In the infinite vastness of the universe, it is likely that many, if not
most, objects escape the realm of human notice completely. Theoretical
physicists, even Einstein, can only do so much in telling us what might and
what might not be out there. To put it simply, there are some things that
theory suggests should not exist, but which may in fact exist anyway. One
such object is the black hole’s theoretical opposite — the white hole.
What is a white hole?
The simplest way to visualize a white hole is basically as a black hole in
reverse.
White holes aren’t just black holes with a new paint job. In fact, despite
their name, they’re thought to look very similar to black holes. Almost
identical, even — a cosmic twin. A crew of astronauts approaching one would
hardly be able to tell the difference.
However, astronauts would have an immediate ‘tell’ for a white hole: there
wouldn’t be a massive gravity well threatening to suck them in upon passing
too close to one in the far reaches of space. That’s because black holes and
white holes can be thought of as functional opposites of each other.
While a black hole might suck in all nearby matter and crush it with enough
force to pull it apart at the atomic level, white holes don’t have any
‘pull’ to speak of.
So in what way is a white hole the ‘opposite’ of a black hole, then? It’s
all in the key characteristic of a black hole. Despite how black holes are
portrayed as sucking nearby matter into a cosmic abyss, their actual
defining characteristic is having a gravitational field so strong that no
matter or radiation, not even light, can escape. Still grim, just in a
different way.
In the case of a white hole, the reverse would be true — nothing could
enter. It’s a cosmic gate that nothing can pass — not light, not matter. In
contrast, a white hole would constantly emit matter and light, but while
material inside a white hole can leave, once it exits there’s no way back
in.
How does a white hole form?
As white holes are hypothesized to be closely related to black holes, there
are several theories as to how they might form.
The theoretical origins of white holes can be traced back to Russian
cosmologist Igor Novikov, in 1964. Novikov came up with the idea of white
holes as a sort of cosmic twin to black holes as part of a solution to
Einstein's field equations, building on the work of German physicist Karl
Schwarzchild, who described the spacetime geometry of empty space
surrounding any spherical mass.
Schwarzchild’s solutions to Einstein’s field equations included the
prediction that if a mass were compressed inside of a critical radius (now
called the Schwarzschild radius), then its gravity would become so strong
that not even light could escape — in other words, it would become a black
hole.
But Schwartzchild’s description also included the possibility of a
theoretical ‘twin’ for the black hole, as well as what we now call wormholes
— folds in spacetime that objects in space can theoretically pass through to
near-instantaneously cross great distances — in between the event horizons
of a black hole and a theoretical ‘negative’ version of the singularity.
In 1960, Mathematician Martin David Kruskal extended Schwarzchild’s work to
include a reflection of the black hole singularity, but it was Novikov who
developed this into the notion of a white hole.
Until more recently, physicists treated the possibility of white holes as a
mathematical exercise — they could be shown to be mathematically feasible,
but were seen as being impossible in “real life.” One reason for this was
that no one could come up with a mechanism for how they would actually form
— a black hole is formed when a star collapses, but the reverse of this — a
black hole erupting into a star, would seem to violate the laws of
entropy.
A different theory suggests that white holes aren’t twins of black holes,
but what happens to a black hole upon its death, albeit for a very very
brief moment.
However, the work of physicist Stephen Hawking demonstrated that black holes
can in fact emit thermal radiation (Hawking radiation) due to the steady
conversion of quantum vacuum fluctuations near the black hole into pairs of
particles and anti-particle. The positive particle escapes, while the
negative anti-particle falls in, causing the black hole to lose mass. Over
time, Hawking radiation reduces the mass and rotational energy of black
holes and could theoretically cause a black hole to evaporate.
This brings up a number of questions, however. One of which is that, if a
black hole can evaporate away, what then happens to the information that it
swallowed? According to general relativity, this information can't escape,
and according to quantum mechanics, it can't be deleted. The answer, for
some theoretical physicists, is that it disappears down a wormhole and
emerges from a white hole.
Some physicists suggest that once a black hole grows small enough, it could
transform into a white hole. This white hole would, Tardis-like, be
minuscule on the outside but the inside would contain much of the
information swallowed by the black hole, which would then emerge over time.
Still, others have suggested that the explosion of the Big Bang might in
fact have been the emergence of information from a white hole.
Do white holes exist?
Currently, there’s no evidence pointing to the existence of white holes in
the universe. As of now, the white hole is a purely theoretical concept.
The closest thing we’ve seen to a potential sighting of a white hole in
space came from a
paper published
in 2011. Scientists speculated that the known gamma-ray burst GRB 060614 may
be the remnants of a white hole.
Aside from that, everything we’ve seen written on white holes is purely
theoretical. Despite this, there’s hope by some in the scientific community
that the existence of white holes will be proven eventually. After all,
Einstein published his General Theory of Relativity in 1915, which predicted
the existence of black holes, but it was 1971 before the first black hole
was actually identified.
While many scientists view white holes as a purely mathematical exercise,
others are hopeful that we’ll be able to spot this markedly rare
astrological event eventually. Although, we may not recognize it when we do.
Stephen Hawking pointed out that white holes and black holes could behave in
an identical manner, making them virtually indistinguishable.
Much of the uncertainty with white holes comes from our current
understanding of astrophysics. White holes are, by nature, thought to be
incredibly unstable. There’s no way a cosmic event expelling that much
matter could sustain itself long enough to be caught in an astronomer’s
telescope.
Some speculate that when white holes begin to expel matter, once the
expelled matter collides with any matter in orbit, the system would
immediately collapse into a black hole, possibly creating an infinite loop
of white holes turning into black holes and vice versa.
White hole gravity
Much like how what goes on at the center of a black hole’s singularity
requires one to stretch their understanding of classical gravity, white
holes may also need to be looked at through a special theoretical lens in
order to be proven.
The closest thing to this we’ve seen is the idea of loop quantum gravity –
currently a far-out theory on the fringes of mainstream physics.
According to this theory, space-time — the fundamental concept of Einstein’s
groundbreaking work on relativity – is made up of a series of ‘loops’ at
their fundamental level tying everything together in a neverending network
of nodes. These loops tie together space, understood as blocks under this
theory, and could prevent dying stars from collapsing into points of
infinite density, instead recoiling and turning into white holes.
Should the approach of loop quantum gravity to white holes be demonstrated
as possible, then many of the supernovae astronomers have observed over the
years could turn out to be markers of a white hole’s formation and death,
much like some of the theories around GRB 060614.