A NASA space telescope recently recorded two black holes shining in ultra-bright X-ray light in the galaxy Caldwell 5, 7 million light-years from Earth. Scientists speculate that these are intermediate, rather than supermassive black holes.
Black holes start from massive stars which exert tremendous gravitational force. Gravity and black holes could help us travel in time. But the kind of time travel we could do ranges from a 50% reduction in rate to an acceleration of billions of years per second.
A star exists because gravity compresses a massive cloud of hydrogen gas and heats it up so much that the nuclear force overcomes the natural repulsion of protons, which are hydrogen nuclei, so that they fuse to form helium. This fusion releases energy, which is one of the reasons that stars shine and why stars become nuclear furnaces. But this release of energy also burns hydrogen and makes helium waste. When the helium waste builds up over billions of years, it causes the furnace to shut down. Then gravity reappears like a pouncing arch-demon and crushes the star into a white dwarf. The helium waste can still be burned, given sufficient heat, like the ash from a camp fire. This is the reason that the white dwarf continues to burn, converting the helium ash into lithium and other elements, until it burns down into iron. Iron nuclei cannot fuse and release energy, so that, in medium sized stars such as our sun, the collapsing star has run out of fuel, and the white dwarf turns black. However, for stars that are several times the mass of our sun, gravity continues its labors of concentration, causing the iron core to collapse and the outer layer of the dwarf to be released as a supernova. What’s left after the explosion is a dead star, a neutron star. However, in really massive stars—ten to fifty times the size of our sun, the force of gravity continues to squeezes the neutron star into a black hole.
In an article in Science Digest (September 1982) Isaac Asimov suggested a method of time travel related to black holes. He pointed out that, in the extra-large stars, the black hole has a radius of about eighteen miles. This radius forms a spherical surface known as an “event horizon.” But beyond the event horizon, the black hole continues to collapse until its mass is squeezed into an infinitely small point, known as a singularity. We know that black holes exert such a gravitation force that even light cannot escape. But time flows at a different rate at a black hole than in the rest of space. Assuming that a human traveler could resist the gravitational forces, the traveler would discover that, just before crossing the event horizon, time would accelerate to billions of years per second, until, after crossing the event horizon, the remaining life of the universe would have passed. Unfortunately, Asimov notes, this is a one-way trip.
Another method for time travel around a black hole, offered in the same edition of Science Digest, utilizes the “light cone” first developed by the German mathematician Hermann Minkowski to visualize space and time. In actuality the time cone looks more like an hourglass, with diagonal lines to represent the speed of light and the circular top and bottom signifying the time coordinates. Light, traveling along the vertical plane, comes up through the bottom half of the hourglass, which is the past, to emerge at the top, which is the future. Light would travel through space on a horizontal axis that splits the two halves of the cone (past and future). One of the two cones would also possess the same coordinates of space and time. One cannot go outside the limits of the cone, because one would be traveling faster than light, which contradicts the law of relativity.
How the cone is involved in time travel is derived from the fact that light can be bent by gravity. The light cone, nearing a massive object such as a black hole, would start to revolve around it. The light cone could then tip into negative time. A traveler could travel along any path within the light cone. A timid voyager that faithfully adhered to the space axis would end up at the beginning, while a more adventurous soul could choose a path below the space axis (but still within the light cone) and would journey down a helical road into the past. After enjoying an earlier age of the universe, the traveler could hop onto another helical conveyor to the present.
Another less dramatic method was suggested by Stephen Hawking (see http://bit.ly/wkDEdH). It is based on the relativistic principle that time slows and appears to stop for bodies approaching the speed of light. He postulates that, for travelers orbiting a black hole at a sufficient distance to avoid its powerful gravitation, time would slow down by half. Therefore five years spent by the travelers would translate to ten years on earth, and the passengers would return to a future Earth.
In his book, A Brief History of Time, Hawking echoes Asimov in observing that a spaceman falling through an event horizon would be confronted by a singularity and the end of time. Again, a one-way trip.
We on Earth are aware that time travel does not require a black hole. We leap forward and fall back an hour every year, in obeisance to the daylight savings commandments. And time certainly slows down when we microwave our dinner or build up our abdomens in a regimen that ostensibly lasts no more than fifteen minutes a day.