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Who really said it

But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future

Spoken by Stephen Hawking 1942–2018 · theoretical physicist; Lucasian Professor of Mathematics, Cambridge, 1979–2009
Verified In copyright

Confirmed word for word in Hawking’s essay “The Future of the Universe,” which the book footnotes as a Darwin lecture given at the University of Cambridge in January 1991 and prints as essay 13 of Black Holes and Baby Universes and Other Essays (Bantam Press, 1993). He made the same joke, more drily, in the closing line of his 1992 Physical Review D paper “Chronology protection conjecture.”

Last verified 17 Aug 2026 · how we verify · spot an error?

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“But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future” Verified. “But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.” — Stephen Hawking, “The Future of the Universe,” a Darwin lecture given at the University of Cambridge in January 1991, collected in Black Holes and Baby Universes and Other Essays (Bantam Press, 1993). — Quotle.info

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In context

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Provenance

Where the line actually comes from

Work
“The Future of the Universe”
Occasion
Darwin lecture given at the University of Cambridge, January 1991
Collected in
Black Holes and Baby Universes and Other Essays
Edition read
Bantam Press, London & New York, 1993 — essay 13

There seems to be a Chronology Protection Agency that makes the world safe for historians by preventing travel into the past. … The Chronology Protection Hypothesis is supported by some recent calculations that I and other people have done. But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future. Trimmed at the ellipsis

From essay 13, “The Future of the Universe.” The famous sentence is the punchline of the paragraph: Hawking sets out the physics he thinks forbids travel into the past, then offers the empty tourist trade as the clincher.

Black Holes and Baby Universes and Other Essays (Bantam Press, 1993) — catalog record

The paper trail, in order

  • January 1991 — Hawking delivers “The Future of the Universe” as a Darwin lecture at the University of Cambridge, closing a passage about a “Chronology Protection Agency” with this sentence (Quote Investigator ↗).
  • 28 January 1992 — the Chicago Tribune runs the line in its “Quotes of the Day” column, dropping the opening “But” — the clipped form most reposts still use (Quote Investigator ↗).
  • 15 July 1992 — Hawking puts the joke through peer review. The closing sentence of “Chronology protection conjecture” in Physical Review D reads “There is also strong experimental evidence in favor of the conjecture from the fact that we have not been invaded by hordes of tourists from the future” (Quote Investigator ↗, transcribing p. 610).
  • 1993 — the lecture is printed as essay 13 of Black Holes and Baby Universes and Other Essays, with the footnote naming the January 1991 Darwin lecture; the collection’s contents list is on the Internet Archive catalog record ↗.
In copyright

The sentence comes from a lecture Hawking gave in January 1991 and from Black Holes and Baby Universes and Other Essays, first published in 1993 — far too recent for the US public domain, where the line in 2026 falls before 1931. Quoting the sentence for attribution and comment is ordinary fair use; reproducing the essay is not.

Rights status is our research, not legal advice — and copyright varies by country and edition. For commercial or published use, confirm with the rightsholder. See terms.

Fact-check

Right man, wrong book — and usually the wrong words

The attribution is sound: this really is Hawking. What travels badly is the wording and the source. The line is routinely pinned to the wrong book, quoted with its first word amputated, or replaced altogether by a snappier sentence he does not appear to have written.

Credited to Stephen Hawking, A Brief History of Time Paraphrase

This tightened rhetorical question is what most quote sites serve up — Goodreads ↗ attributes it to A Brief History of Time. The wormholes-and-time-travel chapter of that book does discuss the absence of visitors, but in different words; the question form is a later compression, not a sentence of Hawking’s that anyone has produced.

Physical Review D, “Chronology protection conjecture” Different wording

The famous sentence is often footnoted to Hawking’s journal article. The article’s actual closing line is drier: “There is also strong experimental evidence in favor of the conjecture from the fact that we have not been invaded by hordes of tourists from the future” — transcribed from p. 610 by Quote Investigator ↗.

Newspapers and quote sites Clipped

Hawking’s sentence begins “But the best evidence…” because it follows a paragraph of actual physics about the Chronology Protection Hypothesis. The Chicago Tribune printed it without the “But” in its “Quotes of the Day” column on 28 January 1992, and that decapitated version is the one that circulates (Quote Investigator ↗).

Robert Silverberg, The Masks of Time (1968) Earlier phrase

Quote Investigator notes the phrase itself turns up in Silverberg’s 1968 novel — but without Hawking’s argument that the absence of such tourists is evidence against time travel. The phrase predates him; the joke does not (Quote Investigator ↗).

Hawking’s own wording, in full: “But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future” — from “The Future of the Universe,” a Darwin lecture given at the University of Cambridge in January 1991 and printed in Black Holes and Baby Universes and Other Essays (Quote Investigator ↗).

Context

Why a physicist reached for a joke

Hawking was not only being flippant. By the early 1990s time travel had briefly become a respectable research topic: his own paper opens by noting the suggestion that “an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear,” and it works through cosmic strings, Cauchy horizons and the energy-momentum tensor before answering with the chronology protection conjecture — the proposal that the laws of physics step in to stop a time machine ever switching on.

The tourists line is what happens when a physicist reaches for the one piece of evidence a lay audience already holds. In the lecture he called it simply the best evidence we have; in the journal he upgraded it to “strong experimental evidence.” Eighteen years later he ran the experiment properly, throwing a party for time travellers at Gonville and Caius College on 28 June 2009 and posting the invitations only afterwards, so that nobody without a time machine could have known. Nobody came (Wikipedia ↗).

There seems to be a Chronology Protection Agency that makes the world safe for historians by preventing travel into the past.

Why ‘experimental evidence’ is a joke told with a straight face

The argument is the Fermi paradox pointed down the time axis rather than across space: if the thing were possible, where is everybody? Hawking never pretended it was a proof. In the 1992 paper it arrives after nine pages of Cauchy horizons and energy-momentum tensors, as the very last sentence of the body before the acknowledgements.

The standard objections write themselves: a time machine can only carry you back as far as the moment it was switched on, so an era before the first machine would see no tourists at all — and visitors who could warp spacetime could presumably also avoid being noticed. Hawking’s serious case rested on the calculations, not on the empty tourist trade.

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Quote Origin: The Best Evidence That Time Travel Is Not Possible Is That We Have Not Been Invaded By Hordes of Tourists from the Futurequoteinvestigator.com

The definitive investigation: the January 1991 Darwin lecture, the Chicago Tribune printing of 28 January 1992, the Physical Review D wording at p. 610, and Robert Silverberg’s earlier use of the phrase.
Chronology protection conjecture — Phys. Rev. D 46, 603 (1992)doi.org

The registered record for Hawking’s peer-reviewed paper, whose closing sentence makes the same joke in journal prose.
Black holes and baby universes and other essays — catalog recordarchive.org

Library record for the Bantam Press 1993 edition (ISBN 0593034007), with the contents list showing “The future of the universe.”
Hawking’s time traveller partyen.wikipedia.org

The 28 June 2009 experiment at Gonville and Caius College, Cambridge, where the invitations were sent out only after the party had ended.
Black Holes and Baby Universes and Other Essaysen.wikipedia.org

Background on the 1993 Bantam collection of essays, lectures and an interview in which this line was printed.
Stephen Hawkingen.wikipedia.org

Life dates (8 January 1942 – 14 March 2018), the Lucasian Professorship 1979–2009, and his conjecture that travel into the past is effectively impossible.

The author

About Stephen Hawking

Stephen Hawking

1942–2018 · English theoretical physicist; Lucasian Professor of Mathematics at Cambridge, 1979–2009

Stephen Hawking worked on black holes, singularities and the quantum structure of spacetime, and held the Lucasian Professorship of Mathematics at Cambridge from 1979 to 2009. A Brief History of Time (1988) made him the most recognisable physicist alive, and the deadpan one-liner was one of his standard instruments — the 1992 journal paper in which he argued that the universe forbids time machines ends by calling the absence of visitors “strong experimental evidence.” The same 1993 collection that carries this quotation also contains his essay “My Experience with ALS.”

For writers & researchers

Cite this

Documented source

Stephen Hawking, “The Future of the Universe,” in Black Holes and Baby Universes and Other Essays (London and New York: Bantam Press, 1993), essay 13 — footnoted in the book as a Darwin lecture given at the University of Cambridge in January 1991. The same argument, differently worded, closes S. W. Hawking, “Chronology protection conjecture,” Physical Review D 46, no. 2 (15 July 1992): 603–611.

This verification page

Quotle.info. “Who really said ‘But the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future’?” Last verified 17 Aug 2026. https://quotle.info/who-said/but-the-best-evidence-we-have-that-time-travel-is-not/

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