Imagining warp drives often conjures up scenes from Star Trek, where ships enter ‘warp speed’ accelerating up to and past the speed of light to zoom from one destination in the cosmos to another. The term warp drive refers to how the system generates the propulsion, allowing the spaceship to achieve its superluminal travel. The common ingredient to all warp drives however, is the warping of spacetime, through a particular mass and energy configuration. This is what gives the idea its name. To read more about spacetime and what I could possibly be talking about when I say it can be warped, refer to some previous posts here and here. This idea lived purely in the realm of science fiction until 1994, when Miguel Alcubierre constructed the Alcubierre Drive.
The Alcubierre Drive was the first theoretical construction for a warp drive, with all the mathematical underpinnings necessary for something to move from the realm of scientific fiction to theoretical non-fiction. However, despite the mathematical underpinnings of its construction, it was wholly unphysical and was recognised as so by the community pretty quickly. As such, we shall not go into the detail of the proposed warping of the Alcubierre Drive but rather outline its problems. Firstly, the drive assumes the ability to accelerate beyond the speed of light barrier. However, Einstein’s theory of relativity does not allow objects to break the speed of light barrier, doing so would require an infinite amount of positive energy. The second problem is that, to attempt to get around the previous issue, the construction guiltily introduces negative energy. And not just a little, an immense amount of this negative energy is built into the formulation. Negative energy, to the best of our knowledge, does not exist in our universe. So the idea of the warp drive returned to live in the wishful world of science fiction, until earlier this year.
A paper ‘Introducing Physical Warp Drives’ was released in February 2021, by Alexey Bobrick and Gianni Martire at Applied Physics in New York – what seems a somewhat mysterious yet promising new independent research institute. Let me attempt to explain simply the construction of this warp drive. The warp drive is, in essence, the spaceship itself. The spaceship can be imagined like a bubble, where its walls, referred to as the ‘warp shield’, are made of an incredibly dense material. The inside of the bubble, i.e the ship, is open and constitutes the passenger area. The ship then travels through the external spacetime. In mathematical phrasing, the geometry of the ship is then embedded into the geometry of the external universe.
The first key idea is that the passengers are in the reference frame of the ship as a whole. A common analogy for reference frames in relativity is being aboard a train here on Earth. When you are sitting on a train and it is moving with a smooth constant speed, you can often feel like you are stationary and it is the world outside the windows that is moving. You are in the reference frame of the train (here, the spaceship). The second key idea is how mass warps spacetime, this is a fundamental pillar of Einstein’s theory of general relativity. Like I have explained in previous posts here at RTU, high densities of mass cause spacetime to bend or warp. For observers residing near these high mass densities, their passage of time slows relative to other observers. This effect is known as time dilation. If Alice travels close to a black hole (an area of extreme mass density), leaving Bob at a far distance, her passage of time will be far slower relative to his. By the time she returns to Bob, 4 days may have passed for her whilst 4 months or even 4 years may have passed for Bob (depending on the mass of the black hole!). This is a pretty huge idea so if you’re coming across it for the first time I’d recommend some extra reading, it won’t help your sanity but over time it might help normalise the phenomena for you.
The construction of Bobrick and Martire’s warp drive is built on the exploitation of this phenomena. The ships high density walls warp the surrounding spacetime, causing time to pass slower for the passengers inside relative to outside, allowing them to perceive a (relatively!) quick journey from A to B. If you are undertaking a warp drive journey, you have to make sure you’re happy with the fact that you don’t plan on seeing the friends that you left behind on Earth again. (By the time you return from your interstellar voyage they will be long gone.)
This physical warp drive does not break the speed of light barrier and does not require negative energy. Although superluminal travel is thrown out, the construct would still be exciting – given how far we have currently travelled as a species is our hop across to the Moon. However, to achieve substantial warping the material of the walls would have to be incredibly dense. “If we take the mass of the whole planet Earth and compress it to a shell with a size of 10 metres, then the correction to the rate of time inside it is still very small, just about an extra hour in the year,” says Bobrick. Obviously we as a species do not currently have the ability to acquire, let alone manipulate such resources but the key difference here is that the construction is not unphysical with respect to what we know about the universe – unlike the Alcubierre Drive. The mathematical underpinnings therefore can continue to be explored, without a feeling that the work is done in total vain. For example things like the geometry of the spaceship can be experimented with, as some configurations will require less massive materials to achieve the same time dilation effects. Bobrick and Martire have showed that a flattened ship, in the direction of travel, would require less energy (i.e. less mass in the warp shield) for the same degree of warping. This makes intuitive sense, making things aerodynamic in classical dynamics seems to echo the same reasoning. I read a quote in Popular Mechanics which I thought summed up the difference between the new construction and the Alcubierre drive very neatly – “this new concept uses floating bubbles of spacetime rather than floating ships in spacetime.”
Now that the warp drive construction has been explained, I’d like to end on one aspect that has always troubled me when discussing the overall topic. Warp drives allow the spaceship’s passengers to travel interstellar distances at sub, yet near light speeds. In the standard visualisations of warp drives you see the millions of stars flying past your view, as you hurtle through the cosmos. What I’ve never understood is how are the passengers always so relaxed during the journey, how can they be sure their trajectory isn’t going to collide with one of the intermediate stars (or planets, or asteroids)? Such a collision would without a doubt result in entire disintegration! I understand the space is incredibly sparse but if you’re travelling these interstellar distances your odds build up… To be confident of plain sailing you’d have to have a map of the entire space between destination A and B before embarking and set your trajectory accordingly. Not feasible if you’re using warp drives to explore new parts of the universe! Furthermore, as I laid out, time is passing slowly for you inside the ship but outside the processes are continuing as usual. Planets are orbiting stars, stars are orbiting the centres of their galaxies and asteroids are whimsically flying around. So not only do you need to map out your trajectory, you need to account for the movements of the celestial bodies over what will be the millions of years that pass for them whilst you are inside your ship. This seems like a very complex calculation indeed requiring huge amounts of galactic data. Maybe by the time we will have acquired the ultra dense resources for the warp drive construction we will have super computers to perform these cosmic calculations for us…
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