A wormhole is a special solution to the equations describing Einstein's
theory of general relativity that connects two distant points in space or
time via a tunnel. Ideally, the length of this tunnel is shorter than the
distance between those two points, making the wormhole a kind of shortcut.
Though they are a staple of science fiction and have captured the popular
imagination, wormholes are, as far as we know, only hypothetical. They are
legitimate solutions to general relativity, but scientists have never
figured out a way to maintain a stable wormhole in the real universe.

### Who discovered wormholes?

The simplest possible wormhole solution was discovered by Albert Einstein
and Nathan Rosen in 1935, which is why wormholes are sometimes called
"Einstein-Rosen bridges." Einstein and Rosen started with the mathematical
solution of a black hole, which consists of a singularity (a point of
infinite density) and an event horizon (a region surrounding that
singularity beyond which nothing can escape). According to The Physics of
the Universe, they found that they could extend this solution to include the
polar opposite of black holes: white holes.

These hypothetical white holes also contain a singularity, but they operate
in reverse to a black hole: Nothing can enter the event horizon of a white
hole, and any material inside the white hole gets ejected immediately.

Einstein and Rosen found that, theoretically, every black hole is paired
with a white hole. Because the two holes would exist in separate places in
space, a tunnel — a wormhole — would bridge the two ends.

### What makes a wormhole traversable?

However, a wormhole created from a pair of black and white holes wouldn't be
very useful. For one, white holes would be unstable. If you were to drop a
particle toward the event horizon of a white hole, the particle would never
reach the event horizon, because nothing can enter a white hole. So the
energy of the system would continue to increase to infinity, eventually
blowing up the white hole, according to the University of Colorado physicist
Andrew Hamilton.

Second, even if white holes could exist, the only way to enter this kind of
wormhole would be to cross the event horizon of the black hole on the other
side. But once an object crossed the event horizon, it could never leave. So
objects could enter the wormhole but never escape.

Lastly, the wormholes themselves would be unstable. A single photon, or
particle of light, passing through the wormhole tunnel would introduce so
much energy to the system that the tunnel would snap apart, destroying the
wormhole, according to the European Southern Observatory.

In the 1970s, however, physicists worked out the math needed to make a
stable, or "traversable," wormhole, according to University of California,
Santa Barbara physicist Diandian Wang. The trick is to move the entrance of
the wormhole tunnel beyond the event horizon of the black hole and to
stabilize the tunnel itself so that matter passing through doesn't cause
immediate catastrophic collapse.

The key ingredient for stabilizing wormholes is so-called exotic matter, or
some form of matter that has negative mass. Unfortunately for such
wormholes, scientists have never found evidence for negative mass, and it
would violate conservation of momentum, which states that the momentum
should remain constant if no force is applied; a negative-mass object placed
next to a positive-mass object would immediately accelerate, with no source
of energy.

### What do wormholes look like?

If such a wormhole did exist, it would look very strange. The entrance would
be a sphere, like the surface of a planet. If you looked into it, you would
see light coming in from the other side. The wormhole tunnel could be any
length, and while traveling down the tunnel, you would see distorted views
of the region of the universe you came from and the region you were
traveling to.

### Wormholes and time travel

In theory, a wormhole could also act as a time machine. Special relativity
dictates that moving clocks run slowly. In other words, someone racing
around at nearly the speed of light would not advance into their own future
as quickly as someone standing still.

If scientists could somehow construct a wormhole, initially the two ends
would be synchronized in time. But if one end were then accelerated to
nearly the speed of light, that end would start to lag behind the other end.
The two entrances could then be brought together, but then one of the
entrances would be in the past of the other, according to MIT physicist
Andrew Friedman.

To travel back in time, you'd simply walk through one end. When you exited
the wormhole, you would be in your own past.

### How do wormholes form?

There is currently no known way to construct a wormhole, and wormholes are
purely hypothetical. Although exotic matter is unlikely to exist, there may
be another way to stabilize wormholes: negative energy.

The vacuum of space-time is filled with quantum fields, the fundamental
quantum building blocks that give rise to the forces and particles that we
experience, and these quantum fields have an intrinsic amount of energy.
It's possible to construct scenarios in which the quantum energy in a
particular region is lower than its surroundings, making that energy
negative at a local level. Such negative energy exists in the real world in
the form of the Casimir effect, in which the negative quantum energies
between two parallel metal plates cause the plates to attract, according to
University of California, Riverside mathematician John Baez.

But no one knows if this negative quantum energy can be used to stabilize a
wormhole. It may not even be the "right" kind of negative energy, since it's
only negative relative to its surroundings, not in an absolute way.

Wormholes might occur naturally at microscopic scales in the quantum foam,
the roiling nature of space-time at the very tiniest of scales due to those
same quantum energies. In that case, wormholes might be popping in and out
of existence constantly. But again, it's not clear how to "scale up" those
wormholes to sizes big enough for you to walk through, and keep them stable.

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