Fumfer Physics 7: Quantum Unitarity, No-Cloning, and Information Capacity in Cosmology

Scott Douglas Jacobsen: Now we are doing digital physics. All right, unitarity and no-cloning. In enclosed quantum systems, information is preserved by unitary evolution. So you cannot perfectly copy or delete unknown quantum states. No cloning, no deleting. How does that strike you?

Rick Rosner: Where does this show up? What do you mean? All right, expand the definition. 

Jacobsen: You can mathematically describe quantum states. Unitarity means that in an isolated system across evolution, probability always sums to one, and no information is lost—it is a mathematical guarantee. The other principle is that even though data is preserved, it cannot be perfectly copied or destroyed. You cannot make a clone. That is interesting.

Rosner: So, where do these situations and issues show up?

Jacobsen: Cryptography, black hole physics, and quantum computing.

Rosner: Okay.

Jacobsen: What are your thoughts on this from a cosmological perspective?

Rosner: I do not know. I have not thought about this aspect. Quantum mechanics is about the flow of information, but that information is always incomplete. In its incompleteness, it is a set of probabilities. Any quantum system you examine is not deterministic—probabilities of future outcomes describe it. Even that probability set is fuzzy because you are not entirely sure what system you are dealing with, unless you go to great lengths to characterize it completely. You can design experimental systems where the probability sets are pinned down because you have controlled the experiment precisely. However, overall, what you are describing sounds like quantum behaviour in general: uncertainty, incomplete knowledge, and systems defined by limited information.

Jacobsen: We can add another piece to that and make it more robust. There is a finite information capacity to any region of spacetime. There is a maximum number of bits in a bounded spacetime volume.

Rosner: Yes, that is one example, the event horizon. 

Jacobsen: But more generally, any volume of spacetime has a finite information capacity. It is not infinite. Even black holes are collapsed matter in IC—extremely dense, but not infinite—as opposed to the old idea of singularities.

Rosner: Here are the issues I think matter. For the universe to function as an information processor, it needs a way to dispose of entropy. I do not believe the universe has constantly increasing entropy; it is the opposite. The universe tends to increase information. 

Its physics allow it to sequester entropy. Closed systems obey the rule of increasing entropy, but most of the macro-universe is not closed. Our planet, our solar system—these are open systems that can easily dispose of waste heat into space. Waste heat is entropic. If you can radiate it away, you have negentropy—an increase in order, which we see. Electromagnetic radiation loses energy as it travels billions of light-years. 

That lost energy translates into a slight reduction in curvature in space, which contributes to the net information in the universe. Over tens of billions of years, that means a more ordered universe. 

Burnt-out galaxies collapsing and later reigniting might be part of the same process: local collapse balanced by energy released to fuel expansion elsewhere. A cycle that eliminates entropy. The universe has memory. The accumulation of information is the opposite of entropy. I do not know.

Thank you for your dedication and your insane ethic.

Jacobsen: Insane?

Scott Douglas Jacobsen is the publisher of In-Sight Publishing (ISBN: 978-1-0692343) and Editor-in-Chief of In-Sight: Interviews (ISSN: 2369-6885). He writes for The Good Men Project, International Policy Digest (ISSN: 2332–9416), The Humanist (Print: ISSN 0018-7399; Online: ISSN 2163-3576), Basic Income Earth Network (UK Registered Charity 1177066), A Further Inquiry, and other media. He is a member in good standing of numerous media organizations.