Does simultaneity have any meaning?

 Someone on Quora asked me a question about simultaneity with respect to Einstein’s special theory of relativity (SR), so I referenced a 30min video of a lecture on the subject, which I’ve cited before on this blog. It not only provides a qualitative explanation or description, but also provides the calculations which demonstrate the subjectivity of simultaneity as seen by different observers.
 
Below I’ve copied exactly what I posted on Quora, including the imbedded video. I’ll truncate the question to make things simpler. The questioner (Piet Venter) asked if there is experimental evidence, which I ignored, partly because I don’t know if there is, but also because it’s mathematically well understood and it’s a logical consequence of SR. Afterwards, I’ll discuss the philosophical ramifications.
 
Does the train embankment thought experiment of Einstein really demonstrate relativity of simultaneity?
 
Actually, there’s a very good YouTube video, which explains this much better than I can. It’s a lecture on the special theory of relativity (SR) and you might find the mathematics a bit daunting, but it’s worth persevering with. He gives the perspective from both a ‘stationary’ observer and a ‘moving’ observer. Note that he also allows for space-contraction for the ‘moving’ case to arrive at the correct answer.

 
To be specific, he uses the Bob and Alice scenario with Bob in a spaceship, so Bob’s ‘stationary’ with respect to the light signals, while he’s ‘moving’ with respect to Alice. What I find interesting is that from Bob’s perspective, he sees what I call a ‘true simultaneity’ (though no one uses that term) because everything is in the same frame of reference for Bob. The lecturer explains both their perspectives qualitatively in the first 6 mins, before he gets into the calculations.
 
When he does the calculations, Bob sees no difference in the signals, while Alice does. This infers that Bob has a special status as an observer compared to Alice. This is consistent with the calculations if you watch the whole video. The other point that no one mentions, is that Alice can tell that the signal on Bob’s ship is moving with respect to her reference-frame because of the Doppler shift of the light, whereas Bob sees no Doppler shift.

 
I commit a heresy by talking about a ‘true simultaneity’, while physicists will tell you there’s no such thing. But even the lecturer in the video makes the point that, according to Bob, he sees the two events recorded by his ‘clocks’ as happening at the same time, because everything is stationary in his frame of reference. Even though his frame of reference is moving relative to others, including Alice, and also compared to anyone on Earth, presumably (since he’s in a spaceship).
 
I contend that Bob has a special status and this is reflected in the mathematics. So is this a special case or can we generalise this to other events? People will argue that a core tenet of Einstein’s relativity is that there are no observers with a ‘special status’. But actually, the core tenet, as iterated by the lecturer in the video, is that the speed of light is the same for all observers, irrespective of their frame of reference. This means that even if an observer is falling into a black hole at the speed of light, they would still see any radiation travelling at the speed of light relative to them. So relativity creates paradoxes, and I gave a plausible resolution to that particular paradox in a recent post, as did David Finkelstein in 1958. (The ‘special status’ is that Bob is in the same frame of reference, his spaceship, as the light source and the 2 resultant events.)
 
In another even more recent post, I cited Kip Thorne explaining how, when one looks at the curvature of spacetime, one gets the same results if spacetime is flat and it’s the ruler that distorts. If one goes back to the Bob and Alice thought experiment in the video, Alice sees (or measures) a distortion, in as much as the front clock in Bob’s spaceship ‘lags’ his rear clock, where for Bob they are the same. This is because, from Alice’s perspective, the light signal takes longer to reach the front because it’s travelling away from her (from Bob’s perspective, it’s stationary). On the other hand, the rear clock is travelling towards the light signal (from her perspective).
 
When I was first trying to get my head around relativity, I took an unusual and novel approach. Because we are dealing with light waves, it occurred to me that both observers would ‘see’ the same number of waves, but the waves would be longer or shorter, which also determines the time and distance that they measure, because waves have wavelength (corresponding to distance) and frequency (corresponding to time).
 
If I apply this visualisation trick to Alice’s perception, then the waves going to the front clock must get longer and the waves going to the rear must get shorter, if they are to agree with the number of waves that Bob ‘sees’, whereby from his perspective, there’s no change in wavelength or frequency. And if the number of waves correspond to a ‘ruler’, then Alice’s ruler becomes distorted while Bob’s doesn’t. So she ‘measures’ a longer distance to the front from the light source than the rear, and because it takes longer for the light to reach the front clock, then it ‘lags’ (relative to Bob’s recording) according to her observation, using her own clocks (refer video).
 
So, does this mean that there is a universal simultaneity that we can all agree on? No, it doesn’t. For a start, using the thought experiment in the video, Bob is travelling relative to a frame of reference, which is the spacetime of the Universe. In fact, if there is a gravitational gradient in his space ship then that would be enough to put his clocks out of sync, so his frame of reference is idealised.
 
But I would make the point that not all observations of simultaneity are equal. While observers in different locations in the Universe would see the same events happening in different sequences; for events having a causal relationship, then all observers would see the same sequence, irrespective of their frame of reference. Since everything that happens is causally related to past events, then everything exists in a sequence that is unchangeable. It’s just that there is no observer who can see all causal sequences – it’s impossible. This brings me back to Kant, whom I reference in my last post, that there is an epistemological gap between what we can observe and what really is. If there is a hypothetical ‘universal now’ for the entire universe, no single observer within the universe can see it. Current wisdom is that it doesn’t exist, but I contend that, if it does, we can’t know.

What would Kant say?

Even though this is a philosophy blog, my knowledge of Western philosophy is far from comprehensive. I’ve read some of the classic texts, like Aristotle’s Nicomachean Ethics, Descartes Meditations, Hume’s A treatise of Human Nature, Kant’s Critique of Pure Reason; all a long time ago. I’ve read extracts from Plato, as well as Sartre’s Existentialism is a Humanism and Mill’s Utilitarianism. As you can imagine, I only recollect fragments, since I haven’t revisited them in years.
 
Nevertheless, there are a few essays on this blog that go back to the time when I did. One of those is an essay on Kant, which I retitled, Is Kant relevant to the modern world? Not so long ago, I wrote a post that proposed Kant as an unwitting bridge between Plato and modern physics. I say, ‘unwitting’, because, as far as I know, Kant never referenced a connection to Plato, and it’s quite possible that I’m the only person who has. Basically, I contend that the Platonic realm, which is still alive and well in mathematics, is a good candidate for Kant’s transcendental idealism, while acknowledging Kant meant something else. Specifically, Kant argued that time and space, like sensory experiences of colour, taste and sound, only exist in the mind.
 
Here is a good video, which explains Kant’s viewpoint better than me. If you watch it to the end, you’ll find the guy who plays Devil’s advocate to the guy expounding on Kant’s views makes the most compelling arguments (they’re both animated icons).

But there’s a couple of points they don’t make which I do. We ‘sense’ time and space in the same way we sense light, sound and smell to create a model inside our heads that attempts to match the world outside our heads, so we can interact with it without getting killed. In fact, our modelling of time and space is arguably more important than any other aspect of it.
 
I’ve always had a mixed, even contradictory, appreciation of Kant. I consider his insight that we may never know the things-in-themselves to be his greatest contribution to epistemology, and was arguably affirmed by 20th Century physics. Both relativity and quantum mechanics (QM) have demonstrated that what we observe does not necessarily reflect reality. Specifically, different observers can see and even measure different parameters of the same event. This is especially true when relativistic effects come into play.
 
In relativity, different observers not only disagree on time and space durations, but they can’t agree on simultaneity. As the Kant advocate in the video points out, surely this is evidence that space and time only exist in the mind, as Kant originally proposed. The Devil’s advocate resorts to an argument of 'continuity', meaning that without time as a property independent of the mind, objects and phenomena (like a candle burning) couldn’t continue to happen without an observer present.
 
But I would argue that Einstein’s general theory of relativity, which tells us that different observers can measure different durations of space and time (I’ll come back to this later), also tells us that the entire universe requires a framework of space and time for the objects to exist at all. In other words, GR tells us, mathematically, that there is an interdependence between the gravitational field that permeates and determines the motion of objects throughout the entire universe, and the spacetime metric those same objects inhabit. In fact, they are literally on opposite sides of the same equation.
 
And this brings me to the other point that I think is missing in the video’s discussion. Towards the end, the Devil’s advocate introduces ‘the veil of perception’ and argues:
 
We can only perceive the world indirectly; we have no idea what the world is beyond this veil… How can we then theorise about the world beyond our perceptions? …Kant basically claims that things-in-themselves exist but we do not know and cannot know anything about these things-in-themselves… This far-reaching world starts to feel like a fantasy.
 
But every physicist has an answer to this, because 20th Century physics has taken us further into this so-called ‘fantasy’ than Kant could possibly have imagined, even though it appears to be a neverending endeavour. And it’s specifically mathematics that has provided the means, which the 2 Socratic-dialogue icons have ignored. Which is why I contend that it’s mathematical Platonism that has replaced Kant’s transcendental idealism. It’s rendered by the mind yet it models reality better than anything else we have available. It’s the only means we have available to take us behind ‘the veil of perception’ and reveal the things-in-themselves.
 
And this leads me to a related point that was actually the trigger for me writing this in the first place.
 
In my last post, I mentioned I’m currently reading Kip A. Thorne’s book, Black Holes and Time Warps; Einstein’s Outrageous Legacy (1994). It’s an excellent book on many levels, because it not only gives a comprehensive history, involving both Western and Soviet science, it also provides insights and explanations most of us are unfamiliar with.
 
To give an example that’s relevant to this post, Thorne explains how making measurements at the extreme curvature of spacetime near the event horizon of a black hole, gives the exact same answer whether it’s the spacetime that distorts while the ‘rulers’ remain unchanged, or it’s the rulers that change while it’s the spacetime that remains ‘flat’. We can’t tell the difference. And this effectively confirms Kant’s thesis that we can never know the things-in-themselves.
 
To quote Thorne:
 
What is the genuine truth? Is spacetime really flat, or is it really curved? To a physicist like me this is an uninteresting question because it has no physical consequences (my emphasis). Both viewpoints, curved spacetime and flat, give the same predictions for any measurements performed with perfect rulers and clocks… (Earlier he defines ‘perfect rulers and clocks’ as being derived at the atomic scale)
 
Ian Miller (a physicist who used to be active on Quora) once made the point, regarding space-contraction, that it’s the ruler that deforms and not the space. And I’ve made the point myself that a clock can effectively be a ruler, because a clock that runs slower measures a shorter distance for a given velocity, compared to another so-called stationary observer who will measure the same distance as longer. This happens in the twin paradox thought experiment, though it’s rarely mentioned (even by me).

The role of prejudice in scientific progress

 I’m currently reading Black Holes and Time Warps; Einstein’s Outrageous Legacy by Kip A. Thorne, published in 1994. Despite the subject matter, it’s very readable, and virtually gives a history of the topic by someone who was more than just an observer, but a participant.
 
What I find curious is how everyone involved, including Einstein, Oppenheimer and Wheeler, had their own prejudices, some of which were later proven incorrect. None of these great minds were infallible. And one shouldn’t be surprised by this, given they were all working on the very frontier of physics and astrophysics in particular.
 
And surely that means that some of my prejudices will eventually be proven wrong. I expect so, even if I’m not around to acknowledge them. Science works because people’s prejudices can be overturned, which always requires a certain cognitive dissonance. As Freeman Dyson remarked in one his Closer-to-Truth interviews with Robert Lawrence Kuhn, every question answered by science invariably poses more questions, so it’s part of the process.
 
Of course, I’m not even a scientist, but a self-described spectator on the boundary of ideas. So why should I take myself seriously? Because, over time, my ideas have evolved and I’ve occasionally had insights that turned out to be true. One of these was confirmed in the reading of Thorne’s book. In a not-so-recent post, The fabric of the Universe, I attempted to resolve the paradox that an external observer to someone falling into a black hole sees them frozen in time, whereas the infalling subject experiences no such anomaly. I concluded that space itself falls into the black hole at the speed of light.
 
It so happens that a little-known postdoc, David Finkelstein, wrote a paper effectively coming to the same conclusion – only a lot more rigorously – in 1958, when I was still in primary school. The thing is that people like Penrose, Oppenheimer and Wheeler were convinced, though it had stumped them. In fact, according to Thorne, Wheeler took longer to be convinced. Thorne himself wrote an article in Scientific American in 1967, describing it by using diagrams showing a 2-D ‘fabric’ dragging ants into the hole, while they 'rolled balls’ away at the speed-of-light. At the event horizon the balls were exactly the same speed as the fabric, but in the opposite direction. Therefore, to the external observer, they were never ‘received’, but to the ants, the balls were travelling at the speed-of-light relative to them. Paradox solved. Note it was solved more than 60 years before I worked it out for myself.
 
And this is the thing: I need to work things out for myself, which is why I stick to my prejudices until I’m convinced that I’m wrong. But, to be honest, that’s what scientists do (I emphasise, I’m not a scientist) and that’s how science works. I contend that there is a dialectic between science and philosophy, where philosophy addresses questions that science can’t currently answer, but when it does, it asks more questions, so it’s neverending.