I haven’t been blogging much lately. It’s not that I have anything against blogging – I haven’t been doing much of anything lately, unless breathing and digesting count.

In another universe, however, I’ve been blogging up a storm while hiking through Alabama with Gwyneth Paltrow and Vladimir Putin. That’s what many physicists seem to believe anyway. (In that other universe, Vlad promised Gwyn and me that he’s going to stop interfering with Ukraine.) 

For example, Max Tegmark of M.I.T. has written a book that, according to the New York Times, suggests that he was almost hit by a truck while riding his bike in Stockholm, and the truck hit him, resulting in some slight injuries, and the truck really clobbered him, which meant he didn’t live to write the book he later wrote: 

He endured every possible outcome, happy and unhappy, that can befall a bicyclist who encounters a speeding truck. All of these happened, he argues, because everything that can happen does happen — in at least one of an infinite number of universes.

This extremely large set of parallel universes is called the “multiverse”. Our universe, the particular one that I’m experiencing now, in which I’m not pals with Gwyn and Vlad, is merely one universe among many – no more real than the others. 

Very smart people like Max Tegmark and Stephen Hawking accept the multiverse theory, which is also known as the “many-worlds interpretation of quantum mechanics”. They think this apparently crazy idea best explains the truly crazy stuff that happens at the quantum level of reality, like the double-slit experiment and the situation with Schrödinger’s cat. We’ll probably never know for sure, since verifying the existence of another universe is supposed to be impossible.   

Still, something has been bothering me since I read that article in the Times. When people talk about parallel universes, they usually talk about universes branching off from each other. What supposedly happens is that whenever there’s more than one possibility in a given universe, that universe somehow splits into two or more separate universes. You start out in one universe and get hit by a truck but in another universe you escape injury. In one universe, you order pork chops, in another you have a salad and in a third you go somewhere else to eat. The examples in these discussions are almost always familiar events or decisions, the kind of possibilities we can all relate to.

Physicists, however, don’t usually concern themselves with what people have for dinner. Quantum physics, in particular, concentrates on very small-scale events. Will an atom of carbon-14 decay or not? How frequently do quantum fluctuations (the so-called “appearance and disappearance of virtual particles”) occur? In theory, each of these small-scale, apparently random quantum events marks a divergence in the history of the universe. If an atom decays, the universe goes one way. If it doesn’t decay, the universe goes another way. According to the many-worlds theory, as it’s almost always explained, each event that could have happened differently results in the creation of separate universes.

Of course, since the universe is a very big place, there is room for lots of events to occur, especially the tiny, random ones. Here’s a quote from The Many-Worlds Interpretation of Quantum Mechanics by the physicist Bryce DeWitt: 

This universe is constantly splitting into a stupendous number of branches, all resulting from the measurement-like interactions between its myriads of components. Moreover, every quantum transition taking place on every star, in every galaxy, in every corner of the universe is splitting our local world into myriads of copies of itself [161].

Wow! Is it really possible that equally gigantic, almost exact replicas of the universe, with all its particles, planets, galaxies and so on, are springing into existence zillions of times a second, whenever a sub-atomic particle somewhere in the universe sneezes? And that those replicas immediately start replicating themselves? As that old TV commercial said, that’s one spicy meatball!

The effect seem way, way, way out of proportion to the cause. Where does all the energy come from to create fully-formed copies of previously existing universes? Assuming that these new universes immediately pop into existence, how do they get so big and so detailed so quickly? Or maybe universe creation takes a long “time”, but we don’t notice any breaks in the action because we keep coming into existence with our memories and instrument readings intact, as if nothing weird has happened (remember that you and I have lived through zillions of such universe creations – it’s not as if we’ve always lived in the nice, stable universe and everybody else is off living in copies).

I don’t know enough math or physics to criticize the many-worlds theory for real, but I was pleased to see that one of the things bothering me is a standard objection to the theory. From Wikipedia:

Conservation of energy is grossly violated if at every instant near-infinite amounts of new matter are generated to create the new universes.

To which, proponents of the theory are said to have two responses:

First, the law of conservation of energy says that energy is conserved within each universe. Hence, even if “new matter” were being generated to create new universes, this would not violate conservation of energy. (That doesn’t seem like a very good answer to me, since it amounts to saying, “It’s very strange that there’s an exception to this fundamental law, but that’s what happens”.)

Second, conservation of energy is not violated since the energy of each branch has to be weighted by its probability, according to the standard formula for the conservation of energy in quantum theory. This results in the total energy of the multiverse being conserved. (Which seems to mean that if there’s a 50/50 chance of some atom decaying, each new universe has half as much energy as the last one. Wouldn’t that eventually result in new universes having no energy at all?)

So it was with some relief that I turned to a helpful website called “Ask a Mathematician”. It should really be called “Ask a Mathematician or Physicist”. because it’s apparently a mathematician and a physicist answering questions, most of which have to do with physics (they don’t identify themselves, they just answer questions). A few years ago, they got this question:

According to the Many Worlds Interpretation, every event creates new universes. Where does the energy and matter for the new universes come from?

Here’s some of the physicist’s (rearranged) answer:

If you go online (or read some kind of book or something), you generally find the Many Worlds Interpretation presented as the universe “splitting”. Something along the lines of “everything that can happen will, just in different universes”. Supposedly, every time any kind of quantum event happens that could have one of several results (which is essentially every moment for every thing, which is plenty) the entire freaking universe splits into many universes. But, the universe contains a lot of energy….So, whence does this energy come?

[However,] there is no new energy or matter (or even new universes)…The universe doesn’t split or spawn new universes…. The universe doesn’t branch so much as it meanders and intertwines….If you want a picture to work with, rather than thinking about the universe as an ever-branching tree, think of it as an intertwining (albeit, very complex) rope.

The many (like: many, many) different versions of the universe branch apart, and come together all over the place. That is: one event can certainly lead to several outcomes, but in the same way, several causes can lead to the same event.  Everything that could happen will happen (given the present) and everything that could have happened did happen (given the present)….

A particle comes along with some amount of energy. When it has a choice of two paths it takes both.  The energy of the particle is divided in proportion to the probability of the path taken.  So, for example, a 50% chance of each path means equal division of the energy and matter of the particle.  Before the fork all of the energy is on one path, and afterwards, despite the fact that the particle is behaving as though it’s in two places, the same amount of energy is present, just spread out.

So, while it’s fun to talk about “other quantum realities” and “different universes”, it’s more accurate to say that everything is happening in one universe. One, stunningly complex, weirdly put together, entirely counter-intuitive universe.

Clear? Despite the standard explanation, we’re all living in the same universe, but it’s a universe that has lots of its contents in strange, probabilistic quantum states. Since these various states (like when a photon is 50% likely to be here and 50% likely to be there) are equally real, they can be thought of as parts of different universes, but that’s just a manner of speaking.

Whether or not this way of understanding the multiple universe theory is correct (is it, since Max Tegmark apparently suggests otherwise?), it makes me feel better. For one thing, it’s less mind-boggling. Big branching universes seem both implausible and terribly wasteful. Secondly, I think there’s zero probability that one of me is hiking through Alabama with Gwyneth and Vladimir (while wearing a blue shirt, and a red shirt, and a green shirt, and no shirt, etc. etc.) while also breathing and digesting here in the Garden State.

(Note, however, that if the universe is infinite in time and space and configured a certain way, it’s possible, maybe even a sure thing, that everything that isn’t contradictory happens over and over again. But that’s something to wonder about another day.)