In a way it's a bit more surprising Einstein didn't get the Nobel prize earlier for his Brownian motion paper from the same year as his special relativity and photoelectric effect papers. It's the most beautiful of his three great papers of 1905. [pdf: http://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/einstein_... ]
General Relativity was under active development in 1920-1921, and it is so fundamentally different from Special Relativity that it is unfortunate that each theory shares half a name with the other.
(Even more unfortunate is that it took a few decades to arrive at a mathematical and conceptual understanding SR in a GR context as simply one of an infinite number of possible hyperbolizations of a first order quasilinear system of PDEs along the lines of the Einstein Field Equations, or alternatively as a low energy effective field theory in the sense of Kenneth G Wilson (which is what GR itself might be in turn) -- that all became possible only because of mathematical and calculational discoveries made well after Einstein's death).
Special Relativity was surely worthy of a Nobel prize once its relationship with Minkowski space and the Lorentz and Poincaré isometry groups became fully understood (all by the mid-1910s), but General Relativity had also just become Nobel worthy thanks to the 1919 expedition.
So a Nobel prize for a special-case theory which may have seemed like it was about to be overthrown by a generalization by its originator? Or push the award off into the future after General Relativity had developed further? Or possibly two Nobel prizes, one for each theory? In 1920-1921, nobody could have known that it would take four decades for GR to become useful for working scientists, nor that SR and Newtonian mechanics would remain in use as (or in) effective theories even in 2016.
I wonder how much that line of thought might have weighed on the prize committee's mind, rather than objections raised by a non-physicist philosopher.
This is a kind of thing that have always fascinated me: People from past epochs (Bergson in this case) trying to convey a though and we being unable to parse it.
My favourite example is one of Robert Musil's essays. There are multiple versions of it, showing that he desperately tried to convey a thought and make it as precise as possible, but the meaning escapes me.
The reason for fascination is that even in the past there must have been clever people, Einsteins born 1259, that had valuable thoughts but were able to express them only within the contemporary context, using contemporary means.
It may even be that those thought could possible lead to whole new areas of knowledge but were never followed simply because the world have moved in a different direction in the years that followed.
A sad thing is that these alternative thoughts can be spotted only if they occurred not that long ago (for me, personally, the limit is maybe 1850). When you go further into the past, the things get simply weird and undecipherable (angels dancing on the tip of the needle, anyone?) Thus, the possibility to explore alternative branchings of the river of thought disappear forever as we move forward.
Some of these people caused similar revolution against their predecessors. We may not be aware that some of their toughs are interesting deviations or improvements for the previous state of things.
Freud is perfect example. He caused huge revolution in the philosophy of mind and changed our culture in many visible ways, but his theories did not survive.
Freud's theories are based on outdated concepts of how the brain works. The evidence was usually incomplete or questionable. Freud based his theories on anecdotes from his practice and his patients were mostly upper class women who suffered from sexual repression. Freud overgeneralized from this very odd set of patients very strange theories.
Still his basic themes are considered correct even today and they were revolutionary at the same way as Einstein's. They seem so basic and obvious to us that we don't think there was controversy or revolution involved.
Five still valid _general_ ideas that are left from his psychodynamics theories after most was debunked:
1. There are unconscious processes. Parts of cognition, motivation and emotions can be hidden from consciousness.
2. There are competing processes and conflicts in the brain.
3. Personality is significantly shaped by childhood
4. Our mental representations of other people shape our social relations.
5. There is a sequence of different developmental stages that create internal psychological conflicts.
While I get what you mean, there's absolutely no (soft or hard) limit in 1850 or any date in the past for that matter.
People have always been people, and most of their concerns and theories were more or less similar. Even the most alien ones can still be relatable (in fact that's already an ancient, roman, idea: "Homo sum, humani nihil a me alienum puto", by Terence).
Until very recently (up until the seventies or so) when humanities still reigned supreme in universities and schools, being able to read, enjoy and understand anything from Homer and Plato to Shakespeare and Kant was nothing special for higher education people.
That said, often being able to parse those things is question of being accustomed not to an era, but to a way of thinking. (E.g. Anglo-saxon vs continental philosophy).
The meeting was recounted in the Bulletin de la Société française de Philosophie, vol. 22, no. 3, July 1922,
pp. 102–113. It was reprinted in Bergson, Ecrits et Paroles, vol. 3, pp. 497, and in Henri Bergson, ―Discussion
avec Einstein,‖ in Mélanges (Paris: Presses Universitaires de France, 1972).
of which i took the Bulletin de la Société française de Philosophie, vol. 22, no. 3, July 1922,
Einstein has been described as a philosopher, and in a way he was - one of a new kind, both able and unafraid to follow reason through a veil of paradox into a disorienting world of postmodern uncertainty. It was an inflection point for philosophy, which could either follow Einstein or sink into self-referential introspection. Bergson had no clue as to what was happening.
Einstein himself stumbled over quantum mechanics, but there were others to lead the way.
I think Einstein did pretty well with quantum mechanics, as judged against his contemporaries. At least he knew something was wrong with the statistical/Copenhagen view. I think if he were told the Many-Worlds interpretation, he would find it beautiful and in line with what the character of natural law is supposed to look like.
The opposition didn't feel rational. For example, the EPR paradox was presented as a failure of quantum mechanics, when quite the opposite was true - and indeed quantum entanglement has been proposed for commercial applications. Simply put, in his later years people were already using QM to solve real world problems, while he was telling them their theories were wrong.
Consonant with the theme of deposing humanity from the center/top e.g. geocentric to heliocentric.
Bergson's perspective, which I interpret as 'things don't matter until they matter to someone', strikes me as more like engineering or economics: use not truth.
OTOH there's that quantum mechanics idea (that I don't understand) that a waveform only collapses into particles when observed by a conscious "observer"... seems very anthropocentric. Surely it can't be right.
> OTOH there's that quantum mechanics idea (that I don't understand) that a waveform only collapses into particles when observed by a conscious "observer"... seems very anthropocentric. Surely it can't be right.
It's not right; it's a nonsensical (yet common) misinterpretation. It's not possible to measure quantum things without affecting what happens - no consciousness needed, just really tiny experimental subjects that are affected by influences from outside the system being analysed. See https://en.wikipedia.org/wiki/Observer_effect_(physics) and its Quantum mechanics section.
I always wondered about the observer and how the fuck the particles "know" (or detect) that they are being observed or what is even the criteria - is a human being an observer? What about a measuring device a la Schrodinger? Does the device measuring count as an observation, or is it when a human looks at the device? Turns out we're not the only ones confused:
Critics of the special role of the observer also point out that observers can themselves be observed, leading to paradoxes such as that of Wigner's friend; and that it is not clear how much consciousness is required ("Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer - with a PhD?"[3]).
Mmmm, sort of. It's probably easier to understand the why of Bergson's theories in their historical context. In the early 20th century, General Relativity and Special Relativity had space and time 'figured out' and quantum mechanics wasn't really a thing yet.
The common train of thought post-Einstein but pre-quantum mechanics was that physics was close to a theory of everything: that the universe could be described with a set of deterministic equations and everything, including human behavior, could be successfully predicted from the beginning of time to the end of time.
Bergson's objections to Einstein are rooted in the concept of free will. They centered on Einstein's handling of time as another spatial concept. Physics would never be able to quantify human behavior, according to Bergson, because Einstein used the wrong model of time. Time (again, according to Bergson) isn't a countable and finite dimension like space is - and thus Einstein was wrong.
Bergson was also had no small amount of mathematical understanding, although he certainly wasn't at Einstein's level. Prior to this debate, he wrote an entire book about Einstein's Twins Paradox, and why it the premise it started from - that of a countable, space-like time, was wrong.
One reason that the Bergson-Einstein debate impacted the Nobel committee to such a degree was academic politics. At the time, many thought that physics had everything figured out and it wasn't long until everything, including human behavior, could be predicted using the scientific methods of physics and relativity.
Not unsurprisingly, a LOT of non-physicists had a problem with this idea.
Now off to read to the article, so I can see what was actually discussed there....
If, after merely a few centuries of industrial revolution, we are nearing to be able to make a good enough physics simulation, and thus it's conceivable that in th future (100-1000 years) we could create a very fine simulation of a world (with simulated humans in it too, a la Sims).
Then the same (that they themselves will be eventually able to create a simulation) can be said for the inhabitants of the simulated world we've created.
This means that in the end, there might be thousands or millions of such simulations inside a prior simulation.
This begs the question: why would then our world not be a simulation itself? Certainly if there are 1000's of simulations, it's more probable that we're somewhere in the middle than on the top of the stack.
Now consider the simulation above us: they create our universe as a simulation.
It would then make sense that:
1) the simulation would be in final analysis discreet.
2) it would only spare computation cycles when it was worth it.
So, (1) would involve something like Planks length (and related constants).
And (2) would make it so that "reality" (active simulation) exists only when one of us (humans/consciousness) look at it.
MWI is just the unitary, deterministic, local, continuous, linear evolution of a partial differential equation. There's no extra semantics that need to be added to tell you how to interpret it.
It's the only interpretation of quantum mechanics that has all of these properties. Others fail most of those qualifications. I'm particularly bothered by the thought that we don't live in a deterministic universe. This is what I mean by "more elegant".
As for whether our universe is a simulation, I'm familiar with Bostrom's argument, which you outlined. It's certainly an interesting idea and might be correct, but orthogonal to the original question of interpretations of quantum mechanics.
I'm not sure what's more amazing - the fact that Bergson was feted as a philosophical genius in the first place, or the way that Einstein dissected his pretensions so economically.
Sure, some Internet guy summarily dismissing a philosopher whose oeuvre spans decades and who is widely recognized and admired. That must be all there is to it.
I know next to nothing about philosophy, bur I've just looked up what Will Durant has to say at the very end of the subchapter "Criticism" about Bergson:
"Yet, of all contemporary contributions to philosophy, Bergson's is the most precious. We needed his emphasis on the elusive contingency of things, and the remoulding activity of mind. We were near to thinking of the world as a finished and pre-determined show, in which our initiative was self-delusion, and our efforts a devilish humor of the gods; after Bergson we come to see the world as a stage and the material of our own originative powers. Before him we were cogs and wheels in a vast and dead machine; now, if we wish it, we can help to write our own parts in the drama of creation."
Seems like the last gasp for philosophy as a publicly influential field of work. I'd never heard of Bergson before, but I've certainly heard of Einstein.
When was the last time a philosophical argument or book was front-page news? I can't remember it happening. Stories of scientific discovery make the news pretty frequently.
Perhaps that is the fault of a deficient classical education, rather than the merit (or lack thereof) of Bergson's ideas? To extrapolate from your example argument, there are thousands of historical figures you have never heard of. And yet, their decisions and actions have had a profound effect upon trajectory of history, and the way the world appears to you today. Are they unimportant just because you have yet to personally learn about them?
Or perhaps it is a lack of imagination: Bergson was certainly a large part of the intellectual milieu in which Einstein was working. His thoughts pushed Einstein in certain directions Einstein might not have gone otherwise. Even if Bergson is unremarkable for his own work, surely he is important for no other reason than his impact on the thought of the remarkable and memorable theories of Einstein?
Taking 'I haven't hear of them' as your starting point of historical importance seems like an intellectually lazy argument to me. Each to their own, however, and you are certainly entitled to your opinion.
> Perhaps that is the fault of a deficient classical education, rather than the merit (or lack thereof) of Bergson's ideas?
My point was not about merit, it was about cultural impact.
But having had some time to think about it, I'm probably wrong. Kuhn's concept of the "paradigm shift" is more recent than Bergson. While not directly challenging a specific theory the way Bergson did, it has certainly captured the imagination of the public and of scientists.
That said, it seems that Bergon's star has faded over time. I looked up a few lists of the most influential modern philosophers and he is not usually highly ranked.
In many cases, post-modernist philosophers (theorists) don't make arguments; on the other hand the effect their ideas have had on mass culture has been immensely pervasive. I would rather use the word insidious but that's another matter.
So, I worked with Bergson's texts quite a bit in grad school, as he is heavily in vogue at the moment in certain disciplines. To break down his argument to its essentials, the whole concept he is railing against is the spatialization of time. That is, for Bergson, time cannot be subdivided into the "mechanistic time" of the ticking clock, and the idea of a timeline is an abomination.
Hence Bergson's framing of time as duration: for Bergson the essence of experiential time is that our consciousness is always experiencing the latest moment sliding smoothly into the next. Time, he says, cannot be spatialization and counted as can space. Bergson railed against the idea of time being extrapolated to just another metric dimension like the 3 dimensions of space. The spatial dimensions, to him, were static, fixed, dead. It is only duration that gives our existential experience of the lived experience that we know. Spatialization, to Bergson, was a dirty word; it was the spatialization of our lived experiences that rendered industrial life dead, static, mechanistic and uninteresting. Bergson was railing against the idea of a physics that could predict everything, a popular thought in the early 20th century.
After WWII, Bergson was largely forgotten until Deleuze & Guatarri ressurected him. Deleuze in particular was an enormous fan of Bergson and promoted his ideas heavily.
But what was revolutionary about Deleuze's handling of Bergson was his incorporation of post-war complexity/chaos theory and quantum mechanics to recover space as a dynamic and mutable. Influenced by such concepts as Reimman mana olds and fractal theory, Deleuze recognized that space wasn't a static and mechanistic concept at all, but instead, like Bergson's duration, can give rise to all types of unpredictable behaviors, experiences, and mathematics.
Rather than focus on one concept of "space" - the abstracted Euclidean grid - they classified space in two broad classes, the smooth and striated. Smooth spaces are spaces that are analogous to Bergson's duration: the experienced space of the journey, nomadic spaces, spaces that unfold rather than increment, that are uncountable and unexpected. Striated spaces are the class of spaces Bergson focused on exclusively: coordinate spaces, the countable spaces of the Euclidean grid and the map, or that of the timeline.
Essentially, D&G 'recovered' mathematical space as an exciting and unpredictable philosophical concept. All spaces arise from continuously recapitualtion of smoothing and striation, and counting spaces always give rise to the uncountable and to emergent behavior.
A good example is Conway's Game of Life: a simple set of rules played out on a metric space in countable time (striation) gives rise to emergent organizational patterns and a higher level of emergent behavior that simply cannot be predicted or quantified using the original simple set of rules alone (smoothing). Or, to take another case, the Mandlebrot set: a simple pattern gives rise to a recursive, self-similar-yet-never-identical structure that persists to infinity. For D&G it the uncountable always arises out of the act of counting.
This comment is somewhat outside the normal domain of HN, I know, so I hope you will excuse it. I rarely get to show off the hundreds of hours I dumped into D&G and Bergson in gradschool in my day job. :-D
Interesting. The vocabulary disconnect with General Relativity (which is the more relevant theory of relativity here, I think) is pretty frustrating, although one thing that struck me is that at the time Bergson was making these arguments, there was a lot of GR jargon yet to be invented. Also, crucially, a formal process for foliating a "block universe" spacetime was decades off (the 3+1 Arnowitt-Deser-Misner formalism arose in the late 1950s), so a late 1910s criticism of GR as treating the timelike axis as "dead" like the spacelike ones was almost reasonable.
Other important and relevant tools were either extremely fresh (e.g. Noether's first theorem) or had yet to be formalized (e.g. gauge theory), and these put practical limits on conceptual attacks on dynamical spacetimes (that's one reason why externally static vacuum metrics, like Schwarzchild's, were popular at the time). Numerical relativity wasn't even a dream in the 1920s.
However, in spite of not-yet-existing tools, it was pretty clear that General Relativity's coordinate freedom combined with diffeomorphism-invariant models of matter would accomodate standard approaches to time-series evolutions of field content (e.g., initial values surfaces and physical laws). Additionally, "ticking clocks" that appeared in Einstein's and others' GR papers were meant as shorthand for much more general objects -- basically anything that has some state that isn't time-translation-invariant. Ideal gases and other thermodynamic composite "objects" count, as do fundamental particles, as does an entire expanding or contracting universe. "Ticking" is simply the application of some arbitrary coordinates (not necessarily linear or even uniform ones; in GR they only have to admit a diffeomorphism) on those "clocks".
One of the interesting things that was pretty fresh prior to Einstein's Nobel was the resolution of the hole argument, which essentially abandoned manifold substantialism. Spacetime without a clock is simply an irrelevance; it's only the presence of at least one (or more) "ticking clocks" that gives meaning to any system of coordinates one puts down on the manifold -- and in particular it's the "ticking clock" or clocks that generate the metric; it is not something that is a property of wholly empty space, and that in turn led to a deeper understanding of the G_{\mu\nu} + \Lambda g_{\mu\nu} side of the Einstein Field Equations (i.e. the curvature of spacetime determined by the metric).
There was undoubtedly some "philosophy" going on in the early days of General Relativity, but frankly most of the work was on modelling gravitational collapse in general, which was both fairly difficult technically and also a deep well of unexpected consequences that were even more strikingly different from Newtonian gravitation than the Kepler problem in GR.
I'm fairly confident that the ideas raised related to this Bergson-Einstein debate were uninteresting (and possibly even mostly unknown) to most of the scientists exploring the golden age of General Relativity (1960s & 1970s mainly). GR, especially post-Einstein, racked up some extremely precise quantitative predictions of the behaviour of large bodies (and small things near large bodies) that matched later observations with high precision.
By the 1980s, the space for thinking about the philosophy of General Relativity was already mainly at inaccessible energy-densities or at almost pointlessly timelike-separations from us (e.g. the earliest we could see the consequences of black hole evaporation is about a hundred billion years in the future), so what's more interesting (I think) is the study of the mechanisms that generate the metric and the exploration of non-exact solutions, rather than picking at the scabs of GR's unremovable background.
This... was an amazing reply. Thank you for offering it, and taking the time to lay out such a long and thoughtful response. I think you are pretty much dead on, and your reply helped me connect some dots in my own mind about the meta-history of relativity.
Regarding the lack of interest in Bergson during 70s and 80s, I think you are precisely right, and the untestable nature of the time-like ramifications of relativity weren't something I had previously considered. Of course, by that time Einstein was so obviously right, and Bergson so obviously wrong, I think those physicists can be forgiven for not knowing, or for not giving a shit if they did know.
One of Bergson's chief objections to the Twin's Paradox was the idea of time slowing down for the twin sent on the relativistic journey. Such a thing made no sense to him, giving how he framed time: as an unrolling now that could not be subdivided into metric units.
Bergson's objections to time-like relativity are certainly understandable, I think, given the historical context. As you pointed out, the notion of a physics without a background of absolute space - the concept of the ether or an absolute background metric against which space time is measured - were the 'standard' model of the time. I would go even further, and say that many physicists at the time either had severe difficult in coming to terms with physics based on frames of reference, or they rejected it outright. So I don't think Bergson's objections to a relative experience of time are unreasonable, nor do I think you can fault him for his objections, given the difficulties physicists themselves had coming to terms with the implications of relativity. Something I hadn't really considered, however, is that we didn't have the laboratory apparati to test the hypothesis that time passes differently under acceleration until decades after Bergson himself was dead.
Regarding clocks, I certainly understand that a 'clock' in physics is a shorthand for a physical system undergoing periodicity: whether it is an actual clock, a cesium atom, or a gas, etc. For Bergson, however, it was the act of reducing the dimension of time to a countable metric itself that was problematic. For him, the idea that time can be subdivided like space was simply a trick of memory, not actual experience. If we focus only on the unfolding 'now' - something difficult enough to do Bergson wrote whole books on it - we only see one moment elide seamlessly and smoothly into the next.
Bergson had no problem with pointing out that metric time worked quite well in modeling physical systems; his objections were to using this approach to model human experience (particularly with regards to free will and the implications of determinism inherent in relativity). Bergson was a proto-postmodernist, and was trying to get at the idea that the 'map is not the territory.' Hence Bergson's focus on the Twins Paradox. Relativity allows for a space-like time that can be 'run in reverse,' but actual time isn't space-like, in the sense that it can be traversed in one direction only. So despite what Einstein's equations predicted, Bergson objected that the notion of the Twins experiencing time differently was non-sensical.
What I hadn't realized prior to reading your comment is the similarity of Bergson's objections to the objections/difficulties physicists themselves had in abandoning the idea of a fixed, background metric space. He is essentially arguing for a fixed background of indivisible non-metric time that everyone experiences universally and that unrolls at a fixed rate for all observers.
On a side note, I've always thought Bergson (and pretty much the entire history of the philosophy prior to Einstein) had it precisely backward. Thousands of works have focused on and prioritized time as a cornerstone philosophical concept. Bergson was not alone is his obsessive focus on it. And yet, time is the most ephemeral and intangible concept of them all. You can't see it, you can't hold it, there is nothing there. 'Time' as we know it is merely the periodic spatial change repetition of some physical phenomenon: the vibration of an atom; the periodic steps of a watch hand; the filling of a fixed volume of space with water (as in a water clock).
Perhaps it's only the fact that I take living in a post-Einsteinian space-time for granted, but I always found it strange that people -including Bergson - so obsessively abstract 'time' as something distinct from itself, when what they are really seeing is space itself unfolding into... well, more space I suppose.
Thanks again for the thoughts, it was a great read with my morning coffee!
"He is essentially arguing for a fixed background of indivisible non-metric time that everyone experiences universally and that unrolls at a fixed rate for all observers."
Right, that pre-Einsteinian picture has proven to be wrong. Accurate clocks at different altitudes and moving at different groundspeeds bear this out, even if people living on mountaintops or flying in jets don't notice the parts per billion difference in their day from the people living at sea level. The GPS tools they have with them do, though.
Penultimately, there are some theoretical physicists who think time is "real" in the sense that it is fundamental rather than just emergent. I think you are taking an emergentist position (which I agree with) when treating it as arising from observed periodicity. (Remember that your observation of something's period -- like the bouncing light pulse between the parallel mirrors -- is not necessarily the same as another person's observation of the same something.)
Finally, just to bend your brain a bit, in General Relativity in any universe which is even close to being like ours, you cannot have a system where a pair of mirrors with a light pulse bouncing between them can be forever parallel. The parallel mirrors and light pulse are a system of mass-energy that source very slight (but nonzero) curvature. That curvature means that the parallel mirrors, if close to one another, are on a converging path even in empty space far from all other matter. If far from one another, the metric expansion of space means that the parallel mirrors are on diverging path. In a completely empty universe with a finely tuned dark energy, one can set up a classical system in which the system is extremely finely balanced so that the mirrors will stay the same distance apart (measured locally by a notional mass-energy-less observer moving with the mirrors), but real mirrors and light, made out of parts of the Standard Model, will break that fine balance, and the mirrors will move onto either a converging or a diverging path eventually (maybe bet on diverging because of the relative strength of the electromagnetic interactions with the light pulse compared to the gravitational potential energy, and because real mirrors are imperfect reflectors so some photons will "leak away").
On top of that, a really long (approximately "straight-line") Twin Paradox journey in an expanding universe can put a cosmological horizon between the Twins, so they'll never be able to compare their wristwatches in person. Each will see the other slow down and grow dimmer, but only the one moving at near the speed of light (still locally constant everywhere) will live to see her twin disappear completely across the horizon.
(Of course a similar journey confined to the neighbourhood of the Milky Way, e.g., by zipping to and fro many times, will not involve a cosmological horizon.)
"post-Einsteinian space-time"
Well, we call it post-Newtonian. General Relativity's fundamental theory (and in particular the Einstein Field Equations) is very much Einsteinian still. We just understand it better than he did, mainly because we have newer calculational tools (and newer mathematical innovations), and because we have the advantage of access to many thousands of relativists' work over the sixty years or so since his death.
> Thanks again for the thoughts, it was a great read with my morning coffee!
The sequence of discoveries or formalisms weighs heavily on how we teach students; it's not just because earlier formalisms are necessarily easier or more intuitive, although they certainly appear to be when it comes young people who have grown up very close to the surface of the earth when it comes to classical mechanics and Newtonian gravity versus the post-Newtonian extensions.
Certainly lots of physicists took varying amounts of time coming to terms with Special Relativity; few today are au fait with General Relativity. Indeed, even relativists who are will tend to prefer to cast problems as Special Relativity ones, using (or even deliberately abusing) the approximately flat spacetime close by the strict definition of "local", because even when they are comfortable with General Relativity, it is faster to use SR where one can, even in cases where one has to manually put in corrections arising from slight curvature.
In an SR setting one usually teaches Lorentz transformations by trying to impart understanding about three things: firstly, the constancy of the speed of light for all observers in uniform motion everywhere, and secondly, thinking of a "clock" that is a pair of parallel mirrors with a pulse of light continuously bouncing back and forth between them. An observer moving with the parallel mirrors will see the pulse "forever" moving perpendicularly back and forth at the same frequency. An observer in any other uniform motion will see the pulse follow a non-perpendicular path (try it with your thumb and forefinger on one hand held parallel and representing the mirrors, with your index finger on your other hand pretending to be the front of the pulse of light -- hold your hands at a fixed distance in front of your face and watch, then try moving your arms left and right, or towards and away from you, or up and down.). The third thing to understand is that the zig-zagging of your finger between your moving thumb-and-finger appears to be a longer path because it is a longer path (think of a set of coordinates on a wall you see past your hands -- bricks or a wallpaper pattern may help). Moving-with-mirrors twin sums up the length traversed by the pulse of light and arrives at something shorter than not-moving-with-mirrors twin's sum, since the latter sees the pulse travelling along a zigzag between the moving mirrors. Since light always travels at a fixed speed, the longer zig-zagging path must take more time than the shorter always-parallel path. That is, each zig-zag "bounce" takes longer, i.e., the zig-zag bounce frequency is lower, or equivalently, the moving-with-mirrors twin's time is passing more slowly.
Einstein wrote about light bouncing between parallel mirrors, but unfortunately almost always in technical settings. I wonder if that would have helped people like Bergson.
'map is not the territory' -- funnily that's exactly what General Relativity is about; diffeomorphism invariance means that you can have arbitrarily many maps of the same matter, all exactly equivalent, and that you can apply arbitrary coordinates over the configuration of matter.
'Relativity allows for a space-like time that can be 'run in reverse'
Well, sorta. Flat spacetime is time-symmetric; since the symmetry group of flat spacetime is fundamental to the Standard Model, all Standard Model interactions are time-reversible.
BUT... the Hubble volume is extremely curved and in an expanding universe, time-reversibility is far from clear. Indeed, there is a pretty clear thermodynamic arrow-of-time, since the earlier universe, being hotter and denser, had less entropy than the later universe (which has lots of almost wholly empty space, and space with a tiny tiny tiny energy-density can be arranged in all sorts of ways and look the same macroscopically). As the metric expansion of space continues, entropy increase because of all that extra new practically empty space. The empty space can pop up all over the place and in almost any sort of configuration, and we get the same overall picture of the cosmos (in particular everything on Earth looks fundamentally, if not absolutely exactly, the same). Reversing the "movie" of the expanding universe with lots of galactic clusters in it requires very careful positioning of all the "almosts" in the empty space as it disappears, otherwise the overall picture of the cosmos diverges dramatically from our history of it. So at that scale, time-symmetry appears to vanish.
(You could also think of it this way: if you blow up the earth you get a cloud of dust and rocks and stuff. Following Boltzmann's definition of entropy as above, one cloud of dust and rocks and stuff can be pretty indistinguishable from another. But if you reverse the explosion (say, via gravitational collapse), you aren't going to get the dust and rocks coalescing into cities and coral reefs and the Himalayas as we know them unless you are very very precise. So even at that scale, time-symmetry vanishes.)
I am not certain that our brains are actually sensitive to those sorts of time-symmetry violations. Maybe we don't reconstruct the future as well as we reconstruct the past because some part of our brain was lost during the evolutionary periods in which we lost various features found in our common ancestors with birds (e.g., the ability to synthesize vitamin C in our own bodies; tails; nictitaing membranes on our eyes; ...). It'd be interesting to have a conversation with a corvid or a grey parrot or something, or a cetacean. Maybe they have a more symmetrical view of "past" and "future", in that they can remember both. Maybe we are good at playing catch because our brains actually "remember" where the ball will be, rather than doing some sort of calculative prediction.
General Relativity is not quite silent on these points; the theory is a "block world" one in which the whole of spacetime is fully determined. Formalisms that do a 3+1 foliation to look more like pre-Einsteinean physics can produce surprisingly bogus results, even though the "block world" suggests that if we know the entire configuration of the universe at any "slice", we know the configuration of the whole "block" history of the universe. (Why is the subject of a substantial amount of current research).
'actual time isn't space-like, in the sense that it can be traversed in one direction only'
So, above, I said that in an expanding universe, or in the presence of curvature near planetary masses, time-reversal fails, but it fails globally. The individual local interactions within atoms and within molecules are all fully time-symmetric (and we can more-or-less show this in labs).
Again, this is a hot topic in physical cosmology. However, I think everyone agrees that no humans are known to have travelled backwards in time, even if subatomic parts of humans may have (due to e.g. the presence of positrons from radionuclide decays within their bodies, or the uncertainty principle).
The issues of simultaneity in consciousness are different from the issues that relativity address. I get the impression that Bergson thought his intuitions about time, arrived at (at least in part) by introspection, could somehow invalidate Einstein's evidence-based reasoning.
Could it be that our brain simply overclocks/underclocks in certain situations, leading to different perceptions of time? Such a construct would probably never occur to either Bergson or Einstein.
General Relativity was under active development in 1920-1921, and it is so fundamentally different from Special Relativity that it is unfortunate that each theory shares half a name with the other.
(Even more unfortunate is that it took a few decades to arrive at a mathematical and conceptual understanding SR in a GR context as simply one of an infinite number of possible hyperbolizations of a first order quasilinear system of PDEs along the lines of the Einstein Field Equations, or alternatively as a low energy effective field theory in the sense of Kenneth G Wilson (which is what GR itself might be in turn) -- that all became possible only because of mathematical and calculational discoveries made well after Einstein's death).
Special Relativity was surely worthy of a Nobel prize once its relationship with Minkowski space and the Lorentz and Poincaré isometry groups became fully understood (all by the mid-1910s), but General Relativity had also just become Nobel worthy thanks to the 1919 expedition.
So a Nobel prize for a special-case theory which may have seemed like it was about to be overthrown by a generalization by its originator? Or push the award off into the future after General Relativity had developed further? Or possibly two Nobel prizes, one for each theory? In 1920-1921, nobody could have known that it would take four decades for GR to become useful for working scientists, nor that SR and Newtonian mechanics would remain in use as (or in) effective theories even in 2016.
I wonder how much that line of thought might have weighed on the prize committee's mind, rather than objections raised by a non-physicist philosopher.