Why do we perceive a 4-dimensional universe as 3?

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Re: Why do we perceive a 4-dimensional universe as 3?

Postby Sizik » Mon Jan 06, 2014 1:02 pm UTC

Meteoric wrote:(IANAP, but would it also have problems explaining the occurrence of more than one electron-positron annihilation in the history of the universe?)


Annihilation is what we see when the electron "turns around" before it reaches the end of the universe.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby Copper Bezel » Mon Jan 06, 2014 3:58 pm UTC

That's bizarrely cute, really. If that were the case, though, where would the photons come from?
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby eternauta3k » Mon Jan 06, 2014 5:02 pm UTC

From the annihilations.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Mon Jan 06, 2014 6:46 pm UTC

For the single-electron idea, it's worth noting that the universe is likely* quite a bit bigger than the visible universe. Hey, we may just find ourselves in a "matter" neighborhood, one of a great many matter and anti-matter neighborhoods each a very tiny hundred-trillion times the size of the visible universe, not like the interesting bit of the universe 1036 light years that way --> where all the good parties are!

*Well, "likely" is maybe the wrong word, but there's no apparent constraint on it's total size, so no reason to think it's anywhere close to visible-universe size, unless the CMBR turns up evidence of a smallish closed universe, and chances of that are pretty slim at this point, given it would be sort of obvious from existing data.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby eternauta3k » Mon Jan 06, 2014 9:07 pm UTC

It's interesting how you can just throw that around but the person who disproves it has to plug some actual quantities to prove it's unlikely.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Mon Jan 06, 2014 9:35 pm UTC

Well, group possible universe sizes into these categories:
  1. Smaller than the visible universe (and closed) - this will be evident in the CMBR, eventually, if true
  2. Exactly the size of the visible universe
  3. Just a bit bigger than the visible universe
  4. Much bigger than the visible universe
  5. Infinite, open universe
For some reason, belief in c seems pretty common (at least in discussions I've had), but it seems to violate the Copernican Principle to assume that. Without evidence for a, or a strong theoretical grounding for b or e, d is what's left.

In my personal cosmology, based on nothing but amusement-value-if-true, the universe is finite and has a distinct (and exciting) edge, we're just not in a place where we can see the edge. :lol:
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Mon Jan 06, 2014 10:32 pm UTC

c is not at all common among practicing cosmologists. I'd say infinite universe is very widely favored, or if you want to say our universe is a finite bubble in a multiverse, you're looking at d.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Mon Jan 06, 2014 10:59 pm UTC

lgw wrote:For some reason, belief in c seems pretty common (at least in discussions I've had), but it seems to violate the Copernican Principle to assume that.
There is good reason not to believe c, but it would only violate the Copernican Principle if there was also a distinct boundary, which I'm pretty sure no one who really knows what they're talking about believes.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Mon Jan 06, 2014 11:17 pm UTC

gmalivuk wrote:
lgw wrote:For some reason, belief in c seems pretty common (at least in discussions I've had), but it seems to violate the Copernican Principle to assume that.
There is good reason not to believe c, but it would only violate the Copernican Principle if there was also a distinct boundary, which I'm pretty sure no one who really knows what they're talking about believes.


Well, if the ratio of observable to total universe changes over time, it would be quite special if we happened to live just when the two were nearly the same size, no?
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Tue Jan 07, 2014 1:01 am UTC

That's... not what the Copernican Principle is about?
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Tue Jan 07, 2014 1:16 am UTC

The Copernican Principle: here is not special, now is not special, or "be skeptical of any theory which requires us to be alive as just the right place or time to get the observed results" to say the same with more words.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Tue Jan 07, 2014 1:24 am UTC

That may be true of location, but less so of time. We're alive during the fairly narrow period when both annular and total eclipses are possible. Should we doubt the claim that the moon is moving away from Earth?

There are plenty of things about our existence that are only possible at approximately this point in the universe's evolution (where "approximately" is to be understood on a universal time scale).
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby doogly » Tue Jan 07, 2014 1:28 am UTC

Being able to observe other galaxies springs to mind as well. They ain't gonna be around for long (cosmically).
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Tue Jan 07, 2014 6:59 pm UTC

gmalivuk wrote:That may be true of location, but less so of time. ...


Now you're just perceiving a 4-dimensional universe as 3 ... sneaky mod bringing the thread back on topic.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Tue Jan 07, 2014 11:13 pm UTC

Yeah, well time *is* different. There's only one timelike dimension, and it has at least one endpoint, compared to three spacelike dimensions with no endpoints. Of course time is going to be different.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Thu Jan 09, 2014 12:04 am UTC

gmalivuk wrote:Yeah, well time *is* different. There's only one timelike dimension, and it has at least one endpoint, compared to three spacelike dimensions with no endpoints. Of course time is going to be different.


Well, those are all sort of open questions. Is space bounded, infinite, or cyclic? Probably not the first, but if it's big enough the universe could be any of those and we'll never know for sure (which is why looking for evidence of cycles in the CMBR is cool IMO - the only way we'll know for sure is if it's small and closed). Is time (semi-)bounded, infinite or cyclic? Could be any. But the distinction become pretty arbitrary near a singularity, and that itself may be profound if the universe started from/as one.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Thu Jan 09, 2014 2:37 am UTC

Nobody believes that space is bounded, and the fact that you say both small and closed suggests to me that you're not using "closed" the same way as a topologist or cosmologist.

Even if space may be cyclic and time may be cyclic, it's still really obviously not in the same way. According to every going theory, the Big Bang was still a thing that definitely happened, more or less. Like, maybe it was a Big Bounce or something more exotic, but there was still a time when everything we currently observe was all super close together. There is at least something like an endpoint for time, and there is something like an arrow of time, with causality and a past that's separate from the future in a way that cannot happen with three spatial dimensions. Time is objectively distinct from space, and there's no reason to suppose that therefore general principles that hold for space should also hold for time.

In addition, as previously mentioned there are already *plenty* of things that are only true now which weren't true in the early universe and won't be in the late universe. We are a life form that could only have evolved in the presence of heavy elements and a particular type of star, and those aren't things that have always existed or will always exist. On top of that are all the additional coincidences like the previously mentioned two-kinds-of-eclipse and visible-non-local-group-galaxies that weren't true in the distant past and won't be again in the distant future.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby lgw » Thu Jan 09, 2014 7:32 pm UTC

gmalivuk wrote:Nobody believes that space is bounded, and the fact that you say both small and closed suggests to me that you're not using "closed" the same way as a topologist or cosmologist.


Hmm, perhaps "compact" is a better word? Not sure. What I mean to say is that "all points in space are no more than some maximum distance from one another", either because there's a boundary or because space is "cyclic" (dang, what the heck's the right word for "cyclic" in topology??). If the universe is much larger than the visible universe, it could have almost any topology and it would be unlikely we'd have direct evidence of which. However, if the universe is smaller than the visible universe, we can know.

If the universe is "small" (smaller than the most distant visible object), and bounded, then it could only be a simulation because it sure doesn't look that way. I suspect we could eventually discover if we were in a simulation, but that's a different thread.

However, if the universe is "small" and "cyclic", then there will be evidence in the CMBR. There is a whole group of topological cosmologists who have cataloged a vast array of possible ways the universe could be "cyclic", with each sort of curvature and many different kinds of "tiling" (more than I would have ever imagined), and what the CMBR would look like in each case. From what I hear, there was no obvious match to anything in the CMBR, but they haven't given up hope yet and analysis is ongoing.

gmalivuk wrote:Even if space may be cyclic and time may be cyclic, it's still really obviously not in the same way. According to every going theory, the Big Bang was still a thing that definitely happened, more or less. Like, maybe it was a Big Bounce or something more exotic, but there was still a time when everything we currently observe was all super close together. There is at least something like an endpoint for time, and there is something like an arrow of time, with causality and a past that's separate from the future in a way that cannot happen with three spatial dimensions. Time is objectively distinct from space, and there's no reason to suppose that therefore general principles that hold for space should also hold for time..


Well, I don't completely agree. There is an interesting event in time and space that we call the "big bang", to be sure. There's some evidence the 4D topology of spacetime has a boundary there, or at least narrows to a point or nearly so if it's cyclic, or if it extends past infinitely in both directions. But AFAIK the universe could still have the topology of a 4-sphere, with the big bang being just a spot on the sphere that everyone can agree on. Sure, the topology could be different along the time axis than the space axes, but that's not necessarily so.

Sure, you can assign a "direction" to time in a way that distant observers can agree on, as long as they're not separated by an event horizon, and that's interesting. But event horizons (and singularities) are also interesting, and if you don't consider the universe as bounded by event horizons, then I'm not sure the direction of time or even the axis in spacetime we call "time" is universal. I think everyone locally can agree that one axis is special and assign it the negative sign in Minkowski space, sure, but approach a singularity and not so much. I also believe that if time exists "before" the big bang, then the arrow of time point uniformly away form the big bang, not in a consistent direction "through" it (hmm, I should start a thread about that - I'd love to discuss that in more depth).

gmalivuk wrote:In addition, as previously mentioned there are already *plenty* of things that are only true now which weren't true in the early universe and won't be in the late universe. We are a life form that could only have evolved in the presence of heavy elements and a particular type of star, and those aren't things that have always existed or will always exist. On top of that are all the additional coincidences like the previously mentioned two-kinds-of-eclipse and visible-non-local-group-galaxies that weren't true in the distant past and won't be again in the distant future.


There are plenty of things that are true if your location is in the core of a star that aren't true here as well. We are a life form that could only have evolved at the appropriate distance from a star, and that's a rare set of locations. But that's not really what the Copernican Principle is about, is it? It's the rules that are the same everywhere/when, not e.g. the temperature. So, yes, it was stretching things a bit to suggest that the universe is "just that special size", that it's a very specific level of tired, but it's not the time aspect that makes it a stretch. BUt it would still be, like the size of the Moon, one heck of a coincidence to be so exact.
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Re: Why do we perceive a 4-dimensional universe as 3?

Postby gmalivuk » Thu Jan 09, 2014 10:29 pm UTC

Coincidence, sure. My point was simply that such coincidences in no way violate the Copernican principle.
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