In my universe, set in the future, a device allowing inversion (of the arrow of time) has been invented, which is effectively a larger version of the turnstile from Tenet. In fact, this device is so large that a spaceship which can reach 80% of light-speed (as measured from Earth’s reference) can fit inside and be inverted. As it turns out, general relativity is extended in such a way that the laws of physics also apply equally to inverted people or objects from their perspective.

Naturally, a stationary non-inverted observer in Earth’s frame of reference would observe expected relativistic effects when this spaceship (in its reference) is flying at maximum speeds. What would this same observer see if the ship was inverted while flying at maximum speed (in its reference)?

P.S. This universe uses Novikov self-consistency to handle potential time-travel paradoxes.

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    $\begingroup$ I like this question. Perhaps explain what the turnstyle is -- for people who have not seen TENET and also why you think the spaceship going from Earth to Mars might look different from the flipped ship going from Earth to Mars. $\endgroup$
    – Daron
    Commented Apr 7, 2022 at 0:44
  • $\begingroup$ @Daron The best I can do to explain is that instead of moving forwards at 1 second/second, the inverted object moves backwards, instead moving at -1 second/second. Since the end time would be before the start time, the speed (calculated by distance divided by time) would appear to be negative and it’s unclear to me how relativistic effects might be different. $\endgroup$ Commented Apr 7, 2022 at 0:49
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    $\begingroup$ @ReinstateMonica3167040 It wouldn't be any different. Relativity is invariant under time symmetry. It works exactly the same with either arrow of time. (As does quantum field theory, and pretty much everything else, which is why figuring out where the arrow of time comes from is such a thorny theoretical issue.) $\endgroup$ Commented Apr 7, 2022 at 1:21
  • $\begingroup$ @LoganR.Kearsley I thought it was established that the arrow of time points in the direction it does because of entropy? $\endgroup$
    – Tom
    Commented Apr 7, 2022 at 1:39
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    $\begingroup$ @Tom That is one popular theory, but not universally accepted. Either way, though, special and general theory both remain invariant under exchange of sign in the time axis. $\endgroup$ Commented Apr 7, 2022 at 1:57

1 Answer 1


The only real difference that the arrow of time makes is where the eyeballs are pointed. From our perspective, the people are watching where they've been instead of where they're going.

Tennet got most of the physics seriously wrong. It made for some interesting weirdness, though, so I suspended my disbelief. For instance, time-inverted bullet shells to jump into your hand, you would be able to feel the impact on the table combining to throw the shells upward. Since the table itself wasn't time-reversed, it wouldn't cooperate. This leads to thinking of the energy/momentum itself being time-reversed instead of the material, and that would end pretty much as soon as the object was dropped by the time-reversed entity whose metabolism was providing the time-reversed energy. The chemical energy stored in the bullets would work this way, pushing the bullets back in time, but the shell would just be an empty shell until you loaded it into a time-reversed gun.

Think in terms of a seismic disturbance throwing a base ball off of the ground, where a bunch of air pushes the ball in an arc to where a batter is ready to catch the ball with just enough force to bounce it back to the pitcher.

With a spaceship, a time-reversed spaceship would theoretically look like it was passing through an area of space that had been cleared of debris in anticipation of its passing. You would see thrust being sucked back into the engines, slowing the ship down.


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