# The Universe In a Nutshell by Stephen Hawking

• RuroumiKenshin
In summary, "The Universe In a Nutshell" by Stephen Hawking discusses the concept of rotating Einstein universes and their relation to time travel and time loops. These rotating universes are mathematically equivalent to other backgrounds that allow for time loops, such as universes with two expanding space dimensions and one periodic dimension. Hawking also mentions the idea of a "tolaroid" universe, which is an open universe with an increase of entropy occurring in one direction. He suggests that our universe may be a sub-universe within a larger, infinite universe, similar to how atoms make up larger structures. The concept of a limit on the size of the universe is also discussed, and Hawking compares our perspective of the universe to viewing a
RuroumiKenshin
"The Universe In a Nutshell" by Stephen Hawking

I'm reading the book "The Universe In a Nutshell" and I've become confused with a couple of parts...
"What do rotating Einstein universes have to do with time travel and time loops? The answer is that they are mathematically equivalent to other backgrounds that do admit time loops. These other backgrounds are unvierses that are expanding in two space directions. The universes are not expanding in the third space direction which, is periodic. That is to say, if you go to a certain distance in the this direction, youget back to where you started. However, each time you do a circuit of the third space direction, your speed in the first or second directions is increased."

I don't quite understand what he's talking about (esp., his diagram doesn't quite make sense either).

Thanks a bunch!

Space news on Phys.org
Perhaps its possible to illustrate the second kind of universe he mentions in three dimensions if we just ignore one of the expanding dimensions he mentions and have one expanding dimension and one periodic dimension (these are all spacelike dimensions, the timeline dimension is not part of this discussion).

Perhaps we can model the second type of universe he mentions by ignoring one of the expanding dimensions. So we get a two dimensional object lying in our three dimensional space, with one expanding dimension and one periodic dimension. What we have is a finite cylinder, like a tin can with the ends taken off. The two dimensions are its length, and a circular latitude dimension that goes around its curve. The latitude is periodic, that is, it's a circle. You go around it and come back to where you started.

The length dimension is the one that is expanding, the cylinder is getting longer.

Now if the circular dimension is timelike, then you certainly have time travel. In fact you just keep going around and around in the same time loop like the guy in Groundhog Day. This is called a Closed Timelike Curve (CTC) and is the gr-speak version of time travel.

Hawking is saying he can map a rotating universe into one of these (the full dimensioned one, of course, not the model) in a one-to-one nice manner in such a way that if the cylinder universe has CTCs then so does the rotating universe. I don't know if this is Hawking's own idea or if it goes back to Goedel, who showed rotating universes have CTCs back in the 1930s.

Is this what you call a tolaroid? I've heard about it before. It suggests an open universe, does it not?

Does the increase of entropy in this tolaroid universe occur in one direction? (i'm guessing yes?)

MajinVegeta, aren't you like 15? I got to say I'm impressed. Most people don't even begin to understand a concept like entropy until college (if ever).

Oh, I wish I were 15! I've got 2 more years to go, russ. I'm only 13.

anyway: When the tolaroid is an open universe, so this implies that this universe is a sub universe since if our universe is open, it has to be open relative to somewhere else; the real universe. Does anyone think, like Stephen Hawking does, that there could be other universes? That the infinity of the universe (in the aforementioned context about how we're in a sub universe according the tolaroid universe) is infinite in the sense that it is non-terminating, endless and there will always be something "outside". Just like atoms. There are protons, neutrons, electrons. Protons are made up of 2 up quarks, one down quark. Neutrons are exactly the opposite (fascinating, isn't it?) and for all we know, the quark could be made up of something else. The Planck length sets a limit on the smallest size we could observe.
This further causes me to inquire whether or not the universe has a limit of size (this time a BIG one) that we could...observe it.
My dictum(a weird one) that corresponds is as follows, the universe is to us as the Earth is to an atom. We are unable to observe the universe as it is withoutp painstakingly making the big picture by bits of information. Another anology will shorten what I have to say (so you don't have to read a lot). The Lily Pond, by Monet is REALLY big, like 20 meters in length or something (it's really long). Whe you're close up to it,the brush strokes sort of make it hard to make out the painting, but when you view the painting from a distance, the painting takes form. So, in conclusion, I will state that we are viewing the universe from a microscopic perspective relative to the size of the universe.

## 1. What is the main concept of "The Universe In a Nutshell"?

The main concept of "The Universe In a Nutshell" is to provide a comprehensive and accessible explanation of the universe, from its origins to its potential future, using scientific theories and evidence.

## 2. Who is the author of "The Universe In a Nutshell" and why is he considered an authority on the subject?

The author of "The Universe In a Nutshell" is Stephen Hawking, a renowned theoretical physicist and cosmologist. He is considered an authority on the subject because of his groundbreaking contributions to our understanding of the universe, including his work on black holes and the origins of the universe.

## 3. Is "The Universe In a Nutshell" suitable for non-scientists?

Yes, "The Universe In a Nutshell" is written in a way that is accessible to non-scientists. Hawking uses simple language and analogies to explain complex scientific concepts, making it easier for readers without a scientific background to understand.

## 4. What are some of the topics covered in "The Universe In a Nutshell"?

"The Universe In a Nutshell" covers a wide range of topics, including the Big Bang theory, black holes, the laws of physics, the search for a unified theory, and the potential end of the universe. It also delves into philosophical questions about the nature of time and the meaning of our existence.

## 5. Is "The Universe In a Nutshell" based on scientific evidence?

Yes, "The Universe In a Nutshell" is heavily based on scientific evidence and theories. Hawking draws on the work of other scientists, as well as his own research, to support his explanations and arguments about the universe.

Replies
19
Views
5K
Replies
3
Views
1K
Replies
9
Views
2K
Replies
2
Views
2K
Replies
5
Views
3K
Replies
13
Views
2K
Replies
2
Views
2K
Replies
3
Views
1K
Replies
37
Views
7K
Replies
10
Views
3K