You "need another concept of movement" for what? — Luke
Put simply, 3D objects move; 4D objects don't. — Luke
↪Luke
I'm done with the thread, I said what I have to say. — ChatteringMonkey
But at this point i'm starting to repeat myself again. — ChatteringMonkey
You need to adjust the concept of motion to the 4D frame, — ChatteringMonkey
... saying a 4D object doesn't move, doesn't make sense because there is no 5th dimension in relation to which it could move. The term movement just doesn't apply, because motion is change in position over time. There is no 'over time' for a 4D object as a whole. — ChatteringMonkey
This is likely my ignorance, but the MST seems to imply a privileged history. — Kenosha Kid
The whole point of spacetime is that it is invariant. — Kenosha Kid
The passage of time for a woman flying to a neighbouring solar system at 0.1c is not that of her Earthbound twin, and no objective duration for comparison is necessary or possible. — Kenosha Kid
So either the MS adds nothing, or it adds something that makes a nonsense of the world around us. — Kenosha Kid
Or does every observer have her own spotlight? — Kenosha Kid
why would that be smuggling in religion? — jorndoe
MST is a hybrid of Presentism and Eternalism. Presentism does indeed entail a "privileged history". However, I'm not here to defend Presentism, but to point out the nature of the (block) universe according to (B-theory) Eternalism, which, by definition, contains no temporal passage. — Luke
I'm not sure whether that's the "whole point of spacetime", but the laws of physics are considered to be invariant in spacetime, yes. — Luke
That only makes sense if there is a passage of time. — Luke
when does motion occur according to Eternalism? — Luke
My point was that there is no concept of absolute simultaneity. There is no "now" that you and I share, unless we're co-moving. — Kenosha Kid
But you are presenting MST for consideration. — Kenosha Kid
My point was that there is no concept of absolute simultaneity. There is no "now" that you and I share, unless we're co-moving. So I was wondering how a spotlight illuminates "now" across many bodies moving at different speeds. Is there a basis for choosing? — Kenosha Kid
It absolutely is. (I used to teach this stuff at uni, so I'm not totally pulling this out of the air.) — Kenosha Kid
That only makes sense if there is a passage of time.
— Luke
Yes, that's the problem. The MST seems to reintroduce a passage of time — Kenosha Kid
Simplest of kinematics is velocity: change in position / change in time. This, and all higher orders of motion, are retained in four dimensions. It's just that "change in time" is not special. Let's say you're due south of the summit of a mountain. As you move toward the summit, you're moving north. But you're also moving upwards as you ascend. There's a relationship there: the gradient change in altitude / change in latitude. "Motion" in the usual 3D+1 way of thinking is now just equivalent to that. — Kenosha Kid
I presented the MST mainly for comparison with the block universe. Perhaps that was an error on my part. — Luke
Does this imply that you acknowledge and find it unproblematic that Eternalism (i.e. the block universe) has no passage of time? — Luke
If so, then how do you answer my question of what it is that changes position over time if 4D objects (and/or their subdivided 3D parts) remain fixed at their spatiotemporal locations according to Eternalism? — Luke
According to the principle of relativity, laws of physics don't privilege any reference frame. But that doesn't mean that a reference frame cannot be privileged in some other sense - like in the sense of indicating the absolute now. The absolute now would not be part of the known laws of physics if it existed; it would come as an extra fact about the world. But that's old news - it was as true for Galileo and Newton as it is for Einstein. — SophistiCat
When you say "changes... over time", in the block universe that means "what changes in the rest of the worldline as we move along the time axis in a particular direction". — Kenosha Kid
Do you mean simply tracing out a path on a map — Luke
Yes, just this. It would be easier if I could draw it, or write equations. But if you can imagine it, groovy. The question is: what changes (other than the time coordinate) as you follow the path of the helix? The answer is the spatial coordinate of the helix.
To anticipate the follow-up question, or the similarity of what I'm suggesting to the spotlight, it is not necessary to do this for the "change" with time to be there. — Kenosha Kid
It's merely a means of illustrating that the change is already encoded in the worldline. — Kenosha Kid
Motion is conventionally defined as change in position wrt time (which also works for presentism). Motion exists in the block (if anything exists whose spatial position changes as its time position changes). The answer is inevitably geometric, since the block universe is geometric, and inevitably kinematic given the (imo only sensible) kinematic definition of motion. — Kenosha Kid
That is, everything exists at all times. — Luke
The block universe in this case is not a model along which you can trace time with your finger — Luke
To anticipate the follow-up question, or the similarity of what I'm suggesting to the spotlight, it is not necessary to do this for the "change" with time to be there. It's merely a means of illustrating that the change is already encoded in the worldline. — Kenosha Kid
The block universe in this case is ... the actual universe in which nothing changes its spatiotemporal location. — Luke
Caveat to follow, but: everything exists at all times, but not necessarily at the same place at all times. This variance of position with time is motion. — Kenosha Kid
Imagine a 3D universe consisting only of an eternal stationary ball. In the block universe depiction, this is a straight line parallel to the time axis. Now boost to a frame of reference in which the straight line now has a gradient. i.e. is no longer parallel to the time axis. There's your motion: I just moved the whole universe for you, Luke, and you're still not happy!!! — Kenosha Kid
The ball has motion, though. Informally, the ball "is moving", i.e. if instead the worldline of the ball is sometimes parallel to the time axis and sometimes not, I could say "the ball is sometimes moving" and you'd understand me, right? — Kenosha Kid
You still seem to be presuming that an object can change its temporal position (i.e. move through time). Eternalism rejects this. — Luke
Taking the broader point, I agree that the existence of things that cannot even be indirectly observed is possible. I'm less convinced that it's meaningful to talk about them. Which I guess is what I was saying earlier: what is the explanatory power of the spotlight? If we accept that a) it is a privileged frame, not shared by all of us, and b) makes no difference to observable phenomena, it can't explain, say, the psychological passage of time, which is subjective, i.e. relative. — Kenosha Kid
An "absolute now" is not a concept that makes sense to me though. "Now" now is not "now" exactly a year ago: it is not absolute. But a privileged moment (e.g. 13.7 billion years ago) wrt which "now" can be referred and seen to change would be absolute and sensible, even if it has no obvious descriptive power. — Kenosha Kid
"change in t" does not mean "some passage of some objective present moment from the first value of t to the last".
That does not mean that the worldline itself, which we may write as a function of (z,y,z,t) is moving with respect to some other time t2. — Kenosha Kid
While relativistic laws are reference frame invariant (up to coordinate transformation), the same cannot be said about those things that the laws do not fix, such as the distribution of matter and radiation in the universe. — SophistiCat
If we take those other things into account, we can identify reference frames that are special in some way, such as the frame in which the cosmic microwave background radiation has the same energy profile in all directions. — SophistiCat
All the same,the need for a privileged frame is not fatal to presentism, although as you point out, no observation can help us identify this frame. — SophistiCat
You seem keen to saddle me with Presentist assumptions. I have not mentioned an objective present moment or a second time dimension. I am using the same definition of motion as you. — Luke
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