A Defense of the Reality of Time
quantamagazine.org
quantamagazine.org
For the most parts people (except maybe Julian Barbour) are arguing about whether the the concepts of "now" and "change" come from outside of physics, in such a way that the present is in some sense more real that the future or past.
If so we can and must think of physics as how "real things now, change".
The alternative is that time is part of physics, and that every part of the history of universe is equally real, and that "now" and "change" are perceptions of observers within that history.
Both views accept the reality of time. Things don't become less real just because they are physical facts rather than metaphysical necessities.
Well, there's not really a deep disagreement because facts about left/right are observer-relative. Furthermore, we might reasonably conclude that left/right orientation isn't part of the fundamental structure of the world.
Now, if I looked at a timeline of events and said "Trump became president after Obama" and you (or any other observer from any other point in time) said "Trump became president before Obama", who is right?
Barbour is saying that this disagreement is like the left/right dispute. Maudlin is saying that it isn't.
I think it's fair to characterize this disagreement as being about whether time is really part of the fundamental structure of the world. And it's clear that there's a meaningful disagreement here.
But I understood him as disagreeing, not just with Barbour, but with a whole lot of physicists (including me) who are "B-theorists" which is a horrible term that means you consider the whole history of the universe as the fundamental reality and see change as a thing observed by beings within it.
But that doesn't mean I think physics, or the history of the universe are time symmetric. My own view is there is some fundamental (but not very well understood) time asymmetry in physics. Yet I am still a B-theorist.
But the alternative called "A-theory" (which I think Maudlin believes) is that time, and change are more fundamental than physics, so the fundamental reality is "the state of the universe _now_" and the laws of physics tell us how it has changed and will change.
A-theory is also perfectly cogent, but it is separate from time symmetry. The only link being: IF physics is time symmetric (which it isn't) THEN B-theory implies there is no fundamental difference between time directions, while A-theory still allows it.
My above analogy to the left/right issue is the example I use to introduce students to the A- vs. B-theory.
The A- vs. B-theory debate is, of course, complicated, and you point to one possible complication: can we separate the issue of temporal asymmetry from the issue of temporal change?
As I see it, the answer to this question is "No".
We can talk about how some physical object 'changes' its spatial properties as we move from one spatial slice to another. For instance, as we move from the pointy-end of a cone to its base, each circular slice of the cone 'changes': they keep getting bigger until you reach the base.
This use of 'change' is symmetrical. There's no preferred direction of 'change' here. It makes equal sense to speak of changes "from bigger circle-slices to smaller-circle slices" as it does the other way around.
But in the context of the temporal dimension, talk of 'change' is not symmetrical. It's fine to say "The President changed from Obama to Trump", but you can't say "The president changed from Trump to Obama".
The point of all this just is: the issue of whether (temporal) change is symmetrical is tied up with the issue of whether time itself is symmetrical. You can't have your cake and eat it too. There's either an objective temporal ordering of physical events from past to future (as A-theory posits) or there isn't (as B-theory posits). If you're a B-theorist, you have to say that "Trump came after Obama" is like saying "The cup is on the left".
This is not a refutation of B-theory. But it, I hope, makes clearer what the stakes are. And when those stakes are made clearer, B-theory, in my view, looks less attractive.
So please help me check I am understanding the terms of that debate correctly.
(1) Even if it is not the way you explain it to students, do you accept my claim that A-theory considers time and change as being more fundamental than physics?
(2) If I further said "... as opposed to considering time change to be properties of the history of the universe", then would you accept the implied dichotomy?
[1] And I say the A-B is in turn not truly a debate reality of time, which was just an unfortunate framing by McTaggart.
That's likely still unclear, but that's to be expected. We typically can't ask questions about fundamental structure without invoking that fundamental structure itself. That's why I find the analogy to left/right so helpful. We have a pretty good idea of what it means for left/right orientation not to be built into the fundamental structure of nature. And so that can provide a way of testing various claims about the fundamentality of asymmetrical temporal ordering.
2. I'm not sure I have a grip on what this additional thing is supposed to add. But I'll note that you reference 'history' and one might reasonably think that that is a temporal notion. If you meant 'history' to mean something like "the universe's extension in the temporal direction", that doesn't quite seem to allow for the distinction I took you to be trying to make.
(And, yes, McTaggert's framing is highly unhelpful.)
If/how non-determinism can be rectified with relativity without invoking uncountably infinite realities, which is ontologically hard to swallow, is unclear.
Now if our bitmap were determined by a sufficiently deterministic 1-D cellular-automaton then our one scan-line would determine the rest of the picture.
But if the bitmap was actually a 2-d picture drawn by an artist who had some freedom of choice, but also was constrained by some geometrical rules, then our scanline would not determine the picture, though it would correlate with other pixels.
Yet the entire picture is still just as real as the scan-line we picked.
He lays out his theory in his book, The End of Time.
I like how this guy thinks.
This is something I've never understood about including time as just another dimension.
So, that's essentially where, "time is a dimension" comes from, and that's where it goes.
Aside, if you're wondering what coordinate-invariant length really is: just think about the fact that looking at a house from different angles will not change the distance between the doors. This length is calculated with the Pythagorean theorem involving xx+yy+zz, and when you extend it to lengths which are also invariant under different choices of the time coordinate you arrive at xx+yy+zz-tt.
To complete what has turned into a brief introduction to SR, note how differences in the most natural choice of the direction of time might arise:
We want the path of a stationary object to involve no movement through space, only the required advance into the future (through time). So, we note that time is parallel to the path of a stationary object. However, people moving relative to each other will disagree about which objects are stationary. Therefore, they will end up thinking differently about which way time points.
Returning to my earlier analogy, this disagreement about which direction time points can be compared to the different viewers of the house, who may disagree about which direction forwards or left points.
It isn't too unnatural when you think about it, which is a good thing because it is, well, natural.
As whatshisface mentioned, the sign of the time coordinate is different when computing the interval. What this means in practice is that you can't 'rotate' from a motion forwards in time to backwards in time. Instead you get what is basically a shearing transformation. The amount of rotation is limited by the speed of light (c).
So SR, I think, doesn't really say that time is just another dimension like the spatial dimensions. But it does mix the two together with a shearing transformation.
In fact, I would say that special relativity doesn't have much in particular to say about what time is. It merely specifies the mathematical transformations between reference frames, including between the local times in those frames. You have to go to metaphysics to say what this really means. In this sense it's a bit a like quantum mechanics.
There's actually a trilogy by Greg Egan based on the premise that time is just like a spacial dimension (i.e., where the Minkowski metric is instead xx+yy+zz+tt). It radically changes physics and chemistry, and the books are set in this different universe.
Does time exist for systems with no events?
If a tree falls in the forest, and no one is around to hear it, does it make a sound?
https://en.wikipedia.org/wiki/Zero-point_energy
Particles will still be created and annihilated there and will interact with your system, so you cannot have no events.
http://www.sciencedirect.com/science/article/pii/S0167278998...
The reference that I did see was to here: http://blog.rongarret.info/2014/09/are-parallel-universes-re...
And that page has what I would regard as an appropriate level of skepticism for it's own position. It says things like "IMO". It says what it thinks is correct, but it recognizes that it could be wrong. (Reasonable enough, when you're saying that the Copenhagen interpretation of QM is wrong. That may in fact be correct, but dogmatism is not warranted yet, and may never be.)
The links you present show a paper that puts forward a theory. Again, I'm not saying it's wrong, and it seems to make some things easier to understand. But where's the experiment that tests a prediction of this paper, which other interpretations of QM can't explain? Without that, it's just a nice theory. There are lots of those.
I am nonplussed by this question. Did you even bother to read the first post I referred you to? Because that is all about a (thought) experiment.
Yes, we can gain insight from a thought experiment. If the underlying assumptions are correct, we can even learn something true about reality. It's that "if" that I'm questioning. And without an actual experiment, you don't know.
And with that assumption for a foundation, your experiment becomes a way to experimentally distinguish which interpretation of QM is correct. I'm not saying that such an experiment is impossible, but it's going to be really hard.
Yes, obviously. But special relativity was developed almost exclusively on the basis of thought experiments. So basing a scientific conclusion on a thought experiment has a venerable history.
BTW, the EPRG experiment is not a thought experiment. That experiment has actually been done (despite the fact that the outcome was never in doubt, which is why it is not particularly noteworthy).
> And with that assumption for a foundation, your experiment becomes a way to experimentally distinguish which interpretation of QM is correct.
No, it's not possible to experimentally determine which interpretation is correct. But this is not unusual in science. There are an infinite number of theories that are consistent with any finite data set. To choose among them you need some criterion other than consistency with the data.
Even at absolute zero, there are still quantum fluctuations which give rise to phenomena such as quantum vibrational energy in molecules.
Within the nucleus and among the electrons, a system would still exchange energy, even if there were no low energy photon emissions. Photons would still be in play between the electrons and the nucleus.
Within the individual subatomic particles themselves, there is also activity. As far as we know protons don't decay, but it doesn't mean they are without events, even when no one is looking.
Mostly I'm just trying to poke holes in the concepts and analogies that get bandied about, because when one attempts to apply common-sense, folksy wisdom to these topics, understanding frequently goes wildly off the mark.
That the nucleus locks away energy, which can be set loose, is basically the fundamentals of fission and fusion, and that peculiar varieties of standing matter could produce heat for no outwardly obvious reasons, was the genesis of the idea for the atom bomb.
Feynman diagrams and rules, within the scope of quantum electrodynamics [0], provide for how a stable molecular system would still represent components exchanging energy at the subatomic level. An object would be regarded as cold and stable in deep space at absolute zero, but other activity could still occur. Maybe a population of stray photons bouncing around in some kind of stablized trap, forever... I'm not saying that such a thing exists, but somehow it doesn't sound totally unreasonable.
Either way, my example is a bit facetious, since there doesn't seem to be any escape from galactic star light at any distance, if Hubble space telescope images are to be believed. So we don't know that a system could exist which does not experience at least some photons falling onto it, from outer space. That photons can travel for 13 billion years, uninterrupted, and still produce a signal with any degree of fidelity, should provide at least a clue as to how long a photon can keep doing the same thing among non-decaying protons, for a pretty long time.
As for whether protons decay, I'm not being 100% accurate. Recent research set "lower bounds" on the period or likelihood for natural decay [1], and it's an exceptionally long amount of time: 5.9 × 10^33 years [2]. More than the billions of years we ballpark as the entire existence of the universe, so far.
But if I'm remembering correctly that number is regarded as a safe number, simply because for the duration of the longitudinal study, they have not yet recorded a spontaneous decay event for any of the protons in the experiment, which is a huge number, and so, statistically, based on no observations across N protons (50,000 tons of not-just-any water), over N units of time (three experimental research phases from 1996 to 2008?), they've stated an interpretation that one proton is expected to last at least so many years. When that appeared in the news, the sentiment was expressed as "nothing yet, boss!"
Does this mean perpetual motion exists within protons? Don't say that out loud, in front of the wrong people, geeze... But, if perpetual motion is not possible, then does time eventually stop?
[0] https://en.wikipedia.org/wiki/Quantum_electrodynamics
[1] https://en.wikipedia.org/wiki/Super-Kamiokande#Proton_Decay
This uncertainty in energy over small time-scales provides a "floor" for the energy of empty space.
Yes, haven't you been to any pointless meetings? :)
> If a tree falls in the forest, and no one is around to hear it, does it make a sound?
Obviously yes, sound is just resonating air and if physics is not broken it does make a sound.
I always thought the same and guess most practical scientists do as well. There is nothing in thermodynamic puzzles that indicates there is some problem or insight to be gained on the 'nature of time', whatever that is.
Analogously, we can use a yardstick to measure space in a "static" way, but don't have a timestick – we have to actually step off each unit.
(As a teacher it's always so fun seeing how an example gets interpreted entirely reasonably in way other than you intended.)
When they say time isn't real, they mean something more like "not ontologically fundamental" -- i.e. you can produce a model with no explicit reference to time, and it can produce all the same predictions as a "timeful" model -- of course, you'd have to do some final transformation at the end to express the time values if you want it to compare to a timeful one.
Yes, you can measure it, but that measurement is wholly implied by all the other (ontologically fundamental) ones.
Compare to a word in which people used four dimensions to describe space. In that case, you could correctly argue that, "hey, space is only three dimensions. Yes, you're measuring a fourth one, but that's redundant with the first three; once I know the first three, I can tell you what your measurements of the fourth one will be."
Barbour's argument, then, is that you can derive the laws of physics in terms of a rule for "which universe configurations are possible", and make that your primary model; time then just becomes a label for a measure you apply between different configurations (the "Machian distinguished simplifier"), but is wholly implied by them.