Current views on what happens at largest and smallest scales

[Tanelorn]

This was one of my earliest childhood conundrums, and I am still not much clearer on it; what happens at the largest and smallest scales of space or space time? The infinite number of Russian dolls problem..

How do we deal with the infinite? (or if we take Guth's 10^32 times the size of the observable universe what could lie just beyond?)
Similarly how do we deal with infinitesimal distances? Quantized space? Or does space cease to exist?

If we do not know the answers to these questions yet (or never expect to) that is ok.

 

[PeterDonis]

All of our current theories model spacetime as a continuum. That means there is no largest or smallest distance scale in our current theories.

It might be that, when we have a good theory of quantum gravity, it will model spacetime some other way, as emerging from something else, in a way that involves some smallest or largest distance scale. But we don't have a good theory of quantum gravity yet.

[Btw, if you label a thread as "A", that implies that you have a graduate level understanding of the subject matter. This question is really an "I" question, and I have relabeled the thread accordingly. It might even be a "B" question, but I'm not sure whether the properties of the continuum as a model are covered in all high school curricula.]

 

[Tanelorn]

High School? ok I get the message. Thanks Peter, sorry to have caused such a problem, I will refrain from posting here. Thanks again.

 

[Drakkith]

High School? ok I get the message. Thanks Peter, sorry to have caused such a problem, I will refrain from posting here.

No need to leave! We haven't even gotten the six-layer bean dip out yet!

 

[PeterDonis]

High School?

The "I" level is undergraduate. "B" level is high school, but, as I said, that wouldn't be correct for this thread.

The levels actually serve a couple of purposes. One is to signal the level of knowledge required to understand the subject matter of the question; the other is to signal the level of knowledge at which answers are desired. The two aren't necessarily the same in principle; but in practice, it's extremely difficult to ask a question for which they are actually different.

In any case, no negative connotation is intended when a moderator changes the level of a thread. The intent is only to more accurately reflect the above intent for the levels. (One very good rule of thumb is that, if there is no math in your question, it's extremely unlikely that it is an "A" level question.) Moderators try to give feedback when a question's level is changed to help posters in judging the correct level of future questions.

 

[Tanelorn]

ok well it seems this was more important than the question.
I am a graduate in Physics so that's as far as I went with that selection, I was more interested in trying to get my question to make sense. I don't think you fully understood me anyway, which could also be my fault.

At the infinitesimal end of the scale, I thought I recalled that the Planck length was the limit, and that this implied space was quantized. But at the opposite end of the scale I am not clear on what is going on, perhaps no one is. I have read that infinite space cannot be possible (infinity is a mathematical abstraction only), and terms like pseudo infinite is preferred, which begs the question what happens if space is not infinite, what is the next abstraction layer in the Russian Doll so to speak. I suspect no knows for certain? As I said I am only a part timer.

 

[PeterDonis]

I don't think you fully understood me anyway

I might not have. As far as I can tell, I think the answer to the question you are trying to ask is "we don't know yet".

 

[Tanelorn]

Peter, thanks for confirming this.
I believe that this is the biggest question in Cosmology, well it is for me.
Everything we are used to in our lives happens inside something else, I just wonder at what point that ends.

At least the U is temporally finite, or maybe not.

 

[Tanelorn]

Smolin's idea that the U is a black hole inside another U is an interesting idea, are there other alternative ideas?
I don't know if ideas like this are mainstream or fringe, but they are at least trying to answer the core questions about Cosmology.
Would space look flat and pseudo infinite in Smolin's model?

 

[PeterDonis]

Would space look flat and pseudo infinite in Smolin's model?

I'm not sure because I have not looked at the actual math of Smolin's model. Do you have a reference?

(The answer for an ordinary black hole would be "no". But I don't know if what Smolin is referring to as "the universe being a black hole inside another universe" is mathematically the same spacetime as an ordinary black hole.)

 

[Tanelorn]

Cosmological Natural Selection discusses his black hole idea:
https://en.wikipedia.org/wiki/Lee_Smolin

I only dropped by to ask if much had changed regarding whether we live in an infinite or non infinite Universe, and the implications of each. I will ask again next year!

 

[PeterDonis]

Cosmological Natural Selection discusses his black hole idea

Ah, ok. This idea is not that our universe is a BH inside another universe. It's that every universe gets spawned from a BH in a previous universe (basically, the singularity at the center of each BH, instead of being a place where spacetime just ends, is where a new universe gets spawned). But the new universe is not "contained" in the previous one; it's a completely separate spacetime in its own right.

 

[Kevin McHugh]

A. But the new universe is not "contained" in the previous one; it's a completely separate spacetime in its own right.

Peter, would that imply parallel dimension? Or is it part of a multiverse theory? Or both?

 

[PeterDonis]

would that imply parallel dimension?

I'm not sure what "parallel dimension" means. There is no interaction between the new universe and the old one, if that helps.

is it part of a multiverse theory?

It's one version of a "multiverse" theory, yes.

 

[Kevin McHugh]

I'm not sure what "parallel dimension" means. There is no interaction between the new universe and the old one, if that helps.
It's one version of a "multiverse" theory, yes.

I was thinking it would be parallel on a time axis, but with different spatial coordinates. Does that make any physical sense?

 

[PeterDonis]

I was thinking it would be parallel on a time axis, but with different spatial coordinates. Does that make any physical sense?

It could if you are talking about two different objects in the same universe that are at rest relative to each other--the history of each object is a timelike curve in spacetime, but the two different curves have different spatial coordinates. But the new universe that gets spawned from a black hole in Smolin's hypothesis doesn't have that kind of relationship to the old one.