Simply put, let's say there is a portal that leads to the past, and that it's "catching up" : an observator from the present looking into the past sees time go faster, and an observator in the opposite direction would see it go much slower. Let's say the speed ratio is 3, for example, or n to generalize, meaning if a TV displaying The Matrix were placed on the past side of the portal, an observer from the present side would see it span 45 minutes if n=3 (original duration is 136 minutes).

What would be the additional visual consequences or artifacts, as seen from either side ?

One evident thing is that the past world would seem very bright to the present observer and that the present would look very dim to a past observer. That's because assuming both sides of the portal are under similar sunlight conditions, the photons flowing through the portal in one way would be "packed" a lot more than in the other.

Would there be a consequence in the chromatic sense ? I somewhat figured there should be an shift to the blue, like a Doppler effect, when seen from the present side, but I may have this the wrong way. If I was correct about that, would the blue things appear black, the red things appear blue, and the infrared things appear red ? What would be a rule-of-thumb relation between the time flow ratio between the two sides of the portal, and the color/brightness measurable effects, for example as what color (of what wavelength) would a given color of a given wavelength appear ?

Assume the past side is always the one where time goes faster, and prefer observations from the present side to observations from the past side.

EDIT: adding some details that were misunderstood or ommitted.

There is no causality concern, because the portal's ends are actually two different worlds, I'm using the theory where going back in time branches another universe, coupled with wormholes. But I guess it's not a wormhole in the classical sense of the term then - much less in the sense of general relativity.

Also, do not wonder about the causes of the time flow desync, as the gravity-themed answer does (with very interesting notions), only on its effects. Maybe it doesn't deserve a hard-science tag then, I don't know. Just assume that it's a Portal-like portal, with no matter passing through (yet), just photons.

Come to think of it, and setting aside the brightness difference I mentioned above, wouldn't it behave just like between two elements which conduct light at different speeds, so with refraction-like effects ? At an extreme level, of course, since a factor of 1/3 the speed of light in void is massive.

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    $\begingroup$ I think you've hit all the nails already. Frequencies would be shifted by a factor of n. And that's not only frequency of light, but frequency of events. The rate of seconds appears faster, and movies shorter. $\endgroup$
    – BMF
    Commented Jul 21, 2023 at 23:13
  • $\begingroup$ @BMF Thanks, that's a very simple way of seeing it (pun intended). $\endgroup$ Commented Jul 22, 2023 at 2:14
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    $\begingroup$ With a speed factor of about 2, the reddest light humans can see becomes the bluest light they can see. With 3, all the visible light from the fast side is shifted up to ultra-violet for the slow side, and the visible light on the slow side is the infrared from the fast side. Colours will make no sense at all, but you'll still be able to make out shapes in infrared. $\endgroup$ Commented Jul 22, 2023 at 7:20
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    $\begingroup$ I think you are right about the refraction thing. You could probably calculate it as if the fast side has a refractive index of 1/3 (i.e. light travels 3 times as fast as in vacuum) or alternatively do the calculations from the fast side, say the fast side is 1 and the slow side is 3. $\endgroup$ Commented Jul 22, 2023 at 7:21
  • $\begingroup$ @JBH I wanted answers based on real physics, and the answers I got do just that, talking about frequency shifts, refraction index and all that seems ground-based enough. $\endgroup$ Commented Jul 22, 2023 at 22:59

2 Answers 2


I'm far from an expert in general relativity, but I think your portal will have another property that you haven't considered: An extremely strong gravitational field. The field doesn't need to extend beyond the mouths of the wormhole; keeping it in the "throat" of the wormhole is fine; but it'll need to be there.

The way that wormholes are normally thought to work, time passes at the same rate on both sides, as seen by observers on either side of the wormhole.

That is, say I have two synchronized clocks.

I set one on a table by one mouth of the wormhole, then I pick up the other and carry it through the wormhole. I wait a bit on the other side of the wormhole, then I haul the clock back through. When I compare it to the first clock (still sitting on the table), they will still be perfectly synchronized. The second clock's trip through the wormhole won't cause its timekeeping to diverge from the first clock at all.

This holds even if the exit mouth of the wormhole is located on a spaceship zipping away from Earth at a significant fraction of the speed of light, and also if both mouths are on Earth but one is located thirty years in the past.

Your wormhole does not behave this way.

Say I repeated the experiment with the synchronized clocks, starting on the "present" side of your wormhole. I pick up one clock, step through to the past, wait thirty seconds, and return. Now, the clocks are out of sync. The clock I carried measured thirty seconds, while the clock that remained in the present measured only ten seconds. They're off by twenty seconds, relative to each other.

So... clearly there's some sort of time dilation going on.

The kinds of time dilation seen in special relativity don't help here. As mentioned earlier, "normal" wormholes don't desynchronize clocks, even if one of them is in a relativistic spaceship or orbiting a black hole.

You might be able to get this effect is the exit portal is accelerating extremely rapidly (i.e. on a spaceship that's burning a hideously powerful rocket engine, or sitting on the surface of a stellar remnant- the effect is the same)... but you've also specified that your exit portal is on Earth. So your wormhole must be doing something else.

Which leaves just one thing that I know of: Gravitational time dilation.

Time passes slower the deeper you are in a gravitational field.

According to that Wikipedia page, the Earth's core is about 2.5 years younger than its surface.

If you synchronized two very precise clocks at the Earth's core four billion years ago, shipped it up to the surface, waited until the present day, and brought it back down to your lair in the core, you'd find that the the clock that was on the surface is now 2.5 years ahead of the clock that stayed in the core.

That's the same result we got from when I took a clock into the past using your wormhole. The magnitude of the effect is different, but the effect is the same.

Therefore, your "past" portal must be "above" your present portal, with respect to travel through the wormhole.

Going to the past will require climbing against an extremely strong gravitational field. I don't know how strong precisely, and I imagine you could reduce the strength of the gravity if you make the wormhole very long. Either way, I hope you've brought a ladder. Or, better yet, a winch on the "past" side with a very strong cable.

Returning to the present would require going "down" through the same gravitational gradient. A parachute would help... if all the air in the wormhole weren't getting sucked through to the present with the same acceleration. Once again, a ladder (or a winch, or a fireman's pole, etc.) would probably be your best bet.

On that note, there would also be a great gale of wind rushing into the "past" side of the wormhole and out the "present" side. Watch out for falling debris. (Unless, of course, somebody built a vacuum chamber around the "past" side. In which case, you'll have to bring your own air when you climb the ladder.)

Looking into the portal from the "present" side would be like looking up from somewhere near a black hole. You're right that all the photons coming from the past would be all bunched together. They'd also be blueshifted quite a lot, probably well into the ultraviolet. (Bring sunglasses.) Infrared light might get shifted into the visible spectrum, enabling you to see heat without needing a fancy IR camera.

Also, if you look at the edges of the wormhole, you may see a lot of distorted reflections of your own face and of other things on the "present" side of the wormhole. Gravity does bend light, and the gravity in your wormhole is no exception.

Looking into the wormhole from the "past" side would be the opposite. Not a whole lot of light would make it through, and what does would be redshifted, probably well into the infrared. If the Sun is shining on the "present" side, some of the Sun's ultraviolet light may be shifted into the visible. Or maybe not, I'm not sure.

You may also see reflections of yourself and things behind you on the "past" side. It depends on the exact geometry of the wormhole, but they probably wouldn't be as prominent, and would be relegated to the edges of the wormhole more so than on the "present" side.

All in all, not a method I'd prefer to use in order to get to the past. I'll stick to my nice, well-behaved Ellis wormholes, thank you very much.

  • $\begingroup$ So your wormhole must be doing something else. It could also be experiencing extreme angular acceleration (going around in circles really fast), which would also produce the desired effect. $\endgroup$
    – BMF
    Commented Jul 22, 2023 at 0:01
  • $\begingroup$ Thanks for the very complete answer, though that wasn't exactly what I was looking for. I edited the question to add more details. $\endgroup$ Commented Jul 22, 2023 at 2:12
  • $\begingroup$ @BMF I'm not even entirely certain that having the far end of the wormhole accelerating constantly (whether by rocket, gravity, or centrifuge) would work to give the OP's desired effect at all. After some more thought, I'm leaning toward "probably not", but I'm still not certain. If it does work, though, then yes, a centrifuge would work just as well as a rocket or stellar remnant. $\endgroup$ Commented Jul 22, 2023 at 3:13
  • $\begingroup$ @Gouvernathor If you want to explicitly ignore the gravity gradient, then yeah, you've pretty well already got it covered. The "present" side effectively has a refractive index of 3, close-ish to zinc oxide and on par with cinnabar (en.wikipedia.org/wiki/List_of_refractive_indices). From the "past" side, it'd look a lot like glass. From the "present" side, it'd be like looking out of glass, or out of water, except the "Snell's window" would be smaller. Also, having your body halfway through might be... disruptive to your biology. $\endgroup$ Commented Jul 22, 2023 at 3:33
  • $\begingroup$ So, under high angles from the present, it would act like a mirror ? And from the past, no matter the angle (almost), you would always see what is in front of the portal on the present side and barely what's on the side ? $\endgroup$ Commented Jul 22, 2023 at 4:17

So, the answer is difficult.

I think your conception is wrong. I think both ends of the Wormhole would just look like a Black Event Horizon.

Lets imagine a scenario. We have a Wormhole, one end of which is 1 year in the past. Alice and Bob are our observers. From Alices point of reference, Bob is one year in the future. While for Bob, Alice is one year in the past. What do they see ?

If we apply fairly standard logic to it, Alice could not see anything other than a Event Horizon duo to causality. If she could see light from the Future, that would break causality. So just conceptually, she cant see anything. Alternatively you might say the light she sees is redshifted so much it cant be detected anymore.

For Bob, the answer is a lot harder. Conceptually him seeing the past shouldn't be an issue. We see the past all the time when looking at the stars. So causality is fine. But, we have a different problem. What happens when he enters his side of the Wormhole ?

Well, he cant arrive in the past, because Causality. But he also cant enter the Wormhole at all, because the moment he does his time coordinate would have to decrease. Essentially, he has a White Hole in front of him, where nothing can enter. Within the Wormhole, time Time Coordinate flows "out", away from the center. So there would be an asymptotic relationship at play. Where it is impossible for Bob to ever enter his side of the Wormhole. While for Alice it would be impossible to exit once she entered her Horizon.

You may have noticed that i am just describing an altered version of the Cosmic censorship hypothesis. In this set up it would be strictly impossible for either the past or future to interact with each other. So there would be Horizons on either side. On one side you cant exit, on the other you cant enter. Which also means Bob couldn't actually see the past. He would just see a weird spacetime distortion with light bending around it, maybe glowing. It could also just be black as well.

Probably not the answer you want, but i really dont see how it should work any other way in a Hard Sci-Fi context. Causality is just one of those "unnegotiable" terms.

There is the concept of Retrocausality which might be a saving grace here. But i honestly dont know enough about it.

  • $\begingroup$ It depends on their spatial distance too, which OP hasn't divulged (because it likely isn't relevant to the Q). Bob might be one year in the future, but Alice may be >1 light year away. $\endgroup$
    – BMF
    Commented Jul 21, 2023 at 23:58
  • $\begingroup$ @BMF i had an initial comment which argued this way. But i don't believe that is correct. I dont think seeing the past would conceptually be a break of causality. My issue is more that if you could see the past, you can by definition interact with it. Because light can exit the Wormhole. If that is possible, entering is as well by definition. Which allows the Future to influence the past. Which is a big no no. $\endgroup$
    – ErikHall
    Commented Jul 22, 2023 at 0:03
  • $\begingroup$ Not sure it works that way. Say you make a wormhole pair and send one off to a distant star at high speed. A 100 light-year journey is knocked down to a few years travel time in the frame(s) of the wormholes, and now after only those few years astronauts are exploring the distant system. To them, Sol is "97 years in the past", and they can go to and from at a whim, but they can't influence the future without attempting to send another wormhole to Sol in the same way. Doing so creates a CTC and the scenario you're thinking of. And perhaps certain chronology protections kick in. $\endgroup$
    – BMF
    Commented Jul 22, 2023 at 0:18
  • $\begingroup$ @BMF If we assume both Wormholes have the same time coordinate, there is no paradox involved. Looking through a wormhole, even 100ly away, doesn't break causality because no observer could measure a communication speed >c. You gotta remember that for the Wormhole itself, the distance between either ends =/= 100ly. it is whatever the diameter of the sphere is. The moment different time coordinates are involved, at least the Past -> Future hole has to be a Horizon, because that breaks causality immediately. The Present -> Past one, well i am very certain it would be a white hole like deal. $\endgroup$
    – ErikHall
    Commented Jul 22, 2023 at 0:23
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    $\begingroup$ Okay so the question is asking "This wormhole exists in my world; what are the consequences?" and your answer is "This wormhole doesn't exist in your world." The question is tagged [hard-science] so maybe this is a valid answer but I don't think it's really useful. We can easily postulate that the wormhole doesn't lead into our own past, but an alternative timeline. $\endgroup$ Commented Jul 22, 2023 at 7:16

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