In response to what I wrote there, Dave Matson asked:
Why should time be defined in terms of relativity physics where, presumably, it means a fixed future? I would think that time, as defined in quantum mechanics, would be equally preferable. As defined in this latter sense, time is presumably compatible with an undetermined future.
If we accept time as defined in relativity, then it seems that physicists would have to accept that there are "wheels within wheels" within quantum mechanics, which most physicists deny. That denial suggests that time as seen in quantum mechanics is defined differently than in relativity physics. One view seems compatible with a fixed future whereas the other does not obviously fit into that mold, if at all.
So arguing for a fixed future on the basis of relativity theory is tantamount to assuming that its definition of time should be preferred. Therein, I see a problem.
In answering this question I will use RT for Relativity Theory and QM for Quantum Mechanics. And I will not repeat what I said in my book. So if you haven't read it, though you don't have to in order to understand what follows, you probably should read it before commenting on any of this.
I've made my answer easier to grapple with by breaking it into four parts, but I have to warn you: Since this is a highly cerebral issue, my answer might give you an ice-cream headache. In fact, I don't recommend anyone bother reading what follows at all, unless you have the patience of Job for scientific brain pain. Because this is the ultra-technical stuff that I deliberately didn't torture readers with in my book.
Okay. Here we go...
- First, it really doesn't matter to me what the answer is, apart from how it alters our physics and cosmology. If, instead, you feel a desperate need for the answer to come out one way or another, you are never going to discover or accept the truth in this matter. Your emotional anxieties will stand in the path of your reason. My advice is to purge your irrational anxiety first before approaching problems like this.
That's right, you will gain nothing from the discovery that the future is not fixed. This knowledge will not change a single choice you ever make, or that anyone ever makes, and therefore the future will remain just as beyond your power to control and just as much within your power to control, as in any other universe. Your power to control (to "cause") certain events in the future rather than others will not be increased or decreased whether the future is fixed or not, because your abilities are the same in both a deterministic and an indeterministic universe. All your knowledge and intelligence and character and desires and everything else remains the same, while any random variables there may be are by definition never in your control anyway, no matter how they turn out (so knowing they could turn out differently won't help you one bit).
I demonstrate this in Sense and Goodness (pp. 97-118), where I also show how this question has no bearing on the validity of assigning responsibility to human actions. Responsibility assignment in the real world actually has nothing to do with the fixity of past, present, or future. Though many people are obsessed with the superstition that it does, I explain in my book why such a belief is illogical. As illogical, in fact, as fatalism, the peculiar disease that results from accepting determinism and then irrationally concluding that you have no control over your life or your future (pp. 115-18).
I won't be discussing any of that here today. I already demonstrate these points in my book. If you have a bug up your butt about free will, or if the "problem" of free will keeps you up at night or worries your little soul, please read my book and think it through. Nothing I say today concerns that question at all.
- Second, the reason I cite RT (among other arguments and evidence) in favor of my hypothesis that the future is fixed, and the universe deterministic, is because ample evidence confirms RT is true, yet the only plausible model for RT entails a fixed future, therefore the current scientific evidence entails a fixed future.
If A speeds away in a spaceship at a high speed relative to B, then time flows slower for A than it does for B. Not only does RT predict this, we've done it, and confirmed it empirically. A does not notice any change in time flow for itself, and in fact sees time flow more slowly for B instead, but because A (and not B) accelerates and decelerates (hence undergoing transformations of its "reference frame"), when A gets back to B, B will actually have crossed more time than A. So, for example, A might return to B and find that B crossed an entire extra hour of time more than A, although now they are both standing next to each other at the same time. So where was A during that extra hour?
The only way to answer that question is to accept that this extra hour existed in A's future, at least in the sense that it had already been crossed by B before A got there, but A ended up crossing fewer points in time to get to the same place. But this means there was a whole hour of A's future already there for B to cross and A to skip around. This is only true for A relative to B, however, since "time" is only a measure of the amount of time (the temporal "distance") between A and B and their conjunctions. But even so, A still crosses less time than B despite ending up at the same place. Therefore, more time has passed for B than A.
Consider A's journey on a space-time diagram. On the diagrams below, the vertical axis is moving up in time and the horizontal axis represents one of the three dimensions in space (the dimension along which A flies away and back). To make it easier, I shall also stipulate that A's acceleration and deceleration are sufficiently rapid to fall below the resolution of these graphs. Okay. If RT were not true, then A's journey would look like this:
But we now know that doesn't happen. The space-time coordinates above show A passing through five hours of time, but in fact A only passes through four hours of time. So A's journey actually looks more like this:
This means something is happening that our first diagram didn't show. For A, the time dimension is warped. As shown on this new diagram, A appears to be moving through a different grid of time coordinates than B, so when B is at 1:30 on the time axis, A is still at 1 pm, half an hour behind. In other words, at half past one, B thinks A is sharing the same point in time (1:30 pm by B's clock). However, B finds that A's clock has been moving in slow motion over the past hour and now shows that it's only one o'clock for A. But it's not as if A is lagging behind, since at the end of A's trip, A catches up with B without crossing the same amount of time. A only crosses four hours of time, while B crossed five.
Scientists can only make sense of this if time is a location, part of a four-dimensional structure, so where you are in time is definable only relative to where everything else is in time. Which means everything else must already be there. So there is no absolute location in time. There is no "now" and thus no "then" or "later," except relative to other things in time (or other parts of you in time, since on RT you are a kind of four-dimensional person-tube, and you only see a cross-sectional slice of you at any given moment in time).
Paul Davies illustrates this relativistic time warping with the following diagram in his book About Time. Again, the vertical axis marks the passage of time, while the other axes mark dimensions in space:
The plane drawn at each moment of time here represents everything in space that you will see as happening simultaneously at that point in time. In other words, it contans everything that (relative to you) is located at the same point in time. The top diagram is how things always look to you no matter how fast you go. But as you start moving faster, space-time folds up in the direction of your motion, so someone still "at rest" (relative to you) will see that you are observing as simultaneous all events on the slanted plane in space, instead of events in a perpendicular plane (like the diagram above). So different events will appear simultaneous to you than for them. In other words, different events will be located at the same point in time for you than for them.
As you go faster and faster, this warping increases, the plane of simultaneity slants further and further up, until, at the speed of light, it folds completely into the time axis, and everything in your direction of motion is what you now see as simultaneous. In other words, your entire future becomes simultaneous with your entire past and present. In effect, you then have no "future" because everything that will ever happen to you already immediately happens to you, all at the same time. This is how a ray of light experiences the universe. Every event in its future happens simultaneously with its present. Relative to photons, all future time is fixed and instantaneous.
If a ray of light still had time to think, it could perhaps talk about "later" events and "earlier" events in its timeline, but only by arbitrarily choosing some event as the dividing line between them. In other words, it can say "relative to what is happening at t = 1 (the first plane marked on Davies' diagram), the points in time above it are in the future and the points of time below it are in the past, but only with respect to that point in time."
Objects moving slower than light haven't folded space-time completely, so they still have a non-arbitrary "present," but only relative to other objects moving through time. Remember, it's all relative: so, relative to a beam of light, you are travelling at the speed of light, so every event in your future has already happened, simultaneously with every other event in your past and present. You are in the same position as the ray of light. You only don't know this because you are moving so slow (at least from your point of view) .
RT further entails that a beam of light passes through all points of space that exist in its path instantaneously, owing to the contraction of space-time in its direction of motion, which reduces even an infinite distance to zero. In exactly the same way, a beam of light passes through all the points in time associated with all the points in space it simultaneously occupies. For some beams of light, which go on into the dead of space forever, this means infinite time is crossed in zero time.
That same beam of light only appears to us to be moving through time more slowly than that, because we are moving so slowly through space ourselves. If we were moving as fast as a beam of light, we would instantly cross the whole of time, too. From the point of view of a beam of light speeding off into the void (or any immortal human who went as fast for as long, if that were possible), the whole life of the universe has already happened--we (us slow pokes) just haven't caught up with it yet.
So far as I have seen, technicalities of wording aside, all RT scientists agree on all this. See my bibliography on the subject (on p. 95), including Davies' book About Time, which, by the way, discusses how QM also suggests a fixed time, as do other books I list there. We already know the two phenomena are somehow related...
Ever wonder how a photon in QM can magically "know" where it will end up? Well, since it instantly passes through every point in space-time it will ever cross, it simultaneously knows everywhere it will ever be--because it's already there. The photon simultaneously occupies its present and future, so the shape and structure of the photon's path is simultaneously determined by all the information on its path. It's only relative to us that it appears otherwise. We're just watching this photon unfold its destiny in slow-motion. But relative to the photon, it's already finished moving. It's already sitting in our future waiting for us to catch up with it.
This means our future already exists before we get there. Since all interactions, all impacts of one object against another (which require photon exchanges between electrons and protons) and all transfers of information (which are also carried by photons, like visible light), since all are produced by photons, and since all photons simultaneously occupy every point of space-time they will ever occupy, there is nothing we can do to change the outcome of the future. To change anything, we would need to move objects (like our arms) or send or gather information (like look or listen or speak), but doing those things requires exchanging photons, yet all the photons in the universe are already done moving. We can't change their paths now. Indeed, relative to these photons, our decisions and actions have already participated in determining their paths, and the paths of all other photons in the universe that are interconnected ("entangled") with those photons, and all objects entangled with any of those photons, and on and on.
Therefore, all future time is fixed on RT. The universe is done. We just haven't caught up with it yet. Even so, regardless of the fixity of the future, our decisions still matter and we still have free will in the only worthwhile sense there is: we can think, plan, learn, change, and more or less do whatever we want. But you'll have to read my book to find out why.
- Third, it is not true that QM entails a non-fixed future. QM is compatible with both possibilities. So far, RT is not. The only scientifically plausible conclusion on present evidence is that RT and QM somehow agree (though we don't know how yet), and since the only way they can agree (so far as we presently know) is for RT determinism to prevail, therefore RT determinism probably prevails.
Moreover, as I explain in Sense and Goodness (pp. 98-99) there are not one, but two ways that QM can be compatible with determinism. Only one of them is what Dave calls the "wheels within wheels" explanation, where some deterministic process actually underlies all QM phenomena, such that the latter only appears indeterministic. It's not true that most scientists "deny" this possibility. Most scientists concede only that we have no evidence yet that this is the case. Those same scientists concede that due to the particular nature of quantum data, we are very unlikely to gain access to that evidence even if it existed. Therefore, we are not in a position to issue denials. It's fallacious to conclude that because we can't look inside the box, therefore the box is empty.
Hence many scientists are still searching for and testing deterministic models of QM phenomena, and no one pooh-pooh's this effort as pseudoscientific or a waste of time. We've already seen how RT entails things about light that actually explain some QM strangeness. Likewise, current theories of antimatter propose that antimatter is normal matter traveling backwards in time. As a result, superluminal causation is becoming an increasingly acceptable hypothesis, and leading theories of quantum determinacy appeal to some form of it. Hence a "wheels within wheels" model (already confirmed as a viable path in the domain of particle physics) remains a viable way to unify RT and QM. It's at least worth searching for. Ultimately, the vote is not in.
But even if we conclude that we ought to have discovered by now some evidence of deterministic underpinnings to QM, and even if we therefore conclude it's "improbable" that there are such underpinnings (though I am not aware of any valid syllogism by which one would establish on present evidence that QM determinism is "improbable"), there is yet another way for QM to be deterministic. Quantum phenomena may be irreducibly indeterministic and still entail only one fixed future. This future would only be indeterministic in the sense that it can't be predicted, no algorithm determines its structure, since the structure of events through time is to some extent fundamentally random. But there is still only ever one structure that is realized, and it's already a fixed fact which structure that will be.
On this view, the structure of the future is not fixed by any set of causes, but merely by the fact that all time has already run its course, and that from a point of view outside the universe, we (you and I) are actually in the past and only observing how that randomly patterned future has already unfolded. And since it can only unfold once, the future for us is fixed and unchangeable--as fixed and unchangeable as our past. After all, even if the whole universe is indeterministic, nearly everyone agrees there is still only one past, and that our past has only turned out one way, which is now fixed and can never be otherwise. No one claims the past is still undecided. Not even quantum indeterminists claim that. Thus, a singular, fixed past is clearly compatible with quantum indeterminism. But according to RT, we are already in the past--someone else's past. For example, we exist in the past of every beam of light that radiates into our future. Therefore, our future, being the universe's already-played-out past, is just as fixed and immutable as our own past, regardless of how undetermined that was.
Take again a beam of light, which instantly knows everywhere it will ever go because it goes through its entire path instantly. What determines what that path will be? If the universe is thoroughly deterministic (if it's all "wheels within wheels" as Dave puts it), then the path the light takes will be entirely determined by the structure of all the space and objects in its path, like a billiard ball on a snooker table. At every point, there is only one way it could have gone. And so it goes.
But if there is anything fundamentally indeterministic about how photons bounce around the universe, then there will be nothing about the space or objects on its path that guarantees it will take one path rather than another. Though QM still limits the options statistically, so it's not just "anything goes," nevertheless there is nothing that determines which available option plays out. Even so, that just means the photon will instantly freeze in a path that is partly random. But it's still frozen. There is still only one path. And that path has already been traveled before we get there. Therefore, even if QM is fundamentally indeterministic, if RT is true, then there is still only one fixed and immutable future. Until you empirically demonstrate that our current RT-model is false, you have no basis for arriving at any other conclusion.
- Fourth, and finally, there is no "different definition" of time between QM and RT that relates to the fixity or non-fixity of the universe.
But this difference has no bearing on whether the future is fixed or not. If the QM definition is true, all this means is that within a minimum unit of time (a Planck time), you can't have any definite location in time, but beyond that, on larger scales of time, you can. This may mean that objects must be spread over, blurred across, an entire Planck time, but this says nothing about whether future time is or is not fixed. However, RT does say this, and RT describes how space-time works on a larger scale.
According to RT's definition of time, where you are in time is only definable relative to other things in time (including other temporal slices of you). This means a future can only exist in RT if there is already a series of future events relative to which we are in their past. If there is something in the future relative to which we are in its past, only then does it remain meaningful on RT to say those events exist at a location in the future. Because their location is only in the future relative to us. Relative to those events, our future is in their past. And that entails all future events already exist--we just haven't gotten there yet.
All this entails a basic determinism: regardless of whether the world is fundamentally indeterministic, there is only one future and it will only play out one way. Because it already has.