| back to the future(expression, science, fiction, franchise) |
| time machine(expression, concept, time, travel, theory) |
| time traveller(expression, arrival, visitor, traveller, newcomer) |
| wormhole(noun, opening, passage, entry, hatch) |
| arrive in the future(expression, time, travel, arrival) |
| astronaut(noun, person, space, explorer) |
| doctor who(expression, science, fiction, franchise) |
| forward time travel(expression, concept, time, travel, theory) |
| portal(noun, opening, passage, entry, hatch) |
| arrive in the past(expression, time, travel, arrival) |
| backward time travel(expression, concept, time, travel, theory) |
| black holes(expression, move, time, space) |
| closed time-like curve(expression, time, loop, theory) |
| cosmonaut(noun, person, space, explorer) |
| grandfather paradox(expression, nonsense, paradox, enigma) |
| hypothetical device(expression, concept, science, device, machine) |
| spaceman(noun, person, space, explorer) |
| spacewalker(noun, person, travel, space) |
| time machine research(expression, time, machine, theory) |
| time travel concept(expression, study, thought, theory) |