Infinite Universe = Impossible

[K^2]

I think, but wait for his own answer, too, that K^2 and you have a very different understanding on what the answer to "what is dark energy" looks like. To you, it is some kind of explanation where it comes from and why it is there, but to him it probably is a full description of what it does. In the end, everything we can research will likely turn down to "how does it behave" instead of "why does it behave that way". The good old example of gravity strikes again: one could say that mass bends spacetime and thus all the conclusions from general relativity; but in the end, you only moved the goal post to "why does mass bend spacetimee¿". Instead, the pragmatic way is to just find all the formulas and such describing its properties and I think that's what K^2 is talking about here.

Essentially. We're still searching for dark energy, because we want to measure it directly, and hopefully map its abundance in the universe, or at least our neighborhood, but we aren't trying to define it. What it does is what it is.

By the way, the answer to the gravity question is that mass bends space-time because stress energy tensor is the conserved charge of the Poincare local symmetry, whose gauge field defines curvature. All of this has to do with the fact that Poincare group is one of the local symmetries of the Lagrangian. (It is a global symmetry of QCD Lagrangian when gravity is not taken into the account.)

So the current goal post is "Why does Lagrangian obey local symmetry rules, and why these particular symmetries?" There are also some very serious questions on quantizing relevant field when gravity is involved, but that's basically the topic of quantum gravity, so it's something we at least have a handle on.

That's all just to point out that goal posts aren't just moving, but moving fast enough for general public, and even specialists from other fields, to be typically a few decades behind.

[AngelLestat]

You know K2, Many times when I look in topics that are clearly your field, I read them first.

But you have a big defect, your science methodology is complety lost when you defend certain theories that are the ones you like.

I never see this behavior in any scientist, even if they are the ones who made the theory.

We know what is dark matter? really? since when we have such certainty?

Show me a link of any respected scientist who made the same claim.

Last time I check, there was like hundreds of theories all pointing in different directions.

EDIT: At first I thought that you said dark matter, but even dark energy is still a mistery.

Edited January 21, 2015 by AngelLestat

[YNM]

I don't really know what's all the thing there was (one night lost and three pages appear full of dimensions, which some I don't understand), but to be sure, you must observe. The good times to observe the night sky is around these months, isn't it (least for those away from the equator) ?

Regarding dark energy : it's required mostly due to the fact that there should be a thing to (in newtonian terms) counter gravitational attraction (like, to avoid seeing the Universe contracts away). Maybe in GR terms is to counter the fact that curvature of spacetime is only and only caused by mass-energy, which all are positive (so tends to contract). Even more required because we observe that the expansion of the Universe is accelerating. Regarding what is it physically, nobody can tell yet; it could be an integral thing to spacetime, it could be a different thing that's continually being produced as space expands. Maybe we should start to see the amount of acceleration, then calculating it against gravitational attraction (well in cosmology GR and newtonian gravity is interchangeable, the potential isn't that huge).

And the question "what is it" can be answered by it's physical form or it's influence (the physical form should affect what does it do).

[EdFred]

Again, you've missed the point. It is easy to understand how his 'truth' would stand under our scrutiny, because we've seen what he has not. Assuming a lack of communication, he'll never know the real 'truth', and have to live with what he knows for the rest of his life.

Right now, we're in his position. We've seen the universe around us contain everything, and we see no obvious 'windows' or 'doors' to to it. We cannot observe or travel to the 'outside' of the universe, at least not now. We are the man who is locked in the house. How is it not logical for one of us to conclude that there are no other universes besides our own?

You may argue that this may not be true. And you may point out that someone better informed in the future could make a better conclusion than what we have. But for now, we have no evidence about the existence of the 'outside' of the universe. While absence of evidence may not mean evidence of absence, in this case, the existence (or lack thereof) of another universe have little visible effect on the workings and mechanisms of our universe, at least as far as one can predict from here, so it is convenient to simplify the hypothesis by stating that there are no other universes out there.

No, I get it. But have problems with absolute statements when it's really just a hunch.

I have as much of problem with the statement, "There are other universes out there," as I do "There are no other universes out there,"

[michaelsteele3]

This just blew my mind

[Technical Ben]

They do. For some special cases. Because it makes computations a hell of a lot easier. When you want to consider gravity just in the neighborhood of a black hole, or when you are working out a design of a theoretical warp drive, you can use embedded manifolds to simplify a lot of the math both computationally and conceptually. It's a great tool. But that's kind of like reducing orbital mechanics problem to 2D. It works in a simple case. It does not solve general problems.

That's why I've mentioned superstrings and bulk. We can't detect these things directly, but we can ask what would be different if these dimensions existed in some way that's topologically relevant. We get some variation of String Theory. Well, String Theory makes predictions. We've been testing these predictions for over two decades on larger and larger accelerators. By now, String Theory has a standing of, "An interesting idea which shed some light on particle physics, but has no direct practical applications." We simply don't live in a universe where String Theory applies. There are still some people who are trying to work out some variation that might work, but essentially, if you need proof that there are no higher dimensions, accelerator facilities at RHIC and LHC have provided it.

The third dimension is not topologically relevant to the people living in a 2d universe. In fact, AFAIK (or understand) it's impossible to observe OR detect it's existence. It could only be theorised and, assuming it exists, any interactions from the 3d dimension into the 2d universe would present themselves as 2d interactions that break "possibility/laws of physics". However, both systems would be valid, and would exist.

No need for philosophical talk or arguments about science, we daily observe systems in isolation. Likewise, if out universe is part of anything "bigger" then we need not be able to detect it, nor should it be required to be detectable to exist. Just as a 2d universe could not detect the 3rd dimension, even though it does exist.

Or would it be required for a 2d universe to be able to detect the 3rd dimension? Would the scientists in the 2d universe argue "the third dimension posses no practical applications so does not exist"?

PS, talking about gravity being the things we use as a calculation to predict it's effect, is like saying Jam is made out of sticky substances attracted to clothing. It's effect and it's construction and it's form are all different things.

Edited January 25, 2015 by Technical Ben

[K^2]

The third dimension is not topologically relevant to the people living in a 2d universe. In fact, AFAIK (or understand) it's impossible to observe OR detect it's existence. It could only be theorised and, assuming it exists, any interactions from the 3d dimension into the 2d universe would present themselves as 2d interactions that break "possibility/laws of physics". However, both systems would be valid, and would exist.

The whole point is that they are observable. Not only that, but if we assume extra dimensions exist, we can predict what these interactions are going to be. We have done that. These interactions would result in certain observables. We have looked for these observables. We have excluded them as possible with countless experiments. Ergo, extra dimensions don't exist by a contrapositive. The alternative is field theory is completely wrong, but gives predictions correct to 12 decimal places by pure chance.

There are some versions of extra dimensions we have not completely excluded. But these can be rolled under an umbrella of holographic interpretations. All of the known ones are impossible, but that's not even the main point. Holographic interpretations exhibit duality with gauge symmetries. In fact, they must, because gauge theory works. So if they exist, they simply perform a function of an external degree of freedom given by the U/SU symmetries, contributing nothing new to physics.

[worir4]

Total Mind blow! We just did this in my philosophy class! Such a coincidence!

[YNM]

The third dimension is not topologically relevant to the people living in a 2d universe. In fact, AFAIK (or understand) it's impossible to observe OR detect it's existence. It could only be theorised and, assuming it exists, any interactions from the 3d dimension into the 2d universe would present themselves as 2d interactions that break "possibility/laws of physics". However, both systems would be valid, and would exist.

Think about the surface of a sphere... What would happen if the creatures living on that surface creates "circles" or "triangles" ?

[Cocox]

At this point scientists do not state that Universe is infinite. It is finite, but does not have a "start" or "end". Think inflated baloon that is really, really big (and still expanding).

Yes, but a baloon needs space in order to expand.

So.. our universe is expanding in a limited or infinite space?

I don't think that what we call tri-dimensional was created with the big bang. Space was there, matter wasn't. Also they say that time appeared at the moment of the big bang. But if there was no time, what conditioned the big bang event since time was "frozen"?

I think space and time are independent of the big bang. But that's just me thinking in my stupidity..

[*Aqua*]

@Cocox

It's a bit different than what you described. Spacetime came into existence 'during' the Big Bang. We know that because our current theories tell us that. But what happened in the 'timespan' 'between' the actual Big bang and the creation of spacetime we don't know. Our theories don't work at that point.

If we somehow create and proof a theory of everything then we'll be able to tell what exactly happened. So far we've got only candidates but nobody knows how to proof them. There are also scientist who say that's impossible to do.

[Technical Ben]

The whole point is that they are observable. Not only that, but if we assume extra dimensions exist, we can predict what these interactions are going to be. We have done that. These interactions would result in certain observables. We have looked for these observables. We have excluded them as possible with countless experiments. Ergo, extra dimensions don't exist by a contrapositive. The alternative is field theory is completely wrong, but gives predictions correct to 12 decimal places by pure chance.

There are some versions of extra dimensions we have not completely excluded. But these can be rolled under an umbrella of holographic interpretations. All of the known ones are impossible, but that's not even the main point. Holographic interpretations exhibit duality with gauge symmetries. In fact, they must, because gauge theory works. So if they exist, they simply perform a function of an external degree of freedom given by the U/SU symmetries, contributing nothing new to physics.

Ok, so how would 2d beings (say those on a computer program) detect the third dimension? Are they required to be able to detect this? Say if we produce a mechanical robot that operates in 2 dimensions only? Would it detect extra dimensions?

As said, being able to say "we can confidently say nothing exists past X" is an interesting statement to make. Any observation on it is important data to know.

Think about the surface of a sphere... What would happen if the creatures living on that surface creates "circles" or "triangles" ?

Good example. Yep, I know of it. But is that always the case? Is it a requirement that any dimension be detectable that way? If it is, then we can confidently state we are able to detect all that can possibly exist? (That there can never be a region just outside of our observation?)

I also cannot see any solution to the "beginning of time" question. We only have 2 possibilities on that one, and only observe one fact on the matter.

Edited January 27, 2015 by Technical Ben

[*Aqua*]

Ok, so how would 2d beings (say those on a computer program) detect the third dimension? Are they required to be able to detect this? Say if we produce a mechanical robot that operates in 2 dimensions only? Would it detect extra dimensions?

The robot would detect it if the extra dimension influences the laws of its 2D world.

Let's assume the robot stands in a valley before a mountain. We also assume the valley is flat. At this moment height is still an unknown dimension for the robot. Its vision is limited to length and width so it would only detect a more or less irregular shaped plane it is in.

Now it moves to the edge of the plane (= moves to the border of mountain and valley). At the exact edge the robot would rotate in an additional space dimension as it is now trying to climb the mountain.

-> The robot now 'sees' that the irregular shaped plane changes.

-> The robot also experiences a kind of drag it never felt like that before. (In the 3D world gravity tries to pull the robot down the mountain.)

-> Other stuff influences the movement and perception, example falling rocks.

If the AI in the robot is smart enough it can now conclude there's something more to the world it knows about. And if it does an Einstein it'll discover that there's a height dimension.

Edited January 28, 2015 by *Aqua*

[rtxoff]

The robot would detect it if the extra dimension influences the laws of its 2D world. *snip*

So the analog conclusion for our world would be: "Gravity is the consequence of an extra dimension in our universe which we don't see."

[YNM]

Good example. Yep, I know of it. But is that always the case? Is it a requirement that any dimension be detectable that way? If it is, then we can confidently state we are able to detect all that can possibly exist? (That there can never be a region just outside of our observation?)

Not really if you happens to actually lives at a site which is flat. But then, gravity clearly drags everything else along with them, even spacetime (it's second to dark energy I guess at the time), so we can state that gravity might be another thing. But say, other universes, you don't see any parts of the universe we live in to be having strange effects due to that other universe, so we can't really observe them at all, directly or indirectly. It remains there as a mathematical curiosity (or well, until a better conclusion in that direction appears).

Observation is easy, and the results are always true; what's hard is concluding them correctly.

[*Aqua*]

So the analog conclusion for our world would be: "Gravity is the consequence of an extra dimension in our universe which we don't see."

I'm not sure if an analog conclusion can be made. What the robot experience isn't analog of what we experience.

[ZetaX]

I'm not sure if an analog conclusion can be made. What the robot experience isn't analog of what we experience.

What if your experience is not what I experience¿ What does that even mean¿ (rhethorical questions!)

Sorry, but these kind of questions are just silly when you want to talk about science and/or reality. This is not a philosophical debate.

Rgardless of experience, the more basic assumption of sharing the same universe implies that, when doing it correctly, you, me and the very advanced robot will end up with equivalent laws of nature (or be wrong).

[Cocox]

@Cocox

It's a bit different than what you described. Spacetime came into existence 'during' the Big Bang. We know that because our current theories tell us that. But what happened in the 'timespan' 'between' the actual Big bang and the creation of spacetime we don't know. Our theories don't work at that point.

If we somehow create and proof a theory of everything then we'll be able to tell what exactly happened. So far we've got only candidates but nobody knows how to proof them. There are also scientist who say that's impossible to do.

So, some say that we live inside a "bubble". What happens when your reach the end, the wall that separates our universe from whatever else is beyond it? We smash our heads on a "brick wall"?

Nah..

First man said that the world is flat and the Sun revolves around us.

Then we realized that we are surrounded by many stars like the sun, probably with planets.

Then man understood that all the stars that we see are our galaxy and said that our galaxy is the Universe.

Then we discovered other galaxies and later we were amazed to find out that there are actually clusters of galaxies.

Evolve technology to allow to see past our Universe and I bet that the model repeats itself. We will see other Universes at mind blowing distances, then clusters of universes and so on..

[Rocketscience101]

Wether or not the universe is infinite, it is still expanding, and expanding at such a rate, that humans will not in the near future reach the edge of the universe, and the actual edge of the universe is not defined, I'm sure it means areas where there are galaxies stars and planets, and while there is an edge away from them, the galaxies are all moving away from each other, so that the empty space is now taken up by the universe.

One of my favorite was something I heard from Neil Degras Tyson, in Cosmos. An old Greek philosopher theorized that, if you stood at the edge of the universe, and fired an arrow, one of two things would happen, 1. It would fly straight on, or 2. it would hit a wall. For 1. if it continued to fly straight, than you had not reached the universe, and if it never stopped, than the universe is infinite, or 2. if it hit a wall, then what you perceived as the edge of the universe was wrong, and you would move to that edge and try the experiment again, and continue with that experiment, with only those two options happening. Which meant that the universe must be infinite.

[Vladthemad]

Hmmm...the universe is finite...but what it's expanding into is infinite...would that be to simplistic an explanation?

[K^2]

Hmmm...the universe is finite...but what it's expanding into is infinite...would that be to simplistic an explanation?

It's not expanding into anything.

[Vladthemad]

It's not expanding into anything.

I understand that there's nothing there, but it's still expanding into...empty "space". What I was getting at is that the material cast off from the Big Bang is something and is limited...even if it's just a stray atom light years from anything else at the edge of the expansion, it's still something. Past that, there truly is nothing (I assume...I'm not a sciency science scientist, but I used to follow the Discovery and History channel before they became reality networks so I know enough to stick my foot in my mouth on occasion) and that nothingness would be unlimited.

Does this remind anyone else of Rockbiter from the Neverending Story talking about the Nothing?

[Robotengineer]

If the universe is infinite it is also perfectly symmetric correct?

[Sirrobert]

I understand that there's nothing there, but it's still expanding into...empty "space". What I was getting at is that the material cast off from the Big Bang is something and is limited...even if it's just a stray atom light years from anything else at the edge of the expansion, it's still something. Past that, there truly is nothing (I assume...I'm not a sciency science scientist, but I used to follow the Discovery and History channel before they became reality networks so I know enough to stick my foot in my mouth on occasion) and that nothingness would be unlimited.

Does this remind anyone else of Rockbiter from the Neverending Story talking about the Nothing?

There is no edge, it's infinite

[Robotengineer]

I understand that there's nothing there, but it's still expanding into...empty "space". What I was getting at is that the material cast off from the Big Bang is something and is limited...even if it's just a stray atom light years from anything else at the edge of the expansion, it's still something. Past that, there truly is nothing (I assume...I'm not a sciency science scientist, but I used to follow the Discovery and History channel before they became reality networks so I know enough to stick my foot in my mouth on occasion) and that nothingness would be unlimited.

Does this remind anyone else of Rockbiter from the Neverending Story talking about the Nothing?

It couldn't be space. If it were space, then it would be a part of the universe. The whole point of nothing is that it is unquantifiable. So it could not be expanding into nothing, because you cannot expand into nothing, to 'into' nothing means it is something, which it would then not be nothing.