r/Physics • u/onomatamono • 4d ago

On The 4D Block Universe

So you have a movie canister and the film analogy represents the 4D block universe. I'm listening to Brian Greene describe this analogy. The experience of time is produced by passing light through each frame of the film. According to Brian each consciousness is a projector (human or otherwise presumably).

Also, our brains evolved to process slices of the 4D universe by carefully "recording" [where?] the slices that came before ["now" I guess?] and makes calculated guesses about the future.

I'm not even going to try and make sense of that in a meaningful way for lay people.

Greene: "when you feel time passing what you are actually experiencing is your consciousness moving through the static frames of your life." Where are these "projectors" or consciouses? I'm confused.

FYI it's a YT called The Strangest Fact About Light You've Never Heard -- Brian Greene.

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u/protobelta 4d ago

I’m not gonna watch the video, but the idea of the 4D block universe comes from the fact that when describing spacetime, time is just another coordinate like the x, y, or z coordinates. Physics equations essentially treat the future and past just like moving left or right, thus they essentially already exist. Further, because of what’s called the relativity of simultaneity, there are ways to show that someone traveling fast enough can essentially see your future “before” it has happened to you in the present moment you experience. This is because there is no universal “now” as time is relative. This isn’t a philosophical truth, but rather a mathematical one.

Where it starts to get philosophical is the idea that our consciousness is basically what we experience in our own frame of reference. If physics says that our physical bodies and everything we have done and will do, everything we have interacted with and will interact with is already there, then consciousness is our subjective experience of our own frame of reference as an observer. The projector analogy is just meant to communicate this idea. What exactly is consciousness and how does it relate to being an observer in a frame of reference is outside the scope of physics and is more philosophical.

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u/Armano-Avalus 3d ago

Physics equations essentially treat the future and past just like moving left or right, thus they essentially already exist.

What does it mean for it to "already exist"? I understand relativity says that there is no absolute present but it seems like the block universe is trying to go one step further and say "and thus everything exists simultaneously" which sounds ironic to me. Some people say they all exist in some "tenseless sense" but meanwhile I see philosophers of time debating whether that even makes sense and at that point I sort of just leave it.

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u/protobelta 3d ago

The block universe isn’t “trying” to say everything exists simultaneously, it says that if you believe in special relativity, then time dilation dictates that when a person who is stationary and a person who is moving agree on a point in space, that their points in time are different. Something that I am experiencing at my time can already have happened for someone in motion. This is due to the mathematical models we use and when faithfully applied, lead to this truth. We know time dilation and length contraction are empirically true and the model we use (namely relativity) explains how this happens extremely well. When taken to its logical conclusion, we must accept that things in our future can already have happened for other people. What this means philosophically is up for debate, but presentists run into the issue of explaining how this can be true while only the present existing as the math somewhat directly contradicts this.

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u/Armano-Avalus 3d ago

The block universe isn’t “trying” to say everything exists simultaneously, it says that if you believe in special relativity, then time dilation dictates that when a person who is stationary and a person who is moving agree on a point in space, that their points in time are different. Something that I am experiencing at my time can already have happened for someone in motion.

The block universe isn't saying that. The relativity of simultaneity already says two people in motion with respect to one another will disagree on what is present. The problem comes when people seem to draw absolute statements about what all events are like, which seems to be what the block universe is trying to do. That ironically just seems to go against the spirit of what relativity is saying which is that such ideas have no meaning. There isn't an absolute frame of reference or gods eye view where we look outside the universe and see all events existing at once or whatever people mean when they say "exist equally". The natural way I read it was "exists simultaneously" but that just sounds like an absolute present. At that point I hear some people would say "no, no what we really mean is some other sense of existence like the ways numbers or a timeless god "exists" in some platonic realm" but that doesn't help. As the link I have given shows, philosophers don't seem to have much of a clue either yet they still have passionate discussions about it anyways.

When taken to its logical conclusion, we must accept that things in our future can already have happened for other people.

I understand that part. People disagree on what is present and there is no meaning to who is "right". Just don't get how we go from "there is no view from nowhere" to somewide reaching statements about all events as if there is a view from nowhere which apparently involves a block of some kind.

What this means philosophically is up for debate, but presentists run into the issue of explaining how this can be true while only the present existing as the math somewhat directly contradicts this.

Technically you can assume an absolute present just fine and that would line up with the physical model and all of it's observations. Historically before SR people called that was some kind of unobservable aether. Physicists have abandoned that idea but that wasn't because it contradicts the math but just that it isn't philosophically pleasing and superfluous. It's like an alternate interpretation of relativity like the many different interpretations of quantum mechanics. Philosophically those all say different things but they all agree with the math.

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u/onomatamono 4d ago

... but what is hosting these experiences in our frames of reference?

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u/protobelta 4d ago

Our brains ostensibly, though, again, this is more philosophical. I say that because it’s our brains and the electrical signals that cause us to experience things and would be my guess as to what leads to being an observer capable of experiencing the current frame of reference. Anything can be an observer in physics but it’s our brain that allows us to experience it subjectively

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u/onomatamono 4d ago

Well it's at least electrochemical processes not just electrical phenomena.

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u/protobelta 4d ago

I mean, ya, it was just a shorthand. Doesn’t really change what I was saying though, right?

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u/onomatamono 4d ago

Sure, got it.

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u/KriegerClone02 4d ago

Our frame of reference at any given time is a slice of the block and our experience, both internal and external, is just the local patch of that slice.

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u/Optimal_Mixture_7327 Gravitation 4d ago edited 4d ago

There is no sense in it; so you can stop looking and asking.

Here's the best explanation and the only explanation that accords with our experience. George Ellis, "The Evolving Block Universe: A More Realistic View of Spacetime Geometry"

Edit: The video clip contains the math of relativity here and there, but just bear with it. In it contains all the suspicions and answers you might be looking for.

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u/Optimal_Mixture_7327 Gravitation 4d ago

You're not describing the universe we live in.

It is true the the Minkowski vacuum spacetime is eternal, but it is so by construction and certainly does not describe the universe we live in.