The closest star to Earth is Proxima Centauri. It is about 4.25 light-years
away, or about 25 trillion miles (40 trillion km). The fastest ever
spacecraft, the now-in-space Parker Solar Probe will reach a top speed of
450,000 mph. It would take just 20 seconds to go from Los Angeles to New
York City at that speed, but it would take the solar probe about 6,633 years
to reach Earth’s nearest neighboring solar system.

If humanity ever wants to travel easily between stars, people will need to
go faster than light. But so far, faster-than-light travel is possible only
in science fiction.

In Issac Asimov’s Foundation series, humanity can travel from planet to
planet, star to star or across the universe using jump drives. As a kid, I
read as many of those stories as I could get my hands on. I am now a
theoretical physicist and study nanotechnology, but I am still fascinated by
the ways humanity could one day travel in space.

Some characters – like the astronauts in the movies “Interstellar” and
“Thor” – use wormholes to travel between solar systems in seconds. Another
approach – familiar to “Star Trek” fans – is warp drive technology. Warp
drives are theoretically possible if still far-fetched technology. Two
recent papers made headlines in March when researchers claimed to have
overcome one of the many challenges that stand between the theory of warp
drives and reality.

But how do these theoretical warp drives really work? And will humans be
making the jump to warp speed anytime soon?

## Compression and expansion

Physicists’ current understanding of spacetime comes from Albert Einstein’s
theory of General Relativity. General Relativity states that space and time
are fused and that nothing can travel faster than the speed of light.
General relativity also describes how mass and energy warp spacetime – hefty
objects like stars and black holes curve spacetime around them. This
curvature is what you feel as gravity and why many spacefaring heroes worry
about “getting stuck in” or “falling into” a gravity well. Early science
fiction writers John Campbell and Asimov saw this warping as a way to skirt
the speed limit.

What if a starship could compress space in front of it while expanding
spacetime behind it? “Star Trek” took this idea and named it the warp drive.

In 1994, Miguel Alcubierre, a Mexican theoretical physicist, showed that
compressing spacetime in front of the spaceship while expanding it behind
was mathematically possible within the laws of General Relativity. So, what
does that mean? Imagine the distance between two points is 10 meters (33
feet). If you are standing at point A and can travel one meter per second,
it would take 10 seconds to get to point B. However, let’s say you could
somehow compress the space between you and point B so that the interval is
now just one meter. Then, moving through spacetime at your maximum speed of
one meter per second, you would be able to reach point B in about one
second. In theory, this approach does not contradict the laws of relativity
since you are not moving faster than light in the space around you.
Alcubierre showed that the warp drive from “Star Trek” was in fact
theoretically possible.

Proxima Centauri here we come, right? Unfortunately, Alcubierre’s method of
compressing spacetime had one problem: it requires negative energy or
negative mass.

## A negative energy problem

Alcubierre’s warp drive would work by creating a bubble of flat spacetime
around the spaceship and curving spacetime around that bubble to reduce
distances. The warp drive would require either negative mass – a theorized
type of matter – or a ring of negative energy density to work. Physicists
have never observed negative mass, so that leaves negative energy as the
only option.

To create negative energy, a warp drive would use a huge amount of mass to
create an imbalance between particles and antiparticles. For example, if an
electron and an antielectron appear near the warp drive, one of the
particles would get trapped by the mass and this results in an imbalance.
This imbalance results in negative energy density. Alcubierre’s warp drive
would use this negative energy to create the spacetime bubble.

But for a warp drive to generate enough negative energy, you would need a
lot of matter. Alcubierre estimated that a warp drive with a 100-meter
bubble would require the mass of the entire visible universe.

In 1999, physicist Chris Van Den Broeck showed that expanding the volume
inside the bubble but keeping the surface area constant would reduce the
energy requirements significantly, to just about the mass of the sun. A
significant improvement, but still far beyond all practical possibilities.

## A sci-fi future?

Two recent papers – one by Alexey Bobrick and Gianni Martire and another by
Erik Lentz – provide solutions that seem to bring warp drives closer to
reality.

Bobrick and Martire realized that by modifying spacetime within the bubble
in a certain way, they could remove the need to use negative energy. This
solution, though, does not produce a warp drive that can go faster than
light.

Independently, Lentz also proposed a solution that does not require negative
energy. He used a different geometric approach to solve the equations of
General Relativity, and by doing so, he found that a warp drive wouldn’t
need to use negative energy. Lentz’s solution would allow the bubble to
travel faster than the speed of light.

It is essential to point out that these exciting developments are
mathematical models. As a physicist, I won’t fully trust models until we
have experimental proof. Yet, the science of warp drives is coming into
view. As a science fiction fan, I welcome all this innovative thinking. In
the words of Captain Picard, things are only impossible until they are not.

Written by Mario Borunda, Associate Professor of Physics, Oklahoma State
University.

Originally published on The
Conversation.

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