Is time travel possible?
September 17th 2006 05:15
Based on some very superficial reading. I don't really know anything about physics, nor philosophy of time. If you're keen for more info, contact someone at Sydney Uni's Centre for Time.
Hypertime paradox
People have thought it nonsense to speak of time travel. "Doesn't it imply two times? How can you say, 'it takes 15 minutes to travel 20 minutes'?"
But this objection gets shot down in an Einsteinian world, where there are different frames of reference, where time is relative to the position of the observer. -- My journey can be 15 minutes with respect to my wrist watch, but 20 minutes with respect to the earth.
The mainstream view (among philosophers, scientists) is that travel into the future is possible.
In 1971, Hafele and Keating flew very accurate atomic clocks twice around the world, then compared them to clocks on earth -- and the atomic clocks had run slower.
Grandfather or auto-infanticide paradox
What about into the past? Couldn't you kill your grandfather, or yourself?
Some would argue: since there are no possible worlds with logical contradictions, then there are no possible worlds that allow backwards time travel.
Sci-fi fiction is rarely about travelling to the same time. The normal plot travels to a different timeline, or to something like a different universe. After all, if it's the same time, then what's happened has happened. Either Marty's parents got together in 1955, or they didn't. It's contradictory for both to be the case.
In 1949, Goedel found solutions to the field equations of general relativity that allowed closed causal chains. So "by making a round trip on a rocket ship it is possible in these worlds to travel into any region of the past, present, and future and back again, exactly as it is possible in other worlds to travel to distant parts of space."
But general relativity doesn't say that our world has closed causal chains. Goedel himself thought it didn't, because: (i) the implications are paradoxical; and (ii) there isn't enough fuel in the universe to propel such a trip.
The fuel problem is one for physicists. But there is an easy reply to Goedel's first objection, and to grandfather paradoxes in general.
It's simply this: that the impossibility of changing the past just means that it's impossible to change the past. It doesn't rule out travelling to the past. There might be any number of reasons why you don't succeed in killing your grandfather or yourself, but the fact is that you don't. It's a fallacy to think that you can -- there is no "second time around".
Presentism
So nothing has so far ruled out the possibility of time travel. But there's one final objection.
There are perhaps five main views on what times exist.
1. "Eternalism" or "block universe" -- past, present, and future exist.
2. "Dynamic block" or "growing block" -- only past and present exist (which is why Aristotle thought that statements about the future were neither true nor false).
3. Solipsistic presentism -- only this moment exists (just as Descartes wonders if only Descartes exists).
4. Multiple-worlds presentism -- there are many separate present moments; and each one takes it in turn to exist.
5. Overlapping presentism -- there is some degree of overlap in the many presents, and that's why we can directly experience change.
The "man on the street" opinion is presentist. And perhaps it's only if you believe in block or growing block that you can easily believe in time travel.
How to choose between the five views? Well, pulling in some Quine, any view can be saved if you're desperate enough to save it. The only consideration might be Occam's razor.
And mainstream science, at any rate, seems to think that block universe is the most economical: special relativity rules out presentism.
Some references
-- Kurt Goedel, "Example of a new type of cosmological solutions to Einstein's field equations" (1949) Reviews of modern physics 21
-- Paul Horwich, Asymmetries in time: problems in the philosophy of science, 1987, Ch 10 -- discussion of the objections to backwards time travel
-- Barry Dainton, Time and space, 2001, Ch 7 -- gives the schema I've used for the available views on what time exists
-- Theodore Sider, Four-dimentionalism, 2001 -- discusses the incommensurability of presentism with special relativity
Images
This article is licensed under the GNU Free Documentation License.
The "twins paradox" image was obtained from here.
The image of Einstein and Godel was obtained from here.
The image from the 1960 time machine film was obtained from here.
The TARDIS came from here.
The melting clocks and "Back to the future" images were obtained from Wikipedia.
***
Hypertime paradox
The notorious twins paradox -- click to expand
But this objection gets shot down in an Einsteinian world, where there are different frames of reference, where time is relative to the position of the observer. -- My journey can be 15 minutes with respect to my wrist watch, but 20 minutes with respect to the earth.
The mainstream view (among philosophers, scientists) is that travel into the future is possible.
In 1971, Hafele and Keating flew very accurate atomic clocks twice around the world, then compared them to clocks on earth -- and the atomic clocks had run slower.
***
Grandfather or auto-infanticide paradox
Some would argue: since there are no possible worlds with logical contradictions, then there are no possible worlds that allow backwards time travel.
Sci-fi fiction is rarely about travelling to the same time. The normal plot travels to a different timeline, or to something like a different universe. After all, if it's the same time, then what's happened has happened. Either Marty's parents got together in 1955, or they didn't. It's contradictory for both to be the case.
But general relativity doesn't say that our world has closed causal chains. Goedel himself thought it didn't, because: (i) the implications are paradoxical; and (ii) there isn't enough fuel in the universe to propel such a trip.
No fuel problem
It's simply this: that the impossibility of changing the past just means that it's impossible to change the past. It doesn't rule out travelling to the past. There might be any number of reasons why you don't succeed in killing your grandfather or yourself, but the fact is that you don't. It's a fallacy to think that you can -- there is no "second time around".
***
Presentism
So nothing has so far ruled out the possibility of time travel. But there's one final objection.
There are perhaps five main views on what times exist.
1. "Eternalism" or "block universe" -- past, present, and future exist.
2. "Dynamic block" or "growing block" -- only past and present exist (which is why Aristotle thought that statements about the future were neither true nor false).
3. Solipsistic presentism -- only this moment exists (just as Descartes wonders if only Descartes exists).
4. Multiple-worlds presentism -- there are many separate present moments; and each one takes it in turn to exist.
5. Overlapping presentism -- there is some degree of overlap in the many presents, and that's why we can directly experience change.
The "man on the street" opinion is presentist. And perhaps it's only if you believe in block or growing block that you can easily believe in time travel.
How to choose between the five views? Well, pulling in some Quine, any view can be saved if you're desperate enough to save it. The only consideration might be Occam's razor.
And mainstream science, at any rate, seems to think that block universe is the most economical: special relativity rules out presentism.
Einstein and Goedel
***
Some references
-- Kurt Goedel, "Example of a new type of cosmological solutions to Einstein's field equations" (1949) Reviews of modern physics 21
-- Paul Horwich, Asymmetries in time: problems in the philosophy of science, 1987, Ch 10 -- discussion of the objections to backwards time travel
-- Barry Dainton, Time and space, 2001, Ch 7 -- gives the schema I've used for the available views on what time exists
-- Theodore Sider, Four-dimentionalism, 2001 -- discusses the incommensurability of presentism with special relativity
Images
This article is licensed under the GNU Free Documentation License.
The "twins paradox" image was obtained from here.
The image of Einstein and Godel was obtained from here.
The image from the 1960 time machine film was obtained from here.
The TARDIS came from here.
The melting clocks and "Back to the future" images were obtained from Wikipedia.
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Comment by Ahmed
Video Gamer Kids
Little Green Foosballs
PolyKicks
So let's say you go into the speed of light (though it is impossible) for ten earth years of someone who isn't travelling at the speed of light.
When you stop travelling at that speed and disembark from your spaceship you will find you have not aged (or even moved!) even a tiny bit, however everyone else who waved goodbye who are now waiting for you are 10 years older, they are the same people who waved goodbye as you went off into that super fast vehicle.
Comment by Adrian
Philosophy Blog
The ageing-oddity you mention is sometimes called the "twins paradox", and there's an image in my post that illustrates it.
Mind you, I don't fully understand how ths is explained in relativity. Is it simply (as your comment suggests) to do with the subject's acceleration? Or is it also to do with objects of large mass curving time-space in particular ways?
Comment by Ahmed
Video Gamer Kids
Little Green Foosballs
PolyKicks
I think the excessively large gravity created by black holes for instance are supposed to stop time (or even tear it apart, but I think thats just sci-fi) because they don't let anything escape, light included.
Comment by Damo
However there has been no experiments I know of that show evidence of going backward in time. A multiverse made up of many universes make great science fiction and theory but still not proof of that either. So sorry to say it, but we are trapped in an journey the can only go forward.
Theories are great and we can dream but proof is like a cold shower.
Comment by Ahmed
Video Gamer Kids
Little Green Foosballs
PolyKicks
Comment by Harmony Rocks
head for threads
Harmony's Forum For You
Harmony's Forum For You
Comment by Adrian
Philosophy Blog
There's a sense, you know, in which there's always time delay.
... like light from a distant star...
Your lips move, there's pressure in the air, my ear detects the pressure, my brain makes sense of the words.
(This is to explain "hearing" without appealing to consciousness. As to where consciousness fits into the picture, I can't say.)
So, I've heard it suggested, in one sense we never experience the present. Our reality is necessarily the past.
Comment by Adrian
Philosophy Blog
Incidentally: (1) I seem to remember Hawking speculating about time travel in A brief history; something about astronauts and wormholes; will have to revisit and see what he says; (2) if time travel were possible, this wouldn't mean that changing the past is possible.
Strap yourself in for the ride of a lifetime to past
An eminent scientist says time travellers won’t need a spinning police box, just a set of mathematical equations.
It may take more than a nuclear-powered De Lorean or a spinning police box, but time travel could actually be a possibility for future generations, according to an eminent professor of physics.
Professor Amos On has set out a theoretical model of a time machine which would allow people to travel back in time to explore the past.
The way the machine would work rests on Einstein’s theory of general relativity, a theory of gravity that shows how time can be warped by the gravitational pull of objects.
Bend time enough and you can create a loop and the possibility of temporal travel. Professor On’s theory, set out in the prestigious science journal Physical Review, rests on a set of mathematical equations describing hypothetical conditions that, if established, could lead to the formation of a time machine, technically known as "closed time-like curves".
In the blends of space and time, or spacetime, in his equations, time would be able to curve back on itself, so that a person travelling around the loop might be able to go farther back in time with each lap.
In the past, one of the major challenges has been the alleged need for an exotic material with strange properties what physicists call negative density to create these time loops.
"This is no longer an issue," he said. "You can construct a time machine without exotic matter. It is now possible to use any material, even dust, so long as there is enough of it to bend spacetime into a loop." Even though Professor On, of the Technion-Israel Institute of Technology, believes his new work strengthens the possibility of a real Tardis, he would not speculate on when a time machine would be built, or even if it would ever be possible.
"There are still some open questions," he said.
The main remaining issue is the stability of space time, the very fabric of the cosmos, in time travel scenarios. But overcoming this obstacle may require the next generation of theory under development, called quantum gravity, which attempts to blend general relativity with the ideas of the quantum theory, the mathematical ideas that rule the atomic world.
Time travel has long been a fascination. HG Wells grappled with the scientific issues in his 1895 science fiction classic The Time Machine, Dr Who is still fighting the time war and Hollywood insisted all that was needed for time travel was a De Lorean and a good flash of lightning. But more serious work on general relativity first raised the astonishing possibility of time travel in the 1940s. In the half century since, many eminent physicists have argued against time travel because it undermines ideas of cause and effect to create paradoxes so that a time traveller could go back to kill his grandfather so that she is never born in the first place.
In 1990, the world’s best known scientist Professor Stephen Hawking proposed a "chronology protection conjecture", which flatly says the laws of physics disallow time machines. Three years later, Professor On concluded that the possibility of constructing a time machine from conventional materials could not be ruled out.
Professor Hawking fought back with his Cambridge University colleague Michael Cassidy and they concluded that time loops were extremely unlikely. Tongue in cheek, Professor Hawking added that there was experimental evidence that time travel doesn’t exist. "We have no reliable evidence of visitors from the future," he said. "I’m discounting the conspiracy theory that UFOs are from the future and that the government knows and is covering it up. Its record of cover-ups is not that good." But now, in Physical Review, Professor On has provided some more advanced solutions to the problems of time travel outlined by the likes of Professor Hawking, helping to realise an idea that dates back millennia and appears in 18th century literature, Harry Potter, Dickens, sci-fi movies and much more besides.
Professor On’s theory is highly mathematical and arcane but, at its simplest, his time machine has to create a "doughnut" of spacetime.
Set out in one direction and you return to your past and then to your departure point: thus, by travelling around this loop of spacetime, time travel has been achieved.
The time travel theory proposed by Professor On and his student Dana Levanony in Physical Review shows that the loop would form within an empty, doughnut-shaped region of spacetime enveloped by a sphere of normal matter.
"The machine is spacetime itself," Professor On said. "If we were to create an area with a warp like this in space that would enable timelines to close on themselves, it might enable future generations to return to visit our time. We, however, could not return to previous ages because our predecessors did not create this time travel infrastructure for us."