Holes in space and time travel

How many ‘holes’ are there in the spacetime fabric of the universe? One, two, three, or more? If you answered three, you are correct. However, only one of the holes is real and the other two are imaginary – yet to be discovered. The hole most of us are familiar with is ‘black hole’. Some of us may have heard of ‘wormholes’ from science fiction stories or movies. But only a handful, mostly physicists, know that there is another hole called ‘white hole’.
Einstein’s general relativity, albeit complex, is rich in content and fascinating in its predictions. One of the predictions is the existence of black holes - literally unfathomable holes drilled in the superstructure of the universe. They are created when stars die catastrophically in a supernova explosion. Black holes are gravity’s last hurrah. If a star has a mass greater than three solar mass, there is no force in the universe that can prevent gravity from crushing it into zero volume and infinite density, known as singularity. Inside a black hole, the immense strength of gravity distorts the structure of space and time so severely that not even light can escape.
And time slows to a crawl at the event horizon of a black hole. Often referred to as an ‘exit door from our universe’, event horizon is a kind of invisible membrane that marks a point of no return. Once a star crosses the event horizon, its fate is sealed. It will go inexorably forward toward the singularity and vanish into oblivion, leaving behind black hole as relic of its past life. Black holes can, therefore, be thought of as a star’s one-way ticket to eternity.
Black holes are more than fine points of general relativity. Although they confound common sense, they are real, albeit oddballs, in the stellar ‘zoo’. Their presence has been observed by their effects on the orbits of other stars and on gas and dust near them. More of them are being discovered all the time.
In the 1930s, Einstein and Nathan Rosen discovered that just as general relativity equations predict that matter can disappear forever into a black hole, they can also predict that matter can do the opposite and emerge from a white hole – a hypothetical region in a parallel universe. Clearly, a white hole is the opposite of a black hole. While a black hole sucks matter into a single point with no escape route, a white hole expels matter from a single point with no entryway. There are suggestions that the so-called Big Bounce model of our universe, an interpretation that the Big Bang was the result of the collapse of a previous universe, is in fact the outcome of just such a phenomenon.
Thus, in principle, mass that disappears in a black hole may emerge somewhere else. If the somewhere else is in a distinct region in our universe, the connection is called a wormhole – a speculative topological tunnel with two ends or mouths connected by a throat. Consequently, wormholes could be a shortcut connecting two separate points in spacetime. If the somewhere else is in another universe, the wormhole is called the Einstein-Rosen Bridge (ERB).
Since wormhole is a consequence of general relativity, its mouths are black holes. However, a naturally occurring black hole, formed by the collapse of a dying star, does not itself create a wormhole. Instead, it is formed by two entangled black holes pulling each other apart.
Do wormholes really exist and, if so, could we travel through them? Because wormholes are a conduit through four-dimensional spacetime, British astrophysicist Stephen Hawking and others hypothesise that if we could travel through wormholes to another place in our universe, then we could travel back to another time, too.
But the paradoxes we encounter when we think about time travel are severe and seem to have no resolution. This is because time travel is fundamentally illogical and irrational. It violates causality, the notion that effects must follow their causes. To date, we have never seen a violation of causality. On the contrary, the universe seems remarkably rational.
Even if time travel becomes possible, detailed calculations reveal a major obstacle of travelling through a wormhole. The powerful gravity of a black hole would cause the wormhole to collapse as soon as it forms. As a result, to get from one side of a wormhole to another, we’d have to travel faster than the speed of light, which is not possible.
Wormholes and white holes are just ideas that are not subject to observational verification. After all, we could never get inside a black hole to see the white holes or wormholes. But maybe in our vast universe or in a parallel universe, there’s space for them. For now, all the holes are favourites of science fiction writers and Star Trek buffs. 

The writer is Professor of Physics at Fordham University, New York.

Holes in space and time travel

How many holes are there in the spacetime fabric of the universe? One, two, three, or more? If you answered three, you are correct. However, only one of the holes is real and the other two are imaginary yet to be discovered. The hole most of us are familiar with is black hole. Some of us may have heard of wormholes from science fiction stories or movies. But only a handful, mostly physicists, know that there is another hole called white hole. Einsteins general relativity, albeit complex, is rich in content and fascinating in its predictions. One of the predictions is the existence of black holes - literally unfathomable holes drilled in the superstructure of the universe. They are created when stars die catastrophically in a supernova explosion. Black holes are gravitys last hurrah. If a star has a mass greater than three solar mass, there is no force in the universe that can prevent gravity from crushing it into zero volume and infinite density, known as singularity. Inside a black hole, the immense strength of gravity distorts the structure of space and time so severely that not even light can escape. And time slows to a crawl at the event horizon of a black hole. Often referred to as an exit door from our universe, event horizon is a kind of invisible membrane that marks a point of no return. Once a star crosses the event horizon, its fate is sealed. It will go inexorably forward toward the singularity and vanish into oblivion, leaving behind black hole as relic of its past life. Black holes can, therefore, be thought of as a stars one-way ticket to eternity. Black holes are more than fine points of general relativity. Although they confound common sense, they are real, albeit oddballs, in the stellar zoo. Their presence has been observed by their effects on the orbits of other stars and on gas and dust near them. More of them are being discovered all the time. In the 1930s, Einstein and Nathan Rosen discovered that just as general relativity equations predict that matter can disappear forever into a black hole, they can also predict that matter can do the opposite and emerge from a white hole a hypothetical region in a parallel universe. Clearly, a white hole is the opposite of a black hole. While a black hole sucks matter into a single point with no escape route, a white hole expels matter from a single point with no entryway. There are suggestions that the so-called Big Bounce model of our universe, an interpretation that the Big Bang was the result of the collapse of a previous universe, is in fact the outcome of just such a phenomenon. Thus, in principle, mass that disappears in a black hole may emerge somewhere else. If the somewhere else is in a distinct region in our universe, the connection is called a wormhole a speculative topological tunnel with two ends or mouths connected by a throat. Consequently, wormholes could be a shortcut connecting two separate points in spacetime. If the somewhere else is in another universe, the wormhole is called the Einstein-Rosen Bridge (ERB). Since wormhole is a consequence of general relativity, its mouths are black holes. However, a naturally occurring black hole, formed by the collapse of a dying star, does not itself create a wormhole. Instead, it is formed by two entangled black holes pulling each other apart. Do wormholes really exist and, if so, could we travel through them? Because wormholes are a conduit through four-dimensional spacetime, British astrophysicist Stephen Hawking and others hypothesise that if we could travel through wormholes to another place in our universe, then we could travel back to another time, too. But the paradoxes we encounter when we think about time travel are severe and seem to have no resolution. This is because time travel is fundamentally illogical and irrational. It violates causality, the notion that effects must follow their causes. To date, we have never seen a violation of causality. On the contrary, the universe seems remarkably rational. Even if time travel becomes possible, detailed calculations reveal a major obstacle of travelling through a wormhole. The powerful gravity of a black hole would cause the wormhole to collapse as soon as it forms. As a result, to get from one side of a wormhole to another, wed have to travel faster than the speed of light, which is not possible. Wormholes and white holes are just ideas that are not subject to observational verification. After all, we could never get inside a black hole to see the white holes or wormholes. But maybe in our vast universe or in a parallel universe, theres space for them. For now, all the holes are favourites of science fiction writers and Star Trek buffs. The writer is Professor of Physics at Fordham University, New York.