Why does special relativity predict the future already exists?

Factual Questions
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Brian Greene is not wrong in anything he proposed. That said he is not right either. He is showing a possible way of how things work as far as we understand it today. It makes for good TV because the implications are mind blowing.

And you have repeatedly belittled the double-slit experiment. Not sure why. It has been said most of quantum mechanics and its weirdness can be discerned from that elegant and simple experiment.

But we can do one better on that. Check out the Delayed Choice Quantum Eraser experiment where it seems the experiment reaches back in time to change current results. An indication that maybe past, present and future are not as distinct as you’d like.

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Oh man I am DEFINITELY not belittling the double slit-experiment. It’s, like you said, elegant and brilliant. One of the top 10 experiments in science. And the delayed choice quantum eraser is the pinnacle. It’s just that popularizers make it seem spookier than it is (even though it’s already mind blowing). They try and make it seem as if the universe is consciously conspiring to stymie our attempts to figure out if light is a particle or a wave when, at least according to the physicists I’ve spoken to, the real revelation is if we attempt to detect a particle, we detect a particle and if we attempt to detect a wave, we detect a wave. My only source on the double-slit before researching it at stack exchange was Dr. fricking Quantum.

Regarding Brian Greene, perhaps I’m just not understanding it. Why does the angle of the time slice increase with the distance from the other planet?

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Your are semi in luck because PBS Spacetime, which is a pretty reliable source of non-woo videos that are fairly digestible without math just came out with a video that will explain some of this.

Take special note of when he talks about “proper time”, but remember “proper time” is a function of the observers view, it is not universal.

I didn’t watch the whole thing, but fast forwarding through it looks like he covers simultaneity as related to Lorentz transformations. But it may not be clear that there is no “movement” in spacetime as far as this goes, there is just the worldline.

But I think you are still looking for a way to visualize this in your head, and that is not going to happen without massive simplification. We just can’t intuitively think in 4D space time.

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I didn’t watch the video so I’m not aware of the context, but proper time is a Lorentz scalar and hence independent of any coordinate system or observer. Now confusingly you can talk about the proper time of an observer between two events as long as they are coincident to those two events and another observer who is also coincident to those two events may experience a different proper time (this is in fact the basis of the twin paradox), however proper time is still an invariant as it relates to the worldlines of the two observers which are different.

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The video is actually seems to be an introduction to the invariance of the hyperbole, but it doesn’t cover the implications of it being equivalent to a circle in normal euclidian space. It is wristwatch time with a stationary observer where events are simultaneous.

In GR if you can choose Minkowskian coordinates, and perform a Lorentz boost into whatever rest frame you want. But when talking about aliens in the case provided, locality or flatness is likely an issue. I shouldn’t have broken the spherical cow.

But as the video only brushes on curved space time, and only in local cases yes that invariance holds.

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Yea I’ve seen those videos rat_avatar and one of them has the best illustration of the nature of space time I’ve ever seen. He plots the movement of a light clock from a moving and stationary perspective with space on x axis and and time on Y. He then shows the Lorenz transformation by warping the axes. Really brilliant. Makes it so intuitive.

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Here it is. If there’s a better explanation of spacetime, I haven’t seen it. YouTube would do the world a service by deleting all other explanations. The example starts at about 3:30.

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It’s not that I don’t get the concept of simultaneity, it’s that I think Greene’s example blows. Interesting to note that nowhere in PBS Spacetime do they discuss eternalism, leading me to believe that discussions of SR hinting at eternalism is just another “let’s get the viewer interested by saying mind blowing things that aren’t true.”

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Simple question: what would “future does not exist” look like?
"and in today’s astrophys forecast, the front of no-future is rushing towards us from what used to be a direction, that used to be in the direction of Alpha Centauri.

Just quick mulling the math over, I think you have two possible flavors:
A. Everything ends, so there is some sort of boundary between “future” and “not-future”. Then everyone’s light cones hit not-future eventually, but not simultaneously.
B. Assuming no future is equivalent to “all movement in the time direction stops or ends”, then I think black hole singularities count. Things go in and are never seen again, the universe has ended for that spacetime path.

Quip from one of my classes. “If you think you understand quantum mechanics (or general relativity) then you don’t understand quantum mechanics.” Scientists might have more understanding because they can fiddle with the equations, but Copenhagen Interpretation boils down to “we don’t know how quantum stuff works, just that it follows these equations. Do the math, ignore questions about the how or why”. To explain what they understand requires simplifying, so you skip over any messy details or "only partially correct’. So definitely, science communicators might say some weird-sounding stuff, and stuff that might be wrong, but that is just a side effect of simplifying.

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:slight_smile:

Fascinating thread and made me sign up after lurking for a bit.

I’m working from memory here so please correct me if I am wrong.

Couple of things.

The ‘Twin Paradox’ is nothing to do with the different ages between the twins. Relativity predicts there will be a difference in ages between them.

The paradox arises because Special Relativity (SR) says that any frame of reference is as valid as any other. So to twin A on Earth his twins ship is moving away at say, 99%, of the speed of light.

However on the spaceship it is just as valid for Twin B to say that it is the Earth moving away from the spaceship at 99% speed of light.

The paradox is that they both cannot be ageing at different times with respect to each other. For A on Earth he is getting older as twin B, on the spaceship, time slows and B stays younger.

But twin B, on the ship, see’s Twin A zooming away from him at 99% speed of light and so twin B get’s older as Twin A, on Earth, stays younger as A moves away from the spaceship.

They are both getting younger and older with respect to each other at the same time.

It’s not really a paradox as the frames of reference change during deceleration, changes in vector etc , mostly to Twin B on the ship.

Still pretty cool though.

The thing about the ‘slice through space-time’ changing so you move into the past or the future of an Alien’s ‘Now’ moment is really cool as well, but Relativity says that is only possible because you cannot in anyway communicate or interact with that changing time line slice. So you cannot send a message to the alien warning him he’s about to have a traffic accident. :slight_smile:

Quantum theory may have something to say about that I suppose. You want strangeness in droves try and bone up on that.

Everyone have a good day and be lucky.

PS Speed of light is 186,000 Miles per Second, or 299, 792, 458 km per second for those of us who live by the decimal system. I tried to remember those figures without googling so feel free to check.

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[quote=“KidCharlemagne, post:60, topic:778677”]

Yes but AFAIC, if an event has occurred in the past of anyone else’s time frame, then it can’t be rightly used as evidence of eternalism.
[/QUOTE ]

Why not? Seems to me that’s exactly the most straightforward implication: if some event in my future is in the past as far as another observer is concerned, the it would seem my future is fixed.

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That’s an interesting digression. IOW, What if the entire future was to stop happening someday for everyone everywhere?

But that’s not at all the question under discussion. Which is: “Does the future ‘already’ exist as a fixed reality in the same way that some undiscovered asteroid or seamount really exists as a fixed reality even if we lack the tools to have seen it yet?”

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[quote=“Half_Man_Half_Wit, post:71, topic:778677”]

The moving observer may see an event sooner but it’s not seeing the other person’s future. The observer moving towards the alien planet may see the ships launch sooner, but he can’t predict they’ll invade the earth.

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All exactly correct. It’s “paradoxical” in SR; each sees the other ageing at a different rate. But if one of the ships “turns around” and comes back to meet the other, then acceleration has taken place, and the knarly GR equations have to come in to play. One of the twins is definitely older than the other, with no ambiguity.

Nearly anyone who can do square roots can do SR math, but GR math is just brutal.

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I just wanted to pile on that PBS Spacetime is fantastic and any interested amateur in space stuff and quantum weirdness should be subscribed.

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It’s not about seeing anything sooner or later, it’s about different planes of simultaneity, different present moments.

In the Newtonian universe, there is one single present moment—one three-dimensional snapshot of the universe at a particular time, one set of locations at the same moment. Time then is just a film-like sequence of these moments.

This doesn’t work in a special relativistic world. There, different observers have different notions of present, depending on their relative state of motion. There’s no way to slice up the universe into a global sequence of three-dimensional frames; rather, every observer gets their own such decomposition.

We can, however, keep the interpretation of such a slice as a present moment—i.e. a set of events happening now, at the same time, having the same time coordinate, the same clock reading. But if we do, we find that observer A’s present moment may include events that are to the future of events within observer B’s present moment, while observers A and B are each within the others present moment—since they are at the same position, only in different states of motion.

A way to make sense of this is to imagine the universe as 2d, and glue all the present moments of Newtonian time together, into one long world sausage. You can then slice this sausage at different angles—these slicings correspond to different inertial observers, each slice being a given present moment. Now suppose two slices intersevt—say, one cuts the sausage orthogonally, one at a 30° angle to the first. These are the present moments of two observers, and the intersection corresponds to their meeting. However, each one observer’s present contains events to the future and the past of events in the other observer’s present.

In the analogy, this is only possible because the full sausage was on the table; but translating that back to spacetime just is eternalism.

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I’d disagree, the slicing is not that important that is should suggest eternalism; what us important is the causal structure.

In Minkowski space, for any event p, the rest of the spacetime into 3 disjoint sets: the causal past of p, the causal future of p and those events which are in neither the causal future or past of p. For the purposes of eternalism I would say this is no different to the way Newtonian physics, for a given event, divides all other events into past, future and present which are also disjoint.

Even in general relativity we usually see causality (i.e. that the causal past and future are definable and disjoint) as one of the basic conditions of physicality.

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After reading up on eternalism…

Gack. My question isn’t a digression because there seem to be multiple definitions of “future” and “exist” being thrown around. And frankly, it seems like Physics is just being used to excuse muddy thinking. (IANA philosopher) Trying to figure out how relativity, simultaneity, and future relate, and how to talk about it, gave me a BSOD. I just can’t see any logical way to go from “non-causally connected events can occur before, same time, or after depending on reference frame” to “all causally-connected events are fixed and predetermined”

Using the train thought experiment. Any observer, regardless of speed and reference frame, will always observe that the light turns on before the light reaches any end of the train. Causes still precede effect because light cones. An event is still only caused by events in its’ past light cone. So if “future exists”, it is only in the same sense as before relativity. (if you allow FTL, then you just went past the limits of my knowledge. Good luck)

Or, what Asympotically fat said

So I still ask, what is the opposite of “the future exists”?

(Re two slit experiment, my spooky is the single-photon variation. Build 1000 identical test setups. Send one photon through each setup and mark on a screen where it lands. If both slits are open, you can plot all that onto a single graph and you see an intereference pattern. If only one slit is open, there is no interference. Even though there was only one photon each time, and you used different (but similar) hardware in different locations each time.)

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Couple of random thoughts whilst I wait for lunch.

Causality is transitive and (we currently assume) local in all dimensions. That covers a lot of the basis of the questions.

However. A question of the form “does the future exist now?” Has a really difficult problem without going much further. What does “now” mean in reference to the future? Has the future always existed? What does that even mean? What does always mean? Time has become self referential.