The Best Evidence That Time Travel Is Not Possible Is That We Have Not Been Invaded By Hordes of Tourists from the Future

Stephen Hawking? Apocryphal?

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Quote Investigator: There is an argument against the existence of time travel that can be stated as follows:

If time travel were possible, we would have been overrun by tourists from the future by now.

Did the famous physicist Stephen Hawking say something like this? Alternatively, was it said by another physicist or a science fiction writer?

Quote Investigator: In January 1991 Stephen Hawking delivered a lecture at Cambridge University during which he expressed skepticism about the possibility of traveling backward in time: 1

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.

This quotation appeared in the chapter titled “The Future of the Universe” in the collection “Black Holes and Baby Universes and Other Essays” by Hawking, and the date of the speech was specified in a footnote.

Here are additional selected citations in chronological order.

In 1968 the prominent science fiction author Robert Silverberg released a novel called “The Masks of Time” that mentioned the possibility of a large number of tourists visiting from the future, but Silverberg did not argue that time travel was impossible; indeed, a character named Vornan in his book was a time traveler: 2

Last fall everyone thought the world was about to end, and now everyone thinks we’re going to fill up with tourists from the future. I watch the faces of the people on the screens, the ones who follow Vornan around, cheering, kneeling. He’s like a messiah.

In January 1991 Stephen Hawking presented a lecture with the quotation given previously. He used the phrase “Chronology Protection Hypothesis” to label the conjecture that the laws of physics prevent backward time travel: 3

There seems to be a Chronology Protection Agency that makes the world safe for historians by preventing travel into the past. What seems to happen is that the effects of the uncertainty principle would cause there to be a large amount of radiation if one traveled into the past.

This radiation would either warp space-time so much that it would not be possible to go back in time, or it would cause space-time to come to an end in a singularity like the big bang and the big crunch. Either way, our past would be safe from evil-minded persons. The Chronology Protection Hypothesis is supported by some recent calculations that I and other people have done.

In January 1992 the “Chicago Tribune” printed an instance of the statement by Hawking in a feature called “Quotes of the Day”: 4

ENGLISH PHYSICIST STEPHEN HAWKING: “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.”

In April 1992 the journal “Science” published a news item that included quoted remarks from a draft article by Hawking: 5

In an effort to prohibit time machines of any design, Hawking has just completed a manuscript, called “The Chronology Protection Conjecture,” arguing that the laws of physics forbid closed timelike curves. Somewhat tongue-in-cheek, Hawking cites as “strong empirical evidence” for his conjecture the fact that “we have not been invaded by hordes of tourists from the future.”

In July 1992 Stephen Hawking published “Chronology Protection Conjecture” in the journal “Physical Review D”. The article was originally submitted for publication in September 1991. The following passage communicates the idea of the quotation, but it is does not provide a self-contained pithy expression: 6

Either way, there seem to be theoretical reasons to believe the chronology protection conjecture: The laws of physics prevent the appearance of closed time like curves.

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.

In 1994 “U.S. News & World Report” magazine printed an article that reworded Hawking’s objection by employing the phrase “overrun by tourists”. This version of the statement was similar to the one given by the questioner: 7

The biggest objection to time travel may arise from common sense rather than theory. Physicist Hawking, whose collection of essays Black Holes and Baby Universes (Bantam Books) came out last fall, argues that time travel must not be possible, since if it were, modern civilization would already have been overrun by tourists from the future.

In 2004 “Popular Science” magazine printed a version of the saying credited to Hawking, but the periodical did not use quotation marks: 8

His most famous science-fiction-writer-befuddling question: If time travel is possible, why haven’t we been overrun by tourists from the future?

In conclusion, Stephen Hawking did express the idea described by the questioner in a 1991 lecture and a 1992 scientific article. The statement in January 1991 is the most compact and quoteworthy.

Image Notes: Time Machine image from Wikimedia Commons. Stephen Hawking image from NASA (public domain) via Wikipedia.

(Great thanks to Joel S. Berson for his remark initiating a discussion on this topic. QI formulated the question and answer.)

Notes:

  1. 1993, Black Holes and Baby Universes and Other Essays by Stephen Hawking, Essay: 13: The Future of the Universe, (Asterisk footnote: “Darwin lecture given at the University of Cambridge in January 1991.”), Start page 141, Quote Page 154, Bantam Books, New York. (Verified with scans)
  2. 1968, The Masks of Time by Robert Silverberg, Quote Page: 151 and 152, Berkley Publishing Corporation, New York. (Originally published in 1968; verified with scans of 1978 paperback edition)
  3. 1993, Black Holes and Baby Universes and Other Essays by Stephen Hawking, Essay: 13: The Future of the Universe, (Asterisk footnote: “Darwin lecture given at the University of Cambridge in January 1991.”), Start page 141, Quote Page 154, Bantam Books, New York. (Verified with scans)
  4. 1992 January 28, Chicago Tribune, Evening People: Quotes of the Day, Quote Page 2, Chicago, Illinois. (Evening Update, C Edition)(ProQuest)
  5. 1992 April 10, Science, “Could a Pair of Cosmic Strings Open a Route Into the Past?” by John Travis, Start Page 179, Quote Page 180, Published by American Association for the Advancement of Science. (JSTOR) link
  6. 1992 July 15, Physical Review D, Volume 46, Issue 2, “Chronology protection conjecture” by S. W. Hawking, (Article Received: September 23, 1991), Start Page: 603, Quote Page: 610, Published by The American Physical Society. (Article from American Physical Society website aps.org)
  7. 1994 May 9, U.S. News & World Report, Volume 116 Issue 18, Alternative realities by William F. Allman, Start Page 59, Published by U.S. News & World Report. (Academic Search Elite)
  8. 2004 February, Popular Science, Volume 264, Number 2, Science: The Cliffs Notes, Subject: Time Travel, Quote Page 65, Published by Bonnier Corporation. (Google Books Full View) link