Historical research guide · 0898 · 1996

Warp drive and causality

Allen E. Everett

In one minute

Two years before this paper, Miguel Alcubierre had shown that general relativity itself permits a spacetime in which a ship sits still inside a bubble while the space ahead of it contracts and the space behind expands. The ship never moves faster than light through the space around it; the distance is what changes, so it arrives sooner than light would. Alcubierre added a conjecture — that the same techniques ought to allow closed loops in time as well. Allen Everett, at Tufts, checks that conjecture and finds it correct. His answer needs no extra ingredient: take Alcubierre’s own metric, make one simple modification, and closed causal loops become possible, meaning a path that returns to its own past. Everett is careful to separate his mechanism from the one Gott had found using cosmic strings; the two are not the same trick. Four pages, exactly argued, and they turn the warp bubble from a curiosity into a spacetime whose causal structure has to be worked out.

Connection

Chapter 4 collects the metric-engineering solutions and chapter 13 asks what they add up to; this paper is where the two meet. It says the Alcubierre geometry is rich enough to carry the deepest question in the subject, and it tells anyone designing a warp shell exactly which question they will have to answer.

From the source

The starting point is Alcubierre’s result, stated by Everett without hedging: within the framework of general relativity there exists a spacetime allowing travel at superluminal speeds, provided matter with a negative energy density — that is, a stress-energy below the ambient vacuum level — can exist. Everett takes the geometry as given and asks what else it implies.Author’s abstract, first sentence
Alcubierre had conjectured that similar techniques should make it possible to construct a theory containing closed causal loops, and therefore travel backwards in time. Everett’s paper exists to test that conjecture rather than to assume it, and the abstract says the outcome in one word: we verify this conjecture.Author’s abstract, first and second sentences
The construction is deliberately minimal, which is what gives the result its force. Everett exhibits a simple modification of Alcubierre’s model, requiring no additional assumptions, in which causal loops are possible. Nothing exotic is added on top of the warp metric; the loops follow from the metric already on the table.Author’s abstract, second sentence
The mechanism is a two-bubble argument. Everett and Roman later describe it in their own words while building the analogous case for Krasnikov tubes: two superluminal journeys, arranged so that the second returns along a path whose orientation differs from the first, compose into a trip that arrives before it departed. Their four-dimensional two-tube construction, they write, is closely analogous to the argument given in this paper for the Alcubierre case.Everett and Roman 1997, introduction and section 4, describing this paper
Everett draws one careful distinction. This mechanism for generating causal loops differs in essential ways from the one discovered by Gott using a pair of moving cosmic strings — a separate and much-discussed route to closed timelike curves. Keeping the two apart matters, because the objections raised against Gott’s construction do not transfer to this one.Author’s abstract, third sentence
What to watch: the paper turns superluminal metric engineering into a chronology question, and that question is live and being worked on. Hawking’s chronology-protection conjecture, the quantum inequalities that bound how much below-ambient energy can be gathered and for how long, and the newer positive-energy and constant-velocity warp solutions that avoid the exotic requirement altogether are all answers in progress. The result that would settle it is a warp-shell solution whose stress-energy stays within measured quantum bounds and whose causal structure is worked out, not assumed.Author’s abstract read as a whole, against the quantum-inequality and physical-warp-drive literature this library holds

The way in

https://doi.org/10.1103/PhysRevD.53.7365

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

LICENCE. Published as Physical Review D, volume 53, issue 12, pages 7365 to 7368, 15 June 1996, under the APS default licence, which is a publisher copyright and not a licence to readers. No text of the paper is reproduced here. SOURCE NOT REACHED IN FULL. The APS harvest service, which this library normally uses for APS papers, began answering 401 not authorized on 2026-09-08. Allen E. Everett posted no preprint: an arXiv search by title and an author search of the gravitation and cosmology archive both return nothing for him, INSPIRE-HEP carries no eprint field for the record, and OpenAlex and Unpaywall report the article closed with no repository copy. The four pages themselves could therefore not be read. This sheet is written from two things that could be read on 2026-09-08. First, the author’s complete published abstract, carried in full by OpenAlex and by INSPIRE-HEP. Second, Everett’s own follow-up paper with Thomas A. Roman, A superluminal subway: the Krasnikov tube, arXiv gr-qc/9702049, which this library holds in full at /library/stm-24a207b819 and which describes the 1996 argument twice — once in its introduction and once in section 4, where the authors say their own two-tube construction is closely analogous to the one given in this paper for the Alcubierre case. Locators say which of the two a statement comes from. Allen E. Everett wrote from the Institute of Cosmology, Department of Physics and Astronomy, Tufts University, Medford, Massachusetts. When the four pages can be read, this sheet should be rewritten from them.

How to cite it

Allen E. Everett (1996) Warp drive and causality doi:10.1103/PhysRevD.53.7365

Where it sits in the curriculum

The metric, warp drives and wormholes