Original questions: “Could you use a wormhole to get to another planet?” asked by Cat, “Do wormhole scientists think with portals?”, asked by Tom, “How might we get to a parallel universe?” asked by Paul, and “Could wormholes be real?” asked by Elin
A wormhole is an object that links two different regions of spacetime - Einstein’s theory of special relativity tells us that space and time are not separate, but that time is simply another dimension of four-dimensional spacetime. This means that under extreme conditions (e.g. if something is moving extremely quickly or if it’s in a very strong gravitational field), these space and time dimensions can get mixed up. Compared to stationary onlookers, time can slow down and objects can get smaller.
The standard picture of wormholes and portals that we see in sci-fi and fantasy are of gateways that you can step through and instantly emerge at a different point in space(time). In ‘reality’ (inverted commas as we don’t really know if wormholes could actually exist), wormholes are extended objects - a wormhole big enough to travel though will likely be many kilometres across, distorting the spacetime around it for many hundreds / thousands of kilometres. The inside of the wormhole will also be pretty extended - you would not be able to simply walk through and instantly emerge the other side.
Illustration 1: Real wormholes are likely to be very different to the ones we see in science fiction (e.g. the video game Portal)
This is because we think that wormholes take the form of a sort of ‘double-ended black hole’. Einstein’s theory of General Relativity tells us that all objects in the Universe distort the spacetime around them: how large this distortion is depends on how ‘compact’ or dense the object is. The image below demonstrates how different objects in space bend spacetime. A star like our Sun will bend spacetime a bit, however this effect is quite small – the light from a star passing just past the edge of the Sun on its way to us will be deflected by about 1/2000 of a degree.
Neutron stars form when larger stars collapse at the ends of their lives. They are about 1 or 2 times as massive as the Sun, but have a radius of only around 10km. This means that they are incredibly dense – if you took all the people on the Earth and squashed us all down so that we were the same density as a neutron star, we’d all fit within a ping pong ball. Their gravity will therefore distort the spacetime around them much more than the Sun does.
Illustration 2: An illustration showing how spacetime is distorted by different astronomical objects. Image from sciencenews.org
Finally, a black hole is a region where spacetime has become so distorted by gravity that the very fabric of the Universe collapses down to a single point or ‘singularity’. The gravitational forces about this point are so strong that nothing - not even light - can escape. In the image above, you can see that we can picture the spacetime bending into a bottomless pit.
You can imagine a wormhole to be like two black holes glued together just before the point where spacetime collapses to a singularity: it becomes extremely distorted, but instead of pinching off into a single point, it instead flares out again, emerging at a different region of spacetime.
Illustration 3: An illustration of the spacetime around a wormhole. For Wikipedia Creative Commons.
Wormholes were first predicted as a mathematical solution of Einstein’s theory of general relativity just a year after it was first published. However, just because the maths says they could occur, doesn’t mean that they actually exist in nature. Most of the mathematical models for wormholes are incredibly unstable and completely impractical for the kind of travel we’d hope to use them for. The maths also has very little to say about how they could actually form. So, while there is nothing we know of currently that says they couldn’t exist, there’s nothing that says they definitely do exist either.
It is possible (and indeed very probable) that there exists some physics beyond our current understanding. With a different model of how gravity works, or with some form of exotic matter, it is possible that wormholes could form and remain stable. But that wouldn’t necessarily mean that we’d be able to travel through them…
As mentioned above, wormholes link different regions of spacetime. These regions may be at different points in the same universe or, if multiple universes indeed exist, could be in different universes. If there was a wormhole near enough to the Earth that we could travel there in a reasonable length of time, and the wormhole exit point happened to be near enough to another planet, then we could use it to travel to another planet.
In order for us to be able to travel such wormholes, they would need to be:
Stable enough for us to pass through without it collapsing around us (some solutions are incredibly unstable to small perturbations, and we need it to be stable enough for a great big spaceship to pass through)
The environment in and around the wormhole must be safe enough that we can pass through it unscathed (i.e. it’s no good if we’re ripped apart by tidal forces!)
Short enough that would could get through in a reasonable length of time (i.e. the journey time must not take many lifetimes)
These conditions impose quite a stringent set of constraints on the possible wormhole models. In order for such wormholes to be possible, we require gravity to behave quite differently from how it is predicted to do so by general relativity and / or for the existence of something physicists like to call ‘exotic matter’ (a type of matter with weird properties that is not currently described by the Standard Model of particle physics).
Neither of these things are completely beyond the realms of possibility - many physicists believe that a large part of the Universe is made up of dark energy which has the weird properties needed to hold open a wormhole. In the future, we could develop technology that would allow us to harness this to build our own wormholes to other planets or even other universes. There are also currently a number of theories of ‘alternative gravity’ that have different predictions from general relativity. With further study of these theories and observations of objects with strong gravitational fields (including gravitational wave detections using telescopes such as LIGO), we may find that one of these models is more consistent with our observations of the Universe than general relativity.
Similarly however, it could be possible that an improved understanding of physics rules out wormholes entirely. We just don’t know.
References and Further Reading:
This wormhole FAQ does a pretty good job of answering most questions you can think of to do with wormholes
BBC Future: Will we ever… travel in wormholes gives a nice description of how we might make a wormhole to travel to far off regions of space
If you’re looking for a more technical description of wormhole theory, then this is the paper I used when preparing for the Science Room session. It does get quite technical in places, but it’s a really comprehensive description of the history of wormhole theory and shows how in order for a wormhole to be ‘traversable’ (possible to travel through), we require exotic matter / a departure from general relativity.