Mirror image

One of the hallmarks of science is its falsifiability.  Models should generate predictions that are testable, allowing you to see if they conform to what we observe and measure of the real universe.  It's why science works as well as it does; ultimately, nature has the last word.

The problem is that there are certain realms of science that don't lend themselves all that well to experiment.  Paleontology, for example -- we're dependent on the fossils that happen to have survived and that we happen to find, and the genetic evidence from the descendants of those long-gone species, to piece together what the ancient world was like.  It's a little difficult to run an experiment on a triceratops.

An even more difficult one is cosmology -- the study of the origins and evolution of the universe as a whole.  After all, we only have the one universe to study, and are limited to the bits of it we can observe from here.  Not only that, but the farther out in space we look, the less clear it becomes,  By the time light gets here from a source ten billion light years away, it's attenuated by the inverse-square law and dramatically red-shifted by all the expanding space it traveled through to get here, which is why it takes the light-collecting capacity of the world's most powerful telescopes even to see it.

None of this is meant as a criticism of cosmology, nor of cosmologists.  But the fact remains that they're trying to piece together the whole universe from a data set that makes what the paleontologists have look like an embarrassment of riches.

The result is that we're left with some massive mysteries, one of the most vexing of which is dark energy.  This is a placeholder name for the root cause of the runaway expansion of the universe, which (according to current models) accounts for 68% of the mass/energy content of the universe.  (Baryonic, or ordinary, matter is a mere 5%.)  And presently, we have no idea what it is.  Attempts either to detect dark energy directly, or to create a model that will account for observations without invoking its existence have, by and large, been unsuccessful. 

But that hasn't stopped the theorists from trying.  And the latest attempt to solve the puzzle is a curious one; that dark energy isn't necessary if you assume our universe has a partner universe that is a reflection of our own.  In that universe, three properties would all be mirror images of the corresponding properties in ours; positive and negative charges would flip, spatial "handedness" (what physicists call parity) would be reversed, and time would run backwards.

Couldn't help but think of this, of course.

The idea is intriguing.  Naman Kumar, who authored the paper on the model, is enthusiastic about its potential for explaining the expansion of the universe.  "The results indicate that accelerated expansion is natural for a universe created in pairs," Kumar writes.  "Moreover, studying causal horizons can deepen our understanding of the universe.  The beauty of this idea lies in its simplicity and naturalness, setting it apart from existing explanations."

Which may well be true.  The difficulty, however, is that the partner universe isn't reachable (or even directly detectable) from our own, Lost in Space notwithstanding.  It makes me wonder how this will ever be more than just an interesting possibility -- an idea that, in Wolfgang Pauli's often-quoted words, "isn't even wrong" because there's no way to test whether it accounts for the data any better than the other, less "natural" models do.

In any case, that's the latest from the cosmologists.  Mirror-image universes created in pairs may obviate the need for dark energy.  We'll see what smarter people than myself have to say about whether the claim holds water; or, maybe, just wait for Evil Major West With A Beard to show up and settle the matter once and for all.

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The symmetrical universe

I try to avoid writing about topics I don't fully understand, because that's just too great an opportunity for my sticking my foot in my mouth (and having to write a retraction afterwards).  Because of this reluctance, and because I'm pretty up-front about it when I don't know something, I don't get caught out very often, and I'd like to keep it that way.

So I'm gonna put a disclaimer right here at the beginning of this post: today's topic is one I have only a shallow understanding of.  If you ask me for more information, I'm likely to give you a puzzled head tilt, the same look my dog gives me when I ask him questions he doesn't have a good answer to, like why he chewed up my magazine before I had a chance to read it.  And if you are an expert in this field, and I get some of the facts wrong, let me know so I can fix 'em.

Okay, that being said: have you heard of CPT symmetry?

The initials stand for "charge," "parity," and "time," and the idea goes something like this: if you take any physical process, and reverse the charges (replace particles with their antiparticles), reverse the parity (reverse everything left-to-right), and run time backwards, the two would be indistinguishable.  Such a mirror universe would proceed according to exactly the same physical laws as ours does.

(As far as I know, it would not generate the scientific result elucidated in the Lost in Space episode "The Antimatter Man," wherein the mirror universe had an evil Don West with a beard.)

Initially, physicists thought that there was also CP symmetry -- that processes needed only charge and parity reversal to maintain symmetry, but that was found to be false when CP violations were found, most notably the decay of the particle called a neutral kaon.  The fact that symmetry is not preserved with reversal of charge and parity is thought to be the key to why there were unequal amounts of matter and antimatter produced in the Big Bang.  Fortunately for us.  If the matter/antimatter ratio had been exactly 1:1, ultimately it would all have mutually annihilated, and the universe would now be devoid of matter -- just space filled with photons zinging merrily about.

So CPT symmetry and CP violations are apparently fundamental to the nature of matter.  Which is why physicists have been pushing on the CPT symmetry idea, trying to find out if it holds -- or if there are circumstances, as there were with CP symmetry, where CPT symmetry is not preserved.

The latest test, described in a paper this week in Nature Physics, finds that even one of the oddest particles ever created in a laboratory preserves CPT symmetry.  In "Measurement of the Mass Difference and the Binding Energy of the Hypertriton and Antihypertriton," written by a team of particle researchers at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, we read about bizarre particles that instead of the "up" and "down" quarks (and antiquarks) found in ordinary matter (and antimatter, if there's such a thing as "ordinary antimatter"), additionally have "strange" quarks (and antiquarks), which have higher mass and only form under extremely high energy conditions.  These particles -- the hypertritons and antihypertritons in the title -- have never had their masses calculated accurately before, and the theory is that if the masses are different, it would break CPT symmetry and require a huge rethinking of how matter works on the smallest scales.

The result?  Hypertritons and antihypertritons have exactly the same mass.  CPT symmetry -- the fact that a charge reversed, mirror-image, time-running-backwards universe would look exactly the same as ours -- is preserved.  "It is conceivable that a violation of this symmetry would have been hiding in this little corner of the universe and it would never have been discovered up to now," said study co-author Declan Keane of Kent State University.  "But CPT symmetry was upheld even in these high-energy conditions."

This discovery gives physicists a clue about what might be happening in some of the most extreme and hostile spots in the universe -- the interiors of neutron stars.  The heat and crushing pressure in the core of a neutron star is thought to have enough energy to produce strange quarks and antiquarks, and therefore if those quarks (and the particles made from them) broke CPT symmetry, it would be a lens into a place where the known laws of physics do not hold.

But the symmetrical models won out.  Also, the measured energy of the hypertriton and antihypertriton were higher than expected, which squares with known neutron star masses.  "The presence of hyperons would soften the matter inside neutron stars," said Morgane Fortin, of the Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences in Warsaw.  "Softer neutron stars would more easily collapse into black holes, so neutron stars couldn’t become as massive.  That feature makes hyperons’ potential presence difficult to reconcile with the largest neutron stars seen in the cosmos — which range up to about two solar masses.  But the newly measured, larger binding energy of the hyperon helps keep alive the idea of a hyperon-filled center to neutron stars.  The result suggests that hyperons’ interactions with neutrons and protons are stronger than previously thought. That enhanced interaction means neutron stars with hyperons are stiffer and could reach higher masses.  So neutron stars may still have strange hearts."

Strange indeed.  Mirror universes, neutron stars, and symmetry preserved to the smallest scales and highest energies.  Amazingly cool stuff, even if (1) I don't understand it all that well, and (2) it doesn't involve evil Don West with a beard.

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This week's Skeptophilia book-of-the-week is brand new: Brian Greene's wonderful Until the End of Time.

Greene is that wonderful combination, a brilliant scientist and a lucid, gifted writer for the scientifically-inclined layperson.  He'd already knocked my socks off with his awesome The Elegant Universe and The Fabric of the Cosmos (the latter was made into an equally good four-part miniseries).

Greene doesn't shy away from difficult topics, tackling such subjects as relativity, quantum mechanics, and the nature of time.  Here, Greene takes on the biggest questions of all -- where the universe came from, how it has evolved and is evolving, and how it's going to end.

He begins with an observation that as a species, we're obsessed with the ideas of mortality and eternity, and -- likely unique amongst known animals -- spend a good part of our mental energy outside of "the now," pondering the arrow of time and what its implications are.  Greene takes a lens to this obsession from the standpoint of physics, looking at what we know and what we've inferred about the universe from its beginnings in the Big Bang to its ultimate silent demise in the "Heat Death" some billions or trillions of years in the future.

It's definitely a book that takes a wide focus, very likely the widest focus an author could take.  And in Greene's deft hands, it's a voyage through time you don't want to miss.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]