Spacetime plot (with angular directions suppressed). Credit: arXiv:1405.1283 [gr-qc]

University of Cambridge physicist Luke Butcher has uploaded a paper to the arXiv preprint server suggesting that there might exist some type of wormhole that is capable of staying open long enough for a photon to pass through—which of course suggests the possibility of sending messages backwards or forwards in time.

Time travel, is still of course, the purview of science fiction writers and daydreamers, though physicists believe that at least in theory, it should be possible. It was Albert Einstein who first suggested one possibility: the wormhole—a tunnel through space time that could conceivably have one end in one place in time and the other, in another. There was one serious catch however, preventing us humans from building a vehicle of some sort and running through such a wormhole—they only exist for very short periods of time, not nearly enough for a person, much less a single particle or photon to enter and pass through. Because of that, the possibility of time travel came to a standstill, at least until 1988, when physicist Kip Thorne proposed an idea where a wormhole could be kept open longer.

He suggested that wormholes might be kept open though the use of negative energy, aka Casimir energy. While the idea seemed promising no one was able to think of a way to cause the creation of Casimir energy to occur inside a wormhole, while a human vehicle passed through. In his paper, Butcher suggests that there might be another way—by taking advantage of Casimir energy that exists naturally in some wormholes. He did a lot of calculating and found that if a wormhole was a lot longer than it is wide, the amount of Casimir energy present inside of it would be enough to cause it to stay open longer than normal. Just long enough, Butcher suggests, to send a photon through.

It's all still theoretical, he acknowledges, and there are other parts of wormholes that aren't understood, some of which might prevent the movement of a photon, or human vehicle, for example. So, it's not like time travel is about to become a reality. It's more likely, he suggests, that his idea will spur further research and perhaps initiate new ideas that could possibly someday lead to some form of time travel.

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More information: Casimir Energy of a Long Wormhole Throat, arXiv:1405.1283 [gr-qc] Casimir Energy of a Long Wormhole Throat, arXiv:1405.1283 [gr-qc] arxiv.org/abs/1405.1283v1 Abstract

We calculate the Casimir energy-momentum tensor induced in a scalar field by a macroscopic ultrastatic spherically-symmetric long-throated traversable wormhole, and examine whether this exotic matter is sufficient to stabilise the wormhole itself. The Casimir energy-momentum tensor is obtained (within the R×S2 throat) by a mode sum approach, using a sharp energy cut-off and the Abel-Plana formula; Lorentz invariance is then restored by use of a Pauli-Villars regulator. The massless conformally-coupled case is found to have a logarithmic divergence (which we renormalise) and a conformal anomaly, the thermodynamic relevance of which is discussed. Provided the throat radius is above some fixed length, the renormalised Casimir energy-density is seen to be negative by all timelike observers, and almost all null rays; furthermore, it has sufficient magnitude to stabilise a long-throated wormhole far larger than the Planck scale, at least in principle. Unfortunately, the renormalised Casimir energy-density is zero for null rays directed exactly parallel to the throat, and this shortfall prevents us from stabilising the ultrastatic spherically-symmetric wormhole considered here. Nonetheless, the negative Casimir energy does allow the wormhole to collapse extremely slowly, its lifetime growing without bound as the throat-length is increased. We find that the throat closes slowly enough that its central region can be safely traversed by a pulse of light. Journal information: arXiv

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