How to Build a Time Machine

For reasons that escape me now, I needed to understand time travel when I was seventeen years old. I bought Brian Greene’s newly published The Fabric of the CosmosThis was the first physics book I ever read (and I needed my dad and a whiteboard to help me make sense of it). A wormhole, it said, “is a hypothetical tunnel through space.”

Hypothetical is the key word here. No one has actually observed a wormhole to date, so we don’t know if they really exist. That said, at the recent launch of his book The Order of TimeCarlo Rovelli was asked by an audience member in New York what he thinks about wormholes. He said while there is no evidence for them in the universe, his school textbooks also said there was no such thing as black holes. We can’t exclude it.

Greene’s book compares a wormhole to a tunnel that is bored through a mountain, allowing you to get from one side to the other quickly, without taking the long route up and over the peak. The difference between a mountain tunnel and a wormhole, though, is that “conventional tunnels provide a new route through existing space; a wormhole provides a tunnel from one point in space to another along a new, previously nonexistent tube of space.”







In his book Black Holes and Time Warps: Einstein’s Outrageous Legacy, Kip Thorne recalls Carl Sagan approaching him in 1985:

“Sorry to bother you, Kip, but I’m just finishing a novel about the human race’s first contact with an extra-terrestrial civilization, and I’m worried. I want the science to be as accurate as possible, and I’m afraid I may have got some of the gravitational physics wrong. Would you look at it and give me advice?”

In the novel, Contact, an astronaut needed to be transported to a distant part of the universe and back in just a few minutes. Thorne identified a wormhole as a good plot device to make Sagan’s science sound. But this was just the beginning for Thorne. Now that he had created for Sagan something of a guidebook for wormholes, he would take his ideas on to further the advancements in science in this area (and of course provide the science for the film Interstellar, as depicted in Thorne’s book, The Science of Interstellar).

Marvel films, too, employ the wormhole as a plot device (and, like Sagan, science advisers to help). You may not think that a film about a Norse thunder god would require a science adviser, but physicist and author Sean Carroll is the reason that the characters in Thor call the wormhole an “Einstein-Rosen bridge.” Here is an excerpt from Sean Carroll’s blog:

There is one phrase used in the movie that I think is directly attributable to my input: “Einstein-Rosen bridge.” This came about from a conversation between producer Kevin Feige and me that went something like this:

KF: We need the Bifrost Bridge to provide a way for the characters to travel great distances in space in a very short period of time.

SC: Sure, you probably want to say that it makes use of wormholes.

KF: Well, we can’t call it a “wormhole.”

SC: Why not?

KF: Sounds too Nineties.

SC: I suppose … you could call it an “Einstein-Rosen bridge.” Means the same thing.

So naturally, in the finished film, Jane Foster calls it an Einstein-Rosen bridge, and someone says “what’s that?”, and she replies “it’s a wormhole.”

The term Einstein-Rosen bridge comes from Albert Einstein and Nathan Rosen, who, in 1935, used the theory of general relativity to propose the idea of bridges through space-time.

But, what does a tunnel through space have to do with time travel?


In his book The Science of The Hitchhiker’s Guide to the Galaxy, Michael Hanlon recounts some ideas that astrophysicist Paul Davies suggests in first making a wormhole on Earth, second stabilizing the wormhole, and third turning the wormhole into a time machine. Hanlon summarizes:

Grab one end of the wormhole using an electromagnetic field, and put the other end in a giant particle accelerator and whizz it around for a few years at near-light velocity. Thanks to time dilation, the stationary end slowly moves into a different time zone from the moving end. Whirl one end for say a decade and you will get a wormhole to 10 years in the past.

Unfortunately for whatever purpose my time traveling knowledge was for, Brian Greene in The Fabric of the Cosmos outlines many, many challenges to actually building one of these wormhole time machines: the possibility that tearing the required hole in space is not allowed by the laws of physics, the instability of a wormhole that allows them to stay open for only a fraction of a second, the challenge of generating enough exotic matter to hold a wormhole open, and vacuum fluctuations between the future and past causing a feedback mechanism that fills the wormhole with ever-increasing energy and would destroy the wormhole.

There is still a lot of science to be done.

A point of interest, if you like wormholes, there is an amazing short film called Einstein-Rosen that played at the New York International Children’s Film Festival in 2017. Two brothers discover a wormhole on their property, but we don’t see what they decide to do with the wormhole until we flash forward to years later when the brothers return to the spot, at the other end of the tunnel.




Further reading: The Interstellar Contributions of Kip Thorne
 What is a Wormhole?


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