing my other question: are forward-timetravel factors (gravity + speed) cumulative?
>>33
gravity and speed are the same factor.
Yes, they are cumulative.
Well, it's not as simple as being "cumulative". >>34 has it, they're different sides of the same thing. The important thing is paths through space-time. Figuring out what the result will be with strong gravity and high speed is nowhere near intuitive. You pretty much have to work through the maths to figure it out.
However, in the case of something like GPS satellites orbiting the earth, speed and gravity are both low, and the effects are approximately cumulative. However, the effects also oppose each other.
http://en.wikipedia.org/wiki/Hafele-Keating_experiment has some information on this.
I see. Ok, my next question:
As one's velocity increases, one goes forward in time. The maximum speed is the speed of light, and is a known constant.
As one approaches a strong gravity field, one goes forward in time. I guess there is a maximum for a gravity field too? Is this known?
>>37
black hole?
We are always Time Traveling to the future!!
There is no maximum as such for how strong a gravity field can be. However, a strong enough gravity field can reach the point where spacetime is bent to the point that no particles can escape, even if they move at the speed of light. As a first approximation, you can think of it as a gravity field so strong that the escape velocity is higher than the speed of light (however, you will need to account for relativistic effects when considering this in practice).
As >>38 mentions, a black hole is the prime example of this.
>>40 Something I don't understand here. Light cannot escape a black hole (obviously), but light particles have no mass, which is why they can travel at the speed of light. If a particle has no mass, how come it can be trapped in a gravity well?
Because the simplistic view of gravity as masses acting on masses in the Newtonian model is incorrect.
Actually, that's not the right explanation. Even in the Newtonian model, light is affected by gravity. But it predicts a different result than relativity. Remember, gravitational acceleration is independent of mass: A heavy body falls as fast as a light one. Thus, one without mass entirely falls as fast as one with a tiny, nearly undetectable amount of mass, which in turn falls as fast as a heavy mass.
However, in relativity it turns out this is not actually the case. In relativity, all bodies follow geodesics (that is, locally straight lines in curved space) through space-time, and matter curves space-time. However, light and mass follow different paths.
This was the basis for one of the early experiments to verify relativity: http://sunearthday.nasa.gov/2006/locations/einstein.php
>>42 Is the warping of spacetime temporary?
That is, does a light ray after passing near a strong gravity like a sun (looking bended while doing so), re-take its original trajectory?
In other words, is it just an optical illusion?
Simply put, no.
Optical illusions are created either by the brain misinterpreting information or light passing through a medium like water or air which distorts the image. Neither is happening here.
The Sun's gravity field is actually not all that strong, but it is the strongest we have nearby. And no, it's not temporary - spacetime is warped by the presence of mass. Light moves along straight lines in spacetime. After passing close to a large mass, the direction of the light will have changed.
Ok, I think I get it.
So, near a strong gravity field spacetime is warped, and that includes whatever light ray that passes thru it. Also, time is passing faster.
What if the strong gravity field is inverted? In a mathematical model of an inverted gravity field, is the time factor also inverted?
Time passing faster is not a separate effect. Also, time can pass slower or faster, depending.
Think of it like this: Two people go from point A to point B. one of the passes through an area of warped space-time, and the other does not. When they meet up, they find that less (or more) time has passed for one of them. It is not because "time has moved faster" for one, it is because their paths through spacetime were different.
Also, there are mathematical models for crazy stuff like negative masses creating weird spacetime distortions, but I don't really know anything about them so I will not comment on that.
OK.
Has there been any known instance of black holes vanishing?
Or black holes appearing where there were none before?
blech, you're starting to ask questions that are beyond me. Basically I can answer that stellar phenomenon like the creation of black holes, is not something that occurs in any amount of time we as humans can witness. It basically takes eons to mash stars together or cause them to explode thus creating black holes. I remember reading a short paper by Stephen Hawking that explained that there's a subatomic particle which is created spontaneously in space. If one were to be created on the edge of a black hole (the event horizon) part would spiral back into the black hole while half would break away and return to normal space. This is the only way a black hole can reduce it's mass, eventually decaying over countless millenia. It's been years since I read that article so the science behind it is beyond me now.
We still can't observe black holes directly, only their large-scale effects, and even those can be hard to unambiguously make out. As far as I know we don't have perfectly solid evidence they exist yet (although lots of circumstantial evidence, so to speak).
The main process that can create black holes that we know of are supernovae. There hasn't been one near enough to study closely, which is good, because it'd probably fry us if it was. As >>49 mentions, these things happen over astronomical timescales, which is far longer than humans have been around.
And black holes are thought to disappear by Hawking radiation, but the timescales involved for that are beyond anything you can imagine.
OK. It seems we'll have to wait for the CERN's LH Collider in 2007 to (hopefully) crank out mini black holes to study before we can map fully the black hole's properties.
Before you can ask yourself "is time travel possible?", you first must ask yourslef "what is the nature of time?". We don't know for sure that time really physically exists at all, it may just be a concept that we associate with the changes in the world around us, and use to chart the progress of those changes, a concept we have become so dependant on that we view it as something physical that can be seen or changed or altered. The fact is we don't know for sure that time is a physical thing at all, it may just be all in our heads, a psychological crutch, if you will.
On the other hand maybe it is. And then you have to wonder what it's physical propertys are. Is it a dimension, a fourth axis on which we are constantly moving? Or perhaps it's some other physical thing. The fact is, it is at our current stage of thought impossible to prove or disprove even the EXISTANCE of time, let alone weather we can alter it's course.
Before you can ask yourself "is space travel possible?", you first must ask yourslef "what is the nature of space?". We don't know for sure that space really physically exists at all, it may just be a concept that we associate with the changes in the world around us, and use to chart the progress of those changes, a concept we have become so dependant on that we view it as something physical that can be seen or changed or altered. The fact is we don't know for sure that space is a physical thing at all, it may just be all in our heads, a psychological crutch, if you will.
On the other hand maybe it is. And then you have to wonder what it's physical propertys are. Is it a dimension, a fourth axis on which we are constantly moving? Or perhaps it's some other physical thing. The fact is, it is at our current stage of thought impossible to prove or disprove even the EXISTANCE of space, let alone weather we can alter it's course.
>>53 Um, we're not talking about the philosophy of existence but the observable nature of spacetime. So, please don't bring psychology into a discussion of physics. You may be correct, we could all be brains placed in a jar and fed stimulation to see our reactions but since we can't measure that scientifically we discuss the things we can measure. And by our mathematics and knowledge of physics time travel IS possible.
> The fact is we don't know for sure that time is a physical thing at all, it may just be all in our heads, a psychological crutch, if you will.
Spoken like a true philosopher with no knowledge whatsoever about the actual physics of what he is talking about. When you say "we", you really mean "I", don't you?
Before you sprout statements like this again, maybe you should take some time to study what we actually do know about time?
To understand timetravel one must first understand time and if this 'time' really exists.
Is traveling faster than the light timetravel? We are looking up at the stars and looking back in "time" beacuse the light is extremely slow when it comes to interstellar distances.
When we look at the moon it is 1 second in the past and if there was an astronaut up there we would know he sees us 1 sec in the past aswell.
How is it possible for us to look back in time yet not be able to travel there?
> Is traveling faster than the light timetravel?
No, because there is no such thing as "traveling faster than light".
> How is it possible for us to look back in time yet not be able to travel there?
How is it possible for me to look at women yet not be able to become one?
To understand spacetravel one must first understand space and if this 'space' really exists.
(...wait, I already did this joke earlier in this thread.)
>No, because there is no such thing as "traveling faster than light".
I know we aren't able to travel faster than light.. YET!
Why can you travel @ the speed of light (light does this) but not actually go 1m/s faster than it?
Just because you cant see an object doesn't mean it stops to exist.
We can travel faster than sound, but it doesn't mean that sound stops to exist (pilots still hear themselves breathe and stuff).
and i mean 'faster than the speed of light'.. not light itself -__-;
>How is it possible for me to look at women yet not be able to become one?
Becoming something is not the same as traveling to something.
> Why can you travel @ the speed of light (light does this) but not actually go 1m/s faster than it?
Because "1m/s faster than light" is a meaningless concept. You only think it makes sense because you are used to very low velocities. Your intuitive image of the universe is nowhere near correct, and it is that incorrect image which makes you think moving faster than light makes sense.
It has nothing to do with light itself. Light just happens to move at the maximum possible speed. The structure of the universe is such that moving faster than light is not something that can happen.
> Becoming something is not the same as traveling to something.
Similarly, seeing something is not the same as travelling to it.
>the maximum possible speed. The structure of the universe is such that moving faster than light is not something that can happen.
yeah, and there cant be life without water... lolz
I am telling you the truth, but you refuse to listen, so what can I do? Well, I guess I can tell you what happens if you try to go faster than light. Please try and read and understand at least some of this.
Let's say you build a rocket with a REALLY big and powerful, yet extremely light-weight, engine. This is currently impossible, but that might just be engineering so we can easily disregard it, as you like to do.
In the first case, let's just set off the rocket and watch it go. You'll see it accelerate up towards the speed of light, just as you expect. However, when it starts to get close, you notice it's not accelerating as much as it should any longer. This is pretty confusing, since the engine is still putting out just as much thrust. Investigating closer (with your strange futuristic equipment), you notice that although the engine is still running correctly, the rocket is suddenly heavier. You will also notice that the close to the speed of light you get, the heavier the rocket will become, and the less it will accelerate.
When the rocket gets really close to the speed of light, its mass has grown enormous. The engine still tries to accelerate it, but it's not doing much good any longer. It will never reach the speed of light, but if it did, its mass would be infinite, and the engine could no longer move the rocket.
Incidentially, you will also find that the rocket has become shorter, and that clocks on board it are running slow. If the rocket should ever reach the speed of light, it would be completely flat, and time would stand still on board it.
Now, in the second test, let's jump aboard that rocket and try to ride it up to the speed of light. At first, everything goes well. The rocket accelerates just as it should. From our previous experiment, we might expect that it will soon stop doing this, because of the mass increase. However, to our surprise, everything is working just fine. But when we look out of the window, we suddenly notice that the entire universe around is shrinking along the direction of travel. Even though we keep accelerating, we never seem to get past the speed of light relative to anything outside the rocket - we reach places that used to be far away quickly, but it is not because we ourselves are moving fast, it's because the rest of the universe is shrinking!
We will reach Alpha Centauri in, say, three hours. But we know from our previous experiment that that rocket reached Alpha Centauri in three hours too, by its internal clock, which was running slow!. Sitting outside the rocket and waiting for it to reach Alpha Centauri took the expected four years, but the internal clock had slowed down so much, it only ever counted up three hours.
None of this is fanciful theory - this is all confirmed every single day, by particle accelerators and GPS satellites. This is how the universe works. It seems very strange, and it seems totally wrong compared to our experience, but it is actually our experience that is incorrect.
ok suppose that we have a looooooooooong tube with inside diameter of 1 cm and length of 10 light seconds. and in side this tube is full of small 0.9 cm metallic spheres lined up from one end to the other. so you push the first sphere. with common sense, you might think that on the other end of our tube, a sphere drops instantaneously right? I mean, nothing is moving faster that the speed of light right? you are just pushing one sphere slowly and on the other end, no matter how long the tube is, one should come out of the tube.
sorry pal, but that’s not the case, its going to take 10 seconds. Practical observation: electricity , more or less is the same concept and it doesn’t travel faster than light. if can’t defy space. how are you going to defy time? the two are closely connect as far as I have heard. so nope: no time travel unless you break space and the laws of universe in some manner.
scientists are known to have slowed the speed of light down, and there was an incident in a russian nuclear reactor, i cant remember what it was, but there was some matter that travelet faster than the speed of light.
google for it, it's there..
Light travels at less than "the speed of light" in a medium. When people say "the speed of light", they actually mean "the speed of light in a vacuum", also known as c. Travelling faster than the speed of light in a medium is not only possible, it happens all the time around nuclear reactors. It causes a phenomenom known as Cherenkov radiation, which incidentially is quite pretty: http://images.vertmarkets.com/crlive/files/Images/8B3B7B58-B579-11D4-8C77-009027DE0829/jgglow.jpg
This is why the "light" part of "the speed of light" is not important. Light doesn't always travel at the "speed of light". The constant c exists independently of the fact that light travels at that speed in a vaccuum. Other things travel at that speed too - gravity being one of them.
im not talking about light itself.. just the speed it has.
Didn't Kurt Godel have something important to say about time travel in relative space.
Gödel was a mathematician, not a physicist.
From Wikipedia's article on "Kurt Godel."
"In the late 1940s, Gödel demonstrated the existence of paradoxical solutions to Albert Einstein's field equations in general relativity. These 'rotating universes' would allow time travel and caused Einstein to have doubts about his own theory."
More detailed information about this body of work can be found here:
http://infoshare1.princeton.edu/libraries/firestone/rbsc/finding_aids/mathoral/pmcxgod0.htm
Also, 72GET!!!
Well, you learn something new every day!
almost. it does happen pretty much simultaneously with the first ball. it will simply take 10 seconds to observe the ball fall out of place, because of the distance the light has to travel to reach back to you. an observer at the other end will see the ball fall out of the tube before you push it, in theory. thats also assuming you can even move that many small metal bearings. they should weigh in the vicinity of several tonnes.
That's plain and simply wrong. It will take at least 10 seconds, in practice much, much longer, before the last ball starts to move. The reason it takes longer is that the pressure will move through the system at the speed of sound, not the speed of light. The speed of sound in a metal is pretty high, but it's much less than the speed of light.
>>75
so the energy you apply to item #1 moves to item #2 and so on. but, assuming we has something strong enough to push them all, wouldnt that mean that either 1) the metallic bearings would change shape, or even break under the pressure or 2) two bearings will occupy the same space at the same time. assuming the bearings are indestructable, neither scenario seems likely. but im also imagining a constant application of pressure, not a impact, like you see on those desk-ornaments with the balls on string.
using that balls-on-string example, lets say you have one of those desk ornaments. if you pick up one ball, and let go, the last ball moves. i understand thats because of the energy wave moving, and i understand it happens at the speed of sound. but do you mean to tell me that if, for instance, you had those five balls at rest next to each other, and you pushed one, (not drawing it back and releasing, just pushing it foward) that the balls dont move as if they were one object? the same thing should happen in this tube. if one thing is pushing on these bearings, then all the bearings should move in unison, so long as the force is sufficient enough to move them all.
> wouldnt that mean that either 1) the metallic bearings would change shape, or even break under the pressure
Yes.
> assuming the bearings are indestructable
They are not, and no such material exists, or even can exist.
> but do you mean to tell me that if, for instance, you had those five balls at rest next to each other, and you pushed one, (not drawing it back and releasing, just pushing it foward) that the balls dont move as if they were one object?
Yes, I mean exactly that. It is not noticable under normal circumstances because the speed of sound in the material is so high in comparison to the speed you move them at, which is why your intuitive sense of the world does not accept it easily.
>>59 The only way to travel faster than the speed of light is by cheating. Usually this involves parallel dimensions or creating a "warp bubble" around you spaceship.
The reason you can't travel faster than light is, basically, because you can't.
It's possible. Very likely too if you consider the following "time travel"
You could build spaceship that accelerates at 1g. It would take you 24 years to get to Andromeda galaxy in your time frame, remember you would be going at relativistic speeds (time dilation). But us here on earth, we would see you on your way for millions of years.
http://www.dailygalaxy.com/my_weblog/2007/07/weird-science-2.html
There. I guess it's not just normal people who believe this shit. The project funders must have too much money in their hands.
If time travel to the past was possible, we'd currently be flooded by otakus from the future who want to catch original broadcasts of Lucky Star.
>>82 The RIAA from the future will not allow it for copyright reasons.
Believe Einstein. I think you can go into the future if you can travel at the speed of light. I think it's true that it is relative to the space you are in so as you travel at the speed of light it might not seem like you have aged but you return and everyone else will be older depending on how long you were away. Maybe it is not time travel but age travel.
Semantics.
I'm going into the future right now!
reverse the polarity.
Great Scott! Where are you going to find 1.21 gigawatts??
all time is occurring simultaneously. It's like pictures in a flip-book. All other informationis classified.
no its not possible, period.
It's real, and I can prove it!
I just don't feel like it...
SEE I DID IT!!
Nobody managed to go backward, and so far it seems impossible,...
I can go backward any time, the problem is how to bring my body with me.
I managed to do it at least to years ending in "5", and then I had to go and wreck the delorean... At least now I have something that travels between years ending in "7", so welcome summer of love!
Stephen Hawking once tried to prove time travel was impossible by means of a convoluted argument that took into account several factors, and then tried to argue that all of them didn't work, probably in an effort to explain why he considered such things as the "grandfather paradox" to be impossible. Seems to run afoul of Occam's Razor to me (but I wish I remembered the argument). There is another school of thought: that if people were to travel back in time and change history, they'd wind up in an alternative universe or in another timeline (and THAT has spawned an entire science fiction subgenre itself).
Some physicists (such as Michio Kaku) have argued that time travel is possible in theory, via means such as wormholes and heating up things so much that wormholes appear. Hawking contends that wormholes would collapse as soon as someone attempts to send light (or information) back in time through it. (And some s.f. authors, such as Stephen Baxter, have tried to cash in on such subjects - "The Light of Other Days", along with time travel stories).
>>98 There is a third option. In some of the Philadelphia Experiments (Scifi or truth? We may never know.) they sent some accomplice back in time to kill the father of some guy, but when the killer came back there was no change in the present. The only thing that changed was that the memory of the father of the guy was somehow fading away.
100 Get
