Parallel Universes

Scientific American ^ | May 2003 | Max Tegmark

[Junior]

Is there a copy of you reading this article? A person who is not you but who lives on a planet called Earth, with misty mountains, fertile fields and sprawling cities, in a solar system with eight other planets? The life of this person has been identical to yours in every respect. But perhaps he or she now decides to put down this article without finishing it, while you read on. The idea of such an alter ego seems strange and implausible, but it looks as if we will just have to live with it, because it is supported by astronomical observations. The simplest and most popular cosmological model today predicts that you have a twin in a galaxy about 10 to the 1028 meters from here. This distance is so large that it is beyond astronomical, but that does not make your doppelgänger any less real. The estimate is derived from elementary probability and does not even assume speculative modern physics, merely that space is infinite (or at least sufficiently large) in size and almost uniformly filled with matter, as observations indicate. In infinite space, even the most unlikely events must take place somewhere. There are infinitely many other inhabited planets, including not just one but infinitely many that have people with the same appearance, name and memories as you, who play out every possible permutation of your life choices. Entire Article

Overview / Multiverses

• One of the many implications of recent cosmological observations is that the concept of parallel universes is no mere metaphor. Space appears to be infinite in size. If so, then somewhere out there, everything that is possible becomes real, no matter how improbable it is. Beyond the range of our telescopes are other regions of space that are identical to ours. Those regions are a type of parallel universe. Scientists can even calculate how distant these universes are, on average.

• And that is fairly solid physics. When cosmologists consider theories that are less well established, they conclude that other universes can have entirely different properties and laws of physics. The presence of those universes would explain various strange aspects of our own. It could even answer fundamental questions about the nature of time and the comprehensibility of the physical world.

[Dog Gone]

Great article! It takes awhile to read it all, but it was worth it.

[Physicist]

I am not really an expert on Cosmology, but I suspect that the author of this article isn't one either.

As a matter of fact, he's one of the premier experts on cosmology in the world (and a colleague of mine here at Penn). When it comes to testing cosmological models against hard observational data, there's none better.

The Big Bang theory stipulates that at the beginning the volume of the universe was zero. After that, it expanded (extremely rapidly) and continues to do so, such that the total volume of the universe grew and continues to grow at a high but finite rate.

That refers to our horizon, but not to the whole. In the standard Friedmann cosmology, the volume of the universe can be represented as the "surface" of a 4-dimensional hypersphere. We have a finite horizon, or "Hubble volume"--the volume of the universe that is receding from us at a velocity less than the velocity of light--but the whole is (in principle) finite, too: it eventually curves back upon itself.

[Geek alert: read carefully what I said. In any cosmological model, there will be parts of the universe that are receding faster than light. This does not violate special relativity, because these regions are out of causal contact with us.]

The question remains: what is the radius of that hypersphere? It could be anything, according to Friedmann; it's just something you have to measure. On Earth, for example, you can measure the radius of the Earth simply by noting that an equilateral triangle, 10 million meters on each side, will have interior angles of 90 degrees each, rather than the 60 degrees of a small equilateral triangle.

We now (as of 4 months ago) have a similar measure for the largest possible triangles that can be drawn in our Hubble volume. No matter how big, the interior angles sum to 180 degrees. This corresponds to a hypersphere of infinite radius; in other words, the universe is flat.

This agrees with the theoretical prediction of Inflationary Cosmology.

[Geek alert: the curvature could also have been negative, in which case the universe would have a hyperbolic shape, rather than a hyperspherical shape.]

[Mr. Mojo]

Looking back on my life, there are a number of really stupid things that I almost did. Now I find out that somewhere out there, I actually did do them. How embarrassing is that?

Looking back on my life, there are a number of really smart things that I almost did. Now I find out that somewhere out there, I actually did do them. How cool is that?

[Buggman]

Fascinating article, but the way that the author bandies around terms like "infinite" and "eternal" without really comprehending their meaning bugs me. For example, he speaks of the universe being "infinite" in size because it (in all likelihood) has no borders. However, that's not really what he means. The universe can be finite without ever allowing us to come to an edge just as the inside space of a sphere can be finite without ever allowing the ant crawling around on its inside to come to an opening.

In fact, the volume of the universe must be finite, if Big Bang cosmology or any other cosmology that depends on expansion is true. The Big Bang states that the universe was at one point finite in size (a singularity or near-singularity) before it started expanding. You cannot expand a finite space for a finite period of time (13-15 billion years) and reach infinity. For that matter, you cannot expand something that is already infinite at all.

Likewise, if the volume of space is finite, the mass and energy contained in it must be finite as well, as you cannot fit an infinite amount of mass into a finite container.

I understand that he's writing at the popular level, but he really should be more careful in the use of his terms. The fact is that "infinite" is a purely conceptual word (and a poorly conceived word at that), and not one that can be observationally or even logically sustained. The author here is misusing it, meaning instead "an incoceivably large amount."

I myself tend towards a more Aristotelian worldview for a simple reason: We know that not all mathematical models correspond to reality. For example, suppose my boss asked for an estimate on how many people it would take to complete a certain project. And then suppose that I use a quadradic equation to make my estimate and end up with the numbers 4 and -3 for x. Could I then go to my boss and say, "Well, it'll either take me four people or negative three people"? Is there a conceivable universe in which -3 people would be the correct answer?

To me, the fact that the universe corresponds so well to mathematics is not evidence that the math is the real and my senses are the unreal, but that it was designed by a very good Engineer.

[freedom9]

What about those which have not yet occured?

[<1/1,000,000th%]

LOL!!

What a great idea for a made-for-TV movie!

[gcruse]

Whew. I was a lot smarter before I read that article.
Now, I (we) aren't sure about anything.

[Junior]

Then they're still up in the air. There's also the possibility of a prophesy being vague enough that just about any situation could be covered by it.

[Dead Corpse]

If you can get production on it, recommend me for a Mabden Chieftain. I'd fit right in, as far as looks are concerned, with Corum's pesky Pony Tribes. ;-)

[Physicist]

Read #62.

[mhking]

"Worlds lived. Worlds died. And the universe was never the same again."

- DC Comics, 1985

[Doctor Stochastic]

Then we would have had not only lots of parallel universes, but lots of parallel lines too.

[Argh]

I hope my double has enough money for beer this weekend, because I sure as hell don't.

[mhking]

"Worlds lived. Worlds died. And the universe was never the same again."

- DC Comics, 1985

[Quix]

Could you think of a better way to rear a ruling 'class' to rule countless galaxies over endless ages based on a philosophical and practical expression of LOVE, humility etc.?

[Sam Cree]

Here's another one of these things.

[RadioAstronomer]

Thanks for the ping. :-)

[honeygrl]

"So I actually AM the Beloved Emperor of the Universe somewhere?"

Yeah but unfortunately, when the choice was given for him to have his first cigarette he said no so he would be your evil smoke nazi twin. (kidding!)

[Physicist]

Could you think of a better way to rear a ruling 'class' to rule countless galaxies over endless ages based on a philosophical and practical expression of LOVE, humility etc.?

We have worlds enough, but not time. The same data that revealed the infinite extent of the universe also measured the rate of expansion as a function of time. The expansion is growing ever faster. In a finite amount of time, the universe will expand so fast that any surviving stars will be flung from their galaxies; after which, the planets from their stars; the stones from their planets; the atoms from their stones; electrons from their atoms; protons from their nuclei; quarks from their protons. Perhaps even the quarks will be destroyed.

Love while you can. For humility, we have the WMAP data.

[far sider]

This parallel universe stuff is old news, isn't it?