Tuesday, February 1, 2011

Stellar Objects Part 7: Wormholes

Stellar Objects Part 7: Wormholes

We might, in theory, be able to construct a traversable wormhole, and perhaps travel across the universe or through time, but the engineering of it would be a nightmare.

Is it possible to shortcut through spacetime using a network of wormholes like Dr. Ellen Arroway did in the 1997 movie, "Contact"? This is a NASA image of what one such wormhole might look like as you approached it in your spacecraft.

The name "wormhole" was coined by the theoretical physicist, John A. Wheeler in 1957, and it was based on the analogy that a worm can chew its way through an apple to the other side, creating a shortcut. This analogy describes the 1935 work of Albert Einstein and Nathan Rosen, using their mathematical proof that spacetime is curved. We have no scientific evidence for the existence of wormholes but different types of wormholes are valid solutions to equations of general relativity. Most people think of a wormhole as an Einstein-Rosen bridge. This bridge is actually just one of the solutions to Einstein's equations, this particular one using Schwarzschild metric. This wormhole solution was later proven to collapse almost instantly upon formation so that not even light could traverse it. However, in 1988, two physicists, Kip Thorne and Mike Morris, proposed a stable wormhole solution that contains a spherical shell of exotic matter whose total energy was negative, and which would hold the wormhole's throat open by preventing it from pinching off into infinite density (this exotic matter would have to generate antigravity to counteract the crushing gravity of the wormhole and prevent it from becoming a black hole). Carl Sagan prompted Thorne and Harris to come up with this scientifically sound scenario for his 1997 movie, "Contact." Wormholes containing no exotic matter (Matt Visser, 1989), or even no matter at all (according to Gauss-Bonnet theory, a modification of general relativity in which extra spatial dimensions are taken into account) have also been theoretically devised as solutions. A type of wormhole that could have been naturally created in the very early universe and is held open by negative mass strings has also been proposed (Matt Visser and John Cramer). And, according to quantum dynamics, tiny Planck-length wormholes might pop in and out of existence all the time, much like micro-black holes are thought to do.

Exotic Matter

Lets explore the Thorne-Morris exotic matter wormhole in more detail, because this wormhole has the best chance of being a working wormhole large enough to potentially transport matter. For this to work, matter with a negative energy state must exist and it does, according to quantum theory. Quantum foam has zero-point energy on average in a perfect vacuum yet the very fluctuations (virtual particles popping into existence and disappearing) that compose it have energy. If the fluctuations were dampened then the quantum state's average energy would be less than zero or negative. This dampened quantum foam has been created by destructive quantum interference as well as by what is known as the Casimir effect where a pseudo-negative pressure density is created, and in both cases it has been indirectly measured. However, these effects appear to operate only across microscopic distances. Negative energy also plays a role in black hole theory. Hawking radiation allows black holes to obey the laws of thermodynamics by bringing them into thermodynamic equilibrium with their environment. Hawking radiation is the radiation of positive energy outward from a black hole. This is accompanied by an equal inward flow of negative energy into the black hole. One would expect exotic matter to be mediated by a particle of negative mass and to mediate some kind of antigravity force. So far, the Standard Model of particle physics does not account for any such negative mass particle.

Keep in mind that the exotic matter we are exploring here is not the same as antimatter; antimatter has positive energy.  As well, negative energy is not the energy associated with the universe's early inflation. That is positive energy with negative pressure.  Nor is it the same as dark energy. I know this is tricky and counter-intuitive - it means that the space which exotic matter occupies must have a negative energy, that the space it occupies must be less than absolutely empty. It also means that negative mass would be repelled rather than attracted by gravity and it would accelerate in the direction opposite of an applied force (that is, if gravitational and inertial mass are considered to be identical - a revision of Newton's law of gravity). Remember that spacetime, according to general relativity, is warped by matter and energy. What we feel as gravity is spacetime's distortion produced by a positive energy or a mass (mass has positive energy, E = mc2). What happens when negative mass bends spacetime? Does negative matter create antigravity and therefore a "negative" curvature of spacetime consisting of negative-matter gravity hills mirroring positive-matter gravity wells? What would negative energy manifestations of the fundamental forces look like? These are problems that still need to be worked out. Though quantum theory allows for the existence of negative energy, it also places strong restrictions, called quantum inequalities, on its magnitude and direction in order not to violate the second law of thermodynamics. This law, in essence, states that it is impossible to extract an amount of heat from a hot reservoir and use it all to do work. Some of the heat must be exhausted to a cold reservoir. One of the resulting restrictions pertaining to wormhole construction in particular is that the band of negative energy used in the ring would have to be extremely thin, less than 10-32 m, barely greater than Planck length (the smallest theoretically possible length), 10-35 m (Scientific American, January 2000), a daunting technological feat to say the least.

Time Travel, Warp Drive?

Wormholes connect two unrelated points in spacetime, which means that they can, in theory, connect two points in space as well as two points in time, allowing either faster-than-light space travel or time travel, with one caveat. According to general relativity, one cannot go back to a time earlier than when the wormhole was first converted into a time machine. According to special relativity, faster-than-light travel is equivalent to time travel. And there may be a way around the "nothing faster than the speed of light rule" if one takes into account the fact that spacetime itself can be warped and distorted. It would take enormous matter or energy to create a spacetime distortion intense enough to permit time travel. But it might be possible, albeit rife with problems such as the grandfather paradox and it would be one heck of a massive engineering job! Only energy equivalent to the massive gravitational fields of black holes can create such a distortion. And yet, could wormhole technology theoretically allow us to use spacetime distortion as a method to break the speed of light barrier? The question behind this one is how fast can the fabric of spacetime move? No one knows. This idea is called Alcublerre drive or warp drive. It is based on the idea the spacetime can be expanded behind, say a spaceship traveling very close to light speed, and contracted in front of the ship. This might not seem as far out there as you think. Remember that the fabric of spacetime is thought to have expanded faster than the speed of light during the very early inflationary universe, right after the Big Bang. But, that occurred under conditions of enormous energies, and you still need a ring of negative energy around your warp-drive spaceship to prevent yourself from being crushed under the enormous mass-energy field required to warp spacetime. And last but not least, you have the sticky grandfather paradox issue to content with.

Wormholes, at least for now, remain the stuff of science fiction. There has been some serious research into whether one could exist or not and that has led to some interesting but inconclusive results - wormholes do not have the theoretical certainty behind them that black holes do, and none have been found to exist. But don't despair fellow sci-fi fans. That does not mean we can't imagine time travel or experience some far reach of the universe in our daydreams. And, wormholes, though unproven, haven't been scientifically ruled out either. 

In the meantime, I think you might enjoy this documentary called "Through the Wormhole" narrated by Morgan Freeman, which aired in June 2010. It's available through the Science Channel online.

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