Seeing into the past?

[Joshua Jones]

@Joshua Jones, if you want to continue that discussion (with a new interlocutor), there's a point (or maybe more) I'd like to add to it.



There's a distinction between what different observers see and what different observers calculate. The order in which you see some series of events is based on when your worldline encounters the light cones from those events. This just comes out of, basically, how close you are to each event and is not a consequence of relativity after all. Like, we will see the light from a nearby supernova before we see the light from a distant supernova just based on light travel times irrespective of the order of events.

But what you calculate comes out of the Lorentz transformation, the mathematical procedure by which you change from one inertial reference frame to another moving at a different speed. This transformation lets you take the spacetime coordinates of some event in one frame (over there, at that time) and translate them to spacetime coordinates in another frame (over here, at this time). When you perform this procedure on two events that are spacelike separated--that is, two events that are outside each other's past light cones and cannot causally influence each other--what you find is that the order of these events is entirely dependent on your velocity (through the Lorentz transformation). So there is no objective fact of the matter about which comes first.

Because this only happens with causally separated events, there is no concern about an "actual" internal sequence leading from one event to the next. This problem doesn't come up for, like, two atoms smashing together and fusing in a red giant, because everything is close together and your past light cones are all overlapping. The trouble with FTL is you can't rely on the light cone to tell you two events are causally separate, but it's still the case that the order is dependent on your frame and the Lorentz transformation. So FTL travel can artificially connect two events causally, but different observers will disagree about which event comes first.

Thanks for this! I'm rather out of tea at the moment, so I cannot guarantee that my thoughts here will be either coherent or addressing the nuance of the issue, but I'm going to give it a shot either way...

That makes sense regarding the epistemological ordering of events, but the epistemology and ontology are two distinct matters. More precisely, suppose two stars, 2 light years apart, have supernova reactions 1 year apart from one another. An observer on the side of one which detonated first will observe the first one detonate, then 3 years later, will observe the other. An observer directly and equidistantly opposite will observe the closer one detonate first, followed by the further one a year later. This seems to be the point you are making in paragraph 1. However, neither observation alters the actual ordering of the phenomenon behind the observations.

What I fail to see is how velocity changes this equation apart from altering perception. It seems to me, so long as matter objectively exists, and objectively interacts with other bits of matter, sequencing must exist apart from the order of observation, even when discussing two non-interacting bits of matter. And, if this is true, I fail to see how FTL would permit information to travel backward in time.

Perhaps an example will clarify my point. A 2x FTL spaceship, say, the Gladius, departs from Spaceport Libertas toward an uninhabited supergiant star 10 light years away. As they hit 5 ly distant, they observe that the star has exploded, rapidly spin and return to the Libertas (my argument should hold if they use a 2x FTL communication instead here). The Gladius (or data from the Gladius) would return 2.5 years before the light from the supernova explosion to inform them of the explosion. It would not, however, be 2.5 years before the explosion itself; it would be 7.5 years subsequent. And, should the Gladius depart again toward the remains of the star, it will re-encounter the observation of the explosion at 1.75 years. However, should it continue on toward the star, it will encounter the remains of the star 10 (plus whatever time is required to report the information, refuel, etc.) years subsequent. So, yes, FTL would enable the communication of information not presently observable, but it would not permit the communication of information before the event occurs which created the information.

I'd go further and suggest that, from the perspective of the the Gladius, the observations aft of the direction of travel would appear to be going backward, while the observations fore of the direction of travel would appear to be rapidly advancing. This is due to the Gladius leaving behind the light behind it, and catching up to light in front of it. So, hypothetically, the Gladius could stop and watch it's own departure, but should it return, it would see it's trip in fast forward up to the point it passes itself in the light cone.

It should also be noted that time would likely be experienced much more slowly progressing by the crew of the Gladius, but this would only increase the rapidity of the fast forward/reverse phenomena described above.

However, the crux of this argument is the premise that perception does not equal reality, in spite of it being the only means for measuring reality. Hence, my position that if matter objectively exists and objectively interacts (i.e. in the absence of observers or in spite of them), the communication of information into the past is impossible, even with FTL technology.

Is there anything I'm missing here?

 

[Ori Vandewalle]

What I fail to see is how velocity changes this equation apart from altering perception. It seems to me, so long as matter objectively exists, and objectively interacts with other bits of matter, sequencing must exist apart from the order of observation, even when discussing two non-interacting bits of matter.

Well, as a rule there is no arrow of time baked into the laws of physics; physical processes are in general reversible. It just so happens that some processes are much more likely than others ("how likely" being a statement about the entropy of a system), so there are plenty of processes we don't end up seeing. But in principle, if a star dies and explodes as a supernova, every interaction in that chain of events could have proceeded the other direction, such that a supernova remnant collapsed into a dying star. It's just, well, even saying "astronomically unlikely" is an understatement there. So there's nothing about a particular sequence of physical events that matters in terms of causality. There's only constant conjunction.

A 2x FTL spaceship, say, the Gladius, departs from Spaceport Libertas toward an uninhabited supergiant star 10 light years away. As they hit 5 ly distant, they observe that the star has exploded...

Ah, but what is the Gladius moving as 2c relative to? That's the key question in relativity. Special relativity isn't well equipped to tell us the direct, relativistic consequences of traveling faster than light (the equations spit out imaginary numbers for observable quantities). Time travel only comes into it when you consider events an FTL thing can be present at from two different (slower than light) inertial reference frames.

As far as how your example leads to time travel, well... like I said, the ordering of events only ever gets out of whack when you consider events that are out of causal contact. Here, you're talking about a supernova, then the light form that supernova interacting with the Gladius, and then presumably the Gladius wanting to prevent that light from reaching Libertas. All of these are events connected by light, which means they are in causal contact. So different observers may disagree about the exact times at which events occur (due to time dilation), but they won't disagree about the order.

Funny business comes in only when you consider spacelike separated events. Because Gladius doesn't see the supernova until it's halfway to the star, the event of its leaving the spaceport is causally disconnected from the star exploding. On the other hand, the event of Gladius seeing the supernova is obviously connected to the supernova (by light), but has spacelike separation (because it's moving faster than light) from its own departure from the port.

That means there is some reference frame in which the Gladius is halfway between port and star (witnessing the supernova) before it leaves the port (specifically, some other ship on a parallel trajectory moving close to the speed of light--I can draw very terrible spacetime diagrams to illustrate if that would help). If it gets where it's going before it starts going at all, we can cook up some paradoxical scenarios. I'll note that because we have no idea what FTL travel really means physically, it's possible said scenarios may be prohibited by some as yet unknown physical laws. But a naive depiction of FTL allows for the possibility of paradox.

(Again, this is totally unrelated to when that other ship sees these events; that just depends on where it is in its journey when all this is happening, and it could very well see Gladius leave the port before it reaches the supernova blast. The ship isn't really necessary, only its valid inertial reference frame.)

 

[Joshua Jones]

Well, as a rule there is no arrow of time baked into the laws of physics; physical processes are in general reversible. It just so happens that some processes are much more likely than others ("how likely" being a statement about the entropy of a system), so there are plenty of processes we don't end up seeing. But in principle, if a star dies and explodes as a supernova, every interaction in that chain of events could have proceeded the other direction, such that a supernova remnant collapsed into a dying star. It's just, well, even saying "astronomically unlikely" is an understatement there. So there's nothing about a particular sequence of physical events that matters in terms of causality. There's only constant conjunction.



Ah, but what is the Gladius moving as 2c relative to? That's the key question in relativity. Special relativity isn't well equipped to tell us the direct, relativistic consequences of traveling faster than light (the equations spit out imaginary numbers for observable quantities). Time travel only comes into it when you consider events an FTL thing can be present at from two different (slower than light) inertial reference frames.

As far as how your example leads to time travel, well... like I said, the ordering of events only ever gets out of whack when you consider events that are out of causal contact. Here, you're talking about a supernova, then the light form that supernova interacting with the Gladius, and then presumably the Gladius wanting to prevent that light from reaching Libertas. All of these are events connected by light, which means they are in causal contact. So different observers may disagree about the exact times at which events occur (due to time dilation), but they won't disagree about the order.

Funny business comes in only when you consider spacelike separated events. Because Gladius doesn't see the supernova until it's halfway to the star, the event of its leaving the spaceport is causally disconnected from the star exploding. On the other hand, the event of Gladius seeing the supernova is obviously connected to the supernova (by light), but has spacelike separation (because it's moving faster than light) from its own departure from the port.

That means there is some reference frame in which the Gladius is halfway between port and star (witnessing the supernova) before it leaves the port (specifically, some other ship on a parallel trajectory moving close to the speed of light--I can draw very terrible spacetime diagrams to illustrate if that would help). If it gets where it's going before it starts going at all, we can cook up some paradoxical scenarios. I'll note that because we have no idea what FTL travel really means physically, it's possible said scenarios may be prohibited by some as yet unknown physical laws. But a naive depiction of FTL allows for the possibility of paradox.

(Again, this is totally unrelated to when that other ship sees these events; that just depends on where it is in its journey when all this is happening, and it could very well see Gladius leave the port before it reaches the supernova blast. The ship isn't really necessary, only its valid inertial reference frame.)

Very interesting. I'll be honest; I spent the day writing budget ordinances and my brain is mostly fried right now, so I'm going to chew on this a bit tomorrow and see if it sticks with me. Right now, I'm at a point where everything is running together...

Either way, thanks for posting this! I greatly appreciate the time you're taking to engage me in conversation on this topic!