Hard SF : SF Tech Issues : Wormhole Time Travel

Kip Thorne has proposed a means that might make time travel (within certain constraints) possible. I read of it in Brian Greene' Fabric Of The Cosmos. This article is based on Greene's presentation of it.

It was suggested that if one could create and maintain a wormhole large enough to travel through, one could try the following. If one of the two wormhole ends could be maintained while transporting it in a spaceship at relativistic speeds, time dilation would result in the two ends being at different times. Therefore, a person traveling through the wormhole would emerge at the time the other end was situated. (Time dilation resulting from relativistic travel is a matter of the rate at which an object experiences time passage - but it only involves time passing in the normal direction towards the future. This means any particular wormhole cannot have an end taken to any time early than the moment the wormhole was first established. If we call the moment the wormhole was established as "the present", this method can only connect "the present" to "the future" - it can never connect to "the past" relative to the establishment of the wormhole.)

Brian Greene notes the issues involved in whether wormholes can ever be used for any purpose. However, he does not raise any objections to the suggestion that time dilation would place the ends of the wormhole at different times. Greene is certainly more knowledgeable in physics than I am, the following comments are not intended to suggest differently.

I don't see how this scheme results in the wormhole's ends existing only in different time periods. Suppose, for the sake of argument, scientists are able to find a way to make practical use of wormholes to connect two locations in space - I'm only talking about wormholes for space travel now. Suppose, I go through the wormhole from Point A to Point B on Monday, will the two ends of the wormhole have a presence on Tuesday for me to return to Point A? It seems to me Thorne's proposal depends on this being the case. For time dilation to make the length of time experienced on the ship considerably different from the length of time experienced off the ship requires the ship to fly at relativistic speeds for a substantial amount of time (as experienced either on or off the ship). So, the same wormhole end on the ship must continue to exist and experience the ship's rate of time passage moment by moment. Meanwhile, the wormhole end that isn't on the ship will also continue to exist and experience its particular rate of time passage.

It seems to me, the two wormhole ends should be like the two people in the "Twins Paradox". In the Twins Paradox, you have two identical twins. One rides on a relativistic spaceship in which the passage of time goes "slower". The twin left behind on Earth experiences time passing more quickly, and so ages more quickly. When the traveling twin returns to Earth, he looks young and his brother looks old. However, the two brothers can meet and talk – they are not isolated in different times.

So let's say the two twins were 30 years old when the spaceship left Earth. For simplicity, I'll call the brother who stays on Earth, "Earthsider" and the brother who went on the spaceship "Traveler". In this version of the story, Earthsider keeps one end of a wormhole in his basement and Traveler takes the other end with him on the spaceship. Between the times the spaceship leaves in 2050 and when it returns to Earth, Earthsider experiences 10 years, he looks 40 years old, his calendar says its 2060 and his wormhole end has collected 10 years worth of dust or whatever. Traveler experiences 5 years, looks 35 years old and his wormhole end only shows 5 years worth of wear and tear.

When the spaceship gets back to Earth, the brothers arrange to meet at Earthsider's house and Traveler decides to bring his wormhole end with him. While the twins are at Earthsider's house, Earthsider walks into his wormhole end and comes out the other side. Is there any reason why he should not come out of the wormhole end that Traveler brought with him to Earthsider’s house that day? And if Earthsider comes out of Traveler's wormhole end at some other time (while Traveler was on the spaceship for the last few years), what makes him come out at one year rather than another? After all, that wormhole end was on the ship during the entire trip and experienced each and every one of the moments in those years – none of those moments are inherently unique in a way that should make it the one and only moment at which someone should emerge from the wormhole.

Similarly, if while the brothers are having their reunion after the spaceship journey, Traveler walks into his wormhole end and out the other, what would make him come out at any time other than the same day? Earthsider’s wormhole end has been experiencing every moment of (from Earth’s frame of reference was) 10 years. It has not even had variations in relativistic effects to give variety to those years. Why come out at one moment rather than another?

If it were possible to anchor Earthsider's wormhole end to the year 2050, so it did not move with the "flow of time" and had no presence / existence in the years 2051, 2052 ... 2060, then I might see how Thorne's idea works. Greene did not mention the proposal requiring such a procedure or discuss how doing so might be consistent with the laws of nature. As best as I can figure, this scheme does not work if all wormhole ends are always naturally anchored at a particular time and only exist at one moment of time - otherwise Traveler's wormhole end would not follow him through those years of dilated time on the ship.

It seems to me that either:

- Wormholes are normally unanchored and some artificial procedure would be needed to anchor Earthsider’s wormhole end, or
- Wormholes are normally anchored and an artificial procedure would be needed to unanchor Traveler’s wormhole end.

In either case, Earthsider’s wormhole end would have no existence in any moment after it became anchored in 2050. It would not be in his basement (or elsewhere) during the following years.

Suppose, Earthsider's wormhole end was anchored and only exists at one moment in 2050. Anyone entering Traveler's end can only come out the other end in 2050. Traveler's wormhole end must have been unanchored for years of time dilation -.having an existence at all those moments. Suppose a photon entered Earthsider's anchored end in 2050 - which of the various moments that Traveler's end has experienced does the photon emerge at? And what determines which moment that is?

For that matter, consider this: If it is necessary to artificially anchor Earthsider’s wormhole end, why does the other wormhole end have to go on a relativistic spaceship? If you can build a wormhole in 2050 that will last for 10 years, and you anchor one end and not the other, won’t the unanchored end have an existence in the year 2060? Wouldn’t someone entering that end in 2060 come out the other end in 2050? If that doesn’t work, why should the relativistic travel change things?

So we seem to come full circle. Anchoring seems to eliminate the necessity of relativistic time dilation to separate the two ends into different times. And since Greene did not discuss an anchoring or unanchoring procedure, it seems even more likely that is not part of Thorne’s proposal. If neither end is anchored, what determines when you come out regardless of whether you enter at Traveler’s or Earthsider’s end?

Suppose, once every day starting in 2050 (according to Earth calendars), one person walks into Earthsider’s wormhole end. Suppose, every day (according to spaceship calendars), one person walks into Traveler’s wormhole end. Do all of the hundreds of people who entered at Travel’s end all get squished together as they all appear at the same moment in 2050 at Earthsider’s end? Do all the hundreds who entered at Earthsider’s end all get squished as they come out simultaneously? Or who comes out when?

I can only think of one factor that would determine which moment at one end of the wormhole will lead to which moment at the other end. If the wormhole itself has a “frame of reference”, then that may determine it. Which events should be treated as simultaneous depends on that frame of reference. But, as I understand it, this does not have to do with subjectivity – the laws of physics dictate which events are simultaneous as far as one frame of reference is concerned. If a person is passing through the wormhole, why shouldn’t it be the wormhole’s frame of reference that determines what moment at one end corresponds to which moment at the other end? I have no particular reason to say the wormhole must do it this way, but what seems most reasonable to me is that at any moment in time there is a point in time at each wormhole end that the wormhole’s frame of reference says corresponds with its “now”. I don’t see why at each and every moment the wormhole should always consider 2050 at Earthsider’s end to correspond to its “now”. Rather, think of the wormhole as an observer that has a different “now” every moment. Each moment the laws of physics tell it certain things are simultaneous and others aren’t. At each moment the wormhole associates appropriate “now” moments at the wormhole ends.

Let's try a different angle. Forget about wormholes for a moment. Here's a thought experiment that might be impractical to carry out, but may illustrate a point:

Traveler's spaceship has a huge spool of telephone wire with one end coming out of the ship and being attached to Earthsider's phone. Traveler's spaceship departs in 2050. It reels out a few million miles of telephone wire and zooms around in circles above the north pole at relativistic speeds. The ship lands on Earth at what Earth calendars say is 2060. Traveler picks up the phone on the ship and dials Earthsider's phone number. Is the Earthsider who answers the phone in 2050 or 2060? If he's in 2060, why is it that telephone wire at Earthsider's house progresses through time from 2050 to 2060, but a wormhole end does not? And, as previously said, if Earthsider's wormhole end (or telephone wire) does not progress in time beyond 2050, why does Traveler's wormhole end (or telephone wire) progress through time? (Traveler's relativistic motion is a matter of the rate of time passage, not a difference between time not passing on Earth and time passing on the ship. Actually, time passes slower on the ship, so if Earthsider's wormhole / wire stays in 2050 and Traveler's wormhole end / wire experiences time advancing slower, how can Traveler's end up later than 2050?)

Another thought experiment: Suppose Earthsider does not stay on Earth. Suppose two spaceships leave Earth in 2050. The first blasts off on Jan. 1 carrying Traveler and one end of a wormhole. On Jan. 2, the other ship takes off carrying Earthsider and the other end of the wormhole. Traveler's ship goes at relativistic speeds such that when the ship returns Earth's calendars say it's 10 years later (Jan. 1, 2060) and Traveler is 5 years older. When Earthsider's ship lands back on Earth, Earth calendars say it's 12 years later (Jan. 2, 2062) and Earthsider is 4 years older (the two ships didn't travel at the same speed). On arrival, what will determine the connected moments at the two ends of the wormhole? The date of ship departure? The number of years experienced on Earth? The number of years experienced on the two ships? Or what?

Neither wormhole end has remained in one place, so neither can justify in that way being in 2050. Did Traveler's wormhole end progress through time during the years of travel, but become situated in 2060 after landing and then not progress through time to 2062 afterwards? Does it make sense to consider a wormhole end on Earth stationary in time because it is not traveling at what we tend to call "relativistic speed" - even though it is moving through space (with the Earth) and experiencing imperceptibly small relativistic effects as a result?

Suppose the two brothers can't wait all those years until the ships land back on Earth to see each other. While both ships are still traveling at relativistic speeds, one of the brothers decides to use the wormhole to visit the other brother. What determines what moment at the other end he will emerge at? Neither wormhole end is stationary or on Earth. Neither end is at the location in space where it began. Neither is experiencing time passage at the rate it originally did before the spaceships launched. So none of those would justify either wormhole end being at a moment in 2050. As said before, a wormhole end on Earth would not be "stationary" or entirely free of relativistic effects either – so is it truly reasonable to imagine it has remained in 2050?

If there’s a physicist reading this that sees the “error of my ways”, please let me know.

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