Does the Past Still Exist?
backreaction.blogspot.com
backreaction.blogspot.com
[O'Brien to Winston Smith]: 'Is it your opinion, Winston, that the past has real existence?'
Again the feeling of helplessness descended upon Winston. His eyes flitted towards the dial. He not only did not know whether 'yes' or 'no' was the answer that would save him from pain; he did not even know which answer he believed to be the true one.
O'Brien smiled faintly. 'You are no metaphysician, Winston,' he said. 'Until this moment you had never considered what is meant by existence. I will put it more precisely. Does the past exist concretely, in space? Is there somewhere or other a place, a world of solid objects, where the past is still happening?'
'No.'
'Then where does the past exist, if at all?'
'In records. It is written down.'
'In records. And- ?'
'In the mind. In human memories.
'In memory. Very well, then. We, the Party, control all records, and we control all memories. Then we control the past, do we not?'
https://www.abhafoundation.org/assets/books/html/1984/162.ht...
> Just like a ball doesn’t change if you rotate one direction of space into another, Maxwell’s equations don’t change if you rotate one direction of space into time.
> So, Minkowski said, we just combine space with time to a 4 dimensional space-time, and then we can rotate space into time just like we can rotate two directions of space into each other. And that naturally explains why Maxwell’s equations have the symmetry they do have.
I have read, enjoyed, and understood a lot of popular physics books. But none of that makes a lick of sense to me. Either I’m dumber than I thought (probably), or I’m not the right audience, or this is a poor explanation.
It takes a bit of time with spacetime diagrams to become more familiar with it
What always amazes me about discussions like this is how even physicists who are experts in relativity fail to see the obvious implications of the fact that, in relativity, spacetime is divided into three regions instead of two. Instead of just "past" and "future", there are "past light cone", "future light cone", and "spacelike separated". "Past light cone" and "future light cone" behave more or less like our intuitive concepts of "past" and "future"--the past light cone is what is fixed and certain and can't be changed, and the future light cone is what can possibly be changed based on things we do here and now. But the spacelike separated region is not like anything in our intuitions, since it is not fixed and certain as far as we know here and now, but we also can't do anything here and now to change it.
But instead of recognizing all this and considering its implications, discussions like this article talk as if the only way to divide spacetime up into regions is "past" and "future" separated by a "now" slice (and the "now" slice is of course frame-dependent so it can't have any physical meaning), or to talk as if the only thing that "exists" is the single point in spacetime that is our "here and now". It's disappointing.
Of course you have described a perfectly good way of thinking about the past/future (and to your credit, she surely should've included what you described). But in her defense, what you have described is also the most obvious explanation consistent with special relativity – the "past" and "future" outside one's line cone cannot be objectively defined, as you could always (hypothetically) boost to another frame in which the disconnected past becomes the disconnected future (and vice versa). Undoubtedly the author is aware of this fact, so I assume the purpose of this blog post was simply to motivate other hypotheses about the nature of time.
She describes them, but does not consider their implications for the structure of spacetime.
> you could always (hypothetically) boost to another frame in which the disconnected past becomes the disconnected future (and vice versa).
No, you can't. That's the whole point of looking at light cones instead of spacelike "now" slice. Light cones are invariant; they're the same in all frames. Spacelike "now" slices are not. Also, you can't do a Lorentz boost that turns "future" into "past" or vice versa.
And yes, you can always boost to a frame such that the "order" of spacelike separated events will change (i.e., reverse which event is above and which is below a line of simultaneity). Since you can't objectively say which event happened first, they must be unable to influence each other, hence "causally disconnected".
I can imagine the layman ascribing some mystical quality to ontological anti-realism, but, ironically, the view acts to dispel the mystical quality of uncritical "existence" claims that many take for granted (much like theism, moral realism, etc.).
Well, of course; that's how we already use the term "exist" in ordinary language. We say that our past of today "exists", but tomorrow does not, at least not yet. But tomorrow, we'll say that what we now call tomorrow does "exist". So our ordinary language concept of "exists" already takes into account that what "exists" changes over time.
I'm fine with this formulation, because I think the answer to it is the one I gave: the past light cone of your "here and now".
Special relativity is great because it produces some very non-intuitive conclusions about our Universe, and it's always lovely to explore new mind-bending questions arising from these conclusions, especially in outreach pieces like this. I think conceptual discussions like these can be very enlightening, but its not something that keep a lot of physicists up at night.
At the level of LQG all you can describe is relations between points of spacetime and the idea of past/future becomes ambiguous.
Another modern approach to quantum gravity (energetic causal set theory) does re-impose a sort of global ordering (in the sense reality is a series of states updated with a single rule depending on the momenta of previous realizations of the state elements) but in that theory the past really is gone. This is similar to the (more classical) approach of Shape Dynamics (a general relativity-like theory of gravity) where all that relates a "current" snapshot of time to a "previous" snapshot is that it minimizes a particular mathematical quantity. I am definitely not an expert in LQG or GR or Shape Dynamics, but the story isn't quite so obvious and simple as you make it out to be and that may be why you are unsatisfied with how physicists talk about it.
Only in spacetimes with closed timelike curves, such as the Godel Universe. Those spacetimes are considered by physicists to not be physically realistic.
All physically realistic spacetimes, including all of the ones we use to actually model things in the actual universe, are globally hyperbolic, and in a globally hyperbolic spacetime the same event cannot be in both the past and the future light cone of some other event.
At the very least, the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood (as do the other more outre theoretical avenues I pointed out). I think its pointless to be glib about the reality of the past, given that fact.
Only inside the inner horizon, which virtually all physicists working in the field consider to be unphysical because of the "infinite blueshift" effect at the inner horizon and the fact that it is a Cauchy horizon: you actually can't predict what will be inside it based on any data from outside it. (Note that the outer horizon, like the horizon of a Schwarzschild black hole, does not have these properties.) The prevailing opinion seems to be that the interior of a rotating black hole will end up being qualitatively similar to the interior of a non-rotating black hole, with the inner horizon and region inside it of the idealized Kerr solution being replaced by something that looks more like a Schwarzschild interior. (And of course if we figure out quantum gravity that might change things even further.)
> We certainly use the Kerr black hole solution to model things in the actual universe.
We use its exterior to model things in the actual universe, yes. But none of those models depend on the interior having the properties that the idealized Kerr solution has. They certainly don't depend on their being CTCs somewhere inside.
> the fact that CTCs appear in solutions to GR but seem physically implausible suggest that the basic physics of time are not fully understood
No, it suggests that, as with any physical theory, you have to look at actual data to see which of the infinite number of mathematically possible solutions to the theory's equations describe things that actually exist. All of the solutions that describe things that actually exist are globally hyperbolic and do not contain CTCs or any other weird "time" properties. That suggests that, as far as things that actually exist go, we have pretty good models of "the basic physics of time".
The central assumption is that for a reasonably physical spacetime there exists a unique maximal globally hyperbolic solution to the field equations, justifying the use of the initial value formulation of General Relativity. [aside: [1]]
Once we do that we can take any arbitrary spacelike hypersurface Σ and in ADM-ish vocabulary observe the compact closure conditions (Σ has a spacelike geodesic from every p in Σ to every other p in Σ; every maximal spacelike geodesic has a nonempty open segment in Σ; every maximal timelike geodesic intersects Σ), the "thin" condition (no nomepty segment of a null geodesic is found in Σ), and a variety of causal conditions (e.g. no timelike geodesic connects points in Σ; no maximal timelike or null geodesic intersects Σ more than once).
One can translate these features to your cones-only view. On a Minkowski diagram with p at the origin, Σ can have any shape or orientation compared to the light cones as long as p -- and nothing else in either of p's light cones -- appears on Σ. We can do the same for any other p in Σ, or we can decide whether the origin of any pair of light-cones is in Σ or not. We can also probably build Σ by hand from a principled choice of available light-cones. (One would be tempted to call that "initial data").
Is Σ physical? Of course not! But it can be used as the basis for physics that describes point-coincidences on other spacelike hypersurfaces, which we can then verify.
> The future light cone is what can possibly be changed based on things we do here and now
In which of your three regions do we find the freedom to do this?
Is here-and-now not fully determined by the "fixed and certain" content of its past lightcone?
Is the "we" part of "what we do here and now" describing a point on the manifold or is it describing a set of points on the manifold? Choose a point somewhere in the middle of "we" -- in which of the three regions are its other points?
I struggle to answer that without thinking about an extended "we" with many entries in Σ (including photons (and other carriers of body forces) bouncing around within or even passing through the extended body). Maybe it's much easier for you.
I wonder how you'd do if you were to think of grossly non-Eulerian (and even non-timelike) observers in a Big Crunch universe. In a Big Bang universe all points are in the future light cone at BB; in a BC universe all points are in the past light cone at BC. At every other point, only some points are in the two lightcones at that point. We get a BC from a time-inversion of our BB->approx de Sitter universe (e.g. for an observer with differently-aligned arrow-of-time). Assuming a long period of matter sparseness, in a BB->BC universe, we also get a ~BC universe if we cut away the expanding part; if we cut away the recollapsing part, we get something that's hard to distinguish from a BB->~dS universe like our own. In a BB->~dS universe I think we'd agree that the future light cones of two observers coincident at p can eventually fully separate. Can we agree that in a BC universe the past light cones of two observers at p can be at least somewhat, or even (practically) fully, non-overlapping (our observers need not be either "Eulerian" comovers with the metric contraction, and can even move lightlike, and there can be a cosmic deflation just prior to the big crunch)?
In an ADM(-like) picture we can fairly straightforwardly use ancestor slices of Σ in the foliation and decide that a pair of photons will be at some point p on Σ. I also think it makes a slice through a spacetime-filling electromagnetic field easier to think about, ditching the idea of bare photons as independent objects. Again it doesn't matter about the timelike direction of the slicing, and one might want to recalculate slicing along some other choice of timelike axis to try to spot coordinate artifacts. However maybe you might object that if we don't have full data for the ancestor of Σ, we're somehow not better off than in the final sentence of your second paragraph.
- --
[1] Weaker causality, such as stable causality in the sense that arbitrarily small metric perturbations don't give rise to CTCs, can still be used but at the cost of solution uniqueness, and weaker still at the cost of smearing out the Cauchy surface so that initial data is not all at t=0 but in some function defined over 0 >= t >= -V^{-1} where V is the scale of strong causality violation.
It's simple: any "slicing" is frame dependent. And relativity says that things that are frame dependent do not have any physical meaning.
> The central assumption is that for a reasonably physical spacetime there exists a unique maximal globally hyperbolic solution to the field equations
Yes, but globally hyperbolic spacetimes still do not have unique slicings. There are still an infinite number of possible slicings, corresponding to the infinite number of possible inertial frames in standard SR.
> In which of your three regions do we find the freedom to do this?
If the universe is fully deterministic, then in one sense of course there is no "freedom" to change anything. But we don't know that the universe is fully deterministic (and QM strongly suggests that it isn't). But whether it is or not, the distinction I described is still there and still valid, since that distinction is about what we know at a particular event in spacetime.
> In a Big Bang universe all points are in the future light cone at BB
No, they aren't. The Big Bang is not a single point. It's a spacelike line. There are an infinite number of causally disconnected points one "instant" after the Big Bang.
Similar remarks apply in time reversed form to the Big Crunch in spacetimes that have it.
> QM strongly suggests that it isn't
an argument over which is certainly out of scope here (however it did answer my question); and
> The Big Bang is not a single point. It's a spacelike line
which I think is wrong for the standard cosmology. See Davis & Lineweaver 2004 <https://ui.adsabs.harvard.edu/abs/2004PASA...21...97D/abstra...> notably fig. 1 and the expalanatory text scattered within.
Why would (cosmological) t=0 be a line rather than a point, a 3-volume, or some even weirder non-Lorentzian structure? I can't think of why it would be a line other than (reaching here) nonvanishing vorticity (in either the sense of the Raychaudhuri equation or because of ~axisymmetry with spin), and those aren't features of the standard cosmology.
Obviously I've seen Penrose diagrams with the BB drawn out to look like a horizontal line, but have never taken that to be anything other than for legibility or print-reproducibility reasons, similar to the dimensional reduction of the diagram itself. I would be grateful for a reference with explanatory text if you think this is wrong.
Absent published authority, I think you could also take this up with Pössel (Max-Planck-Inst. Astro. Haus der Astronomie) who by sheer coincidence ran a related twitter thread <https://twitter.com/mpoessel/status/1551119783315251200> in which he writes (among other things): " ... 23/ ... Let's forget about the hot Big Bang phase, pretend that 24/ all the galaxies taking part in cosmic expansion are point particles corresponding to galaxies we see in the observable universe today were at one and the same location. 25/". @johncarlosbaez has been all over that thread on Twitter too, and would probably engage with your it's-not-a-point-it's-a-line point. I am pretty certain that Pössel means we ignore the "hot Big Bang phase" to mean not just the first few minutes (up to the synthesis of helium) (which is a fairly common time frame for "hot Big Bang") but up to early structure formation, and that he'd clarify if asked.
I agree that in the standard cosmology there are an infinite number of causally disconnected points one "instant" after the Big Bang, but don't see how that can't follow from the standard BB singularity.
Of course a BC universe is not standard cosmology, so alternative final shapes don't raise my eyebrows at all, unless you wish to argue that in General Relativity, no BC universe can even in principle have a point wherein all other points are in its past light cone.
All that said, I'm not sure why you're objecting to slicing as having a dependency on choice of coordinates. I did write that \Sigma is not physical. I will now write that it is very convenient to calculate in some set of coordinates, and that it is possible not to fall into holes (pardon the pun) while doing so.
Finally, I'd like to turn on its head your second sentence. Relativity allows some physical distributions of stress-energy to pick out a meaningful set of coordinates. (as has been known in cosmology for many years. cf. <https://arxiv.org/abs/1211.6338> §2 notably p 4)
(That democracy of causal cones paper is surprisingly relevant to some of the conversations you are running here tonight, particularly §2. I think you might especially enjoy the last paragraph on p. 6, notably its last two sentences starting "We note that, quite generally, C is a nonempty, open convex cone of tangent vectors...". In the context of our discussion, you might not enjoy the part starting at the bottom of p. 7 to the start of §3 :-).)
I have no idea why you would think that, since the very paper you link to explicitly shows diagrams in which the Big Bang is obviously not a single point but a spacelike line.
> Why would (cosmological) t=0 be a line rather than a point, a 3-volume, or some even weirder non-Lorentzian structure?
Because that's what you get when you take the appropriate limit.
> Obviously I've seen Penrose diagrams with the BB drawn out to look like a horizontal line, but have never taken that to be anything other than for legibility or print-reproducibility reasons, similar to the dimensional reduction of the diagram itself.
You are mistaken. The point of a Penrose diagram is to show causal structure; that is how it is constructed, so that light cones on the diagram exactly match light cones in the spacetime it's a diagram of. That means that if it were really true that the entire universe was in the future light cone of the Big Bang, the Big Bang would have to be a point, and the diagram as a whole would be an inverted triangle that was the future light cone of that point. Which obviously is not the case for Penrose diagrams of the universe as a whole.
As for "published authority", as I noted above, you gave one yourself: the Davis & Lineweaver paper. It agrees with what I have been saying, not with what you have been saying.
> I agree that in the standard cosmology there are an infinite number of causally disconnected points one "instant" after the Big Bang
Then you are contradicting yourself, since this is impossible if the Big Bang is a point. It is only possible if the Big Bang is a spacelike line.
> but don't see how that can't follow from the standard BB singularity.
It does. The issue is not "the standard BB singularity", but your mistaken belief that that singularity is a point. It's not. It's a spacelike line.
What is a BC universe?
More precisely, with certain distributions of stress-energy, computations become much simpler in particular coordinates. But that does not mean the coordinates have any physical meaning. If there are any actual invariant properties of the spacetime that particular coordinates make more evident, those properties can always be expressed in coordinate-independent terms, for example in terms of Killing vector fields.
> (as has been known in cosmology for many years. cf. <https://arxiv.org/abs/1211.6338> §2 notably p 4)
I don't see anything in that paper that supports any claim stronger than the one I agreed to above. If you think you do, please give a specific quote.
The definition of "now" as when the light hits the mirror on each side is a "constructed" now, a false now if you may. It takes the light longer on the right side than the left side because she is moving through space, all that tells me is that there is no way to communicate a "now" between different observers because there can't be instantaneous communication between observers, not that time itself is relative or physical stuff itself doesn't have a constant time?
She says in the video that we have to find a way to operationalize how to find a now, to measure it empirically but maybe that's not possible without running a complete simulation after the fact. But to me it seems not surprising that without instantaneous communication there is no way to create a "now" empirically but that doesn't mean it extends to the ontology of physical stuff itself or time itself. There would still be a time in there (you should be able to see it in a simulation?)
Also for example, if you use sound instead of light, if I'm far away from a sound source (farther than the speed of sound), and another observer is closer to it, we would not be able to use the sound source as a source "now", because I would hear it later. Is this the same type of situation or different? I feel like I must be missing something fundamental here but I'm sort of going out on a limb so please don't slaughter me :P
As for the "here" part I am not sure how to respond to that but it sounds right - any observer has to have contact in some direct way (whether with a scientific instrument or not) to the thing physical system he is observing and as such claiming anything about anything indirect or far away is very hard if not impossible so you get the "here".
https://www.youtube.com/watch?v=9oX2xFo7JA4
But, ist the other side of the block-universe existing?
Is it growing in that direction?
Are you on the edge of that growth?
Or are you already ghosts of the past talking among themselves?
Again.
And again.
Forever.
this is very summarized, his ideas goes much farther, for him the base of true reality could be consciousness :
and
https://www.facebook.com/VedicCosmology/posts/do-the-past-an...
https://gauragopala.blogspot.com/2019/02/we-are-deep-inside-...
"The hard part about doing propaganda is that the past changes so quickly you have no idea what will happen yesterday"
Q: What is the hardest task for a Marxist historian?
A: To forecast the past.
To see and touch the dinosaurs, visit Einstein, see everything, even our thoughts. An undetectable time machine that might surround us even now. A traversable field of time. A kind of high fidelity afterlife.
(I am clearly not a physicist.)
Perhaps on our first run free will was much more obvious and abundant, in ways that we simply can’t even imagine right now, kind of like trying to imagine light when you’re born blind. However, the end result still ends up being exactly what you experience now, minus the running dialog of free will.
Our body is controlled by our mind, and our mind is controlled by our consciousness, which is capable of observing and altering our mind’s thoughts. But as far as I can tell, we don’t seem to have any mechanism for “observing” our consciousness. That is the missing freewill component, which may have only existed in our first run.
Do you think the "first run" version of yourself would have made your HN comment?
(The invitations were posted after the party had concluded.)
You probably meant descendants, but that just made your comment delightfully coherent.
Events that have no causal relation between all have their own little timeline, and it's not troubling or surprising to consider that they are simultaneous / that we examine them in a kind of superposition.
Or am I saying something stupid? ;-)
The Big Bang is a different story, because the theory says that spacetime is evolving.
Not entirely relevant, but I think that the Buddhist view of time is interesting, and I guess quite relevant to this block universe idea.
In the Buddhist view, all things are "emptiness", shunyata, without self-existence. This means that there is no fundamental basis with which you can distinguish between things. For example, there is no fundamental property that you can use to distinguish between you and I, or an apple and a keyboard, or even your body and your consciousness. The distinctions are just concepts that we make up and assert that the world fits into it. The Heart sutra says that not only is this true for all the elements that make up our experience, but even also for causal connections such as time.
This view of time was most explicitly emphasised by the Soto Zen philosopher Dogen. He called it Uji, which means "time-being". Effectively he said that being equals time, and that really all time is right now:
[quote]
An ancient buddha said:
For the time being stand on top of the highest peak.
For the time being proceed along the bottom of the deepest ocean.
For the time being three heads and eight arms.
For the time being an eight- or sixteen-foot body.
For the time being a staff or whisk.
For the time being a pillar or lantern.
For the time being the sons of Zhang and Li.
For the time being the earth and sky.
"For the time being" here means time itself is being, and all being is time. A golden sixteen-foot body is time; because it is time, there is the radiant illumination of time. Study it as the twelve hours of the present."Three heads and eight arms" is time; because it is time, it is not separate from the twelve hours of the present.
[/quote] (https://www.thezensite.com/ZenTeachings/Dogen_Teachings/Uji_...)
The way we understand this as Buddhists, is that we are both at the very beginning and very end of our path, at the same time we are both deluded beings and also perfectly enlightened Buddhas:
> Time is not separate from you, and as you are present, time does not go away. As time is not marked by coming and going, the moment you climbed the mountains [walked the Buddhist path] is the time-being right now.
So effectively, we believe that all moments are contained in this moment, because there is no fundamental basis by which we can say that any other moment is separated from this moment. Before and after are illusions, they are just ideas that we have right now.
Many Americans are terminally confused about this.
Also, I don't really care. Grammar is incredibly low on my list of priorities
It is not low enough for you to have avoided being Wrong on The Internet after failing to take correction, and then complaining after that.
My experience is the people who say "rude" most frequently have been the most that. I speculate that they imagine so much rudeness around them that they consider their own forgiveable.