*In our fifth online poll to find out what*

*Plus*readers would most like to know about the Universe you told us that you'd like to find out if time travel is allowed. We took the question to Kip Thorne, Feynmann Professor of Theoretical Physics, Emeritus, at the California Institute of Technology, and here is his answer.Kip Thorne

### Travelling forward in time rapidly

*twins paradox*. One twin (call him Methuselah) stays at home on Earth; the other (Florence) travels out into the Universe at high speed and then returns. When they meet at the end of the trip, Florence will have aged far less than Methuselah; for example, Florence may have aged 30 years and Methuselah 4,500 years. (The twin that ages least is the one who undergoes huge accelerations, to get up to high speed, slow down, reverse direction, then accelerate back and slow to a halt on Earth. The twin who leads the sedate life ages the most.)

*event horizon*of a massive black hole (say, one whose gravitational pull is that of a billion suns) and Florence travels down to near the event horizon and hovers just above it for, say, 30 years and then returns, Methuselah can have aged thousands or millions of years. This is because time flows much more slowly near a black hole's event horizon (where the acceleration of gravity is huge) than far above it (where one can live sedately).

### Travelling backward in time: chronology protection

*The laws of physics always conspire to prevent anything from travelling backward in time, thereby keeping the Universe safe for historians.*

*exotic material*that is required in the manufacture of any time machine might be forbidden to exist, by the laws of physics — forbidden to exist in the large amounts that time machines always require. (2) Time machines might always self-destruct, explosively, when one tries to activate them.

**Me crawling through a wormhole**

*Black Holes and Time Warps*, [4], where you can find a more detailed description of this time machine.) whose length is only a few centimetres but circumference is about that of my belly. (From my book

**A wormhole-based time machine:**A wormhole is a hypothetical tunnel through

*hyperspace*that links one place in our Universe (e.g. my office at Caltech) to another place (e.g. the Caltech football field). Each end of the wormhole (each

*mouth*) looks like a crystal ball. Staring into it, one sees a distorted image of objects at the other end. Looking into the mouth in my office, I see the football field, distorted; someone on the football field, looking into the mouth there, sees me and my office, distorted. The wormhole (tunnel) might be only 3 metres long, so if I enter the mouth in my office and then travel just 3 metres through the tunnel, I emerge from the other mouth, onto the football field 300 metres from my office.

**A wormhole as viewed from a higher-dimensional hyperspace.**Our Universe is the two-dimensional sheet. The wormhole is a short cut through hyperspace from one location in our sheet (our Universe) to another.

**Exotic Matter and Vacuum Fluctuations:**We do not know whether the laws of physics permit wormholes. We do know, however, that a wormhole will implode so quickly that nothing can traverse it, unless it is held open by gravitationally repulsive forces that can only be produced by exotic matter. By the phrase "exotic matter" I mean matter that has negative energy and therefore anti-gravitates, i.e. repels.

*do*permit exotic matter to exist, and it has been created in the laboratory in very tiny amounts: in the so-called

*Casimir vacuum*between two electrically conducting plates, and in the so-called

*squeezed vacuum*that is generated by optical physicists using nonlinear crystals.

*vacuum*) is filled with tiny fluctuations of all kinds of matter and fields that exist in the Universe. It is impossible to make these fluctuations go away. They are a consequence of the quantum mechanical

*uncertainty principle*: if, at one moment of time, there are no fluctuations at all of (for example) the electromagnetic field, then the rate of change of the fluctuations must be infinitely large and a moment later the fluctuations will be enormous. The product of the strength of the fluctuations and the magnitude of their rate of change is always bigger than a certain limit, given by the uncertainty principle. As a result, fluctuations are always present. We call them

*vacuum fluctuations*because they are a property of the vacuum, i.e. of otherwise empty space.

The Casimir Vacuum: When two electrically conducting plates are placed very close together, the vacuum fluctuational electric field parallel to the plates is strongly suppressed while that perpendicular to the plates is little affected. The suppression reduces the fluctuational energy between the plates below what it would be in plate-free empty space, so the vacuum between the plates (the Casimir vacuum) acquires negative gravitating energy. It has loaned some fluctuational energy to the electric fields inside the plates. The plates have catalysed this lending. |

Wormhole held open by Casimir Vacuum: Two concentric spherical conducting plates are placed at the throat of a wormhole, with a tiny separation. (One dimension is suppressed here, so our Universe is a 2-dimensional sheet and the plates look like circles rather than spheres.) If the plates' energies (including their mass's energy E=mc) are small enough, then the repulsive gravity due to the Casimir vacuum between the plates can hold the wormhole open. ^{2} |

^{-121}in dimensionless numbers) that it is irrelevant for my discussion of time machines; so I shall say that the quantum fluctuations produce no gravity at all.

*topological censorship*, see [2].)

**Time machine self destruction:**If it turns out that wormholes can be held open, then doing so is not enough to guarantee that an ultra-advanced civilisation can convert a wormhole into a time machine via a twins-paradox trip (carrying one mouth out into the Universe at high speed and then back). There is a second obstacle that must be surmounted — time machine self destruction:

**Time machine self destruction:**As the right wormhole flies back toward the left, at the end of its twins-paradox trip, vacuum fluctuations flow through the wormhole then out through the space between them, returning to their starting point at the moment they left. Their gravitating energy grows extremely large, and perhaps destroys the wormhole at the moment it becomes a time machine. (Figure adapted from my book

*Black Holes and Time Warps*, [4].)

The fate of any time machine?

^{60}; see Hawking's article in my birthday party book, [6]. That's an awfully small probability of surviving the explosion. Given the opportunity to try, I would not take the risk.

**Other time machines:**It is amazing what we can learn from the laws of physics, when we understand them well. One famous example is the laws' absolutely firm insistence that it is impossible to construct a perpetual motion machine, even if one has all the tools of an exceedingly advanced civilisation. Another example is a proof by Hawking that to make a time machine, no matter how one goes about it, one must use exotic matter — matter with negative energy — as an integral part of the device; wormholes illustrate this, but it is true in general. And a third example is the proof by Kay, Radzikovsky and Wald that the laws of physics as we now know them will break down whenever a time machine is activated, no matter how one designs the machine. Again wormholes are just one example. Hawking's theorem, and that of Kay, Radzikovsky and Wald, tell us that the fates of all time machines are held tightly in the grip of the laws of quantum gravity.

*Plus*article What happened before the Big Bang?).

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