You can try to answer these 37 time travel questions below yourself, or else go to the end of the list of questions to see each question immediately followed by its answer.
Left image: Artist’s conception of “Quantum foam” showing the discontinuous nature of space and time at the very smallest distance scales; right image: Artist’s conception of a wormhole connecting two remote regions of spacetime, and a “closed timelike curve” that could make time travel possible.
- Are we not time-traveling right now?
- What are the different types of time travel?
- Which type of time travel is certainly possible?
- What is time dilation?
- How can very large time dilation effects be observed?
- What is the twin paradox?
- How is the paradox resolved?
- How is the twin paradox time travel?
- Would faster-than-light travel result in backward time travel?
- Are faster-than-light speeds possible in relativity?
- Didn’t the very early universe supposedly expand much faster-than-light?
- Do faster-than-light particles (tachyons) exist?
- Are faster-than-light speed spaceships (“Alcubierre warp drive”) possible?
- Why does General Relativity allow time travel?
- Have the effects of gravity on time been observed?
- Where are the greatest effects of gravity on time?
- What kind of entity might make backwards time travel possible?
- Who first suggested the possibility of wormholes?
- What is quantum entanglement?
- Has entanglement been observed?
- Why did Einstein regard entanglement as “spooky?”
- How is entanglement related to wormholes?
- What would be necessary for a wormhole to function as a time machine?
- Do stable wormholes exist?
- Could wormholes masquerade as black holes?
- Why do physicists believe that wormholes exist at the microscopic level?
- At what distance scale does spacetime become “foamy” or discontinuous?
- Could microscopic wormholes be enlarged to allow them to function as time machines?
- Exactly how might a wormhole time machine work?
- What other two structures besides a wormhole might function as a time machine?
- What is the chronology protection conjecture?
- Through what work of fiction did the concept of time travel first enter popular consciousness?
- Why might H.G. Wells be an excellent candidate for being a time traveler?
- When did public interest in time travel really take off?
- What is the grandfather paradox?
- What are some ways the grandfather paradox could be resolved?
- If time travel were possible, where are all the tourists from the future?
Questions with answers
1.Are we not time-traveling right now?
Yes, at the rate of one second every second. We also time travel mentally, when we access our memories or contemplate the future, but neither of those things corresponds to what is normally meant by time travel.
2. What are the different types of time travel?
We can imagine jumping to some future or past time, and doing so in a manner that is either one-way or round trip, making for a total of four types: one-way to the future, one-way to the past, two-way to the future, and two-way to the past.
3. Which type of time travel is certainly possible?
One-way to the future is certainly possible based on the time dilation effect predicted by special relativity.
4.What is time dilation?
When two observers are in relative motion each considers their time to pass normally (based on their own clocks), and they judge time to run slow for the moving observer. This is sometimes expressed as “moving clocks run slow.” The problem with that wording is that for two uniformly moving observers each can consider themselves at rest and the other one in motion.
5. How can very large time dilation effects be observed?
The size of the effect increases as the relative speed increases, and it becomes very large close to the speed of light, which can occur for subatomic particles in an accelerator or for cosmic rays that bombard the Earth from space.
6. What is the twin paradox?
If one twin stays on Earth, and the other travels to a distant star at close to the speed of light, and then returns, the spaceship twin will be younger when they return. It seems to be a paradox, because why cannot Each twin regard time to run slow for the other?
7. How is the paradox resolved?
Only the space-traveling twin feels the large accelerations and decelerations involved in the trip, and there is no symmetry between the two of them. The returning twin really will have less time passed according to relativity, and they will be the younger one.
8. How is the twin paradox time travel?
If the space twin were to travel on average at 98% the speed of light, relativity predicts that when they make a round trip to a star 1.02 lightyears away that the journey will have lasted two years for them, while 10 years will have passed on Earth.
9. Would faster-than-light travel result in backward time travel?
Yes, in principle, but a simple back and forth wouldn’t do it, and an intermediate step apparently is required.
10. Are faster-than-light speeds possible in relativity?
Speeds not involving the actual motion of matter or information are certainly possible. For example, if you sweep your laser pointer across the surface of the moon, the spot on the moon could travel faster-than-light across the moon.
11, Didn’t the very early universe supposedly expand much faster-than-light?
Yes, according to theory there was a period of “inflation” right after the big bang where this occurred. However, this was an expansion of spacetime itself, rather than a motion of matter through space.
12. Do faster-than-light particles (tachyons) exist?
These hypothetical particles were suggested in 1962 as a “loophole” in relativity, but their existence is at this moment uncertain.
13. Are faster-than-light speed spaceships (“Alcubierre warp drive”) possible?
The idea is extremely speculative, but Mexican theoretical physicist Miguel Alcubierre, suggested a solution to Einstein’s General Relativity by which a spacecraft could achieve apparent faster-than-light travel. However, this solution required the existence of negative mass or energy density which may not exist.
14. Why does General Relativity allow time travel?
Because in GR the presence of mass distorts the fabric of spacetime, so that with the appropriate kind and distribution of matter one can find a solution to the GR equations that allow you to travel back to some earlier time by following a loop or “closed time-like curve.”
15. Have the effects of gravity on time been observed?
Yes, time runs a bit slower when gravity is stronger, say in the basement of a tall building compared to the top floor, which is further from the center of the Earth. The effect, though very small, is still large enough so that GPS wouldn’t work if we didn’t account for it.
16. Where are the greatest effects of gravity on time?
Where gravity is strongest, say near a supermassive black hole.
17. What kind of entity might make backwards time travel possible?
The hypothetical wormhole – a solution to Einstein’s general relativity equations in which a tunnel or bridge connects two remote regions in spacetime – see right figure above.
18. Who first suggested the possibility of wormholes?
Einstein and Nathan Rosen in 1935. These Einstein-Rosen bridges were later renamed wormholes by John Wheeler who doubted they would be stable.
19. What is quantum entanglement?
It is the phenomenon that occurs when a pair of particles is generated, and they interact in a way such that the quantum state of each particle cannot be described independently of the state of the other even when they are very far apart. Entanglement was described by Einstein, Podolsky and Rosen (EPR) in 1935
20. Has entanglement been observed?
Yes, for example, if their total spin of a pair of particles is known to be zero, and one particle is found to have clockwise spin about an axis, then the spin of the other particle, measured on the same axis, must be found to be counterclockwise.
21. Why did Einstein regard entanglement as “spooky?”
Because somehow the fact that the first particle had clockwise spin is communicated to the second particle instantaneously no matter how far away it is.
22. How is entanglement related to wormholes?
The two are related because in both cases remote regions of spacetime have a “shortcut” linkage. Moreover, since we know empirically that entanglement occurs, this suggests that wormholes should exist even if they have not yet been observed. It is no accident that entanglement and wormholes were both described in 1935 papers by two of the same authors (Einstein and Rosen).
23. What would be necessary for a wormhole to function as a time machine?
It needs to be stable and traversable, meaning you wouldn’t be torn apart when traveling through it.
24. Do stable wormholes exist?
Physicists used to be highly dubious, but now some are less so. In fact, no one knows.
25. Could wormholes masquerade as black holes?
That is quite possible, at least when observed from a large distance. Some have suggested that what is believed to be a supermassive black hole at the center of our galaxy might really be a wormhole. There are ways to distinguish the two, but they are subtle, and better observations are needed to tell.
26. Why do physicists believe that wormholes exist at the microscopic level?
At very small distances it is believed that spacetime would consist of many small, ever-changing regions in which space and time are not definite, but they fluctuate in a foam-like manner (see left figure above) giving rise to all kinds of structures including wormholes.
27. At what distance scale does spacetime become “foamy” or discontinuous?
This would be the Planck length which is roughly equal to 1.6 x 10–35m or about 10-20 times the size of a proton. If a proton were enlarged to be as big as the Earth, then an enlarged Planck unit would be roughly the size of a proton.
28. Could microscopic wormholes be enlarged to allow them to function as time machines?
This would require some unknown technology vastly beyond anything humans are capable of, but some physicists have suggested that a super-advanced alien civilization might have already figured a way to do it, so the universe is filled with them. It is also possible that microscopic wormholes formed right after the big bang became so enlarged during the time of cosmic inflation without any help from aliens.
29. Exactly how might a wormhole time machine work?
A proposed time-travel machine might work by moving one end of the wormhole by some significant fraction of the speed of light, and then bringing it brought back to the point of origin. Time dilation then causes the end of the wormhole that has been moved to have aged less, or become “younger”, than the stationary end; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This means that an observer entering the “younger” end would exit the “older” end at a time when it was the same age as the “younger” end, effectively going back in time as seen by an observer from the outside.
30. What other two hypothetical structures besides a wormhole might function as a time machine?
Tipler cylinders or cosmic strings – the first being artificially constructed, and the second being a remnant from the big bang. Both would permit closed time-like curve solutions and therefore time travel.
31. What is the chronology protection conjecture?
At a time when Stephen Hawking believed that time travel was impossible, he advanced a conjecture (or guess) that the laws of physics would always conspire to make stable wormholes impossible. However, based on his posthumously published final book, he changed his mind about time travel being impossible.
32. Through what work of fiction did the concept of time travel first enter popular consciousness?
“The Time Machine” written by H.G. Wells in 1895, ten years before relativity.
33. Why might H.G. Wells be an excellent candidate for being a time traveler?
In his science “fiction” writings aside from anticipating relativity, with time being the fourth dimension, Herbert George Wells anticipated many weapons of war including tanks, airplanes, lasers, and atomic weapons. He also anticipated wireless communication (radio), satellite TV, virtual assistants, the moon landings, genetic engineering, and a version of the Worldwide Web.
34. When did public interest in time travel really take off?
According to the google ngram viewer the phrase “time travel” featured in a negligible number of books until the early 1960’s. From then until 2008 its frequency expanded roughly 20-fold reaching 0.00003% of all books. By way of contrast the phrase “space travel” appeared in a negligible number of books until 1940 and was until then also regarded as strictly science fiction. The start of the upsurge in the phrase time travel coincides with the date when the possibility of faster-than-light tachyons were first suggested in a 1962 paper. As the graph below illustrates, the two phrases now appear in books with roughly the same frequency.
35. What is the grandfather paradox?
The paradox considers your going back in time and killing your grandfather as a young boy which therefore would have prevented both your mother and you from being born, in which case how could you have committed the dastardly act?
36. What are some ways the grandfather paradox could be resolved?
Various possibilities include something always happening to interfere with the murder, alternate timelines or universes, or free will being an illusion. The various kinds of time travel with the paradoxes each involves are nicely illustrated in this video.
37. If time travel were possible, where are all the tourists from the future?
This question known as the Fermi paradox, raised by the physicist who first suggested it, also has various solutions, the simplest being the rule that no time travel is allowed to times before the first time machine was invented, which has not yet occurred presumably. It is also possible that the number of time travelers to a given era is strictly limited, and they are usually good at blending in – perhaps H.G. Wells being an exception.