Codebreaker said 'Nazi'! Godwin's Law! Thread's done, I'm going home for beer. ;)

nezumi.hebereke wrote:

I wanted to bring this up, but not in errata, so I'll make my own space for it... My big gripe is p. 16 - time travel. The arguments regarding time travel aren't ones with special relativity and breaking light speed, but with light cones. This section is a strawman argument. I understand that properly addressing this issue would open a huge can of worms, but honestly, I'd rather they not address it at all than address it wrong.

I think you're misreading what was said on page 16. The second paragraph is clearing up a common conflation of travelling through a wormhole and travelling at a superluminal velocity:

"Gatecrashing, p.16" wrote:

[b]Part of the confusion lies with the fact that gates seem to allow superluminal (faster-thanlight) travel.[/b] In other words, a gatecrasher traveling through a gate to a remote destination four light years away will arrive instantaneously, whereas light itself outside the wormhole will take four years to cross this same distance. This is because gate travel itself works by curving space so that two distant points connect. [b]The speed of light is not exceeded through the gate;[/b] any light passing through the wormhole would still travel far faster than the gatecrasher. Special relativity only applies locally.

The part about SR only applying locally is correct. SR operates on the assumption of a flat spacetime and in the highly curved spacetime of a wormhole, SR would apply over very tiny regions indeed. As you move continuously from locally flat region to locally flat region, the fact that light moves away from you at 3.0x10^5 K/s does not change. The text does, however require you to draw the inference from "The speed of light is not exceeded through the gate..." to [b]therefore no causality violations are occurring due to superluminal travel[/b], which the text does not explicitly state. As for whether or not Gates actually [i]allow[/i] time travel, Gatecrashing punts, giving GMs free reign on the issue:

"Gatecrashing, p.16" wrote:

This theory has yet to be verified with actual experimentation... Other theories have been suggested that could circumvent these problems, though all such debate remains abstract and unproven.

Hope this helps clear up some confusion. If not, let me know. Here's the full text of the sidebar in question for anyone who doesn't have access to a copy of the supplement:

"Gatecrashing, p.16" wrote:

One of the common questions that arises with the Pandora gates and the wormholes they create is: do they allow time travel? The answer, as far as transhuman science currently understands, is: no. At least, not exactly. Part of the confusion lies with the fact that gates seem to allow superluminal (faster-thanlight) travel. In other words, a gatecrasher traveling through a gate to a remote destination four light years away will arrive instantaneously, whereas light itself outside the wormhole will take four years to cross this same distance. This is because gate travel itself works by curving space so that two distant points connect. The speed of light is not exceeded through the gate; any light passing through the wormhole would still travel far faster than the gatecrasher. Special relativity only applies locally. There are researchers who have argued, both currently and in the past, that wormholes could be manipulated to allow time travel. In order to do this, one end of the wormhole would need to be accelerated to a high velocity in relation to the other and then brought back. Within the wormhole, time occurs at a rate separate from that outside the wormhole; two clocks at each end of the wormhole will always remain in sync. To an external observer, however, the accelerated end of the wormhole would have aged much less than the stationary end, due to relativistic time dilation. This means that someone entering the wormhole from the accelerated end would exit the stationary end at a point prior to their entry. This theory has yet to be verified with actual experimentation, and numerous counter-arguments have been raised that suggest that quantum vacuum fluctuations in such a scenario would destroy any wormhole that enabled time travel—or at least anything passing through it, including information. This remains in accordance with the chronology protection conjecture, which strongly suggests that the laws of physics prevent time travel. Other theories have been suggested that could circumvent these problems, though all such debate remains abstract and unproven.

Arenamontanus wrote:

(And, as I have said in countless other threads, there is already backwards in time communication thanks to QE comms, so the can is already open and leaking out little monstrosities...)

If you stick a few QE boxes on those relativistic couriers, you can build yourself a time machine of sorts by sending messages to your past self.

icekatze wrote:

hi hi Any time you have two space-time coordinates moved instantaneously, you have faster than light travel. It doesn't matter whether or not the faster than light travel takes place when the person steps through the gate or simply when the gate is formed. So when the wormhole brings the space-time coordinates from a distant location instantly to your doorstep, FTL is happening. There is no preferred reference frame in the universe, so we have to assume that there is some sort of protection built into the gates to prevent time travel. The simple fact is that it would be [b]more[/b] difficult to build a FTL device that [b]doesn't[/b] involve time travel than it would be to build one that does.

Not quite. What happens with a wormhole is that the [i]distance[/i] between two points is shorter than it would be otherwise. A good example of such a thing is gravitational lensing. There are two types of paths across a gravitational field: 1. Straight through and 2. Bent around. Here's a great picture of what I mean: [img]http://upload.wikimedia.org/wikipedia/commons/0/02/Gravitational_lens-fu... Image Credit: NASA [Public Domain] The wormhole is the silver arrows, the normal route is the orange arrows. It's going to take longer for light to travel the path sketched out by the orange arrows than it would to travel via the silver arrow path. This is exactly analogous to a wormhole - at no time are there any masses moving at superluminal velocities. Hope that makes sense. :D

Ok... at what point does a photon traversing the wormhole exceed the speed of light? It does not. That's the point. FTL is not happening with any objects in the universe in which a wormhole exists. Additionally, I fail to see how the lensing analogy is not exactly (at least in the manner we're talking about here) the same as the wormhole in regards to having two separate paths, one shorter than the other to the same location. A particle of light can be sent on one path (through normal spacetime; along the orange arrow) and intercept the location at one point in time, or it can be sent on another path (through the wormhole; along the silver arrow) and intercept the location at another point in time, which is exactly what you described as a wormhole mechanic. One might be tempted to say that a wormhole connects two distant points at a speed faster than the speed of light, but that would also be wrong. Through what is the wormhole connecting? Nothing. The fabric of spacetime cannot be given a 'speed', it can only be transformed in such a way as to create the possibility of casualty violations.

I think it is important to recognize that the local details of what is going on *do not matter* for the "FTL implies time travel" conclusion. If an event A has a causal effect on something B outside its light cone then there are frames of reference where A precedes B, and other frames where B precedents A. This is shown in every relativity textbook. Just google "FTL implies time travel" for a bunch of explanations. A complication is that in general relativity time itself becomes rather messy in multiply connected spacetimes: the technical term is that there may not exist Cauchy surfaces, breaking global hyperbolicity. I am pretty convinced one can prove that the kind of wormholes EP seem to allow produce this non-hyperbolic structure, which means that if time travel is *not* possible then other weird effects are needed to protect the causal structure. Avoid time travel, have FTL or have normal causality - choose two (at most).

hi hi The uncertainty principle is more of a practical limitation of observation rather than an observation about the nature of matter. Simply put, interaction is necessary to gather data about a particle and the act of observing will change the nature of the particle.

icekatze wrote:

hi hi You're looking at this from a 3 dimensional perspective and assuming a universal frame of reference, however, space and time are interconnected so you can't change one without changing the other. [snipped, it's coming later in the reply] The fabric of space time is tied to mass/gravity and does indeed have a speed, and it has been observed being dragged by spinning masses, propagated at the speed of light.

I'm pretty sure we're arguing the same thing from different perspectives. I agree with you that time travel is implied by the existence of wormholes. Really, I do, and I understand most of the math that says why. That said, at no time, and under no circumstances, in any way, shape or form (can I say this any more strongly? :-P) does anything travelling through a wormhole ever attain a superluminal velocity, from any perspective, frame of reference or observation. Period. End of discussion. The whole point of wormholes is that one can send a mass through them. The only way to have a mass that does anything other than outweigh the entire universe is to make sure that it never attains a superluminal velocity. There is no FTL involved in wormhole travel. What there [i]is[/i] is the ability to violate casualty. FTL is one of many ways to violate casualty. It is not the only way but it is one of the ways that is known to be impossible in this particular universe. You can violate casualty with two cosmic strings; spinning cylindrical universes let you do it with ease... in both of those cases, you can travel back in time without ever attaining infinite mass. The reason why wormholes are such a kick-ass option is precisely because they allow one to skirt that whole infinite mass problem: It takes neither infinite mass to create them, nor does it take infinite mass to traverse them. Again, at no time, in either the creation of, or in the transversal of a wormhole are there any particles exceeding the speed of light. As for the fabric of the universe having a speed, when you understand why you cannot give a meaningful answer to "What is the universe speeding into?" you will understand why the universe cannot be said to have one, even locally. Frame-dragging is a particular formation of spacetime that forces movement on an object, just like being at the top of a gravity well does. We do not say that spacetime has a speed of 9.8m/s^2 on the surface of the earth, just like we do not say that the universe is travelling at 72km/s/Mpc and we don't say that frame-dragging imparts a speed upon the universe. It does not. Rotating masses force an acceleration upon an object within a particular set of locations nearby. Again, I'm pretty sure that we're arguing both sides of the same coin, I agree that casualty can be violated with a wormhole. Getting back to the lensing analogy:

icekatze wrote:

The lensing analogy is false because the photons are traveling along different trajectories, and any change to their world line vector will occur at subliminal speeds. In the wormhole analogy, you could send two photons along the same trajectory and they would have different paths, with the change in paths occurring at superluminal speeds.

You seem to be missing something here, or maybe I am. Let me expand upon what I think you're saying and maybe we'll find out which is which. In your scenario, at time t=0 a photon is sent towards a star. At time t=1 a wormhole is generated somewhere along that path that leads to the same star. At time t=2 a second photon is sent along that path and reaches the star vastly sooner than the first photon. So far so good? If this is the case, then yes, you are misunderstanding and it is the same misunderstanding that leads you to think that spacetime can be assigned a speed. The change in path is not 'occurring at superluminal speeds'. It is an instantaneous reconfiguration of local spacetime. Spacetime does not have a speed. It cannot be assigned a speed. What is the speed of the Cartesian plane under deformation? There is no speed. Objects within spacetime either feel accelerations due to the underlying configuration of spacetime (i.e. local spacetime is curved) or they do not (it is flat). There is nothing else. Hope that helps a little bit.

hi hi

Quote:

End of discussion.

Oh, ok, I guess I'll stop now. Kinda disappointed though.

Arenamontanus wrote:

icekatze wrote:

The uncertainty principle is more of a practical limitation of observation rather than an observation about the nature of matter. Simply put, interaction is necessary to gather data about a particle and the act of observing will change the nature of the particle.

No. The uncertainty principle is very fundamental and independent of measurement method. It is not just a practical issue, it is built into quantum theory at the core. The thing is, certain properties of quantum systems are incomensuable: when two operators do not commute, you get a pair of properties that cannot be measured exactly at the same time (like position and momentum, time and energy, total angular momentum and angular momentum along an axis). This is a mathematical effect and not just that our measurements perturb the system because they are done using imperfect devices. It would presumably apply to psi or any new form of observation we ever devise, as long as they obey the rules of quantum mechanics (and if they don't, then new cans of worms will be yawning).

Question: could you build a teleporter that works by precisely measuring the momentum of all particles that are to be teleported, and thereby spreading their wave-function out over the universe, and then another that measures their location at the target location, while measuring their momentum less accurately, so that the object re-integrates at the target location? Would such a teleporter be limited to the speed of light, or would it be instantaneous?

nick012000 wrote:

Question: could you build a teleporter that works by precisely measuring the momentum of all particles that are to be teleported, and thereby spreading their wave-function out over the universe, and then another that measures their location at the target location, while measuring their momentum less accurately, so that the object re-integrates at the target location? Would such a teleporter be limited to the speed of light, or would it be instantaneous?

The problem is that the wave function will collapse *somewhere*. But you cannot choose where by just setting up a detector. So the teleported particles end up with a very uncertain position, but not where you want them. Besides, to get a certain momentum you probably need to cool them a lot, which might make them form an Einstein-Bose condensate (one way particles can get an uncertain position without shooting off to Andromeda). Quantum mechanics is annoying sometimes. Still, there are cool tricks you can play using the quantum Zeno effect and its relatives.

root@Gatecrashing [hr] I am loving this thread. It isn't that often that I get to see a number of eloquent arguments citing sources and slamming down the math, and have no idea what they are talking about. It makes me warm and fuzzy inside.

Ataraxzy r-rep++

icekatze r-rep++

Areanamontanus r-rep++

It that must no... r-rep++

icekatze wrote:

hi hi

Quote:

End of discussion.

Oh, ok, I guess I'll stop now. Kinda disappointed though.

Now why should you let that stop you? This is the internet! I'm quite enjoying our exchange, but if you wish to drop it, I'm happy to do so.