Danny Uncanny7 wrote: In your universe, If something flies off beyond any possible interaction and has no further effect whatsoever on anything else within the universe, it continues to exist, but if it were to magically disappear and have no further effect on anything else it doesn't exist. What if it instantly altered its velocity to beyond the speed of light? Would it exist then? The last two situations have the same result for any observer.
No, if it instantaneously jumped to c it could still interact with objects. If it instantaneously jumped to >c it can still interact. Sketch the diagrams if you're not sure.
Danny Uncanny7 wrote:Your definition describes the universe from the reference point of absolute knowledge about infinite space as though you have that absolute knowledge. With only partial knowledge. For all you know, the edge of our solar system could be a giant holographic screen projecting radiation. Your definition involves describing everything, even though you don't know for sure what that everything is. So how do you know what the universe is? Your definition of the universe is an undefined entity because you don't know everything. It has no succinct description.
No. The definition I am using (which is as close as I can get to how I believe it is usually used in cosmological discussions) merely requires that such information exists. Yours assumes that we alone in the universe produce valid observations which implies privileged reference frames and violation of SR.
If we have an observation which refutes SR the definition used for the universe may well change however it certainly does not make sense to use a definition contrary to what is a very well tested theory.
Danny Uncanny7 wrote:You are the only human on the world. You have no memory. What is the universe you are in? Can you define it in a meaningful way? Do you need to know about the big bang and the cosmos? Do you need to know the laws of physics? Do you have to redefine the universe every time you learn something new about it? But I thought the universe didn't change.
The universe is the n-dimensional manifold containing everything which can/could/will be able to be observed and everything that everything which can/could/will be able to be observed can/could/will be able to observe and so on. It is this daisy-chain-ing which is necessary in order to produce consistent reasonable results. Now, I wouldn't actually use this definition because it is far too general to be useful and doesn't give any good understanding of what's going on), in practice, depending on the context of the discussion (be it classical, relativistic, quantum etc.) I would use something more specific so that my definition was precise and could actually be used to do things. As War Daft said, the cosmos does not have some intrinsic name and doesn't care how we define "universe", "cosmos" or "world" or any similar term, those words are merely conveniences for ease of communication.
This definition holds for an ancient view of the universe (by which I mean earth is a flat(ish) plane covered by a semispherical dome with everything evolving in time), a classical one, a relativistic one and a quantum one even if the exact specifics vary.
The exact specifics:
In the ancient view, the manifold's co-ordinates lie in R3
and some form of hyper-cyclindrical co-ordinates (whereby the co-ordinates in R3
are spherical) seem most appropriate, the manifold is flat and may be closed, open or infinite. In the hyper-cylindrical co-ordinates we take our first angle to be the bearing and the second to be the attitude.
In the classical view, we've realised the earth is round and so what was the universe in the ancient picture is now rolled into a ball and embedded in a new R3
space. The manifold is flat and infinite.
In the relativistic view, the manifold is now in a more complicated space arguably either C4
, it is no longer necessarily flat but is infinite.
In the big bang picture, the universe is not infinite in its extent back in time.
In MWI, you could say that the collection of all worlds is the universe. In other interpretations you might use a slightly different interpretation.
Danny Uncanny7 wrote:
WarDaft wrote:Good job on completely and totally missing the point of my example. No, an object on the opposite side of the ring outside your horizon cannot affect you, but what you cannot do is pick an object, define the horizon around it to be a universe, and then declare that nothing in this universe can ever interact with things outside that universe. Early on, A can interact with B, and B with C, and C with D, all the way round the ring back to A, but it is entirely reasonable that no interaction can pass from D all the way through the chain to A. So by your definition, A and D are in different universes, but there's a chain of objects interacting from A to D, so how can you possibly claim that A has any sort of privileged claim to defining the universe? Even more-so because it is a ring, and every object can be totally identical. There's no way to pick one and say "that's the universe, yep!"
My point is that the universe is relative to your reference. There is no absolute universe, it is a relative concept. That's what I've been saying from the start. If A is your reference, after D hits light speed, it is gone from A's universe. It has disappeared from the universe. Not physically flown out of it, but disappeared in the exact same manner that the future does not exist in todays universe. Get it? Time is relative, existence is relative to time and time is relative to space. So existence is relative. If you say that New York in the year 2013 exists right now, most people would say you're wrong. It's the future. It doesn't yet exist. It has had no effect on us. It's not a part of our universe from today's point of view. It hasn't affected us In that exact same fashion that D does not exist after it achieves the speed of light relative to A. D is in the future relative to A and further more, unless it slows down, it will always be in the future of A for eternity.
This is claiming that our frame is privileged. It is the exact opposite of using a relative concept. If you want that, you need to find some objective reality which can explain all observers' observations otherwise you end up with each observer claiming, as you are, privileged information contrary to each other and you end up proving the inconsistency of your system.
Danny Uncanny7 wrote:
And so far as I know, you still haven't covered how objects spontaneously ceasing to exist is not a massive flaw in your definition of universe. These aren't just nit-picks, these are huge glaring flaws.
I don't see how an object ceasing to exist is a flaw in the definition in the universe. Why can't something cease to exist? We haven't seen it happen but that doesn't mean it can't happen. I don't think the definition of the universe should contain the laws of physics which operate within it. If it happened, what would be the implications for your definition of the universe? If you observed the the sun disappear?
For that matter. Take two future scenarios. In one instance, the the distance between galaxies accelerated and they all went beyond the speed of light so that we cannot observe them. In the other, the distance between galaxies accelerate and they all went beyond the speed of light, and then the galaxies blinked out of existence. Their mass disappeared. If you were to show up in either future and saw the dark sky how would you tell the difference between the two? Why is something vanishing a problem, but an event with the exact same result as things vanishing but a different name not a problem?
To the first paragraph:
It is a flaw because if things can cease to exist, energy (and momentum) are not conserved. If they are not conserved, that implies that the laws of physics are not constant in time (or space). Not only does this not seem to be the case (note, variation in "constants" like c is not variation in the laws of physics because the fundamental laws are the same, it's just the values they spit out which changes) but it would mean that we could not actually predict anything because the laws of physics could change at random (and indeed, would have to, otherwise the changes in the laws would be governed by laws which would be constant in time and energy would be conserved).
To the second:
Because in one it is no longer observable but still exists, in the other, it no longer exists. The first (that I listed) requires only that not all the universe is observable (this should be intuitive, for one thing, objects can block line of sight) and maintains constancy of the laws of physics. The second, doesn't require anything but means it is impossible to make accurate predictions.
Look up Noether's theorem if you don't believe us about what violation of conservation of energy would imply.
Danny Uncanny7 wrote:
gmalivuk wrote:Which definition do you think changes with every observation? Sure, that is what the observable universe does, but is that surprising? It's right there in the name. The advantage of the conventional nomenclature is that we can distinguish between the observable universe and the universe as a whole. Which incidentally *can* be defined rigorously so that each point in the universe has some other point in the universe in its own observable universe. Patch all the observable universes together, in other words.
gahhh, I've said this like a hundred times, you can't have a "whole view" of the universe where time means anything. This whole argument is about a definition that defines things around present time and past and future. You can't have any of those times with a "whole universe" view. Because all of those times are relative to a specific point in space. You can't talk about the future, unless you also have a location. There is no absolute time, so if you want a universe that is dependent on time, where the future doesn't exist alongside the present, then you need to have a location. You can't say what the future or past is without a reference. And if things can exist where nothing existed in the past, like say the big bang, or any of the theoretical ABCD objects, then time is a very important part of defining what exists.
What makes you say that you can't have a holistic view of the universe?
This is the only way to reconcile contradictory observations, you need some way of mapping observations between reference frames. Without an absolute universe which is just viewed differently by different observers, you end up with contradictory observations and no way to reconcile them. This is a bad
As I understand your view of the universe, you have no way of resolving these contradictions, you simply ignore them.
As an example, without an absolute universe how do you reconcile the simple prediction of SR that two trains passing moving away from a station in opposite directions will each see the other's clock ticking slower than theirs?
Danny Uncanny7 wrote:How can you have a universe with no relative observer or point. What defines it? Why should the universe where the Boltzman constant is what it is be THE universe, and not a universe where it is slightly different. You need something to peg reality, otherwise an imaginary universe is not different from our universe. I suggested that we be that reference.
Observation is what we tie reality to. Where we get conflicting observations, we must use some theory to resolve. In the case of SR, the easiest way to do this is to draw the Minkowski diagram.
Wrt your Boltzmann constant example, we know it is what it is because it has been observed to have one value.
If we were to get different values in reference frames, we would find the value when all the equipment is in the rest frame of all the rest of it and then we would also find the way to transform the value from one reference frame to another (in the case of SR this will be the Lorentz transformation).
Danny Uncanny7 wrote:So like I said the universe is everything that interacts with a relative entity that is known to be in that universe. If you want to forget about time, the universe is everything that has an will interact with an entity in that universe. Either way, I am the only one who has actually put up a real definition and I have yet to see a flaw in it.
The flaw is that it requires that the laws of physics are not constant in time or space.
If this is the case, science is a completely futile enterprise because no predictions can be made.
Danny Uncanny7 wrote:Lastly, why why why are you defining the universe by the laws of physics within the universe. Remember that stemmed from a conversation on multiverses. How can you define the term universe and apply it to other theoretical universes when you pigeonhole that the laws of physics have to be a certain way in the universe. The universe should not be subject to the contents of the universe. It's an encompassing definition. If mass is conserved or not, or space is filled with fluffy pink elephants, it should not affect the definition of the term universe. Whatever the rules are, they are the rules within the universe, they don't define what a universe is. Like you said, if you have new observations, you have to change your definition. Doesn't that suggest that maybe you don't have a clear definition of what a universe is? You are trying to define the term universe by what the contents of our universe are. But you don't have a clear definition of what the contents are. The contents are just what are inside the universe, so you are going around in circular logic, defining the universe by what it contains.
Because without the relevant theory we cannot do anything with our definition and, as I discussed above, with the most general definition, we cannot really do anything. In order to be able to use our definition, we must use that theory to describe "reality" as a thing as best we can. This is almost certainly not the actual description of reality (as I've stated before, I am of the opinion that "the" fundamental theory will never be known) however reality doesn't care what we call it or what things we its "name" for.
The reason we're kicking up a fuss about conservation of mass-energy is that, without that, physics (and indeed, all sciences except mathematics) cannot exist because no predictions can be made.
As for defining the universe by what it contains, what else are we to do? Define it by what it doesn't? Define it by embedding it in some other space? The former of these would be ridiculous for obvious reasons and the latter is not necessary due to the theorema egregium.