02/05 Open Thread: Whither Time's Arrow?
We are getting ready to head out on yet another multi-week adventure in the near future. This means that I need to write, post and schedule a whole bunch of OTs for the period of my absence, as well as some to bridge the gap between now and the start date. This means, among other things, that you shouldn't expect too much from them and will need to provide content as well as commentary yourselves (as if you don't already, heh). So, here we go ...
Time, time to gird your mental loins, there is mischief afoot. We are going cosmology diving. No math, no heavy lifting, but frames of reference will probably need adjusting.
My approach to empiricism and reality was more or less summarized Models, Maps, reality and Relevance. (09/19/2018) Reality is but the name I hang on the Model I have made. https://caucus99percent.com/content/wednesday-open-thread-models-maps-re... . It did not really deal with time, time was taken as a given; yesterday the world looked like this, today it looks a bit different, so I update my map.
Time got a brief nod on 01/09/2019 in perception, aging, sentencing, and logs https://caucus99percent.com/content/wednesday-open-thread-perception-agi..., The primary focus or take away there was Weber's law; perception is proportional to the log of the stimulus. (But how true is that about our perception of time?)
I took another peek at time in Our Sense of Time Revisited on 02/20/2019. https://caucus99percent.com/content/wednesday-open-thread-our-sense-time... That discussed a study on our mapping of temporal sequence, which I related to spatial maps and models as explicated in my 9/19/18 column. It also back references a prior column thusly ""- A while back I put up a post concerned with the question of whether or not we ever directly experience the flow of time. (December 12, 2018); https://caucus99percent.com/content/wednesday-open-thread-tempus-fugitin... "" which is probably the first time I posted Erwin Schroedinger's statement that:
“Time no longer appears to us as a gigantic, world-dominating chronos, nor as a primitive entity, but as something derived from phenomena themselves. It is a figment of my thinking.”
It is, from that view, a function of the model.
I wish to now present you with an article that digs a bit deeper into the whole question. Haunted by His Brother, He Revolutionized Physics ( http://nautil.us/issue/66/clockwork/haunted-by-his-brother-he-revolution... ) It isn't really all that long and some isn't at all tricky, but the tricky bits are seriously tricky. It's about a guy named Wheeler, who at one point said :
“Of all obstacles to a thoroughly penetrating account of existence, none looms up more dismayingly than ‘time,’” Wheeler wrote. “Explain time? Not without explaining existence. Explain existence? Not without explaining time.”
Wheeler, fwiw, gave the name wormholes to things previously known as 'Einstein-Rosen bridges' and the name black holes to black holes. The article follows his journey through physics and cosmology starting with the great clash between Relativity and Quantum physics. More from the article:
Wheeler’s first inkling that time wasn’t quite what it seemed came one night in the spring of 1940 at Princeton. He was thinking about positrons. Positrons are the antiparticle alter egos of electrons: same mass, same spin, opposite charge. But why should such alter egos exist at all? When the idea struck, Wheeler called his student Richard Feynman and announced, “They are all the same particle!”
Imagine there’s only one lone electron in the whole universe, Wheeler said, winding its way through space and time, tracing paths so convoluted that this single particle takes on the illusion of countless particles, including positrons. A positron, Wheeler declared, is just an electron moving backwards in time. (A good-natured Feynman, in his acceptance speech for the 1965 Nobel Prize in Physics, said he stole that idea from Wheeler.)
It is vastly easier if you read the article than if I just throw bits of it at you. Also, it is quasi biographical, and I intend to elide all of that.
A fundamental of quantum theory is that observation or measurement is the determiner of reality. Schroedinger's cat is there, neither dead nor alive until the act of measurement determines its fate. Particles have neither location nor momentum until we measure one, establishing it with certainty and thereby making the other quantity undeterminable.
The Famous Double Slit Experiment:
Fire a photon at a time at a screen penetrated by a pair of vertical slits and the pattern that shows up on a film behind the plate is a set of interference fringes, like a moire'. (Toss two pebbles into a still pond and watch where the growing rings of ripples meet.) This, combined with a ton of prior optics, math and other physics, demonstrated that photons are waves, not particles, for the latter would pass through one slit or another, while the photon passes through both at once. More correctly, we might say that it exhibits wavelike properties when fired at a plate containing two or more slits. Its wavicleness can be expressed in a "wave function", but I promised no math and we don't need to see, know or understand that function (Schroedinger's Wave Equation) Know only that it exists and exists for every known particle, not just photons.
Now place a detector at each slit to count or otherwise measure each photon as it passes and the resultant pattern changes into 2 discrete blobs. In this case each photon passes through one and only one slit, like a tiny bullet. Our attempt to determine the whereabouts of the photon as being at either slit determines a preferred slit and a path from the source to that slit. This is mildly problematic. It is expected in the quantum realm that our measurement will determine one or more properties, such as position at one slit or another, but given that the photons traveled through space from the source to the detector, their arrival at a particular slit means that they took a particular path which means that placement of the detector imposes a history upon the particles, retroactively, thereby messing with time. Mull that over a bit.
At this point I'd like to digress, relevantly, just a wee bit. Back in my column on Models, Maps and all that, Henry A Wallace commented, in part:
Does a map show us the world or change it?
That was in a different context, about a different matter, but, in the quantum context, it certainly appears inescapable that the map changes and perhaps even creates the world. Wheeler wondered if it might not be the case that the entire universe was a product of our observations. OK, back to work.
Wheeler had realized that it would be possible to arrange the usual double slit experiment in such a way that the observer can decide whether he wants to see stripes or blobs—that is, he can create a bit of reality—after the photon has already passed through the screen. At the last possible second, he can choose to remove the photographic plate, revealing two small telescopes: one pointed at the left slit, the other at the right. The telescopes can tell which slit the photon has passed through. But if the observer leaves the plate in place, the interference pattern forms. The observer’s delayed choice determines whether the photon has taken one path or two after it has presumably already done one or the other.
Such an experiment was performed in 1984 and it worked exactly as Wheeler had thought it would. Actions, measurements taken in the present determined what had happened in the past. Leave this film in place and it clearly showed that the photons went through both slits at once. Yank it at the last possible nanosecond and the results clearly showed that each photon passed through one and only one slit. Time's arrow had been reversed within the experimental apparatus.
This is almost as far as the article takes us. The morass of quantum theory is mostly ignored for all practical purposes, because of a truism that quantum principles only apply to the quantum universe, and not to the real world of macroscopic objects. This seems sadly akin to saying that relativity only applies in relativistic situations, and not to good old Newtonian reality. We know relativistic effects exist, or should, at all scales, but simply vanish into unmeasurable, and even definitionally unmeasurable (and hence irrelevant) magnitudes. More importantly, the universe, at cosmological scales and distances, isn't something one can hit with a baseball bat or crowbar, it is seen, explored and measured via photons, and photons are quanta. There is a way out and a way to answer Wheelers questions about the universe, but it might not be the most aesthetically or computationally pleasant and might leave us with the same unfulfilled feeling as a cold-water sandwich or rubber biscuit. We are not yet, however, quite ready to get there.
A safe cracking conga drumming physicist named Richard Feynman, confronted with the quantum level problems noted above, as well as others, proposed a simple yet horribly complicated solution. In looking, for example, at the double slit quandary he suggested that perhaps the photon takes all possible paths simultaneously, ethereal thingie that it is, and the detector only determines the instantaneous one and the future ones, all priors "collapsing" into the one measured resultant at the moment of measurement. This is conceptually somewhat elegant, and computational insanity. Computing the resultant of an interaction of two particles is now as simple as computing all possible resultants of all possible paths of both particles, and then weighting each for its probability of occurrence. Thank you Richard Feynman
Back to Wheeler's tale. Steven Hawking, for the moment, has the last word. Actually, 3 words, "Top Down Cosmology". Photons will be photons, folks, so we just have to live with it. Every photon from the sun, Alpha Centauri, or anywhere else has taken all possible paths to get here. Their predecessors did and their successors, already long in flight, are doing so even now. It seems therefore inescapable that the universe has myriad histories and one of the myriad histories is selected for, actualized and brought into being, in and for the instant of measurement, each time an observer makes a measurement.
So where now time's arrow?
Alrighty, now, officially on the road, gone, done gone and, hence, in absentia today.
Title Image is clock_sync
It's an open thread, so have at it. The floor is yours