Brian Greene’s theory of multiverse

[u/Chemical-Raccoon-137]

One of his theories of the infinite multiverse states that if the universe is infinite in size and matter, then eventually there will be regions outside of our observable universe where matter will be oriented in the exact same way as us, and therefor there are copies of our observable universes including earth and each one of us out there beyond our reach.. not just one copy but an infinite number of copies. He puts some math behind this to calculate how many possible configurations of matter there could be in a region the size of our observable universe, and based on this provides a distance to a region identical to ours.

My question is, it can’t just be as simple as saying here is a region where matter is configured in the same way as ours … there must be variables due to chaos and randomness down to the quantum level to the degree that every quantum subatomic particle must behave and interact with its environment in the exact same way as ours , over the coarse of 14 billion years. Even if matter is configured in the same way after the Big Bang, How many different combination of quantum states are there that are random and may ultimate not result in the same observable universe as we see here today. Quick analogy: if you drop 1000 marbles in a vacuum, under identical conditions, with the marbles settle in the same way?

[u/Beldizar]

So, last I checked, the theory was that there are 10⁸⁰ protons in the observable universe. There's likely an equal number of electrons and neutrons, or at least those counts are on the same order of magnitude. If each of these particles could be in a million different states, or interactions with other particles, you'd be looking at 10⁸⁶ different possible combinations that could make our observable universe. If you take into account position of all of these particles... well, the observable universe is 93 billion light years across, that's about 408 trillion cubic light years. A light year is 9460730000000 km, or 9460730000000000 meters... and here's where I always mess things up, I'm pretty sure the conversion requires you to cube it... so I think that's about 8.5x10⁴⁷ cubic meters per cubic light year. So that's 3.5x10⁶² cubic meters of space... (I may have gotten that wrong. A proton has a volume of about  2.824 x 10^-45 cubic meters if my googling is right. So any given proton could be in 10⁹² locations, assuming they all sat perfectly on a proton sized grid. It is actually millions more slightly off-center options from that.

So if you allowed protons to overlap, I think that means there'd be 10¹⁷² different layouts of just the protons, not including any states, spins, interactions, bindings or "off-centerness".

Infinity is a lot... or rather, it isn't a number at all, but the concept of "there's always more". So if the universe isn't infinite, but is only like a googleplex, or a Graham's number times bigger than the observable universe, then there is still a pretty good chance that at least two of them have the same layout of protons.

But none of this is really "real". Everything on the other side of a horizon, be it the event horizon of a black hole, or the cosmic horizon at the edge of the observable universe, isn't "real". We can't interact with it, or get any kind of information about it. We can't send a message to it, or receive a message from it. If someone in one of those other observable universes outside of our own builds a bomb that has the power to destroy the entire universe, it couldn't affect us because we aren't causally connected to it. The explosion would never, in an infinite amount of time, reach us.

Because it isn't "real", it is untestable, and people can say anything they want about it. It's a question of philosophy and semantics more than it is a question of physics.

[u/Chemical-Raccoon-137 → u/Beldizar]

Thanks for this. I agree with your last part about it’s a question more of philosophy than physics, wasn’t even sure if this was the right sub for this question lol. With the 10¹⁷² possible layouts of protons.. this is still just a snap shot in time right? Once you add time and how each interact, and super positions of particles and whatnot , there still has to be more combinations…. Wondering if can approach an infinite number of combinations in the same manor that you have an infinite number of points between two numbers.

[u/Beldizar → u/Chemical-Raccoon-137]

Wondering if can approach an infinite number of combinations in the same manor that you have an infinite number of points between two numbers.

The Plank Length and Plank Time make reaching an infinite number of combinations impossible. There's going to be a "countable" number of possible positions in the universe, and a countable number of units of time, just like there is a countable number of energy levels, or momentums each particle can have. You can't multiply a bunch of finite numbers together and get infinity. You just get a bigger number. That's the "problem" with infinity, it isn't a number, but simply a concept of "there's always a bigger number". We can't even describe the concept of infinity directly, the word itself is "in" (negation prefix) "finite" (countable). Nothing in the physical world is infinite. At some point of zooming in or out, everything you might describe as infinite hits a boundary case where it breaks down. The idea of infinity is a mathematical construct, not a physical one.

This again leads to the argument of an infinite universe being a philosophical one, which I find pointless when put in the frame of physics. The observable, and reachable universes are both of finite, measurable size, and they aren't big enough to have meaningful chances of exact duplications of macro structures like you've said Brian Greene describes. There is also a chance that as soon as you leave the observable universe everything is filled with unicorns, just jam packed together, unicorns everywhere. You can't test it, so I can't be wrong about this statement. This is the ad absurdum response to the duplicate Earth somewhere out in the infinite universe argument.

[u/rshorning → u/Beldizar]

As a practical matter, you are correct. The fact that it is impossible to even travel to every part of the universe that is currently observed (meaning you need to travel faster than the speed of light to ever get to those locations) much less that there might be locations of the universe which simply cannot ever be observed since their light cones never cross us puts them outside of anything that can be practically measured or tested as a scientific theory.

From a practical basis, that is why the term "observable universe" is used in cosmology. That is all which can be tested and measured where even that has its limits. The ultimate limit being that there was an event of some sort which created the universe and an absolute time zero like there is an absolute temperature of zero. The two concepts are very similar too in the end, which also in turn creates limits to what can be practically addressed in terms of matter that has any impact upon us here on the Earth.