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If events are not absolute, then doesn't that make time travel to the past possible even with just one timeline?
No. Events are absolute in the sense that one, and only one, event specifies a location in space-time. (In other words, a particular point of space-time is
that event). What I was saying is that a region of space-time may or may not be accessible to you depending on where you are and the space-time geometry.So this still doesn't allow backwards time travel. An event in the future cannot affect an event in the past, or an event in elsewhere (regions of space-time that require FTL speeds to reach from the source event). Causal influence must be limited to the future light cone.There's a pretty good walkthrough of why this must be the case on PBS Space-time. I also discuss some of the details on the old forum,
though I dunno how clear my explanations are.
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So we are talking about the same thing but in different words then? You could still have multiple timelines in this scenario, but they would have to have existed since the BB.
Possibly. We're all saying that all points in time and space exist, I'm just saying they're not in a structure that appears the same to everyone. The thing that is the same to everyone is the distance (in space-time, or "space-time interval") between all events. Distances in space, intervals of time, and even the relative order (which event happens first) are all relative, but the space-time interval is not.Now if we say that an event has several possibilities, all of which occur (as in the Many Worlds Interpretation), then the space-time is everywhere proliferating into new, causally disconnected space-times. However, this still doesn't allow time travel into the past within or across those space-times. The causal structure forbids it.
On the other hand, literally adding a second time dimension to the space-time would have dramatically different effects. What that does is cause every event to have causal effects in two distinct time dimensions, yet those dimensions are not causally separated, so very quickly you end up with closed time-like curves -- paradoxes. This is one of the common reasons for excluding theories with multiple time dimensions from physics, though there are actually many others.
The only way to get multiple time dimensions to work nicely is to have all but one of them be so chaotic (as in a having an enormous
temperature for particle motions in that time dimension) that there can be no information propagated along a closed time-like curve, thus ensuring no causal paradoxes. (A good paper about this can be found here
by Foster et. al). I think this is a pretty weird way to do physics though -- it's interesting, but doesn't have a compelling motivation or provide much explanatory power. I don't think it's a good way to describe how nature works.
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This could still work with a temporal landscape but the landscape wouldn't be a flat plane. Blah this is easier for me to visualize than it is to state in words, Wat, so I must apologize for that! What I'm visualizing though makes me feel that there is a strong link between time and gravity, much more than either has in common with the other dimensions or the other forces respectively.
Haha, that's okay.
These things are often difficult to describe (or visualize, for that matter).
There is a link between time and gravity (time passes more slowly in stronger gravity field), but also space and gravity (straight lines deviate, areas and volumes work differently, etc). That's just general relativity of course -- gravitation is
a distortion of space-time geometry. But other forces participate, too. A strong electromagnetic field distorts space-time, as do large fluctuations of any other field. Energy itself bends space and time. Or another way of saying this is that all forms of mass-energy produce gravitation.
This may or may not be related to what you're describing but there's a popular idea that gravitation is weaker than the other forces because it "leaks" into other dimensions somehow. I'm guessing you're familiar with it, though I don't know much detail about how it would work.
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What you said about space-time curvature also makes a lot of sense. I think of an event horizon like a closed door. Things that can happen behind the closed door that we would never know about. But just because we don't know about them doesn't mean they didn't happen
I like that! I never thought of a door analogy before, but that actually works pretty well. Imagine there is a house somewhere, and you try sending a bunch of packages. You get the notifications that they were delivered, but nobody took them inside. They just sit there and form a pile right outside the door. Eventually you decide to go there yourself to see if anyone's home. As you walk up to the door you discover that there is no pile of packages at all. Before you have the chance to ponder that the door opens and someone yanks you inside and kills you.
So were the packages delivered? (Did events occur inside the horizon?) Yes, but the price of discovering them is your doom!
Another visualization I like, and one which you can demonstrate pretty easily in nature, is to make ripples in a stream or river. If you can find a place where the water is flowing quickly enough, the whole ripple gets carried downstream with the flow. You might even be able to find a rapid where the flow stars out slower than the speed the ripple spreads, but then gets faster. Then if you make a ripple at the right spot, the part of it spreading upstream just sits there, while the rest gets sucked downriver. This is a very good analogy to a pulse of light emitted at an event horizon.