# How would wormholes and teleportation affect the passage of time?

Time is not a constant, Albert Einstein proved that much. Time dilation is a difference in the elapsed time measured by two clocks, either due to them having a velocity relative to each other, or by there being a gravitational potential difference between their locations. In the vast universe we can agree about distances but not about time.

Here is my question: How would a wormhole and/or teleportation affect time between two observers on different planets? What are the dangers that need to be taken into account? How can this be taken advantage of?

There might not be a definitive answer due to the fact that we haven't invented faster-than-light travel or teleportation. But knowing this helps from a writing standpoint as it allows to know how it will affect the story if not at all. Would a traveler come back home after the exact time he spent on another planet or would he be too late? If a portal was kept open between two planets would time be a constant? If a portal is closed and then opened again will time have passed into another century? These sorts of questions.

• How happy are you with allowing causal violations? Jan 4, 2020 at 17:57
• Also, you are incorrect: Relativity does not just mean time dilation. It also causes space compression - the universe appears to be squeezed in the direction of travel by the same factor that time gets stretched. Jan 4, 2020 at 18:55
• The correct answer is probably "time-travel", but it's apparently impossible to explain why without a deep enough understanding of Relativity to derive it yourself. Ignoring that, I'd assume that clocks are running at different rates on either end of the wormhole / teleportation channel, due to differences in their velocities relative to each other. The trouble is: what time are you teleporting to, when there is no true "now"? And is there one channel, or can there be several? Jan 4, 2020 at 19:01
• I actually have a question about that. As far as I'm aware Einstein shows that the closer to lightspeed you get, the slower an object can experience changes to itself. So it doesnt slow time, but the speed at which time seems to move according to the object moving at that speed. Yet everyone says its about time, can anyone tell my exactly why its about specifically time, and not the speed at which the object changes? Jan 4, 2020 at 19:22
• @CAEJones are you sure? We have experiments with light tunneling passed an object. If you measured the speed of the photon around the area it tunneled it would seem like it traveled faster than light as it crossed more distance in the same timeframe than the speed of light would allow. Are there constant time travel instances whenever things tunnel themselves around? Jan 4, 2020 at 19:25

Note: I'll take about time travel and causality violation, but what I mostly mean is the creation of Closed Timelike Curves. These are the things you use to go back and kill yourself before you travel through time, and are generally considered to be a bad thing.

Wormholes, by themselves, do not necessarily violate causality. You can induce a time difference between one end of the wormhole and the other (by flying it around at relativistic speeds) but various theories suggest that when you try to use this construct a time machine (by moving temporally separated wormhole mouths close to each other) you get Interesting Effects which cause the wormholes to collapse. For more information, read up on Hawking's Chronology protection conjecture and structures such as Roman rings, but for now you can just assume that in a Universe with chronology protection, you can't violate causality with a network of wormholes.

Wormholes in this sort of friendly universe are relatively safe. Such wormholes are shortcuts through spacetime, but light travelling through them still behaves as you might expect and you don't get the kind of issues that "real" FTL travel or signalling might cause. The price you have to pay is that wormhole mouths get moved around at sublight (but possibly relativistic) speeds. This means that the initial journey between two stars takes many years, but subsequent ones are faster.

Teleportation, if it allows travel faster than lightspeed, will ruin all that. All bets are off. Chronology protection in the presence of FTL travel and signalling is a vastly more complex affair. Creating a wormhole where the two mouths are already widely separated (eg. one end is near a different star, or even planet) is effectively teleporting things around and has the same problems.

It is possible to conjure up a universe where FTL does not result in time travel, perhaps as a result of the existence of a privileged reference frame, but discussion of such things are a bit outside the scope of this question. Given the relativity seems to be a real thing, adding special frames like this in a hard scifi setting is likely to be awkward. You might reasonably assume that all soft scifi settings have one of these magical reference frames, even assuming they cared about relativity in the first place, which they probably didn't

Now, with regards to wormholes:

Would a traveler come back home after the exact time he spent on another planet or would he be too late? If a portal was kept open between two planets would time be a constant?

Wormholes link points in spacetime. When you travel through a wormhole, you have travelled in both time and space. If the two mouths of the wormhole were "stationary" relative to each other, then if you travelled through the wormhole, waited 5 minutes and travelled back, you'd arrive 5 minutes after you left, plus however long the journey through the wormhole took (which you could handwave to be more or less instantaneous, but is likely to take a finite amount of time).

If one end of the wormhole was travelling at high speeds relative to the other end, clocks would of course tick at different rates at either end of the wormhole. You could travel out to a relativistic starship, wait a while, but when you've returned more time will have passed at your origin than you spent at the destination.

If a portal is closed and then opened again will time have passed into another century?

It depends how you re-open the wormhole. If you had to create a new wormhole and then fly one mouth to a distant star, then even if you flew it at high relativistic speeds relative to the source many years will pass before it arrives at its destination. If you were in a universe with chronology protection, this is a safe thing to do, and no time travelling will occur.

If you shrunk a wormhole down to impassibility but kept the mouths in place, you could re-inflate it later and it would be as if it had been left open during the intervening time.

If you can teleport the wormhole faster than light, all bets are off, because this is FTL and that means you're in grave danger of time travel. It is up to you what happens then, because all bets are off.

I can recommend reading the Orion's Arm setting article, Wormholes - A Layman's Guide, for a setting with safe wormholes and a decent amount of thought put into the science behind the whole thing.

Another setting is The Verge by Luke Campbell, which also has some hard-sciency wormholes but with fewer restrictions than Orion's Arm. Scrolling down to the wormhole section in the setting's technology blurb, or reading the shorter bit on wormhole warfare might be interesting.

# FTL is a time machine

I don't a hundred percent follow the explanation, but teleportation allows information to travel back in time:

...

According to us, on Earth, the order of events is thus: we see the light from the event hit us. We call Proxima Centauri on the FTL phone. The Proximal Centaurians do whatever they want to do in response to that call, and then they see the light of the event.

What does the ship see? They see the phone call received on Proxima Centauri. Then they see the phone call placed from Earth. Effect precedes cause: causality is violated. In fact, if the ship had a FTL phone set up in the right way, they could call Earth before Earth placed the call. They could even tell Earth "hey, don't make that call to Proxima Centauri we just saw you make." Then what?

• This is what I mean by it not being comprehensible without mastery of the underlying theory. Intuitively, I'd say the message took two paths, and opening up a third does not let you ignore the shorter of the two. It's explained as though you'd be teleporting into the image, like a Cosmic Blue Skidoo. The closest an explanation has come to making sense to me is where everything has velocity through space and time, and changing one requires changing the other, such that FTL through space = negative time. How that interacts with wormholes et al is not so clear. Reference frames are confusing. Jan 5, 2020 at 8:13