Some recent posts on mirror universes and Star Trek reminded me of something I wrote up a while ago; it's on a somewhat wacky idea that touches on some mysteries of the universe that some of you might find interesting.

Towards the beginning of the second season of the original Star Trek there is a classic episode in which a transporter malfunction throws Kirk et al into one of the greatest creations of the series: the mirror universe. They find a bizarre alternate reality where seemingly everything is backwards (the high ideals of the Federation are replaced by an Empire run by fear). This is episode is also particularly famous for showing us the “evil Spock”:



The title of the that episode lends itself well to the name of this thread, which is only appropriate as we're going to examine whether a mirror universe exists right here, all around us.

Broken Symmetry

Symmetry takes on an important meaning when it comes to physics:it means physics stays the same if you change things in some way. You can rotate a system and the physics of what's going on in that system will stay the same; it doesn't matter which way you're facing. Similarly, you can translate that system through space or time and the symmetry holds. Physical laws won't change depending on what spot you're at or what time it is. Or, put another way, Nature doesn't distinguish between different directions or different locations in space or time.

But there's another one, called p-symmetry (p for parity). Suppose we want to talk about mirror images. It's easier to deal with that mathematically if we not only swap left and right (as mirrors do) but also flip the 'image' upside down—that is, invert all the spatial axes. That inversion is called a parity transformation.



If we do that flip, we've changed those axes and thus the whole coordinate system from being left-handed to being right-handed. The question before us is then whether we have a symmetry—does the physics stay the same if you invert the axes? Turn out it does not. Nature does distinguish between lefthandedness and righthandedness. And it looks like she's a lefty. The real problem here is the weak force, the one that governs particle decay. It's the one that doesn't obey the symmetry and helps Nature make her distinction between lefty coordinate systems and righty coordinate systems. For example, neutrinos (which experience only the weak force and gravity) are all left-handed. Every single one.

Through the Looking Glass

If we don't wish to just accept that Nature is so blantantly unsymmetric, we can try to restore p-symmetry. Some intrepid souls have done this by suggesting doubling the number of known particles by giving every particle we know a mirror counterpart. Instead of living in an unsymmetric lefty universe, then, we'd actually be in a symmetric one (even though we only see half of it). Each mirror particle would have the same mass as its regular counterpart, although some models have them being much heavier (we'll get to the significance of their mass in a minute).

Now wait. We've taken a very big step in that last paragraph: we created an entire universe. Where is this mirror universe? All around us. Every interaction between particles is mediated by other particles called bosons (photons for the electromagnetic force, gluons for the strong force, W and Z bosons for the weak force). In order to see or touch something we need to exchange photons with it; something that doesn't socialize with photons thus can't be seen or touched. Neutrinos are known for being able to pass completely through the earth (or any body for that matter) without feeling a thing, precisely for that reason.

In models of this mirror universe, all interactions are mediated by mirror bosons. Mirror matter can “see” and touch other mirror matter if they exchange mirror photons. Our regular universe (or perhaps I should more acccurately say “our half of the universe”) deals in regular photons, not mirror photons. So we wouldn't see or interact with mirror particles, they'd pass right through us. Except in one way. Via gravity.Gravity is the one universal link that acts on everything and betrays the presence of mirror matter.

That there's something (a lot of something) out there that we can't see, something that reveals itself only through gravity is pretty much a given these days (although--as we went through a little in this thread--like Mr. Spock said, there are always alternatives). Perhaps some time we'll go in depth on dark matter but for this thread it will suffice to say that mirror matter is at least a plausible candidate for some or all of it. Certainly if there was no dark matter that would just about kill the idea of mirror matter existing out there in any reasonably large quantities.

Perhaps even more fascinating is the possibility of matter-mirror matter matchups: mirror planets circling regular suns or regular planets circling mirror suns, each invisible to the other. There have been suggestions (by proponents of the mirror matter idea, of course) that some of the “hot Jupiter” exoplanets that've been discovered may in fact be mirror planets. Such planets are usually detected by the gravitational tug they exert on their parent star: this tug manifests itself as a slight Doppler shift in the star's spectrum. We don't actually see the planets themselves since they're generally too far way to resolve. These “hot Jupiters,” as the name suggests, are usually very massive and very close to the parent star. Massive as in over 5 times Jupiter's mass (at least) and close as in maybe 8 times closer to the star than Mercury is to our sun.

A mirror planet that close to a star would not be subject to the difficulties of formation caused by radiation heating so close; it wouldn't feel anything but the star's gravity, receiving no light or heat from it. In fact, from the mirror planet's point of view, it would seemingly be orbiting nothing at all. How about the other side of the coin, a mirror sun and regular planets? In such a situation we'd see what appear to be free-floating planets with no star. Such planets may have been discovered in the sigma Orionis cluster:



There's even been a suggestion recently that the existence of mirror matter could solve a well-known paradox involving astoundingly fast-moving protons called ultra high-energy cosmic rays.

Whoops, I Lied

I said the only connection between regular matter and mirror matter would be gravity. That isn't exactly true. There could be what's called a “photon-mirror photon kinetic mixing interaction”--that is, photons and mirror photons can mix up with each other leading to a very slight electromagnetic interaction between regular and mirror matter. The effect would be to give mirror electrons, for instance, a very slight regular electric charge.

This could affect the lifetime of the exotic-sounding orthopositronium. Normally when an electron meets its antiparticle, the positron, they annihilate to produce photons. They can be made to very briefly form a kind of atom, with the positively charged positron playing the role of proton. This “atom” decays to form 3 photons. In some circumstances, the existence of mirror matter could slightly bump up the decay rate from what theory predicts (the theory, quantum electrodynamics or QED, is the most accurate physical theory in the history of science), accounting for a few early experimental discrepancies.

Mirror Meteorites

Back to that mixing in a moment. It's interesting to speculate that if mirror matter does exist, there could meteorites or even full asteroids of it floating through our solar system. The possibility of a mirror asteroid belt orbiting the sun in any plane at all can't be ruled out. Some have even considered the astonishing notion that even larger mirror bodies could “masquerade as ordinary planets or moons by accreting ordinary matter onto their surfaces” in our very solar system.

That photon-mirror photon mixing could have interesting consequences for such mirror bodies that get pulled in by the Earth's gravitational pull and enter the atmosphere (these quotes, by the way, are from a paper on mirror matter by Robert Foot).



It also turns out such mirror meteorites would fall in about the same way as regular ones: “One important consequence of this is that the air resistance (or atmospheric drag force ) of the mirror matter space-body is actually roughly the same as if it were made of ordinary matter.” Mirror matter, then, has been put forth as a possible explanation for a number of fireballs and atmospheric anomalies, including the Tunguska event that celebrated an anniversary yesterday. Dr. Foot has even suggested comets could be made of mirror matter, suggesting that the extremely low albedo (light reflectivity) of the nucleus of Halley's comet could indicate it is mirror matter with ordinary matter embedded in it.

There is, of course, a flip side to the mirror meteorite entering our normal Earth atmosphere: a regular object passing through what amounts to a gas of mirror matter. Some have suggested this situation has already arisen with the Pioneer probes.

Pioneer 10 and 11 are on opposite sides of the solar system and were, for a time, the most distant man made objects ever launched (depending on how you define it, Pioneer 10 was the first manmade object to leave the solar system). Both have recently been the subject of a bit of attention because of what's known as the Pioneer anomaly—both seem to be experiencing a very small sun-directed force that has not been satisfactorily explained. It is possible this could be a drag force exerted on the spacecraft as they travel through a haze of mirror matter particles.

Mirror, Mirror

All thought-provoking ideas and fun speculation that for all I know may be correct, though I reckon that's not a very common view.

The possibility of a whole nother universe existing all around us (perhaps even literally on top of us), maybe complete with stars, planets, people is pretty awe-inspiring. Somehow I doubt it contains goatee-sporting morally ambiguous versions of ourselves but certainly this puts an interesting spin on the mirror universe of Star Trek. Perhaps Kirk didn't really go anywhere at all but simply converted into all mirror particles for a time, entering a place that was literally next door but also a universe away.

If mirror particles have the same mass as their regular counterpart and on the off chance that all dark matter is mirror matter, that would mean there's quite a bit more mirror matter than the regular variety. The possibilities become literally endless, with little crossovers when photons and neutrinos mix with their counterparts, and of course gravity linking everything together. The final frontier might be a lot bigger than anybody ever imagined.

A few news articles from the past that anyone who finds this idea interesting might enjoy:

Mirror Matter and Interstellar Pioneers

Mirror Matter Mystery:



And some balance:

Scientists Doubt Claims of Invisible 'Mirror' Comets and Asteroids

Next time, warn people when you make long speeches and one more thing...

seu eht etouq...

Added: Look below if you can't figure it out...

Thanks for that.

Yes, the other day I was watching an old rerun of Mystery Science Theater 3000, and started thinking about the impact on toaster production if... IT'S A TV SHOW! IT'S NOT SUPPOSED TO SPAWN INTENSE THOUGHT ABOUT THEORETICAL PHYSICS! When I want to satisfy a desire for blind contemplation, I simply e-mail Stephen Hawking.

With that idea I had a dream where me and John Cleese found a door to this old fashioned room and directly across from us we saw us doing the same thing as us. basicaly we found a room between an exact mirror universe of our own. and I dont really know why John Cleese was in my dream.

Hey, I read that whole post and didn't get bored once, and it's 4 AM!!! I guess I'm sort of a nerd though. I've heard of that stuff before, but didn't think of all the applications of it. Interesting stuff.