According to Ford and Roman (January, 2000), "special relativity says that you cannot outrun a light signal in a fair race in which you and the signal follow the same route." They add however, that, [w]hen space-time is warped, . . . , it might be possible to beat a light signal by taking a different route, a shortcut." "Wormholes" are the alternate route, the shortcut.
Space-Time and Gravity
To create the shortcut, space-time has to be distorted. Ford and Roman explain that what we perceive as gravity is just "the space-time distortion produced by normal, positive energy or mass." But, add Ford and Roman, when something called " . . . negative energy or mass . . . bends space-time, all sorts of amazing phenomena might become possible," including "traversable wormholes, which" might "act as tunnels to otherwise distant parts of the universe." Other possibilities according to Ford and Roman include "faster-than-light travel" and "time machines."
Ford and Roman distinguish negative energy, "less than zero energy" (and of course, less-than-zero mass," since energy is related to mass) from both the energy produced by anti-matter (which is positive) and from the energy associated with the Cosmological constant or "dark matter." Negative energy is energy borrowed from space-time,and is associated with a vacuum. Ideally the borrowing of energy should be able to cool hot objects, except that negative energy is also accompanied by positive energy.
Negative energy's effects have actually been measured, according to Ford and Roman. When you have zero energy (in a vacuum), what you actually have apparently is fluctuating energy, sometimes positive, sometimes negative. This is because, in particle physics at least, according to Ford and Roman, the quantum vacuum is never really empty, but instead is "a roiling sea of "virtual" particles spontaneously popping in and out of existence."
Gravity and Light in Black Holes, Wormholes
Wormholes are similar in some ways to black holes. So what are black holes? No one has seen inside of one. We have however seen some of the effects of black holes. We've observed the incredible warping of the light of nearby stars as these get closer to a black hole, and we've also observed the areas in space that appear black where light begins to be "infinitely red-shifted," as it nears a black hole.
Gravity, or "the space-time distortion produced by" black holes is what bends nearby light. It is also apparently what prevents much light that is inside a black hole's "event horizon" from "escaping" to where we can see it at all.
The Event Horizon
The event horizon is simply an area around the black hole where the black hole's gravity is strong enough that no light can escape, the place where the black hole becomes truly dark and invisible. Hence regardless of the actual temperature, these parts of the universe appear black, because the light remains trapped beyond the event horizon, and cannot travel to where we can see it. Photons may orbit a black hole near its event horizon.
Singularities at the Center?
At the center of some black holes at least there may be what physicists call "singularities." In singularities, supposedly because of the gravity there, atoms and even particles collapse. However not all "black holes" may contain singularities. Possibly some hide from us other parts of the universe. Because nothing, not even light can reach us across the "event horizon," we may not even be able to measure all the matter that's hidden, at least not if the matter is so far past the black hole's event horizon that its effects on the event horizon do not suggest to us the the true amount of matter. Thus the universe where we are goes on, some argue, as if that matter, that part of the universe hidden beyond the black hole, does not even exist.
Tunnels Through "Singularities" and Wormholes
In a singularity, density and gravity approach infinity. What happens to time in the singularity? Physicists seem to argue about this, but as for space you obviously cannot move in space freely when trapped at such a point where gravity won't let anything escape. In space you are at the mercy of where the singularity takes you, so perhaps space in a singularity is a bit like time on Earth. But is time like space on Earth? Can you move through time? ( I tend to think that by the time I found myself inside a singularity, if I ever should do so, I would be able to do nothing.)
It should be noted here of course that the gravity (the space-time distortion) changes as you tunnel through something. Thus, if it were possible to, instead of collapsing into a singularity, tunnel through it without coming into contact with it at all, the gravity experienced would be near zero, just as it would if one were to reach the Earth's center by tunneling. (If you tunnelled through the center of the Earth, the gravity at the center would be close to zero since all the matter would surround you on all sides about equally.) Now you have an idea of what a wormhole is.
Possible or Not?
However it's probably not possible to tunnel through a singularity without coming into contact with it, and some black holes no doubt really are "points in space," small areas, where we do in fact see all that surrounds them, although the light from the surroundings may appear quite bent. For it to be possible to tunnel through a "black hole," the black hole probably must be without a singularity, and thus cannot be quite just a point in space.
As for wormholes, Ford and Roman have explained that wormholes must be created by "negative energy." Why is that? After all, these are just gravity fields, places where space-time is strongly warped.
Electricity and Gravity
Any electric charge, if the particles are moving, has a magnetic field associated with it: electrons, with a negative charges, attract protons or ions, with positive charges, and vice versa. Gravitational fields are associated with such magnetic fields. So yes, it's apparently possible to warp space-time with just an electric current.
If you touch a light bulb (I have a burn scar across my arm to prove this point), in some instances at least you stick to it and cannot readily escape, because of the electric charge difference between you and the bulb, which glues you to it with quite powerful force (to escape I had to smash the bulb). Not only a magnetic field but also a gravitational field can be created by, say, a simple solenoid (Chown, 1996, "Planes, Trains, and Wormholes"). Such a field can draw some matter into it. Some physicists thus believe that a unified approach is needed for understanding both gravity and electricity.
Gravity, Wormholes, and Negative Energy
Nevertheless, even if a wormhole can be created using something like an electric field or current without using negative energy, any "wormhole" that allows travel at a speed higher than that of light collapses because of gravity. Negative energy is needed to repel the gravity, to hold the wormhole open so that matter can continue through it and exit at the other end (otherwise the matter would "converge" inside the wormhole and not exit as the wormhole collapsed; gravity, say Ford and Roman, is a converging force).
To hold the wormhole open requires both negative and positive energy, the latter because, according to Ford and Roman, the energy balance must remain constant, that is the negative energy must necessarily be accompanied by a "positive energy counterpart." For faster-than-light travel, perhaps the enegy from billions of stars or more is needed, with the negative energy confined to an extremely thin band (the amount needed increases as the diameter of the wormhole's throat increases, say Ford and Roman); or else the wormholes must be microscopic ones, with the aperture somewhere around the Planck length; or else the wormholes must be open only for a very very short time.
The drawback, according to Ford and Roman, is that the greater the negative energy used to hold the wormhole open, the more positive energy that's also required, and for each pulse of negative energy, the pulse of positive energy must be still greater.
The Planck Length
The Planck length is a microscopic distance, quite a bit less than a quadrillionth of a centimeter, a hundred billionth of a billionth of the radius of a proton. It and other Planck scale constants (Planck mass, Planck time, Planck temperature) were relevant at the beginning of the universe. A light wave crosses the "Planck length" in the Planck time." These constants are used in describing electromagnetic radiation.
Quantum physicists argue that, if you try to measure a distance smaller than the Planck length, it is supposedly impossible. Once you get to such a small distance, there is apparently no distance at all in any ordinary sense, no left and right they say. If you try to move something a distance smaller than the Planck length, according to quantum physics, which views space as grainy and not a continuum, that something becomes lost forever. Supposedly also space and time become confused at this distance.
No one is building wormholes of any size in any hurry in any case, and wormholes are still mostly speculation.