I’ve just finished reading Max Tegmark’s latest book, ‘Our Mathematical Universe‘, about his views on multiverses and the ultimate nature of reality. This is the first in a series of posts that I plan to do on it. Tegmark postulates four levels of multiverse. This post is about the first, and simplest version, the Level I Multiverse.
No one knows for sure how large the universe is, but the size of the observable universe is actually pretty well known. The universe as we know it started expanding from a hot dense state about 14 billion years ago. Given the fast but finite speed of light, the further away we look, the further back in time we’re looking.
The furthest and oldest thing we can now see is the cosmic microwave background (CMB), which has been traveling for almost the entire history of the known universe, 14 billion years. However, due to the expansion of space, the CMB we are now seeing originated 46 billions light years away, making that the radius of the observable universe.
Cosmologists often consider the word “universe” to be synonymous with the observable universe. The reason for this is it’s all we can observe, and given that the speed of light is the fastest speed that any interaction or effect can travel, the edge of the observable universe is the limit of our causal influence, or of things that could influence us. In other words, we are currently causally disconnected from the regions beyond this point, beyond or cosmological horizon.
Of course, there’s no real reason to think the universe ends at the limits of our observation. Indeed, attempts to measure the curvature of space seem to indicate that the whole universe is at least hundreds of times the size of the observable universe.
Considering the observable universe to be our universe, Tegmark refers to the regions beyond as other universes. Semantically, this seems like a questionable move. These regions seem like more of just the same universe. Initially I suspected this might be an attempt to redefine the multiverse into something non-controversial so the controversial versions wouldn’t be as much of a leap.
However, if the universe is infinite, it leads to strange conclusions, and with enough distance, the phrase other universes starts to make sense. Within each local observable universe, there are a finite number of ways that the atoms can be arranged. (Note: a very large number of combinations, but a finite number.) This means that in an infinite universe, every possible configuration of matter will eventually be realized. Furthermore, if you could look far enough in this infinite space, eventually every configuration will repeat itself, infinitely.
In other words, in an infinite universe, somewhere there would be another observable universe, also with a 46 billion light year radius, that is identical to ours. If so, it would contain a duplicate version of you reading a duplicate of this blog entry. Actually, throughout infinite space, there would be infinite copies of you reading infinite copies of this blog entry. And there would be an infinite number of you in every possible variation of you leading every possible variation of your life.
These duplicate universes would exist within the same space that we inhabit, have the same laws of physics, and the same or varying histories. But even the closest would be unimaginably far way. Tegmark states that current calculations show that it would be 101029 meters from here, which of course is an indescribable distance.
Do these duplicate regions, these parallel universes, exist? If space is infinite, or at least 101029 meters in extent, it seems hard to argue that they don’t. If space is that large. But we don’t know yet whether or not it is. It might be, or it might not be.
Current measurements show space to be flat, but as I said above, the margin of error on these measurements leaves room for space to still eventually loop back onto itself, to make it that if you could travel in a certain direction long enough, you’d end up back at your starting point, similar to what happens on Earth, but in three dimensions.
But if space is flat and infinite, and these duplicate regions do exist, it’s hard to imagine how their existence could ever have an effect on us (unless maybe if someone invented an intergalactic warp drive). This also seems to make the idea unfalsifiable. Still, the concept, while speculative, is a fascinating one.
This is the simplest of the multiverses that Tegmark discusses in his book. In the next post in this series, I’ll discuss his Level II Multiverse.