Einstein's field equations hit physics like a tsunami in 1915, and theorists are still sorting through the wreckage. Beyond describing the force of gravity, his hypotheses also brought a paradigm-shattering message about the nature of reality. More than a rigid backdrop, space and time bend and fold along with the mass of stars and planets. That insight sparked a race to calculate just how much abuse space could take from the matter that drifts through it.
Within a year, physicist and astronomer Karl Schwarzschild found the first exact solution to Einstein's equations, calculating how space-time curves around a single ball of mass. In his answer lay the seeds of what physicists today call a singularity — a spherical mass shrunken down to an infinitely dense point, wrapping space around it so tightly that the region cuts itself away from the rest of the universe. An event horizon marks this fracture beyond which physics begins to behave weird.
Black holes, the most famous singularities, are regions of space so warped that no exits exist. The outside universe can influence the inside of a black hole's horizon, but the interior can't affect the exterior. That is it swallows everything and anything around it.
But most probably you knew about black hole. Yet we just can’t seem to get enough of that. We seem to give black holes a lot of attention. Today we are going to give attention to their bizarre twins, White Holes.
When mathematician Martin David Kruskal extended Schwarzschild’s black hole description in 1960 to cover all domains of space and time, his new picture contained a reflection of the black hole singularity, although he didn't realize its significance at the time. Later, as black holes entered the vernacular, a natural term emerged for their theoretical twins, the white hole.
Let us get a rough idea of what it actually is. Consider a simple kitchen experiment. When you open the water tap water flows directly down to the mouth of drain. Put a drop of food color in the mouth. Does any of this colors flow out of the mouth? No, right? This can be considered as a black hole. That is any information inside the event horizon cannot travel out of it. Now consider the same water falling on a surface. It creates a circle of water on the surface around the point where water from tap hits the surface. Put a drop of color inside this circle it gets spewed out the circle quickly. But put a drop of color outside the circle it can’t travel inside to the circle. This essentially can be thought as a white hole.
Let’s start again with the older sibling. There are two competing theories in physics to explain black holes. One is general relativity where the mass of black hole bends spacetime so much that it becomes one single point of infinite density. One the other hand according to quantum mechanics there can’t be an infinitely small point. General relativity is best description of gravity where quantum mechanics is called the most successful theory ever. But some scientists believe white holes can bring these theories together and explain what happens inside a black hole. The equation of relativity supports the existence of white holes. All you have to do is set the mass of singularity to zero and you will get your mind blown. It turns out to be a point of infinite density that throws out matter. But how can a massless singularity spew out matter. The answer lies in quantum mechanics. With some changes in general relativity by quantum mechanics the inside of a black hole can be turned to a white hole. It can be explained as the curved space time of black hole is bouncing again to form a white hole. While general relativity won’t let information out of the black hole quantum mechanics won’t let this information to get deleted. It pretty much sounds like a wormhole but instead of information travelling through space (time flows but the space remains same) information here travels through time (time remains constant). This again leads to an alternate theory that this information may give rise to another universe and thus big bang was nothing but a giant white hole.
A body once past the event horizon of a black hole travels very slowly towards it future according to an observer who is outside the event horizon. But the body would be travelling way faster. For a massive black hole these times may differ by a quadrillion times that of the age of universe. But for the small black holes like those that were formed after big bang due to chaos may have converted by now. But as their size would be too smaller than wavelength of light they are invisible to our eyes, thus pretending to be the “dark” matter. And this could be another theory supporting the existence of white holes.
In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation (not compulsory). Because a thermal-equilibrium state is time-reversal-invariant, Stephen Hawking argued that the time reverse of a black hole in thermal equilibrium is a white hole in thermal equilibrium. This may imply that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white-hole emission. Supermassive black holes are found at the center of every galaxy and a galaxy cannot be formed without one. Hawking suggests that this supermassive black hole again spawn a supermassive white hole.
Again, let’s understand through a simple thought experiment. A free-falling body will continue to fall freely until it hits a surface. And we know that a singularity has no surface. So, what happens to a trajectory of a body falling into a black hole? In order to satisfy this requirement, it turns out that in addition to the black hole interior region which particles enter when they fall through the event horizon from the outside, there must be a separate white hole interior region, which allows us to extrapolate the trajectories of particles which an outside observer sees rising up away from the event horizon. For an observer outside the event horizon, infalling particles take an infinite time to reach the black hole horizon infinitely far in the future, while outgoing particles which pass the observer have been travelling outward for an infinite time since crossing the white hole horizon infinitely far in the past (however, the particles or other objects experience only a finite proper time between crossing the horizon and passing the outside observer). The black hole/white hole appears "eternal" from the perspective of an outside observer, in the sense that particles travelling outward from the white hole interior region can pass the observer at any time, and particles travelling inward which will eventually reach the black hole interior region can also pass the observer at any time.
Well much about the theory! But why can’t we see them in universe? Well, here comes our old friend, thermodynamics. According to second law of thermodynamics the entropy of the universe always increases through a process. In simple terms things have to be random to be stable. But white hole essentially is the time reversal of a black hole; that is somehow a black hole is converted into a star and thus the randomness decreases which according to thermodynamics shouldn’t happen. But again, General Theory of Relativity in itself never took care of classical thermodynamics and things just turned fine for black hole. But yes, this suggests that if white holes exist they will be quite unstable. In fact, on June 14, 2006 a peculiar phenomenon was observed in the night sky. Gamma ray bursts or GRBs are produced due to violent collision of large masses like neutron stars. On June 14 such a GRB was observed namely GRB 060614. Prior to this detection, it was believed that a long gamma-ray burst, like GRB 060614, was probably caused by gravitational collapse of a large star into a black hole, and would be accompanied by detectable supernova, whilst short gamma-ray bursts were thought to be the merger of two neutron stars. However, the lack of any supernova and the vanishing spectral lags during GRB 060614 are typical of short GRBs, at odds with the long (102s) duration of this event and its origin in a galaxy 1.6 billion light years away in the constellation Indus. In December 2006, an article on the burst was published in the journal Nature, with the editors describing a hunt by scientists to define a new GRB classification system to account for this burst. GRB 060614 was subsequently classified as a "hybrid gamma-ray burst", defined as a long burst without accompanying supernova, and was hypothesized to have been an observation of a white hole.
While we lack conclusive data about existence of white holes we still can’t deny their existence. In fact, black holes, its sibling came only to existence in human knowledge after 40 years of laying its mathematics down. And for white hole we most probably don’t even have the correct math. Nevertheless, the Nature has never stopped amusing humanity and maybe this is the she does it.
Surya Prakash Mishra, IIT (ISM), Dhanbad
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