Many of us
are familiar with black holes. We know that when a star with a mass more than
three times that of our Sun dies, it keeps collapsing under its own
gravitational pull and eventually turns into a black hole. Professor Jamal
Nazrul Islam defined a black hole in his famous book Black Hole in the
following way:
“If the
mass of a star exceeds three times the mass of the Sun, the Fermi pressure of
neutrons and other outward pressures can no longer withstand the inward pull of
gravity. As a result, the star begins to collapse and ultimately reaches an
extremely dense state within a very small volume. According to Einstein’s
theory of gravitation, when the matter of a star is confined within a certain
small volume, it becomes cut off from the outside world, because no light ray,
electromagnetic wave, or material particle can escape from the star. Gravity
pulls everything inward. This state of the star is called a black hole” [1].
Numerous
scientists have studied and continue to study the mystery of black holes.
However, among those who popularized this mystery, the name that comes first is
Stephen Hawking. In his popular books, he humorously described in simple
language how one could throw a television set, a diamond ring, or even one’s
worst enemies into a black hole forever [2].
Nothing—not
even light—can escape from a black hole. Therefore, there is no way to observe
what is happening inside it. The last boundary that can be observed at the
entrance of a black hole is called the event horizon. In 1974, Stephen Hawking
discovered that some radiation is emitted from the event horizon of a black
hole, which later became known as Hawking radiation. But just beyond the event
horizon lies the point of no return. After that, there is no way for anything
to come back. Beyond the event horizon there is only the dark abyss of the
black hole.
But could
the opposite also happen? Is there something that is the reverse of a black
hole—something from which only emission occurs and into which nothing can
enter? Theoretical physicists have been discussing such a surprising
possibility for several years. Opposite to the black hole, it has been named
the white hole. One of the main reasons for calling it a black hole is that
even light cannot escape from it; everything is pitch dark. The name white hole
may therefore seem to be chosen as the opposite of black hole. However, the
surprising fact is this: the term “white hole” was used even before “black
hole.” We will come to that history a little later.
Among
those who are advancing the theoretical possibility of white holes, the Italian
physicist Carlo Rovelli is currently at the forefront. His popular book Buchi
Bianchi, written in Italian in a way accessible to general readers, was
published in English translation as White Holes in 2023 [3]. (The
renowned translator and science writer Abul Basar has translated the book into
Bengali.) In this book, Carlo Rovelli explains in simple language how white
holes could exist as the opposite of black holes. Everything falls into a black
hole; nothing can enter a white hole. Nothing can escape from a black hole;
everything emerges from a white hole. The direction of the flow of time in a
white hole is opposite to that in a black hole. This is precisely where the
reality of white holes becomes questionable. Carlo Rovelli explains how the
laws of physics can, in principle, support this reverse flow of time.
In films
and dramas, we often see present events shown in reverse order, moving backward
into the past. In science fiction, we also see time travel—where changing an
event in the past alters the future. We watch such stories and sigh: if only
this were possible in reality! In reality, it is not possible, because time
cannot be reversed. A recorded event can be played backward on a camera,
allowing us to watch it from end to beginning, but we cannot make what has
already happened “unhappen.” This is our great frustration. The fact that the
arrow of time points only forward is something we all understand intuitively,
without any scientific explanation. But this seemingly simple idea of time was
radically transformed in 1905 by a junior clerk at the German Patent
Office—Albert Einstein.
From
Einstein’s Special Theory of Relativity, we learn that there is no such thing
as “absolute time.” Time is relative; it changes with the motion of the
observer. From this emerged the concept of spacetime. Ten years after the
Special Theory of Relativity, when Einstein published his General Theory of
Relativity, the physics of the entire universe changed. Newton’s centuries-old
laws of motion and gravitation became obsolete overnight. The universe had to
be recalculated anew. Time was no longer just time; it became inseparably
linked with space. Calculations of gravity and cosmic motion began to be based
on spacetime.
The
central equation of Einstein’s General Theory of Relativity is the Einstein
field equation, which can be written succinctly as:
Here, G_μν
is the Einstein tensor, representing the curvature of spacetime due to gravity.
Λ is the cosmological constant,
which denotes the energy density of empty space. g_μν
is the metric tensor, describing the geometry of spacetime curvature. T_μν
is the energy–momentum tensor, representing the distribution of matter and
energy in spacetime. G is the gravitational constant, and c is the speed of
light in vacuum. The core idea of the General Theory of Relativity is that the
curvature of spacetime is proportional to the total energy and momentum of
matter. The greater the energy and momentum of matter, the greater the
curvature of spacetime.
From
Einstein’s theory emerged the theory of black holes. Not only black holes, but
white holes also arise from the General Theory of Relativity. White holes are
often described as the neglected twin sister of black holes—whose possibility
scientists long ignored.
Only a few
weeks after Einstein published his field equations at the end of 1915, in early
1916 a German soldier and lieutenant, Karl Schwarzschild, found an exact
solution to those equations. For a non-rotating spherical body, he calculated
the dimensions of spacetime and showed that as spacetime curvature increases,
the radius of the event horizon decreases. If the radius of the event horizon
shrinks beyond a certain limit, nothing can escape from it. This radius is
known as the Schwarzschild radius.
We can
derive the Schwarzschild radius in a simple way using the escape velocity
equation. If an object attempts to escape the gravitational pull of a planet
with mass M and radius r, the minimum required velocity (vₑ) is:
According
to Einstein’s Special Theory of Relativity, no object can exceed the speed of
light. If we set the escape velocity equal to the speed of light, vₑ = c, then the equation becomes:
From this,
we obtain:
r = 2GM /
c²
This is
the Schwarzschild radius.
Einstein
himself found it hard to accept that stars in the universe could collapse to
such an extent that even light could no longer escape. He rejected the physical
reality of this consequence of his own theory, appealing to common sense. At
that time, the term “black hole” had not yet been coined.
When
Einstein realized that his field equations implied the existence of
singularities in the universe, he began searching for alternative explanations.
In 1935, together with his colleague Nathan Rosen, he published a new theory
showing that before spacetime reaches a singularity, it might connect to
another spacetime through an extremely narrow bridge. Their paper, titled “The
Particle Problem in the General Theory of Relativity,” was published in Physical
Review in 1935 [4]. This concept later became famous as the Einstein–Rosen
bridge. Essentially, it connects two spacetimes—but how?
Einstein
formulated the theory in such a way that although no singularity forms, the
bridge collapses mathematically as soon as anyone attempts to pass from one
spacetime to another. Two years after Einstein’s death, in 1957, the American
physicist John Wheeler, while discussing such possibilities inside collapsed
stars (the term “black hole” was not yet in use), compared the connection of
two spacetimes by a bridge to a worm-eaten tunnel inside an apple. Thus, the
term “wormhole” was born, which we may translate into Bengali as shweto-gahbar
or shweto-bibar. However, such holes do not allow travel from the
present to the past.
The
instability of wormholes and their proximity to singularities make their
observation impossible. Nevertheless, the Einstein–Rosen bridge stimulated
interest in exploring what other possibilities might exist. In 1960, the
American mathematical physicist Martin David Kruskal and the Austrian
mathematician George Szekeres independently discovered a new coordinate system
that allows two spacetimes to be connected beyond the event horizon. This is
essentially another form of the Einstein–Rosen bridge, in which two separate
universes are connected by a bridge. In this construction, it was found that on
one side everything falls in and nothing comes out, while on the other side
everything comes out but nothing can fall in. Clearly, the direction of time
changes along the bridge.
In 1960,
the British physicist Roger Penrose began drawing diagrams representing various
possible causal structures of spacetime, now known as Penrose diagrams. He
showed that from the Schwarzschild radius, two types of event horizons are
possible: one that traps signals from the future, allowing nothing to escape
(corresponding to real time), and another that allows nothing to enter but let’s
everything emerge. This latter case appears unreal—how can something spew
everything out without ever taking anything in?
The Soviet
physicist Igor Novikov also spent many years studying all possible implications
of Einstein’s field equations. Using the Kruskal–Szekeres coordinate system, he
found that a wormhole mathematically connects two worlds moving in opposite
directions. From one direction everything falls in; from the other direction
everything emerges. The world from which everything emerges, he named a white
hole. This was in 1964. At that time, the term “black hole” had still not been
established; it was introduced later, in 1967, by John Wheeler.
Although
the name white hole came earlier, white holes have not received the same level
of attention as black holes. There are several reasons for this. The main issue
is time. While mathematically time can run backward, accepting this possibility
seems unnatural.
The fate
of the universe is essentially the fate of time; therefore, the history of the
universe is the history of time. In his famous book A Brief History of Time,
Stephen Hawking discussed the direction of time in detail. In the real world,
time has several arrows: the thermodynamic arrow, the cosmological arrow, and
the psychological arrow. Among these, only psychological time can move freely
between past and present—we can recall the past from the present. But we cannot
see the future.
The
relationship between time and thermodynamics is extremely strong and
straightforward. Entropy increases with time. As time progresses, entropy
inevitably increases; it is impossible to reduce it. In a white hole, time
flows from the present toward the past. If this were real, entropy would have
to decrease—but that is impossible. This is why the reality of white holes is
questioned.
The
cosmological arrow of time also points forward—the universe is expanding. If
white holes were globally real, then under the pull of backward-flowing time
the universe should contract, eventually collapsing in on itself. But within
the framework of established physics, this is impossible.
Naturally,
no white hole has ever been observed, nor does anyone seriously expect to
observe one. However, theoretically, conditions similar to a white hole may
arise inside a black hole itself. Physicist Carlo Rovelli firmly believes in
the theory of white holes. He believes that white-hole-like conditions can
indeed arise in the universe.
Let us
consider Hawking radiation. If a black hole were to evaporate completely
through Hawking radiation at some point in time, what would happen then? What
would become of the energy and information contained within it? According to
General Relativity, no information can escape from a black hole. On the other
hand, according to quantum mechanics, information can never be destroyed. So,
what happens in that case? Carlo Rovelli, along with several other scientists,
proposes that just as a black hole is formed through the death of a star, the
death of a black hole may give rise to a white hole. A connection between a
black hole and a white hole could exist through a wormhole, allowing all the
information stored in the black hole to emerge from the white hole.
This idea
is undoubtedly fascinating. But without a well-defined and widely accepted
scientific theory behind it, why should we accept it?
Carlo
Rovelli and many other scientists are currently working to establish the theory
of loop quantum gravity. If this theory is successfully established, the path
toward white holes may become significantly clearer. According to General
Relativity, gravity depends on the curvature of spacetime, and gravitational
interaction is treated as a continuous classical force. However, inside a black
hole, when spacetime is compressed to extremely small scales, spacetime itself
becomes discontinuous. In such conditions, quantum physics appears to be more
relevant than classical physics. The primary goal of loop quantum gravity is to
establish a consistent relationship between General Relativity and quantum
mechanics.
To achieve
this, spacetime must be represented as discrete, extremely small quantum units
governed by the laws of quantum mechanics. The Planck length (~10⁻³⁵ meters) is the smallest physically
meaningful length. A “loop” represents a quantum state of the gravitational
field. These loops form spin networks, which describe spacetime at the quantum
level. In loop quantum gravity, spacetime does not require a pre-existing
background; since spacetime is not continuous, it can emerge from quantum
states themselves. The unification of space and time occurs through spin
networks or spin foams. Spin foam represents the quantum dynamics of spacetime.
For a reliable description of spacetime at the Planck scale, John Wheeler introduced
the concept of “quantum foam.” Spin foam is mathematically analogous to quantum
foam.
If loop
quantum gravity is validated, the one-way fate of black holes—the
singularity—may be avoided, and the argument for the transformation of a black
hole into a white hole would become physically plausible. However, mathematical
consistency alone does not guarantee that white holes will be observed. For
that, we may have to wait many more years.
References
[1] Jamal
Nazrul Islam, Krishnobibor (Black Hole), Bangla Academy, Dhaka, 1985, p.
32.
[2] Stephen Hawking, Black Holes: The BBC Reith Lectures, Bantam Books,
London, 2016, p. 17. (Bengali translation by Abul Basar.)
[3] Carlo Rovelli, White Holes (translated from Italian into English by
Simon Carnell), Riverhead Books, New York, 2023. (Bengali translation by Abul
Basar.)
[4] Albert Einstein and Nathan Rosen, Physical Review, 1935, Vol. 48,
No. 1, pp. 73–77.
