There’s a Theory Beyond Relativity That Would Allow You to Fly Through a Wormhole

By Matt Williams

Wormholes are a popular feature in science fiction, the means through which spacecraft can achieve faster-than-light (FTL) travel and instantaneously move from one point in spacetime to another.

And while the General Theory of Relativity forbids the existence of “traversable wormholes”, recent research has shown that they are actually possible within the domain of quantum physics.

The only downsides are that they would actually take longer to traverse than normal space and/or likely be microscopic.

In a new study performed by a pair of Ivy League scientists, the existence of physics beyond the Standard Model could mean that there are wormholes out there that are not only large enough to be traversable, but entirely safe for human travelers looking to get from point A to point B.

The study, titled “Humanly traversable wormholes,” was conducted by Juan Maldacena (the Carl P. Feinberg Professor of theoretical physics from the Institute of Advanced Study) and Alexey Milekhin, a graduate of astrophysics student at Princeton University. The pair have written extensively on the subject of wormholes in the past and how they could be a means for traveling safely through space.

The theory regarding wormholes emerged in the early 20th century in response to Einstein’s General Theory of Relativity. The first to postulate their existence was Karl Schwarzschild, a German physicist and astronomer whose solutions to Einstein’s field equation (the Schwarzschild metric) resulted in the first theoretical basis for the existence of black holes.

A consequence of the Schwarzschild metric was what he referred to as “eternal black holes,” which were essentially connections between different points in spacetime. However, these Schwarzschild wormholes (aka. Einstein–Rosen bridges) were not stable as they would collapse too quickly for anything to cross from one end to the other.

As Maldacena and Milekhin explained to Universe Today via email, traversable wormholes require special circumstances in order to exist. This includes the existence of negative energy, which is not permissible in classic physics – but is possible within the realm of quantum physics.

A good example of this, they claim, is the Casimir Effect, where quantum fields produce negative energy while propagating along a closed circle:

“However, this effect is typically small because it is quantum. In our previous paper [“Traversable wormholes in four dimensions”] we realized that this effect can become considerable for black holes with large magnetic charge. The new idea was to use special properties of charged massless fermions (particles like the electron but with zero mass). For a magnetically charged black hole these travel along the magnetic field lines (In a way similar to how the charged particles of the solar wind create the auroras near the polar regions of the Earth).”

The fact that these particles can travel in a circle by entering one spot and emerging where they started in ambient flat space, implies that the “vacuum energy” is modified and can be negative.

The presence of this negative energy can support the existence of a stable wormhole, a bridge between points in spacetime that won’t collapse before something has a chance to traverse it.

Such wormholes are possible based on matter that is part of the Standard Model of particle physics. The only problem is, these wormholes would have to be microscopic in size and would only exist over very small distances.

For human travel, the wormholes would have to be large, which requires that physics beyond the Standard Model be employed.

For Maldacena and Milekhin, this is where the Randall-Sundrum II model (aka. 5-dimensional warped geometry theory) comes into play. Named after theoretical physicists Lisa Randall and Raman Sundrum, this model describes the Universe in terms of five-dimensions and was originally proposed to solve a hierarchy problem in particle physics.

“The Randall-Sundrom II model was based on the realization that this five-dimensional spacetime could also be describing physics at lower energies than the ones we usually explore, but that it would have escaped detection because it couples with our matter only through gravity. In fact, its physics is similar to adding many strongly interacting massless fields to the known physics. And for this reason it can give rise to the required negative energy.”

From the outside, Maldacena and Milekhin concluded that these wormholes would resemble intermediately-sized, charged black holes that would generate similarly-powerful tidal forces that spacecraft would need to be wary of. To do that, they claim, a potential traveler would need a very large boost factor as they pass through the center of the wormhole.

Assuming that can be done, the question remains of whether or not these wormholes could act as a shortcut between two points in spacetime? As noted, previous research by Daniel Jafferis of Harvard University (which also considered the work of Einstein and Nathan Rosen) showed that while possible, stable wormholes would actually take longer to traverse than normal space.

According to Maldacena and Milekhin’s work, however, their wormholes would take almost no time to traverse from the perspective of the traveler. From the perspective of an outsider, the travel time would be much longer, which is consistent with General Relativity – where people traveling close to the speed of light will experience time dilation (i.e. time slows down). As Maldacena and Milekhin put it:

“]F]or astronauts going through the wormhole it would take only 1 second of their time to travel 10,000 light-year distance (approximately 5000 billion miles or 1/10 of Milky Way size). An observer who does not go through the wormhole and stays outside sees them taking more than 10,000 years. And all this with no use of fuel, since the gravity accelerates and decelerates the spaceship.”

Another bonus is that traversing these wormholes could be done without the use of fuel since the gravitational force of the wormhole itself would accelerate and decelerates the spaceship. In a space exploration scenario, a pilot would need to navigate the tidal forces of the wormhole to position their spacecraft just right, and then let nature do the rest.

A second later, they would emerge on the other side of the galaxy!

While this might sound encouraging to those who think wormholes could be a means of space travel someday, Maldacena and Milekhin’s work presents some significant drawbacks as well.

For starters, they emphasize that traversable wormholes would have to be engineered using negative mass since no plausible mechanism exists for natural formation.

While this is possible (at least in theory), the necessary spacetime configurations would need to be present beforehand. Even so, the mass and size involved are so great that the task would be beyond any practical technology we can foresee. Second, these wormholes would only be safe if space were cold and flat, which is not the case beyond the Randall Sundrum II model.

On top of all that, any object that enters the wormhole would be accelerated and even the presence of pervasive cosmic background radiation would be a significant hazard.

However, Maldacena and Milekhin emphasize that their study was conducted for the purpose of showing that traversable wormholes can exist as a result of the “subtle interplay between general relativity and quantum physics.”

In short, wormholes are not likely to become a practical way to travel through space – at least, not in any way that’s foreseeable. Perhaps they would not be beyond a Kardashev Type II or Type III civilization, but that’s just speculation. Even so, knowing that a major element in science fiction is not beyond the realm of possibility is certainly encouraging!

https://www.sciencealert.com/there-s-a-theory-of-relativity-that-could-allow-you-to-fly-through-a-wormhole

Using black holes to conquer space: The halo drive

by Matt Williams

The idea of traveling to another star system has been the dream of people long before the first rockets and astronauts were sent to space. But despite all the progress we have made since the beginning of the Space Age, interstellar travel remains just that – a dream. While theoretical concepts have been proposed, the issues of cost, travel time and fuel remain highly problematic.

A lot of hopes currently hinge on the use of directed energy and lightsails to push tiny spacecraft to relativistic speeds. But what if there was a way to make larger spacecraft fast enough to conduct interstellar voyages? According to Prof. David Kipping, the leader of Columbia University’s Cool Worlds lab, future spacecraft could rely on a halo drive, which uses the gravitational force of a black hole to reach incredible speeds.

Prof. Kipping described this concept in a recent study that appeared online (the preprint is also available on the Cool Worlds website). In it, Kipping addressed one of the greatest challenges posed by space exploration, which is the sheer amount of time and energy it would take to send a spacecraft on a mission to explore beyond our solar system.

Kipping told Universe Today via email: “Interstellar travel is one of the most challenging technical feats we can conceive of. Whilst we can envisage drifting between the stars over millions of years – which is legitimately interstellar travel – to achieve journeys on timescales of centuries or less requires relativistic propulsion.”

As Kipping put it, relativistic propulsion (or accelerating to a fraction of the speed of light) is very expensive in terms of energy. Existing spacecraft simply don’t have the fuel capacity to get up to those kinds of speeds, and short of detonating nukes to generate thrust à la Project Orion, or building a fusion ramjet à la Project Daedalus, there are not a lot of options available.

In recent years, attention has shifted toward the idea of using lightsails and nanocraft to conduct interstellar missions. A well-known example is Breakthrough Starshot, an initiative that aims to send a smartphone-sized spacecraft to Alpha Centauri within our lifetime. Using a powerful laser array, the lightsail would be accelerated to speeds of up to 20 percent of the speed of light – thus making the trip in 20 years.

“But even here, you are talking about several terra-joules of energy for the most minimalist (a gram-mass) spacecraft conceivable,” said Kipping. “That’s the cumulative energy output of nuclear power stations running for weeks on end… so this is why it’s hard.”

To this, Kipping suggests a modified version of the “Dyson Slingshot,” an idea proposed by venerated theoretical physicist Freeman Dyson, the theorist behind the Dyson Sphere. In the 1963 book Interstellar Communications (Chapter 12: “Gravitational Machines”), Dyson described how spacecraft could slingshot around compact binary stars in order to receive a significant boost in velocity.

As Dyson described it, a ship would be dispatched to a compact binary system where it would perform a gravity-assist maneuver. This would consist of the spaceship picking up speed from the binary’s intense gravity, adding the equivalent of twice their rotational velocity to its own, and is then flung out of the system.

While the prospect of harnessing this kind of energy for the sake of propulsion was highly theoretical in Dyson’s time (and still is), Dyson offered two reasons why “gravitational machines” were worth exploring:

“First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future where engineering on an astronomical scale may be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kind of observable phenomena a really advanced technology might be capable of producing.”

In short, gravitational machines are worth studying in case they become possible someday, and because this study could allow us to spot possible extraterrestrial intelligences (ETIs) by detecting the technosignatures such machines would create. Expanding upon this, Kipping considers how black holes, especially those found in binary pairs, could constitute even more powerful gravitational slingshots.

This proposal is based in part on the recent success of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detected multiple gravitational wave signals since 2016. According to recent estimates based on these detections, there could be as many as 100 million black holes in the Milky Way galaxy alone.

Where binaries occur, they possess an incredible amount of rotational energy, which is the result of their spin and the way they rapidly orbit one another. In addition, as Kipping notes, black holes can also act as a gravitational mirror – where photons directed at the edge of the event horizon will bend around and come straight back at the source. As Kipping put it:

“So the binary black hole is really a couple of giant mirrors circling around one another at potentially high velocity. The halo drive exploits this by bouncing photons off the “mirror” as the mirror approaches you, the photons bounce back, pushing you along, but also steal some of the energy from the black hole binary itself (think about how a ping pong ball thrown against a moving wall would come back faster). Using this setup, one can harvest the binary black hole energy for propulsion.”

This method of propulsion offers several obvious advantages. For starters, it offers users the potential to travel at relativistic speeds without the need for fuel, which currently accounts for the majority of a launch vehicle’s mass. And there are many, many black holes that exist throughout the Milky Way, which could act as a network for relativistic space travel.

What’s more, scientists have already witnessed the power of gravitational slingshots thanks to the discovery of hyper-velocity stars. According to research from the Harvard-Smithsonian Center for Astrophysics (CfA), these stars are a result of galactic mergers and interaction with massive black holes, which kick them out of their galaxies at one-tenth to one-third the speed of light – around 30,000 to 100,000 km/s (18,600 to 62,000 mps).

But of course, the concept comes with innumerable challenges and more than a few disadvantages. In addition to building spacecraft that can endure being flung around the event horizon of a black hole, a tremendous amount of precision is required – otherwise, the ship and crew (if it has one) could be pulled apart in the maw of the black hole. Additionally, there’s simply the matter of reaching one:

“[T]he thing has a huge disadvantage for us in that we have to first get to one of these black holes. I tend to think of it like a interstellar highway system – you have to pay a one-time toll to get on the highway, but once you’re on, you can ride across the galaxy as much as you like without expending any more fuel.”

The challenge of how humanity might go about reaching the nearest suitable black hole will be the subject of Kipping’s next paper, he indicated. And while an idea like this is about as remote to us as building a Dyson Sphere or using black holes to power starships, it does offer some pretty exciting possibilities for the future.

In short, the concept of a black hole gravity machine presents humanity with a plausible path to becoming an interstellar species. In the meantime, the study of the concept will provide SETI researchers with another possible technosignature to look for. So until the day comes when we might attempt this ourselves, we will be able to see if any other species have already made it work.

Read more at: https://phys.org/news/2019-03-black-holes-conquer-space-halo.html#jCp