Can you explain what a “dynamical quantity” is? Spacetime contains all of time as well as all of space, so saying that spacetime “changes” is nonsensical (change implies a difference over time).
I think the main discussion over time can, perhaps, be boiled down to the following question:
Does the past exist?
If you say, no, the past no longer exists, then there are several ‘names’ for you - one being an a theorist.
If you say, yes, the past does exist, then (again, several names), you are a b theorist.
Now, by exist I mean in a similar way that you and I exist:
A theorist:
The past is gone, no longer accessible, ‘poof’.
B Theorist:
The past is still there and if you could somehow sit outside of the cosmos (bear with me for a minute) you could locate it. You could theoretically travel to the past via wormholes, faster then light travel, etc.
The passage of time:
A theorist:
We perceive ourselves going through time, because we are going through time. Somehow the present becomes the past (and no longer exists). We are moving towards the future.
B theorist:
We perceive ourselves going through time, but this is simply a deception. Perhaps similar to how we see faces in the clouds.
Chronos, we’ve had this discussion before and it might have been clearer in the previous thread, here.
I must admit, this topic is very confusing and I’m probably the main peddler at this point of that confusion (not my intention obviously).
The timing of that prior thread is almost eerie…
OK, that prior thread has much better explanations of the two models than here. Or at least, more sensical explanations; I don’t know what the original forms of the models proposed by the relevant philosophers were.
How do you determine exacly when a particle will decay in the future?
Maybe we are thinking of ‘determinate’ in different ways. I mean that the future is set, although it may be literally impossible for us to predict. If you are saying that the future is indeterminate in the sense that we are unable to ‘determine it’ (haha) then I agree, but that doesn’t address the deeper questions.
As to your question, if no one observed a particle decaying in the past, and the evidence of the decay is no longer available, then we can’t ‘predict’ when that happened either - but I think we both agree that this event is not changeable in any sense.
Nah, that’s just one way to parametrise change. The height of the terrain changes with location: it’s higher in the hills than on the beach. The colour of light after going through a prism changes with the angle. (If you think that’s somehow cheating, consider that the block universe view provides exactly the same interpretation for ‘change with time’.)
In general relativity, spacetime is essentially the gravitational field. Different mass (or stress-energy) configurations yield different gravitational fields, which in turn influence the mass configuration (‘mass tells spacetime how to curve, spacetime tells mass how to move’). To think of spacetime as somehow being fixed misses the point of GR; its background independence, i.e. the explicit absence of a fixed stage upon which the dynamics of the universe unfold, is its greatest conceptual strength: at least in this direction, there’s something like a final answer, rather than an infinite regress of explaining each level in terms of a more fundamental one.
The problem is, people tend to use the term ‘spacetime’ without taking to heart the conceptual revolution it brings; they think of it as simply some ordinary three-dimensional spatial coordinate system (x, y, z) indexed by a real variable t that somehow also plays the role of a ‘fourth dimension’. But that’s conceptually only halfway there, since such a decomposition is not in every case possible, and in general, observer dependent – one player’s x may be another’s t, so to speak.
Agreed. I’m gonna take my hundred bucks and hit the road.
All of the examples in your first paragraph boil down to changes over either space or time, IMO. And I don’t understand the sentence in parentheses.
I have to say I still don’t understand what you mean. However conceptually bizarre the GR universe is, how could it not be fixed at least from the perspective of a single point in spacetime?
Ok, were just using different definitions. I’m not sure that the future is determinate in the sense you are speaking of, but I don’t know, or care. But I doubt you could prove it (haha). We cannot predict the future. We can remember our perception of the past. Some of the past can be determined by measurement to a limited precision. I agree that the past cannot be changed, as far as I know, or anyone else knows so far. But I don’t know what everyone else knows either.
As a note, your use of determinate is an arbitrary selection of a term to describe a physical theory, which postdates my use, which is the literal form, something that can be determined. But I assume you were laughing at yourself. My (haha) was laughing at you trying to prove that you can determine the future.
But then I think you are just stating what everyone agrees is our perception of time, eg:
Yes this is how we perceive it, but the question is whether this might be an illusion in some way.
OK, electrical field strength changes with charge. The point is that when we speak about ‘change’, we think about quantities like x(t), position wrt time. But that definition of change is valid for all quantities of the form a(b), and those are conversely all the quantities we can ever observe. Everything we observe is a relation between partial observables (such relations may be called complete observables), i.e. a and b, or x and t; x and t can’t be observed on their own (think about how we measure all lengths with respect to a standard length, or all times with respect to a standard time interval). Only in the nonrelativistic limit does the partial observable t get singled out and acquire some ‘special’ meaning; in GR, it stands on equal footing with the other partial observables.
That’s just the thing – the concept of ‘a point in spacetime’ on its own is in fact physically meaningless in GR. The best we can do is specify spacetime coincidences – intuitively, particles crossing paths. There’s a famous thought experiment known as the hole argument that illustrates this, but it would take us too far to go through it in detail, and I don’t know of any good exposition on the net (wiki isn’t terribly helpful). However, with such path crossings, whose viewpoint should we accept as the ‘real’ one? Even if two particles cross paths a second time, in general, to both, a different amount of time will have elapsed since their last meeting (that’s essentially the famous twin paradox), and hence, at this point, we’d have (at least) two notions of time, standing on equal footing.
It’s the soup du jour. The past is gone and can never be known; the future (probably) won’t happen. Time is a concrete concept that other humans use to plan and schedule our lives.
Oh, that’s easy. Yes, time might be an illusion in some way.
I’m still not sure I understand the point of whether time is an illusion. What new knowledge would come from proving it was an illusion? For the statement that time is an illusion to have scientific value, it needs to have some form of utility.
And if it does not have scientific value, why are we trying to use science to make sense out of it?
Yes, I feel as though I’ve been muddling things all throughout this thread.
I’ve been trying to make sense of both models and it’s very confusing. There’s not just ‘one’ a or b theory, there are multiple ones. Some involve what seems to me to be linguistic arguments (as in, purely).
It’s over my head, but I’m trying to figure it out - which is why I believe my explanations have been muddled throughout this thread.
The utility of it seems questionable. I think that it can be applied to the origins of the universe and that’s where I primarily see it argued.
One camp says that only the present exists and therefore we have to explain how the universe came from nothing.
The other camp says that all time exists and therefore the universe has always existed (finitely though). That is not to say that time goes back infinitely.
Ah, I see the difference. The theory still doesn’t make any sense, however.
Well, one point is that the search for a theory of quantum gravity proves difficult in part because of the different way time is treated in general relativity and quantum field theory. If now time were an illusion, in the sense that it isn’t fundamental, quantum gravity could be formulated in a timeless way, solving the difficulty of unifying the two contradictory viewpoints. Not that this means that it becomes easy to find a quantum theory of gravity, but it gives you at least one principle upon which to formulate it.
Besides, it’s simply an interesting question, and knowledge can be valuable as its own end, even if there’s nothing as such you can ever ‘do’ with it. Is there a tangible benefit to knowing the exact depth of the Marianas trench (or even to having been there), or the different species of butterflies in the Amazon rainforest, or why the dinosaurs died out? I doubt it, and if there is, it’s most likely incidental. Generally, we don’t do things solely because they have utility – we do them because they’re there to be done, because we’re curious, and I think that’s a good thing.
Boy, will Hawking ever be embarrassed when you tell him!
Ok, so time could be an illusion, in the sense that we percieve it (for whatever reason), but actually it doesn’t exist in terms of the physical nature of the universe. Does that mean there would be some way to percieve the future? If our brains are limited somehow, wouldn’t there be a machine which could overcome that? By machine I don’t necessarily mean a physical thing, but maybe just the theory. Or could that just mean time is not a factor in quantum gravity, not that it doesn’t exist?