This makes me so sad…even though I already knew it from things I’ve read…
You Can’t Travel Back in Time, Scientists Say
LiveScience Staff Writer
LiveScience.comFri Mar 9, 8:10 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, â€œThe Elegant Universeâ€ and a physicist at Columbia University.â€œAnd 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.â€
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, youâ€™re traveling through a direction in space, making headway in all the spatial dimensionsâ€”length, width and height. But youâ€™re also traveling forward in time, the fourth dimension.
â€œSpace and time are tangled together in a sort of a four-dimensional fabric called space-time,â€ said Charles Liu, an astrophysicist with the City University of New York, College of Staten Island and co-author of the book â€œOne Universe: At Home In The Cosmos.â€
Space-time, Liu explains, can be thought of as a piece of spandex with four dimensions. â€œWhen something that has massâ€”you and I, an object, a planet, or any starâ€”sits in that piece of four-dimensional spandex, it causes it to create a dimple,â€ he said. â€œThat dimple is a manifestation of space-time bending to accommodate this mass.â€
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 doesnâ€™t share this multi-directional freedom.
â€œIn this four-dimensional space-time, youâ€™re only able to move forward in time,â€ Liu told LiveScience.
Video: Can You Time Travel?
Tunneling to the past
A handful of proposals exist for time travel. The most developed of these approaches involves a wormholeâ€”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.
â€œWormholes are the future, wormholes are the past,â€ said Michio Kaku, author of â€œHyperspaceâ€ and â€œParallel Worldsâ€ and a physicist at the City University of New York. â€œBut we have to be very careful. The gasoline necessary to energize a time machine is far beyond anything that we can assemble with todayâ€™s technology.â€
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 theoryâ€”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.
â€œMany people who study the subject doubt that that approach has any chance of working,â€ Greene said in an interview . â€œBut the basic idea if youâ€™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.â€
Video: How to Time Travel!
Another popular theory for potential time travelers involves something called cosmic stringsâ€”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 theyâ€™re in loops, with no ends, said J. Richard Gott, author of â€œTime Travel in Einstein’s Universeâ€ and an astrophysicist at Princeton University. â€œSo they are either like spaghetti or SpaghettiOâ€™s.â€
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.
â€œThis is a project that a super civilization might attempt,â€ Gott told LiveScience. â€œItâ€™s far beyond what we can do. Weâ€™re a civilization thatâ€™s not even controlling the energy resources of our planet.â€
Impossible, for now
Mathematically, you can certainly say something is traveling to the past, Liu said. â€œBut it is not possible for you and me to travel backward in time,â€ 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 Einsteinâ€™s equations through a wormhole, and see whether time travel is really possible, Kaku said. â€œBut that would require a technology far more advanced than anything we can muster,” he said. “Donâ€™t expect any young inventor to announce tomorrow in a press release that he or she has invented a time machine in their basement.â€
For now, the only definitive part of travel in the fourth dimension is that weâ€™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.
â€œIf you want to know what the Earth is like one million years from now, Iâ€™ll tell you how to do that,â€ said Greene, a consultant for â€œDÃ©jÃ Vu,â€ a recent movie that dealt with time travel. â€œBuild a spaceship. Go near the speed of light for a length of timeâ€”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 Earthâ€™s future.â€
More about Time Travel
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Don’t tell anyone that I’m still using my Delorean…