Here is a really good example I saw yesterday - I watched the Doctor Who episode “The Pyramids of Mars”.
The Fourth Doctor (Tom Baker) has been compelled by an alien Osirian called Sutekh to travel to Mars in the TARDIS to enable the destruction of the Eye of Horus, a transmitter that is maintaining the imprisonment of Sutekh on Earth. The Doctor avoids death by strangulation (respiratory bypass, to go with a second heart), but cannot prevent the destruction of the Eye.
The Doctor and Sarah then return to Earth, and the Doctor uses the TARDIS console to manipulate Sutekhs time/space portal to trap Sutekh in the portal for thousands of years, causing him to die of old age.
The causality violating bit is this - the Doctor realises that the light cone of the Eye of Horus (a radio transmission) will take several minutes (he says 2) to travel from Mars to Earth, so Sutekh is still trapped. The TARDIS, however, can take no time to get from Mars to Earth. So the Doctor can arrive on Earth, remove the TARDIS console, plug it in to Sutekhs equipment and rig the interface protocols before Sutekh is free.
From a relativistic point of view, turning off the Eye of Horus is simultaneous with Sutekh being released on Earth. FTL travel gets you from Earth to a point in time before the Eye is disabled, violating causality.
As an aside, I think it’s important to accept with this stuff that it’s not going to make intuitive sense.
ISTM when people study relativity they want to relate everything back to their everyday experience.
But in our everyday experience, all motion is relative – there are no invariant speeds (though we have planet earth to helpfully use as a reference frame), and everyone agrees on distances and times.
So things like time dilation and invariant c you just have to accept is part of how the universe works but is difficult for us to relate to.
It doesn’t help that most pop sci explanations of why FTL implies time travel describe it as though it is just seeing events in reverse that is the issue (as if just taping a shuttle launch and watching the tape in reverse would break causality).
But this is only because Einstein chose to use time as a dimension - yes? Isn’t it true that he had originally intended to use a relational approach based on Machian dynamics and had he done so there would be no such thing as space-time?
Let’s start out with the idea that at any given moment the clocks on Earth and WLEFFSDR read the same. Because they are 4 light hours apart, if your telescope is on one, and looks at a clock on the other, the distant one will APPEAR to be four hours slow. But that’s simply because it took four hours for the light from that clock to reach you; as I understand the OP’s wormhole, if you look at a clock that is just on the other side of the wormhole, they’ll be the same.
Okay, now let’s say the football game ends at 6 PM. The poor people on WLEFFSDR won’t know who the winner is until 10 PM, because they are looking through telescopes. But someone who has an Ikea(R) Wormhole will know earlier than the neighbors. He could easily place his bet at 7 PM or 9 PM and be guaranteed a win.
But that doesn’t violate causality. All it accomplishes is quicker information about an event in the recent past. I think the OP is asking for a scenario where you could step through the wormhole and actually change the outcome of the game. If this device does violate causality, then there must be a way to do that. But I can’t construct such a case.
The question is: “By what logistic can FTL travel violate causality?” This is a GQ question. If you disagree, you should report it, not try to derail it.
My wormhole doesn’t have that setting. I can only input space coordinates, not a time coordinate.
How do you see this working? Assuming I can sit in an unobtainium bubble to overcome tidal forces, etc., how am I going to violate causality? If I hang out inside an event horizon for fifteen minutes subjective time, will I step back out of my portal twenty years ago? Wouldn’t a black hole’s time dilation merely slow down my subjective time relative to an outside observer, effectively sending me forward in time? Or do I have that backwards?
I agree here. Most of the answers so far suggest using some vessel traveling at an appreciable percentage of c, but I don’t know any such vessels. I can’t figure out a scenario by which my wormhole generator,on its own, violates causality, unless it’s cmyk’s method.
Not to say, of course, that the solution with the spaceships is wrong, or doesn’t prove my wormhole would violate causality. I was just hoping for a solution I can actually use once I get this thing assembled.
On a side note, this question was inspired by a sci fi novel containing similar tech. (They had wormholes, but no fast spaceships.) I was a little annoyed I had heard this described as “hard” science fiction because I knew that the wormholes would violate causality. But then I started to think about how they would actually do that, and I couldn’t come up with anything.
That’s right. Merely having a faster-than-light wormhole by itself does not violate causality. You also need to add in the ability to accelerate to some fraction of the speed of light to shift your reference frame, to take advantage of the fact that different reference frames can disagree on the ordering of events. So you need to pass through the wormhole, then shift either your reference frame or the reference frame of the origin, then pass back through the wormhole and arrive before you left.
It seems that a wormhole could be deliberately made into a time machine, if you had a mind to do such a thing; but the fact that there is a discrete jump between locations in space-time mean that you could also have wormholes which are completely free of any possibility of causality problems.
That is, assuming that traversable wormholes are possible at all, which is far from certain.
Park it in orbit around a deep gravity well, like a black hole. That should sufficiently speed up one reference frame.
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[QUOTE=Randy Seltzer]
How do you see this working? Assuming I can sit in an unobtainium bubble to overcome tidal forces, etc., how am I going to violate causality? If I hang out inside an event horizon for fifteen minutes subjective time, will I step back out of my portal twenty years ago? Wouldn’t a black hole’s time dilation merely slow down my subjective time relative to an outside observer, effectively sending me forward in time?
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That’s right; both you, and the wormhole mouth, are flung forward in time. To the future.
But *someone in the future could walk through your wormhole mouth, back into the past, and affect his *(or her) own past timeline. This is a reversal of normal causality.
It always seemed confused to me. Maybe I’m thinking about it wrong.
You have one clock at your origin of the wormhole.
Another clock is at the far end.
The far end moves close to the speed of light. Time ticks by more slowly there.
Let’s say 10 years here registers 1 minute here.
Scenario 1:
Seems to me if you step through and it takes one minute there to turn around and step back, 10 years have gone by on the clock at the origin.
Now this second scenario is how I think people are trying to squeeze out a causal violation .
Scenario 2:
The far end is moving close to the speed of light, either in a circle or a yo-yo or something (I don’t know) where after 10 years of origin time it ends up in the same spot again.
If you go in a space ship out to that point, and say that took 10 years, and you took the wormhole back you would have gone 10 years in the past to the time when you departed.
I think the thinking is that the clocks on both sides of the wormhole act like they are keeping the same time. Because seemingly you could just step back and forth like it was nothing.
Even given the fantastical benefit of a wormhole, scenario 2 makes no sense to me. The instantaneous travel with no acceleration is confused for a lack of relativity.
Seems to me that two people looking at each other through such a wormhole, one would see the other moving slow, the other seeing the first moving fast.
Scenario two is correct, or at least more correct. You could, hypothetically, remain halfway through the wormhole, with your head in one location, your feet in another. The space inside the mouth of the hole is supposed to be almost completely identical to ordinary space outside the hole, so you would be able to survive the experience; but your head would be in the future, and your feet would be in the past.
see http://www.zamandayolculuk.com/cetinbal/TIMEMACHINE.HTM
I dunno. He’s correct if you’re only talking about one wormhole with ends separated in a spacelike (instead of timelike) fashion. In other words, the two ends are far enough apart that a signal which might otherwise cause a causality violation wouldn’t get there fast enough to cause a paradox (while traveling at light speed in normal space).
I see two problems:
First, the author acknowledges that if you bring the two ends close enough to have timelike separation, then you get paradoxes. The author handwaves this away by saying the wormhole is destroyed by the so-called Visser collapse. That seems to be a made-up effect invented strictly to avoid time travel paradoxes, and doesn’t really explain anything.
Second, it seems to be that a series of wormholes which are individually spacelike can be timelike as a whole. If you have a special no-paradoxes rule, then it starts needing to be very clever to prevent time travel completely without preventing wormholes at all.
No, you don’t need anything close to that. Take the Hafele-Keating experiment, which put atomic clocks on an ordinary plane. The eventual time gain was on the order of 100 nanoseconds. It would be trivial to build a circuit which sends signal X through the wormhole if and only if it does not receive signal X from 100 ns in the past (from the other wormhole).
So, if you are watching in normal space one would see the spaceship clock accelerating away in space and also slowing down.
If one watches through the hole there is no acceleration or movement between the clocks so they appear to be keeping the same time.
or
The one end that didn’t look like it went anywhere moved along the time axis normally.
The other end, even though it went back and forth through lots of space, did not move as far in time.
So it’s important to remember it’s a space-time tunnel who’s ends have now obtained some separation on the time axis.
Now they are back to being close in space.
Good thing that hole has ends that can be moved.
That’s as much pondering as I’ve gotten to so far.
I can’t see any reason why a Roman ring would be exempt from any chronological protection effect, if such a thing exists at all; either an event horizon would form, closing the ring, or the virtual particle merry-go-round effect would kick in, causing the ring to collapse.
Yes; if you have even a tiny reversal of causality you can magnify it by going backwards round the circuit an arbitrary number of times, until you reach the point when the 'hole was first opened.
This quote from the page on the Chronology protection conjecturesums it up for me: …is this apparent censorship of closed timelike curves a global constraint of physics, in the same way as a conservation law, or is it a series of accidental coincidences?
Maybe one day we’ll discover that there is a general rule that disallows acausal behavior while allowing other wormholes, but I’m skeptical. It seems “easier” for the universe to simply disallow wormholes in the first place.
I think they’re a good fit for hard sci-fi because one can imagine situations not excluded by the laws of physics where they work, but I don’t think we’re anywhere close to knowing if that’s actually the case.
Well, either what he would have come up with would have been equivalent to the space-time picture, or wrong; whether or not there are relativistic effects is not really a question of what theory you use to describe physics. In fact, there exists an alternative theory of gravity based (more explicitly) on Machian principles, Julian Barbour’s shape dynamics, which is known to be (mostly) equivalent to general relativity. It’s true that shape dynamics makes do without ‘spacetime’ in a sense (in fact, abolishing time as fundamental is one of Barbour’s recurrent themes), but the physics is nevertheless the same. You could also look to the ADM-formulation of general relativity, which partitions space-time up into three-dimensional spatial slices together with a prescription of how to link them; time only appears as a parameter indexing the individual slices of space.
Well, the standard answer is simply that if you instantaneously travel from point A to point B, there exists a frame of reference in which you emerge at point B before you have entered at point A.
I’ll just lazily adapt my answer from here: if you enter the wormhole at t[sub]0[/sub] and transport to B, arriving there at t[sub]1[/sub], then the time it takes to get there is Δt = t[sub]1[/sub] - t[sub]0[/sub] = (B - A)/s, if s is the speed with which you ‘move’ through the wormhole (if you transport instantaneously, then that speed will be infinite; but in fact, all speeds greater than c work).
Now let’s view this from a frame moving at a certain velocity v. Then, all times get Lorentz-transformed: t’ = γ(t - xv/c²). Thus:
But (1 - sv/c²) is not constrained to be positive; in fact, it will be negative if sv > c². Thus, since in special relativity, all v < c are allowed, if s exceeds c only by a smidgen, from some frame of reference, Δt’ will be smaller than 0, and you will arrive at your location before you left.
If you want to travel truly backwards in time, i.e. arrive back on Earth before you left, then that can be done, too, but you need to shuffle back and forth a bit. Let’s say there’s some celestial object, a Planet X somewhere, that moves relatively to the Earth at a good fraction of c (such a planet might, for instance, be found in a galaxy receding from ours due to the expansion of the universe). Fix some time before your start as t[sub]0[/sub] on both the Earth and Planet X. Let some time elapse. Now, due to time dilation, if, say, Planet X is moving relatively to the Earth at 0.87c, only half as much time will have elapsed there; i.e. if you waited ten minutes, five minutes will have ticked by on Planet X. Now, you instantaneously transport yourself there, arriving at t = 5 minutes.
However, neither the frame of Planet X nor that of Earth is preferred in any way, so from the point of view of Planet X, the Earth is moving at a speed of 0.87c, and thus, time there ticks by only half as fast. Thus, when you arrive at Planet X at the five minute mark, only 2.5 minutes will have passed on Earth. You then re-open the wormhole and step back to Earth, arriving 7.5 minutes before you left. Then you destroy the wormhole generator, preventing you from leaving in the first place, and presto, causality violation!
I saw a monopole last week. I ran after it in an attempt to engage it in conversation but it saw me coming, quickly hopped on a bus and got away from me.
OK, remind me, what’s a monopole? Is it a theoretical beast or do scientists know that these things must exist somewhere?
An ordinary magnet has both a north pole and a south pole. Cut the magnet in half, and you’ll create two new poles at the cut, so each piece still has a north pole and a south pole. A magnetic monopole, though, is a north without a south, or vice-versa. Obviously, creating one of these is more difficult than just cutting a dipolar magnet. The laws of physics as we understand them make it pretty clear that they should exist (or at least, it should be possible to create them), but creating them is very difficult, beyond the capability of any phenomenon we know of. A monopole detector (that is to say, a device that would tell you if a monopole did happen to pass through it) is very easy to build, and many have been built, but in all the years we’ve been looking for them, we’ve only once made a detection of something that appeared to be one (February 14, 1982, at Stanford). What exactly caused that event is the topic of some debate, since it’s never been repeated: Maybe it was a real monopole and they’re just very rare and we were lucky to get even one, maybe it was an equipment glitch of some sort that just happened to exactly mimic the signature of a monopole, maybe it was a prank that nobody has yet owned up to, etc.
