Thought experiments about time travel
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Thought Experiments About Time Travel — Daily trivia Friday, June 5, 2026: questions and answers
All 10 questions with answers and explanations. Play this daily
Q 01Which car does Doc Brown use to travel through time in Back to the Future?
- AA Ford Mustang
- BA Chevrolet Corvette
- CA DeLorean
- DA Pontiac Firebird
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A DeLorean
A DeLorean. An early draft used a refrigerator instead, until Steven Spielberg vetoed it, worried that children might climb into one.
Q 02Which H.G. Wells novel sends a traveller far into the future to meet the Eloi and the Morlocks?
- AThe Time Machine
- BThe War of the Worlds
- CThe Invisible Man
- DThe Island of Doctor Moreau
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The Time Machine
The Time Machine. In the year 802,701 the childlike Eloi live above ground, while the Morlocks live below and feed on them.
Q 03Which view of time holds that only the past and present exist, with the future yet to come?
- AGrowing block theory
- BEternalism
- CPresentism
- DTemporal determinism
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Growing block theory
Growing block theory. On this view reality keeps getting bigger, as each new moment is added to a past that still exists.
Q 04In the 'grandfather paradox', what contradiction arises if a time traveller kills their grandfather early?
- AYou were never conceived
- BYour grandfather ages backwards
- CYou are duplicated
- DThe machine cannot return
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You were never conceived
You were never conceived, so nobody makes the trip, the grandfather survives, and you are born after all: the loop contradicts itself.
Q 05In the 'Hitler paradox', what is the logical snag once the time traveller stops the dictator's rise?
- AThe traveller ages by the years removed
- BThe reason for the trip no longer exists
- CTwo copies of the traveller now coexist
- DHistory rewrites itself around a substitute
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The reason for the trip no longer exists
The reason for the trip no longer exists, so no one travels back, the dictator rises after all, and the trip is needed again.
Q 06Which Russian physicist's 'self-consistency principle' says only paradox-free trips into the past are possible?
- AAndrei Sakharov
- BIgor Novikov
- CLev Landau
- DYakov Zeldovich
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Igor Novikov
Igor Novikov. He formulated it in the mid-1980s: any event that would change the past has zero probability of happening.
Q 07What is the time-travel loop called in which an object, like a watch passed to its own giver, has no origin?
- AA twin paradox
- BA Fermi paradox
- CA bootstrap paradox
- DA Newcomb paradox
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A bootstrap paradox
A bootstrap paradox. The name borrows the idiom about pulling yourself up by your bootstraps, as in Robert Heinlein's 1941 time-loop story 'By His Bootstraps'.
Q 08Which idea did Stephen Hawking propose to explain why physics seems to forbid travel into the past?
- AThe chronology protection conjecture
- BThe temporal isolation theory
- CThe causal loop principle
- DThe no-boundary proposal
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The chronology protection conjecture
The chronology protection conjecture. Hawking argued in 1992 that laws beyond general relativity must rule out time loops, because building one would take negative energy.
Q 09Why did Stephen Hawking announce his time travellers' party only after it ended?
- AAvoiding crowds
- BDodging reporters
- CPranking colleagues
- DSpotting future visitors
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Spotting future visitors
Spotting future visitors. Only a time traveller could have known to attend, and on 28 June 2009 nobody came.
Q 10Which logician found a rotating-universe solution to Einstein's equations that allows travel into the past?
- ADavid Hilbert
- BJohn von Neumann
- CAlan Turing
- DKurt Gödel
Show answer
Kurt Gödel
Kurt Gödel. He and Einstein were daily walking partners at Princeton's Institute for Advanced Study.
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