By Sara Goudarzi
LiveScience Staff Writer
posted: 07 March 2007
09:01 am ET
The urge to hug a departed loved one again or prevent atrocities are among the compelling reasons that keep the notion of time travel alive in the minds of many.
While the idea makes for great fiction, some scientists now say traveling to the past is impossible.
There are a handful of scenarios that theorists have suggested for how one might travel to the past, said Brian Greene, author of the bestseller, “The Elegant Universe” and a physicist at Columbia University.“And almost all of them, if you look at them closely, brush up right at the edge of physics as we understand it. Most of us think that almost all of them can be ruled out.”
The fourth dimension
In physics, time is described as a dimension much like length, width, and height. When you travel from your house to the grocery store, you’re traveling through a direction in space, making headway in all the spatial dimensions—length, width and height. But you’re also traveling forward in time, the fourth dimension.
“Space and time are tangled together in a sort of a four-dimensional fabric called space-time,” said Charles Liu, an astrophysicist with the City University of New York, College of Staten Island and co-author of the book “One Universe: At Home In The Cosmos.”
Space-time, Liu explains, can be thought of as a piece of spandex with four dimensions. “When something that has mass—you and I, an object, a planet, or any star—sits in that piece of four-dimensional spandex, it causes it to create a dimple,” he said. “That dimple is a manifestation of space-time bending to accommodate this mass.”
The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.
Mathematically one can go backwards or forwards in the three spatial dimensions. But time doesn’t share this multi-directional freedom.
“In this four-dimensional space-time, you’re only able to move forward in time,” Liu told LiveScience.
Tunneling to the past
A handful of proposals exist for time travel. The most developed of these approaches involves a wormhole—a hypothetical tunnel connecting two regions of space-time. The regions bridged could be two completely different universes or two parts of one universe. Matter can travel through either mouth of the wormhole to reach a destination on the other side.
“Wormholes are the future, wormholes are the past,” said Michio Kaku, author of “Hyperspace” and “Parallel Worlds” and a physicist at the City University of New York. “But we have to be very careful. The gasoline necessary to energize a time machine is far beyond anything that we can assemble with today’s technology.”
To punch a hole into the fabric of space-time, Kaku explained, would require the energy of a star or negative energy, an exotic entity with an energy of less than nothing.
Greene, an expert on string theory—which views matter in a minimum of 10 dimensions and tries to bridge the gap between particle physics and nature's fundamental forces, questioned this scenario.
“Many people who study the subject doubt that that approach has any chance of working,” Greene said in an interview . “But the basic idea if you’re very, very optimistic is that if you fiddle with the wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time.”
Cosmic strings
Another popular theory for potential time travelers involves something called cosmic strings—narrow tubes of energy stretched across the entire length of the ever-expanding universe. These skinny regions, leftover from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them.
Cosmic strings are either infinite or they’re in loops, with no ends, said J. Richard Gott, author of “Time Travel in Einstein's Universe” and an astrophysicist at Princeton University. “So they are either like spaghetti or SpaghettiO’s.”
The approach of two such strings parallel to each other, said Gott, will bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.
“This is a project that a super civilization might attempt,” Gott told LiveScience. “It’s far beyond what we can do. We’re a civilization that’s not even controlling the energy resources of our planet.”
Impossible, for now
Mathematically, you can certainly say something is traveling to the past, Liu said. “But it is not possible for you and me to travel backward in time,” he said.
However, some scientists believe that traveling to the past is, in fact, theoretically possible, though impractical.
Maybe if there were a theory of everything, one could solve all of Einstein’s equations through a wormhole, and see whether time travel is really possible, Kaku said. “But that would require a technology far more advanced than anything we can muster," he said. "Don’t expect any young inventor to announce tomorrow in a press release that he or she has invented a time machine in their basement.”
For now, the only definitive part of travel in the fourth dimension is that we’re stepping further into the future with each passing moment. So for those hoping to see Earth a million years from now, scientists have good news.
“If you want to know what the Earth is like one million years from now, I’ll tell you how to do that,” said Greene, a consultant for “Déjà Vu,” a recent movie that dealt with time travel. “Build a spaceship. Go near the speed of light for a length of time—that I could calculate. Come back to Earth, and when you step out of your ship you will have aged perhaps one year while the Earth would have aged one million years. You would have traveled to Earth’s future.”
Gender: Male Location: Welfare Kingdom of California
From a science fiction view...I can think of millions of ways to travel back in time. From gadgets to energy portals....we can go back all the way back to the big bang....
Gender: Male Location: Welfare Kingdom of California
I get what you saying Shaky. I was just pokin fun with ya.
On topic...well, what can you argue with? They're the law right now. Until some other theory or evidence pops up now or the future...you can change what they're saying at the moment.
I can't help but agree with this point... "The urge to hug a departed loved one again or prevent atrocities are among the compelling reasons that keep the notion of time travel alive in the minds of many."
But it's not just a matter of "Can it be done?" but also, "What would actually happen if it could be done?"
Our popular scifi notion of time travel is exceedingly simplistic and likely more a reflection of the Universe-as-Film-Reel idea (just rewind) than reality. We don't even know what Time is. Though on that count, if we do figure out one day what Time is and how to travel through it, my guess is we'll discover it is far more complex an agent of the physical universe than our scifi fairy tales suggest.
Time travel may very well be possible but useless insofar as using knowledge of the future to acquire wealth or prevent disasters, this due to the paradox barrier.
If time travel is possible / worthwhile, then to paraphrase the Fermi Paradox, with regard to travelers from the future: "Where are they?"
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Shinier than a speeding bullet.
Gender: Male Location: Southern Oregon,
Looking at you.
I have heard the story of a Greek goddess who could see into the future, but she could not tell anyone the future because the vision she saw would change as soon as she told any one. So, if I foresee that the dice role will be 3, as soon as I communicate that in anyway, it will have been 4. This idea connects the past and the future in such a way that any change to the future, changes the past; time travel can exist, but paradoxes cannot. So, if you traveled back in time and killed your father, you would become someone else who killed the wrong person’s father.
Okay, that article makes me want to go watch Doctor Who because yay time travel!
But anyway.
I was always under the impression that (from what I understood from my physics class) that due to the fact that time is not actually a constant, that time travel into the future actually IS possible, just not something we could do with our current technology, but it wouldn't be possible to do the reverse and travel into the past.
Not to mention that even if it were possible, damn would it open up a ton of paradoxes.
This seems to concur with what I mentioned, that time / time travel may not be just a simple matter of rewinding some temporal filmstrip.
Also, the paradox barrier would prevent me from killing my father by depositing me in a parallel history, virtually identical to my timeline of origin, the difference being that I am in this second timeline's past, able to kill my parallel timeline father, but not my father from the same timeline as I.
In this manner, there are no paradoxes.
I once wrote a short story suggesting that, just as the main character could do damage to a parallel history (Timeline #1), but not her history of origin (Timeline #2), a parallel copy of her from Timeline #3 was able to affect Timeline #2...in effect, a transverse wave running perpendicular across all Timelines.
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Shinier than a speeding bullet.
Mine too (I think), though when one gets into time travel, the whole notion of cause-n-effect is up in the air: does the parallel timeline come into existence with the time traveler's actions, or does the traveler's actions simply fulfill the parameters defining that particular (pre-existing) parallel timeline?
I always liked what quantum cosmology suggests: the universe has a wavefunction, and this wavefunction posits an infinite number of universe-states ("parallel timelines") coexisting.
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Shinier than a speeding bullet.
They would exist seperately until such time as interaction took place?
Makes me wonder if something was changed in this timeline, would we even know? if that makes any sense?
Gender: Male Location: Southern Oregon,
Looking at you.
It couldn't be gravity or time because gravity is the curvature of space-time, and we are talking about separating space-time when we say separate universes.
KMC Community Forums
Mar 7th, 2007 04:22 PM
I'm telling a MOD to stay on topic. 
