Like, it might apply on a "moving through local space faster than light" sense because physics is weird like that, but the whole idea behind wormholes is to shrink the actual distance traveled (from the traveler's point of view).

So, if you're at A, you can travel faster than the speed of light and arrive before B happens. Then, after B happens, you can travel back to before A happens.

> if you're at A, you can travel faster than the speed of light and arrive before B happens

But if we assume A and B happen "at the same time" and your frame of reference is closer to A than B right? As the light showing B happening is still traveling to the observer while A has already arrived.

If that's the case,

> Then, after B happens, you can travel back to before A happens.

Doesn't make sense, If the following happens:

> Traveler starts at A, jumps the wormhole to point B and immediately jumps back to A, arriving 10 seconds after leaving A.

The observer would see:

> Traveler starts at A, arrives back at A + 10 seconds and a while later, they are seen exiting and entering the wormhole at B while also being visible at A (Assuming they didn't do anything after exiting there)

> But if we assume A and B happen "at the same time"

Is something you can’t do in relativity. If you see A happen before B, at the same time as B, or after B, depends on your motion relative to A and B.

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

Suppose you have a wormhole between points A and B. So from A to B - and from B to A - you can go by two different roads, one is pretty long (say, 1 light month), and another is very short (negligible distance).

Next, suppose B end swiftly rotates around point A, so fast that time in B goes slower than time in A. This means clock in B is going slower than in A, and over 20 years in A only 10 years will pass by clock in B. That can be confirmed by observation - telescopes in A will see slower clock in B, telescopes in B will see faster clock in A (it's B going around A, not vice versa).

That is, if we're talking about telescopes, looking through "regular" space. Since A and B are also connected by the wormhole, looking through wormhole will show us that clocks are synchronized. Over the wormhole, A and B are not moving, so clocks don't deviate from one another.

We can stop B flying around A now. Now suppose B clock has accumulated 10 years of difference from A clock - again, looking via "regular" space. We're leaving point A, flying to point B over the "regular" space and spending 5 years - way slower than speed of light, so our clock is practically synchronous to A clock. At the start of flight, A clock show 20 years, and B clock is 10 years, and by the end of flight A clock is 25 years and B clock is 15 years. After arriving at B, we jump into wormhole and get back to A, when A clock shows the same as B clock - that is, 15 years. According to A clock, we left at 20 years and came back at 15 years. Time travel.

Wormhole technology on a ship sent in a relativistic journey circling the solar system, used to bootstrap humanity and influence it in the past.

There is a catch, this sort of time travel like many others only allows one to go back as far as the creation of the wormhole. Sort of like in Primer, it isn't a general form of time travel. You can take the wormhole forward, or from the other end backwards, but not prior to the time it was created.

Both through telescope as through the wormhole, the clocks can be observed to go slower. After 20 years at A, both the telescope as observation through the wormhole show 10 years have passed at B.

If we don't stop B from spinning: The regular space traveler takes 5 years, as observed from A, to reach B, so reaches B at A:25 years, B:12,5 years. If they leave immediately after arrival, they'll end up again at A at A:30y, B:15y. Otoh, the wormhole travel leaves at A:20y ends up at B at 10y and gets back to A immediately: A still at 20y and B at 10y. Mixed travel:regular travel starts at A:20, B:10y, reaches B at A:25y, B:12,5 and jumps back through the wormhole at the same time as seen from A and B.

I've heard of FTL=time-travel a couple of years ago for the first time and I would love to be able to argue with someone knowledgeable about it to understand. I hoped this HN thread would have answers. However all the answers here seem to have holes in same.

I'm starting to wonder if FTL=time-travel isn't like Schrödinger's cat: a hypothetical thought experiment terribly misunderstood by the masses.

Can you please elaborate on that a bit more? That's the part I don't understand. Why would you observe synchronized clocks when looking through the wormhole?

> Why would you observe synchronized clocks when looking through the wormhole?

I think it's the same question as why we observe diverging clocks when looking via the "regular" space. Einstein provided answers to that - in "regular" space (that is, outside the wormhole - the space in wormhole is also space, with properties of space, including relativistic ones) we have accelerated motion, which is absolute and slows down clock in B, but not in A. In "wormhole space" we don't have that, so there is no reason to have, or observe, that difference. I think, other than entrance to the wormhole, you can't really say which of paths is "regular space" - the long one or through the wormhole, as you may argue that in fact A and B are close by, but they also have a wormhole with really long path - and the rest of the Universe - inside.

This allows different observers to reasonably disagree about which came first. Basic relativity.

Alice sees A happen before B. Bob sees B happen before A.

Everyone will agree that once they saw one event, it was too late to reach the other event before it happened, even at the speed of light.

But faster than light? Then you could get there before it happens.

> How can A come before B come before A from, from one observer?

An observer wouldn't see that.

What they might see is someone exiting a wormhole before they entered it.

Let's say Alice observes A, then wormholes over to cause B. And Bob sees B, then wormholes over to cause A.

Anyone watching from the outside will see a logical-seeming series of events: Alice and Bob exit wormholes and each cause an event. A "different" Alice and Bob watch the events and enter wormholes.

Some observers will see Alice exit before entering. Some will see Bob exit before entering. Some will see both. That's obviously weird. But they'll see nothing weird about A and B. A and B are perfectly normal.

We are talking about observers seeing wrong order (leaving the wormhole at point B before entering at point A), but that's because they are observing everything at a delay or something like that, but I don't see any time travel happening in this example or any order of flying through wormholes that I can come up with.

That means that when you analyze the whole system, A causes A. There is a time loop. You could easily make this into a paradox, too. What if Bob goes and prevents A instead? What if events A and B are actually the birth of Alice and Bob, and after they leave the wormhole they land on the other planet and become each other's parents?

that happens all the time. Lets set aside the Earth being curved for a moment - I'll use real cities but these could be asteroids floating in intergalactic space at the same distance.

I'm in London, I see an event happen in London, Moscow, New York and LA all at the same time, at 12:00:00.500 UTC according to me.

  London-Moscow 9ms
  London-NY 66ms
  London-LA 103ms

The events actually occurred in the follow order, at the following milliseconds past 12:00:00 UTC, according to our NTP synced clocks.

  LA: 397ms
  NY: 434ms
  Moscow: 491ms
  London: 500ms

  LA: 397ms
  NY: 471ms
  London: 603ms
  Moscow: 603ms

From NY's POV LA: 434ms NY: 434ms London: 566ms Moscow: 566ms

So LA thinks LA was first, NY was second, London and Moscow co-timed third.

NY thinks LA and NY were co-timed first, then London and Moscow co-timed third.

Moscow thinks Moscow was first, everywhere else was second.

London thinks everywhere was first

So we disagree. So what.

Now sure, if your wormholes are in different reference frames you can get confusions, but why would a wormhole with two ends fixed in the same reference frame and neither end accelerating allow passing information backwards in time?

I know relativity says there is no universal clock everyone agrees on, but isn't that only applicable for points in different reference planes (and even then you can surely adjust if you know your relative velocity to the universal point)?

Back to cosmic scales. Point A, B, C, D, all 10 light years apart, same reference frame

Point A event happens at t+0

B sees at t+10, C and t+20, D at t+30.

D also has a wormhole to A, so sees the event in the wormhole at t+0, and causes a separate event (light up a sign saying "A event just happend")

C would see that D is flagging the event at t+10, but have to wait until t+20 to see it. Why is that bad?

How would D get a signal back to A before t+0. I guess a spaceship in a different reference frame (say v=0.99c) at D could see (old fashioned looking out the window) the "event just happened" message, then use a separate wormhole to send a message to a spaceship at A travelling in the same reference frame as spaceship D. Spaceship A could then light up a message, which planet A could see, but would planet A see that before t+0 on their local clock?

Wormhole from 1 light minute south of A, to 1 light minute south of B, travelling at 0.99c northwards according to the starbase reference frames, both ends in the same reference frame as each other, but in a different one to the starbases.

Everyone looks at A for the time, it says "time = 1200h" at A, B sees "time=1100h", C sees "time=1000h", D sees "time=900h", but they know the distance so can work out that it is currently 1200h, so all set their clocks, which run at the same time as they are in reference frames.

Event occurs at 1200h at starbase B. Event is seen at starbase A and C at 1300h.

How can you use the wormhole to pass information back in time. Assume both ends of each wormhole are in the same reference frame, but aren't changing reference frame (no acceleration)

If we arrange the starbases on the north-south line, I believe that the roaming wormhole would allow a ship to observe the event at starbase B, then jump through and pop out next to starbase A at roughly 1115h. (If that number is wrong, then adjust the wormhole velocity I guess.)

In this scenario, you can't violate causality. You're 100 light hours from B, and the A<->D wormhole is too far away to help.

But if you changed it so that B, C, and D are only 10 light hours from each other, you could then take the wormhole to D. Now it's roughly 1116h, and you're only 20 light hours from B. You can send a signal about the event at 1200h, and it will arrive at 1136h. You sent information back in time.

120h on the wall at starbase A

starbase B looks at starbase A, sees it's 110h, but the distance is 10h, so sets their clock to 120h

starbase C sees starbase B's clock as 110h but knows it's 10 light hours away and A as 100h and 20lh away, so sets their clock to 100+20h or 120h

starbase D also sets their clock at 120h

event occurs at 200h wall clock in starbase B

event is seen at 210h wall clock in starbase A, 210h in C, 220h in D

OK, your spaceship could tell starbase D the event has occured when it pops out at t=201h, but so what. How does starbase D get a message back to starbase B back before to get them to prevent the event from occuring?

But I can make something work with the new numbers. Here:

At 120h according to the starbases, everyone sets their clocks, just like you said.

Event occurs at 200h in starbase B.

A near-light-speed wormhole is passing by B and D in its own reference frame. As far as it can tell, the starbases have their clocks synced really badly. In this wormhole's reference frame, it's simultaneously "200h" at starbase B and "182h" at starbase D.

A messenger ship launches from B right after the event, goes through the wormhole, and lands on starbase D at 183h.

So far, we're paradox-free. We can tell D the results of the event before the light reaches them, but that's not new. People do that all the time with the normal wormhole. And since we're 20 light-hours away from B, any signal we send from here would arrive too late to affect anything.

But then the ship takes the normal wormhole, the one that's always connecting A and D. That wormhole always agrees with the clocks onboard the starbases. The ship enters at 183h, and exits at 183h, now at starbase A.

Then the ship sends a signal toward B. The signal arrives at B at 193h. This is the same location as the event, 7 hours before it happens.

The notion of what is simultaneous to what gets weird. If you and I are moving relative to each other, we're going to disagree on which distant events happen at the same time. If that happens, then whether something's happening now might change depending on reference frame. The ordering can be t(A) = t(B) or t(A) != t(B). Since no frame is privileged, we can also change the other way around, so t(A) != t(B) can be t(A) < t(B) or t(A) > t(B) depending on reference frame.

It's only flexible like that for things that could be simultaneous in some frame, which at least in special relativity means light from one can't have gotten to another yet. I never took general relativity, so who knows how it works in that setting.

https://en.wikipedia.org/wiki/Relativity_of_simultaneity#Tho...

But yes, generally speaking, there is no known means to travel back in time before the moment of creation of the time machine itself.

My notion of simultaneity can be different from that somebody's. If they think event A and B are at the same time, I might think they're different. Waving hands, no frame is privileged, so I could think they're different as (A < B) if I'm moving this way, or (A > B) if I'm moving that way (relative to this person).

That means that the events this person thinks are simultaneous (you leaving and you arriving) can appear in either order to others depending on reference frame. It can look like you arrived when you left (same time), you arrived after you left, or you arrived before you left.

Say you had a faster than light transmitter that can send signals with (from your perspective) twice the speed of light (2c). If the receiver has no relative motion, everything seems to work fine. But if the receiver moves away from you fast enough (I think 0.5c should do it, but don't quote me on that one), then, from the receiver's perspective, the transmission will happen instantaneously. If the receiver moved even faster than that, the signal would seem to arrive before it had been sent(!), and if they had their own faster-than-light transmitter, they could relay the message back into the emitter's past.

Why? I mean, I get that time is progressing more slowly for the receiver because it's traveling at relativistic speeds, but instantaneous arrival of anything doesn't seem to follow from that, and therefore...

> If the receiver moved even faster than that, the signal would seem to arrive before it had been sent(!)

...neither does this.

Further, the whole point of wormholes (in the context of superluminal travel/communication) is that neither the signal nor the receiver is actually moving at relativistic speeds, but rather that the signal is instead taking a shorter path between two points in spacetime specifically to avoid having to move at such speeds from a local perspective.

I'm on the lookout for an explanation of why FTL=time-travel that I'll finally understand. However in your blogpost my limited knowledge of Special relativity fails to make sense of the whole argument.

A day before departing you look and you see your spacecraft in spacedock and, peering through a telescope, your two-year-old spacecraft in orbit around your destination. You’ve been watching it for a year already.

From your viewpoint prior to launch, there’s two of you, and the distant one is old.

EDIT: I’m wrong, and stand corrected. Relativity is so very slippery!

However, when you arrive at your destination, you can look back at Earth and see an image of yourself 1 year prior to your departure.

Also, in regard to your edit: this doesn’t really have anything to do with relativity. Newton would’ve come to the same conclusion.

As for the two images, it’s well covered on PBS Spacetime’s overview of the Alcubierre Drive from several years back that if a bystander were to observe a FTL craft traverse their field of view laterally, they’d indeed see two images, each heading in opposite directions, as you indeed allude to.

You can look at the equations, but you will literally go back in time if you do this. This is simple I believe. Go there, go back, you've gone back in time, up to the SoL time slows then after it reverses. But you can't go 10 seconds to travel 10 light years, speed of light is absolute so no time travel for you.

People believe wormholes are a loop hole because they puncture space or something something Event Horizon.

This I also believe is untrue. Go through a wormhole, either it takes the equivalent of the speed of light to get there for the true distance, or it's not possible to send information through.

If it was simple then I wouldn't be asking.

> Go there, go back, you've gone back in time

How? If it takes me 10 seconds to travel 10 lightyears, then I should've gone forward in time by 10 seocnds, no matter if I'm measuring by my own frame of reference or an observer's (i.e. if it's my stopwatch v. an observer's stopwatch that's counting the seconds).

> But you can't go 10 seconds to travel 10 light years, speed of light is absolute

And it's never been adequately explained to me why that is.

People also thought that time is universal and common for all places.

According to history of physics, theories usually have limits to their applicability. For relativity theory, currently known limits are in the presence of large gravity centers; but it doesn't mean that those are the only limits. Even putting aside elegant, in my view, proposals to circumvent speed limit, like Alcubierre drive, we don't know if this speed limit is "absolute". It's part of very well working theory, yes, but we don't know what we can learn yet.

You get into trouble if there are two wormholes like this that connect points in different reference frames. You could go through the first wormhole at (t=0, x=A) to (t=0, x=B), then the next wormhole at (t'=1, x=B) to (t'=1, x=A), where I'm using t' as the time in this second wormhole's frame. Even if (t=0, x=B) is before (t'=1, x=B) in all frames, there could be some t' frames where (t'=1, x=A) is before (t=0, x=A), letting you return before you left. But at least there's some scenario (only one wormhole exists) that doesn't have tricky causal implications.

And there are other setups where even going back in time isn't paradoxical. Take this viewpoint: You just can't kill your father when you go back. "It didn't happen" is the same as "It won't happen," so it's a non issue. If you go back and try to kill him, then instead of you succeeding, whatever happened will happen (which we already know wasn't your father dying). Mathematically, it's a loop. You can't continue forwards around a loop to get somewhere that isn't behind you. As a tautology, the only loop-containing solutions to the physical equations would be the ones containing self-consistent loops.