A celebrated astrophysicist is intently studying
the skies in search of his elusive quarry, combing through the thousands of images coming
to him from the state-of-the-art International Event Horizon telescope. Finally, after months
and months of searching, he thinks he may have found what he’s been looking for all this time
- in the images he sees the telltale signs of a mysterious phenomenon called a black hole. But
as he scrutinizes the images captured by the powerful telescope, something doesn’t seem quite
right. There, right in front of his very eyes, the black hole appears to be … burping!? The
scientist knows that this should be impossible: nothing can escape from a black hole, not even
light - that’s why they’re so hard to find - but here is photographic evidence of matter coming
out of a black hole! Could it be that this is not a black hole at all, but the black hole’s
neglected twin - a white hole? Could this be his chance to once and for all answer the questions
that have been nagging at him throughout his whole career - What is a white hole? How do they form?
How do they work!? And, do they even exist at all? In 1915, Einstein’s field equations turned
the world of physics on its head. His theory of relativity described the force of gravity
and shattered the prevailing paradigm of the nature of reality - rather than being rigid,
space and time can actually bend and fold, along with the mass of stars and planets.
Within a year, scientists had calculated how space-time curves around a single ball of mass -
the seeds of what today is called the singularity. Physicists were able to describe how a spherical
mass shrunken down to infinitely dense point could wrap space around it so tightly
that a region of space is effectively pinched off from rest of universe, creating
a no-mans land beyond the event horizon where the laws of physics no longer apply and the
link between cause and effect is shattered. A black hole is an incredibly dense
area of space where all matter has been squeezed into an impossibly tiny space,
called the singularity. This creates such an intense gravitational pull that nothing, not even
light, can escape from the black hole’s clutches. A tiny black hole might be the size of a single
atom, but have a mass equal to a large mountain. Stellar black holes, formed when a dying
star collapses in on itself in a supernova, can have a mass up to 20 times greater than
our sun. The largest black holes are called supermassive black holes, and they can be found
at the center of every galaxy in our universe. The supermassive black hole at the center of our Milky
Way galaxy, named Sagittarius A*, has as much mass as 4 million of our suns, all condensed into a
tiny ball only as big as a few million Earths. A black hole’s event horizon is what we would
consider the surface of the black hole, although it’s not a surface in the true sense of the word
- it’s not a membrane or barrier, but rather, the threshold beyond which there is no going back. The
event horizon is the point of no return - nothing that crosses the event horizon can ever come back.
Even light cannot escape the black hole once it’s passed the event horizon. Once something -
or someone - has crossed the event horizon, they will begin the inevitable process of
falling towards the black hole’s singularity, eventually dissolving into the singularity
itself. We can only guess what happens after that. Physicists have been studying black
holes for decades and are only just beginning to understand them. Only recently have they turned their attention to the black
hole’s neglected twin - the white hole. From afar, a white hole would appear
identical to its better-known cousin, a black hole. Like a black hole, a
white hole might be big or small, might spin or remain stationary, and might be
electrically charged. A white hole would also be surrounded by a ring of dust, and a cloud of
gas and debris would gather at its event horizon. The key difference between a black hole
and a white hole is that white holes burp. Yes, burp. Unlike a black hole,
from which nothing can escape, matter actually can cross the event horizon and
come out of a white hole. It’s only in these moments, when objects emerge from the white
hole, that scientists can definitively say that what they are looking at is a
white hole, and not a black hole. If a black hole’s event horizon is the point of
no return, then the event horizon of a white hole could be described as the point of no admission
- nothing can ever cross the event horizon of a white hole and reach the interior. In a black
hole, objects in the space outside can cross the event horizon and affect the interior of the black
hole, but matter inside the black hole can never again interact with space outside. In a white
hole, the reverse holds true - objects from inside the white hole can cross the event horizon and
interact with objects in the space outside of it, but nothing on the outside can ever enter
the white hole or affect the inside. This is because a white hole is a black
hole’s time reversal, according to physicists. A black hole’s singularity exists in the future,
whereas a white hole’s singularity exists in the past. Since the interior of the white hole is
cut off from the universe’s past via its event horizon, no outside object or event will ever
affect the inside of a white hole. James Bardeen, a black hole pioneer and professor
emeritus at the University of Washington explains the magnitude of this difference:
“Somehow it’s more disturbing to have a singularity in the past than can affect
everything in the outside world”, he says. Scientists had theorized about the existence
of black holes for hundreds of years before Einstein’s theory of relativity paved
the way for physicists to prove their existence - theoretically, at least. Since
no light escapes from a black hole, they are invisible to the naked eye. Until very recently,
the only way scientists have been able to find evidence of black holes has been to look for
signs of their impact on the surrounding universe. Stars, gasses and other space objects behave
differently near a black hole than they do elsewhere in the universe as the black hole’s
intense gravity pulls on them. Using telescopes equipped with special tools, scientists can
pick up a type of high-energy light emitted by objects that interact with a black hole’s
gravitational forces, and reverberation mapping can measure the radiation given off by the ring
of debris that surrounds the black hole, helping physicists pinpoint the location of a black hole,
even if they can’t see the black hole itself. Finally, in 2019, scientists made a stunning
breakthrough in the study of black holes when the International Event Horizon telescope
captured the world’s first image of a black hole. Technically, what they captured was the
black hole’s shadow, since the absence of light reflecting from a black hole makes the black
hole itself impossible to see, but nevertheless, this was the world’s first solid, photographic
proof of the existence of black holes. If black holes have finally been proven to be
real, does that mean that white holes are a proven fact of the universe, too? Well, not exactly.
While Einstein’s theory of general relativity does describe the existence of both black and
white holes, it doesn’t explain how a white hole might actually form in space. A black hole
forms when a dying star implodes in a supernova, collapsing all of the star’s matter into an
impossibly tiny area cordoned off from the rest of space. The reverse doesn’t quite make
sense - the idea of a white hole exploding into a fully-functioning star would be a bit like
unscrambling an egg: it just wouldn’t work. This idea also violates the statistical
law that entropy must increase over time. Furthermore, if a white hole did form,
the matter it releases when it “burps” would collide with the matter in orbit
around the white hole. These collisions would cause the entire system to collapse into
a black hole. Perhaps if white holes do exist, they don’t remain as a white hole for long. Hal
Haggard, a theoretical physicist at Bard College in New York, has said that “a long-lived
white hole, I think, is very unlikely.” Other scientists have different
theories about white holes that help explain some of the inconsistencies.
Steven Hawking discovered back in the 1970s that black holes leak energy, which
led him to wonder - how do black holes die? And what happens to everything that’s been
trapped inside of a black hole when it dies? The theory of general relativity holds that
nothing can get out of a black hole, but quantum mechanics prevents any information inside a black
hole from being deleted. So where does it go? Some have taken this to mean that a white hole is
actually the result of the death of a black hole. As a black hole dies, it may become so
small - as small as one microgram in size, about the mass of a human hair - that it would no
longer obey the laws of physics as we know them. This infinitesimally tiny object would
be so small that it would defy gravity, but inside it would hide a cavernous
interior full of everything it swallowed in its previous life as a black hole.
It’s small size and gravity-defying behaviour could allow it to remain stable
enough to eventually spit out information and matter that had been swallowed by the black
hole, becoming a “burping” white hole instead. If this theory holds true, the universe could
one day come to be dominated by white holes. After all of the stars in the universe have burnt
out and imploded into black holes, and then after all of those black holes themselves have all died,
the universe might be nothing but a sea of burping white holes. Thankfully, this could only happen
in a universe countless trillions of times older than our universe currently is, so it’s not a
scenario we need to worry about any time soon. There are many more questions than answers when
it comes to white holes, and that leaves room for plenty of imaginative theories about what a white
hole actually is. Some scientists actually think that we are currently living inside the ultimate
white hole. To these black hole physicists, the behavior of a white hole looks suspiciously
similar to a little thing we call the Big Bang. The explosion of matter and energy resulting
from the Big Bang that created our universe is remarkably similar to the way theorists
suspect that white holes release matter. “The geometry is very similar in the two cases,"
Hal Haggard, the physicist from Bard College, has said. "Even to the point of being mathematically
identical at times." This theory has attracted plenty of criticism, but Haggard intends
to follow this rabbit hole to the very end, saying “Why wouldn’t you investigate whether white
holes have interesting consequences? It may be that those consequences aren’t what you expected,
but it would be foolhardy to ignore them.” We may still be a long way off from being able
to look into a telescope and watch with our own eyes as a white hole burps out matter into the
surrounding universe. Although we’ve only just gotten our first glimpse of a black hole - and
though we have yet to even lay eyes on a white hole - scientists will undoubtedly discover more
about these mysterious phenomena in the future. If the past has taught us anything, it’s
that just because we can’t see something doesn’t mean it isn’t out there. Only time
will tell which theory about white holes will prove to be correct - or if we had it
completely wrong all along. One day we may get an answer to the question “What is a
white hole?” but until then, it remains yet another of the countless as-yet-unsolved
mysteries of our vast and unknowable universe. If you thought this video was fascinating,
you’ll definitely want to check out “What Would Happen to Your Body in Space?”, or,
you might like this other interesting one!
