In this mostly hard-science universe I am using a theoretical "Hyperstable Gateway" to travel between planet X and Y; The wormhole has a ~5m diameter, The "Bridge" itself is negligible and a physical ring is used to seal it for use, and there is no "Barrier" or other film over the edge/plane or other obfuscation.

How do you solve the time travel issue between 2 rapidly diverging (~0.08c) bodies? Would there be visual lag across the gate? How would I treat the difference in gravity across the gate? Is potential energy maintained across the gate? Is there anything I'm forgetting?

P.S. I made this account for the sole purpose of asking this question, I hope I worded it clearly!

  • $\begingroup$ Well I don't buy the time travel from a moving wormhole idea. I think if you look through the wormhole that is not moving to the one that is, you will see time run very slow on the other side, and the opposite from the other side. I think the idea that you would see time run at the same pace on either side is very flawed. And, once the perception of time is equalized this way, time travel evaporates as an option. $\endgroup$ Mar 13, 2018 at 6:41
  • $\begingroup$ @ArtisticPhoenix A time differential will be created by special relativity. The velocity involved is too, too small to be significant, so no time running slow. Because both ends are moving with respect to each each other, it require high precision instruments to notice the effect. It doesn't matter which end you look through, the other end will be time dilated but only by a tiny bit. However, you're absolutely right. There's no time travel with the OP's E-R bridge set-up. $\endgroup$
    – a4android
    Mar 13, 2018 at 6:56
  • $\begingroup$ I'm not saying there wont be a time difference, in fact the argument of time travel assumes that you would see time progress at the same rate on both sides of a wormhole, and I think that is fundamentally flawed. $\endgroup$ Mar 13, 2018 at 7:07

2 Answers 2


Let's define time travel through a moving worm hole. This is something I found that is credited to Kip Thorne (or the idea is)

Kip Thorne takes his wormhole and puts one end in his living room, and the other aboard a spaceship parked in his front yard. Thorne's wife, Carolee, hops aboard the spaceship to prepare for a trip. The two don't have to say goodbye, though, because no matter how far away Coralee travels, they can see each other through the wormhole. They can even hold hands, as if through an open doorway.

Carolee starts up the spaceship, heads into space and travels for six hours at the speed of light. She then turns around and comes back home traveling at the same speed — a round trip of 12 hours. Thorne watches through the wormhole and sees this trip occur. He sees Coralee return from her trip, land on the front lawn, get out of the spaceship and head into the house.

But when Thorne looks out the window in his own world, his front lawn is empty. Coralee has not returned. Because she traveled at the speed of light, time slowed down for her: What was 12 hours for her was 10 years for Thorne back on Earth.

Now, as Thorne and Coralee hold hands through the wormhole, they are each traveling in time. Coralee has landed on Earth 10 years after she left, and there she will meet Thorne, 10 years older. But she can still reach through the wormhole and find Thorne, who is only 12 hours older. Thorne can step through the wormhole and find himself 10 years in the future, or his future self can step back 10 years into the past.

At first this sounds possible, but this makes the assumption that the perception of time will be the same to the observer no matter what side of the worm hole they are on.

This breaks down if when Thorne looks in the wormhole he sees Carolee moving very slowly, not only her but everything on the other side of the worm hole. When she is looking through at Thorne's world, she will see everything moving very rapidly.

But she can still reach through the wormhole and find Thorne, who is only 12 hours older.

If his time was speed up when she looked in the wormhole, she could not find him only 12 hours older as he would have aged at an appropriate rate. If she waited 1.2 hours then went through the wormhole, he will have already aged apx 1 year.

Thorne can step through the wormhole and find himself 10 years in the future

If her time was slowed when he looks in the wormhole, stepping though after 12hours would be only minutes after she left. If he wants to step though when she arrives on earth he has to wait the 10 years.

This is where the fundamental assumption takes place

Thorne watches through the wormhole and sees this trip occur. He sees Coralee return from her trip, land on the front lawn, get out of the spaceship and head into the house.

If his view of her through the wormhole was slowed in proportion to her speed, this would never happen. In order to see her finish her trip and land he would have to wait 10 years, because everything on her side of the wormhole moves at an incredibly slow rate.

Now there is no reason to think time would appear to move at a constant rate from the point of view of both observers looking through the wormhole. Well unless I missed something. In fact we know the opposite ,which is that the perception of time can change.

As to what happens across the interface of the wormhole, that can get very sticky. There are many different ideas about that, I would suggest to be consistent in whatever you choose.

  • Would there be visual lag across the gate?

This would depend on the speed of the wormholes relative to each other, as I describe above. It might also depend if it takes some amount of time to cross the wormhole. I could see how that is useful in a story. You would be at a slight disadvantage when crossing. Say it takes 15 minutes to cross, you would be working off information 15 minutes old.

  • How would I treat the difference in gravity across the gate?

Gravity in GR is a feature of the geometry of space time. A wormhole is a topological feature of space time. I think one could argue that a wormhole can cancel out differences in those geometries (from one side to the other). If it couldn't then it probably wouldn't exist.

  • Is potential energy maintained across the gate?

This is different then conservation of energy.

But If you talk about energy (just in general), I can see some things where stuff could get weird. Say you put a wormhole on the floor and it's other end on the ceiling (or some combination of wormholes). Then you drop a ball into it. It would pop out on the ceiling and fall picking up speed go back in the wormhole, pop out on the ceiling and fall picking up an infinite amount of speed. Basically you have a perpetual motion machine (you actual have better because it's accelerating). You could probably harness this unlimited gravitational potential to generate unlimited power. Honestly only way I can think how to square that is that you could put a limit on the energy something has when it passes through the wormhole. Maybe if it has to much energy it can deform space time enough to disrupt the wormhole.

This might have implications on moving a wormhole at high speed, because the more you deform space time the more unstable the wormhole would become.

In reality you can do whatever you want, whatever fits your need. You just have to be consistent and come up with a plausible reason. The big thing is thinking of other ways those rules can affect things, like I just did.

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    $\begingroup$ That was the exact article I was thinking of! That is an elegant dissection of the argument for time discrepancy. But the Conservation of energy issue still stands uncontested, any advice? $\endgroup$
    – Belqinor
    Mar 13, 2018 at 8:29
  • $\begingroup$ Think what happens if you put your arm though in that case, if she did a few minutes then her arm would age days ... lol. Got a cut, just put your arm in the wormhole for a second and it heals instantly (from her point of view) $\endgroup$ Mar 13, 2018 at 8:56
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    $\begingroup$ Another thing is, if you jump into an accelerated frame you could burn up from air friction ( maybe ). ... lol ... this could be reasoned away somewhat, but it's fun to think about. You could run a generator with a wire that goes through the wormhole, for every 1.2 day (on the ship) it's ran 1 year. Make a wonderful source of power. $\endgroup$ Mar 13, 2018 at 8:58
  • $\begingroup$ Thanks for the added explanation! I'll have to introduce rules to prevent "abuse" of the laws of physics. Perhaps a there a field or effect that would produce work against it/ strain on the connections based on mass etc., etc.. I just want a tightly built world with few logical inconsistencies, and this does more than fix them! $\endgroup$
    – Belqinor
    Mar 13, 2018 at 9:40
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    $\begingroup$ @ArtisticPhoenix Not so much healed instantly, but more like it falls off as a rotten lump (and she likely gets blood poisoning when she pulls it back in). $\endgroup$ Mar 13, 2018 at 14:57

The time travel issue isn't an issue. The two planets, I presume, are moving apart at 24,000 km/s (0.08 c), which is remarkably fast for two planets, but we can take that as read. This means there will be an extremely small amount of time dilation relative to each end of the hyperstable gateway. In fact, the amount of time dilation will be effectively miniscule. Although it will increase with time. But it is small enough that it can be ignored for practical purposes. Gateway engineers may be concerned about it, but only in terms of precision engineering.

Will there be a visual lag? No the gateway has a diameter of five metres. The only visual lag will be the same lag in the time it takes light to travel a distance of five metres. Effectively, in ordinary practical terms, it's non-existent.

Difference in gravity between the two ends of the gateway will need to be accommodated by placing each end of the gateway somewhere on the surface or even under their surfaces where the gravitation at both ends is the same. Two planets with the same surface gravity practically solve this for themselves.

Let's assume there is a corridor through the gateway. If there are differences in gravitation between the two ends of the gateway, then depending on which direction someone passes through the gateway they will going up or down hill. Basically the same as walking up or down an inclined floor over a distance of five metres.

The query about potential energy cannot be answered unless we know what potential energy you had in mind. Gravitational potential energy has been dealt with in this answer's previous paragraph. But to clarify this further.

Each end of the gateway will subject to the local gravity of either respective planets this will create a gravitational potential difference between the ends. That's why it is necessary to place at locations where the potential difference will be low.

  • $\begingroup$ "Gateway engineers may be concerned about it, but only in terms of precision engineering" I'd enjoy trying to follow that chain of thought in the future! The gravitational potential energy was what I had in mind, micro fluctuations happen at all times as the internal mass of a planet moves; as such there will be a (albeit minor) discrepancy. How does this not violate the law of conservation of energy? $\endgroup$
    – Belqinor
    Mar 13, 2018 at 8:22
  • $\begingroup$ @Belqinor It doesn't violate conservation of energy. Because, trivially, it can't. Just a matter of going up or down a potential gradient. Whatever goes through either gains or loses kinetic energy. $\endgroup$
    – a4android
    Mar 13, 2018 at 9:14
  • $\begingroup$ How so? By moving into a (even minorly) deeper gravity well without expressing work on the mass not lead to the "infinite kinetic energy" issue expressed so wonderfully in the video game portal? Is there a field or effect that would produce work against it? Or is this an issue of classic misrepresentation from media? $\endgroup$
    – Belqinor
    Mar 13, 2018 at 9:35
  • $\begingroup$ @Belqinor Media misrepresentation. Moving into a deeper gravitational well only adds finite kinetic energy. This is normal physics. Infinite kinetic energy is a fantasy. $\endgroup$
    – a4android
    Mar 13, 2018 at 12:57

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