Timey-wimey light

I don't always need to understand things to appreciate them.

In fact, there's a part of me that likes having my mind blown.  I find it reassuring that the universe is way bigger and more complex than I am, and the fact that I actually can parse a bit of it with my little tiny mind is astonishing and cool.  How could it possibly be surprising that there's so much more out there than the fragment of it I can comprehend?

This explains my love for twisty, complicated fiction, in which you're not handed all the answers and everything doesn't get wrapped up with a neat bow at the end.  It's why I thoroughly enjoyed the last season of Doctor Who, the six-part story arc called "Flux."  Apparently it pissed a lot of fans off because it had a quirky, complicated plot that left a bunch of loose ends, but I loved that.  (I'm also kind of in love with Jodie Whittaker's Thirteenth Doctor, but that's another matter.)

I don't feel like I need all the answers.  I'm not only fine with having to piece together what exactly happened to whom, but I'm okay that sometimes I don't know.  You just have to accept that even with all the information right there in front of you, it's still not enough to figure everything out.

Because, after all, that's how the universe itself is.

[Nota bene: Please don't @ me about how much you hated Flux, or how I'm crediting Doctor Who showrunner Chris Chibnall with way too much cleverness by comparing his work to the very nature of the universe.  For one thing, you're not going to change my mind.  For another, I can't be arsed to argue about a matter of taste.  Thanks.]

In any case, back to actual science.  That sense of reality being so weird and complicated that it's beyond my grasp is why I keep coming back to the topic of quantum physics.  It is so bizarrely counterintuitive that a lot of laypeople hear about it, scoff, and say, "Okay, that can't be real."  The problem with the scoffers is that although sometimes we're not even sure what the predictions of quantum mechanics mean, they are superbly accurate.  It's one of the most thoroughly tested scientific models in existence, and it has passed every test.  There are measurements made using the quantum model that have been demonstrated to align with the predictions to the tenth decimal place.

That's a level of accuracy you find almost nowhere else in science.

The reason all this wild stuff comes up is because of a pair of papers (both still in peer review) that claim to have demonstrated something damn near incomprehensible -- the researchers say they have successfully split a photon and then triggered half of it to move backwards in time.

One of the biggest mysteries in physics is the question of the "arrow of time," a conundrum about which I wrote in some detail earlier this year.  The gist of the problem -- and I refer you to the post I linked if you want more information -- is that the vast majority of the equations of physics are time-reversible.  They work equally well backwards and forwards.  A simple example is that if you drop a ball with zero initial velocity, it will reach a speed of 9.8 meters per second after one second; if you toss a ball upward with an initial velocity of 9.8 meters per second, after one second it will have decelerated to a velocity of zero.  If you had a film clip of the two trajectories, the first one would look exactly like the second one running backwards, and vice versa; the physics works the same forwards as in reverse.

The question, then, is why is this so different from our experience?  We remember the past and don't know the future.  The physicists tell us that time is reversible, but it sure as hell seems irreversible to us.  If you see a ball falling, you don't think, "Hey, you know, that could be a ball thrown upward with time running backwards."  (Well, I do sometimes, but most people don't.)  The whole thing bothered Einstein no end.  "The distinction between past, present, and future," he said, "is only an illusion, albeit a stubbornly persistent one."

This skew between our day-to-day experience and what the equations of physics describe is why the recent papers are so fascinating.  What the researchers did was to take a photon, split it, and allow the two halves to travel through a crystal.  During its travels, one half had its polarity reversed.  When the two pieces were recombined, it produced an interference pattern -- a pattern of light and dark stripes -- only possible, the physicists say, if the reversed-polarity photon had actually been traveling backwards in time as it traveled forwards in space.

The scientists write:

In the macroscopic world, time is intrinsically asymmetric, flowing in a specific direction, from past to future.  However, the same is not necessarily true for quantum systems, as some quantum processes produce valid quantum evolutions under time reversal.  Supposing that such processes can be probed in both time directions, we can also consider quantum processes probed in a coherent superposition of forwards and backwards time directions.  This yields a broader class of quantum processes than the ones considered so far in the literature, including those with indefinite causal order.  In this work, we demonstrate for the first time an operation belonging to this new class: the quantum time flip.

This takes wibbly-wobbly-timey-wimey to a whole new level.

Do I really understand what happened here on a technical level?  Hell no.  But whatever it is, it's cool.  It shows us that our intuition about how things work is wildly and fundamentally incomplete.  And I, for one, love that.  It's amazing that not only are there things out there in the universe that are bafflingly weird, we're actually making some inroads into figuring them out.

To quote the eminent physicist Richard Feynman, "I can live with doubt and uncertainty and not knowing.  I think it's much more interesting to live not knowing than to have answers which might be wrong.  I have approximate answers and possible beliefs and different degrees of certainty about different things, but I'm not absolutely sure about anything."

To which I can only say: precisely.  (Thanks to the wonderful Facebook pages Thinking is Power and Mensa Saskatchewan for throwing this quote my way -- if you're on Facebook, you should immediately follow them.  They post amazing stuff like this every day.)

I'm afraid I am, and will always be, a dilettante.  There are only a handful of subjects about which I feel any degree of confidence in my depth of comprehension.  But that's okay.  I make up for my lack of specialization by being eternally inquisitive, and honestly, I think that's more fun anyhow.

 Three hundreds years ago, we didn't know atoms existed.  It was only in the early twentieth century that we figured out their structure, and that they aren't the little solid unbreakable spheres we thought they were.  (That concept is still locked into the word "atom" -- it comes from a Greek word meaning "can't be cut.")  Since then, we've delved deeper and deeper into the weird world of the very small, and what we're finding boggles the mind.  My intuition is that if you think it's gotten as strange as it can get, you haven't seen nothin' yet.

I, for one, can't wait.

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The stubbornly persistent illusion

I was driving through Ithaca, New York a while back, and came to a stoplight, and the car in front of me had a bumper sticker that said, "Time is that without which everything would happen at once."

I laughed, but I kept thinking about it, because in one sentence it highlights one of the most persistent mysteries of physics: why we perceive a flow of time.  The problem is, just about all of the laws of physics, from quantum mechanics to the General Theory of Relativity, are time-reversible; they work equally well in forward as in reverse.  Put another way, most physical processes look the same both ways.  If I were to show you a short video clip of two billiard balls colliding on a pool table, then the same clip backwards, it would be hard to tell which was which.  The Laws of Conservation of Momentum and Conservation of Energy that describe the results of the collision work in either direction.

There are exceptions, though.  The Second Law of Thermodynamics is the most commonly-cited one: closed systems always increase in entropy.  It's why when I put sugar in my coffee in the morning and stir it, the sugar spreads through the whole cup.  If I were to give it one more stir and all the sugar molecules were to come back together as crystals and settle out on the bottom, I'd be mighty surprised.  I might even wonder if someone had spiked the sugar bowl with something other than sugar.

In fact, that's why I had to specify a "short clip" in the billiard ball example.  There is a time-irreversible aspect of such classical physics; as the balls roll across the table, they lose momentum, because a little of the kinetic energy of their motion leaks away as thermal energy due to friction with the surface.  When they collide, a little more is lost because of the sound of the balls striking each other, the (slight) physical deformation they undergo, and so on.  So if you had a sensitive enough camera, or a long enough clip, you could tell which was the forward and which the reverse clip, because the sum of the kinetic energies of the balls in the forward clip would be (slightly) greater before the collision than after it.

But I am hard-pressed to see why that creates a sense of the flow of time.  It can't be solely from our awareness of a movement toward disorder.  When there's an energy input, you can generate a decrease in entropy; it's what happens when a single-celled zygote develops into a complex embryo, for example.  There's nothing in the Second Law that prevents increasing complexity in an open system.  But we don't see those situations as somehow running in reverse; entropy increase by itself doesn't generate anything more than expected set of behaviors of certain systems.  How that could affect how time is perceived by our brains is beyond me.

The problem of time's arrow is one of long standing.  Einstein himself recognized the seeming paradox; he wrote, "The distinction between past, present, and future is only a stubbornly persistent illusion."  "Persistent" is an apt word; more than sixty years after the great man's death, there was an entire conference on the nature of time, which resolved very little but giving dozens of physicists the chance to defend their own views, and in the end convinced no one.

It was, you might say, a waste of time.  Whatever that means.

One of the most bizarre ideas about the nature of time is the one that comes out of the Special Theory of Relativity, and was the reason Einstein made the comment he did: the block universe.  I first ran into the block universe model not from Einstein but from physicist Brian Greene's phenomenal four-part documentary The Fabric of the Cosmos, and it goes something like this.  (I will append my usual caveat that despite my bachelor's degree in physics, I really am a layperson, and if any physicists read this and pick up any mistakes, I would very much appreciate it if they'd let me know so I can correct them.)

One of the most mind-bending things about the Special Theory is that it does away with simultaneity being a fixed, absolute, universal phenomenon.  If we observe two events happening at exactly the same time, our automatic assumption is that anyone else, anywhere in the universe, would also observe them as simultaneous.  Why would we not?  But the Special Theory shows conclusively that your perception of the order of events is dependent upon your frame of reference.  If two individuals are in different reference frames (i.e. moving at different velocities), and one sees the two events as simultaneous, the other will see them as sequential.  (The effect is tiny unless the difference in velocities is very large; that's why we don't experience this under ordinary circumstances.)

This means that past, present, and future depend on what frame of reference you're in.  Something that is in the future for me might be in the past for you.  This can be conceptualized by looking at space-time as being shaped like a loaf of bread; the long axis is time, the other two represent space.  (We've lost a dimension, but the analogy still works.)  The angle you are allowed to slice into the loaf is determined by your velocity; if you and two friends are moving at different velocities, your slice and theirs are cut at different angles.  Here's a picture of what happens -- to make it even more visualizable, all three spatial dimensions are reduced to one (the x axis) and the slice of time perceived moves along the other (the y axis).  A, B, and C are three events, and the question is -- what order do they occur in?

[Image licensed under the Creative Commons User:Acdx, Relativity of Simultaneity Animation, CC BY-SA 4.0]

As you can see, it depends.  If you are taking your own velocity as zero, all three seem to be simultaneous.  But change the velocity -- the velocities are shown at the bottom of the graph -- and the situation changes.  To an observer moving at a speed of thirty percent of the speed of light relative to you, the order is C -> B -> A.  At a speed of fifty percent of the speed of light in the other direction, the order is A -> B -> C.

So the tempting question -- who is right? what order did the events really occur in? -- is meaningless.

Probably unnecessarily, I'll add that this isn't just wild speculation.  The Special Theory of Relativity has been tested hundreds, probably thousands, of times, and has passed every test to a precision of as many decimal places as you want to calculate.  (A friend of mine says that the papers written about these continuing experiments should contain only one sentence: "Yay!  Einstein wins again!")  Not only has this been confirmed in the lab, the predictions of the Special Theory have a critical real-world application -- without the equations that lead directly to the block universe and the relativity of simultaneity, our GPS systems wouldn't work.  If you want accurate GPS, you have to accept that the universe has some seriously weird features.

So the fact that we remember the past and don't remember the future is still unexplained.  From the standpoint of physics, it seems like past, present, and future are all already there, fixed, trapped in the block like flies in amber.  Our sense of time flowing, however familiar, is the real mystery.

But I'd better wrap this up, because I'm running out of time.

Whatever that means.

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