Does the past of an observer still actually exist in their "now"?

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In summary: I removed from the spot at 12:01pm from the past. So the apple that I removed from the spot at 12:01pm still exists at the detector in the past even though I removed it from the immediate vicinity of the detector.Now, what about if I never remove the apple from the spot at 12:01pm? Does the apple still exist at the detector in the past even if I never touch it?
  • #36
Dale hit it on-the-nose. This is a question of philosophy. There is no scientific answer.(Compare to a similar example: "What happened before the Big Bang?" This is a question beyond science; it is philosophy.)
 
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  • #37
oxbaker said:
after re-reading what you said, do you know if it means the past events still exist in some real sense or just logically on paper?
I don't know what "exist just logically on paper" means, but it doesn't seem like what block universe proponents mean.
 
  • #38
oxbaker said:
I appreciate the response. In terms of an hypothetical experiment, is space-time an actual block in the sense that if I put an apple on the table and then tomorrow I move through the block of space time to the point where I put the apple down, does that event still exist for me to be able to go back to? Or does it only exist in the sense that the light from the event is passing through space to be seen by a future observer in another location?
According to Special Relativity, there is no "same spot as yesterday". The "put the apple onto the table" event occurred at a specific place and time. However, as we move forward in time, we have no absolute claim to be in the same spot as that event.

If you wish to measure yesterdays apple event, it may make sense to put your detector on or near the "same" table. But your strategy for doing so would not be that it is the "same place" as measured by some absolute space/time coordinate system - because there is no such system. Instead, your purpose in choosing that location would be that the room and many of the objects in that room were "witnesses" to the event - and therefore may carry definitive evidence of yesterdays table-related events. In the luckiest of situations, your detector might discover that there is a camera in the room pointed straight at the table and that had been recording all of yesterday.

I think I can make this point more clear by picking a different "same location". I'm sitting on a comet and I note the position of the Earth around the sun and the table on the Earth's surface of the table at the moment of the apple event. I then wait 24 hours and place my detector at that "exact location" - which now is in a sunny part of the vast vacuum of space. Chances are my detector will find no evidence of any table, any apple, perhaps any planet.
 
  • #39
oxbaker said:
TL;DR Summary: If I put an apple down yesterday, does that event still continue to exist in some way at this future point in my time if I stay at the same point in space where the event occurred? Do past events of an observer still continue to exist for them and not just for an observer at another distant point in space time?
Events can't "continue to exist". They exist at a particular clock-reading but last for a length of time equal to zero.
 
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  • #40
Mister T said:
Events can't "continue to exist". They exist at a particular clock-reading but last for a length of time equal to zero.
The trouble I have with this take on it is thus: It doesn't work if you rotate it 90 degrees through 4-space, and use the spatial components of an event.

Event [x, y, z, t] surely exists in conjunction with event [x', y, z, t], yes?; it's just that it's next door in space.

Both events exist with a "length" of x "equal to zero".
 
  • #41
DaveC426913 said:
The trouble I have with this take on it is thus: It doesn't work if you rotate it 90 degrees through 4-space, and use the spatial components of an event.
An event does not have spatial components. An event may have spatial components with respect to a particular coordinate system.

DaveC426913 said:
Event [x, y, z, t] surely exists in conjunction with event [x', y, z, t], yes?; it's just that it's next door in space.
You lost me here. Are you rotating an event? Or rotating a coordinate system that was used to assign coordinates to the event. If the former, what does that even mean? If the latter, then you only have one event.

And what does "next door in space" mean? The set of events in 4 space [or in any space-like three dimensional slice] is dense. Given points A and B, there is always a point C that is closer to A than B is. Accordingly, there is no such thing as an event next door.

DaveC426913 said:
Both events exist with a "length" of x "equal to zero".
Huh? You had ##x## and ##x'## as coordinates for events. How did ##x## get converted to a distance?
 
  • #42
Yeah, mea culpa.
 
  • #43
Mister T said:
Events can't "continue to exist".
The block universe interpretation does not say that events "continue to exist". It just says the entire 4D spacetime continuum of events exists.
 
  • #44
PeterDonis said:
The block universe interpretation does not say that events "continue to exist". It just says the entire 4D spacetime continuum of events exists.
Yes, but in a purely metaphysical sense. There is no experimental or physical sense of this existence.
 
  • #45
Dale said:
Yes, but in a purely metaphysical sense. There is no experimental or physical sense of this existence.
That's not what block universe proponents claim. I understand that there are issues with their claims (I've discussed them myself), and of course one big one is that there is no way to experimentally test the claim, but the claim itself, according to its proponents, is a claim of "physical" existence.
 
  • #46
PeterDonis said:
of course one big one is that there is no way to experimentally test the claim
That is precisely the thing that makes it a purely metaphysical claim.

PeterDonis said:
but the claim itself, according to its proponents, is a claim of "physical" existence.
Claims of existence are metaphysical claims, not physical claims. See https://en.m.wikipedia.org/wiki/Metaphysics

But indeed, many or even most of the proponents don’t recognize that fact and mistakenly believe that it is physical rather than metaphysical
 
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  • #47
Ibix said:
Lorentz transforms tell you that different frames describe pairs if events as simultaneous or not, true, but this says nothing about their existence.
The question of whether something "really exists" is as old as pointless. What is truth? Materialists and idealists will always disagree. Why should the Lorentz transformation solve the puzzle and say something about "real existence"?

A punctual sensory impression (a measurement result) at a certain place at a certain time is an expansionless event that the majority of physicists assume to exist.

So the existence of a measured event is a postulate, not a truth. A physicist wants to measure and calculate, not philosophise.

In this sense, two measured (spatially distant) events "exist", from the point of view of certain observers even simultaneously.

In my opinion, it is not a physical statement but a philosophical speculation when some physicists assume a "block universe" and assume that all events that ever happened and will happen in the future "exist" in some sense. As long as this speculation takes place in a materialistic way of thinking, the term "existence" will certainly cause some confusion.
 
  • #48
It is only philosophy if you cannot provide empirical test. See #2 by @Dale.

If you have a model in mind, you can still evade philosophy and enter into the domain of mathematics.

But if you want to eschew agreed upon definitions and talk about "existence" from vague first principles then you are well and truly in the domain of philosophy.

Note that the native "exists" for mathematicians is not a predicate. It is normally phrased using quantifiers and set membership. We are a bit gun shy in the wake of Russel's paradox. If you want to define a different notion for existence, you'll need a notation. Such as...

Definition 1:

Let A and B be events in a spacetime. Boolean function E(A,B) is true iff A and B are spacelike separated.

Definition 2:

Let A and B be events in a spacetime. Boolean function E(A,B) is true iff A is in (or on the boundary of) the past lightcone of B.

Definition 3:

Let A be an event in spacetime. Boolean function E(A) is true.
 
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  • #49
The past has passed. Thta's why we call it "the past".

I don't see how this has anything to do with relativity (one can have a very similar discussion in a Newtonian framework) or for that matter science. What is subject to experimental verification?
 
  • #50
While the block universe is purely philosophy, I appreciated the OP’s willingness to look scientifically. Because of the experiment they suggested it was possible to analyze the concept and come to a definitive conclusion. So the OP’s approach was scientific, and not the same as the purely philosophical block universe.
 
  • #51
  • #53
Since the notion that "an event is a point in time-space" has become a major thesis in this thread, let me knit-pick that "point":

There are some General Relativity / QM / HUP constraints on that "point".
To our best understanding, the universe is not able to be so specific as to allow anything to exist at a "point".

If, as a construct, you consider a tiny time-space hypersphere of Planck's distance (1.616×##10^{−35}## m) diameter rather than a point, you will be in better conformance with what Alden Meads uncovered in 1964.

Alternatively, if we presume that an event is minimally one bit (it happen or it didn't happen), then we can calculate a restraint to an events minimal radius with the Bekenstein Bound:
## R/m \ge3.88\times10^{-44} / (M/kg)##
Where R is the minimum radius in meters and M is the mass of the system in kilograms.
If I wanted to spend another 20 minutes on this, I would try to get an exact answer by matching a photon wavelength to the resulting bound diameter - but I'm guessing that would be pretty close to the Planck's distance.
 
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  • #54
.Scott said:
Since the notion that "an event is a point in time-space" has become a major thesis in this thread, let me knit-pick that "point":
This is not valid. "An event is a point in spacetime" is a fundamental definition of the term "event" in relativity. Trying to redefine it as the rest of your post does is personal speculation and is off limits here.
 
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  • #55
PeterDonis said:
This is not valid. "An event is a point in spacetime" is a fundamental definition of the term "event" in relativity. Trying to redefine it as the rest of your post does is personal speculation and is off limits here.
I will make a couple of edits to put it into strict accordance, but ...

My issue with a physical zero-radius "point" is hardly "personal speculation". It is so "text book" that the papers that back it up go back to 1964 and earlier. Any assertion that a physically identifiable thing can be, in theory, spotted to a better precision than Planck's distance would be speculation. According to the currently accepted theories there is, as a matter of principle, nothing see-able beyond that precision. My statement "The universe is not able to be so specific as to allow anything to exist as a point" was intended to accurately expresses this same notion - though on review, saying "at a point" instead of "as a point" would have been more narrow but still sufficient.

In reference to discussions on relativity, I would treat the term "event" as a construct. Those discussions sometimes involve QM/HUP effects, but most often they do not presume, one way or another, that actual events can actually have precise physical dimensions of zero.

The textbook nature of this notion is demonstrated in a Fermilab article, a portion of which is quoted here:
In 1964, C. Alden Mead published a paper in which he determined the effect of gravity on a phenomenon called diffraction, which describes what happens to light when you send it through a small aperture. Because gravity is so incredibly weak compared to the force that governs the behavior of light (the electromagnetic force), its effect is completely ignored in diffraction calculations. But Mead was curious about quantifying gravity's negligible effect. When you scatter a particle of light off another particle — say an atom — the atom's gravitational attraction to the light particle causes an intrinsic uncertainty in the atom's location. Mead used the uncertainty principle and the gravitational effect of the photon to show that it is impossible to determine the position of an object to a precision smaller than the Planck length.

So why is the Planck length thought to be the smallest possible length? The simple summary of Mead's answer is that it is impossible, using the known laws of quantum mechanics and the known behavior of gravity, to determine a position to a precision smaller than the Planck length. Pay attention to that repeated word "known." If it turns out that at very small lengths, some other version of quantum mechanics manifests itself or the law of gravity differs from our current theory, the argument falls apart. Since our understanding of subatomic gravity is incomplete, we know that the statement that the Planck length is the smallest possible length is on shaky ground. Still, until a better theory of quantum gravity is devised, the Planck length is the best estimate we have for a minimum length.

In this discussion, I think this intrusion of HUP into relativity is warranted. Time is not just philosophical and relativistic, it is also very QM and thermodynamic. So a construct such as "point", well-accepted in relativity, can use a footnote when included in a broader discussion.
 
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  • #56
.Scott said:
My issue with a physical zero-radius "point" is hardly "personal speculation".
It is in this forum. See below.

.Scott said:
It is so "text book" that the papers that back it up go back to 1964 and earlier.
Papers that are not mainstream physics but speculations about quantum gravity. Discussion of such speculations, to the extent they are backed up by mainstream literature, belongs in the Beyond The Standard Model forum, not here. This is the relativity forum and we are discussing the mainstream theory of relativity, which defines "event" exactly as I said and does not contain any of the speculations you mention.

.Scott said:
In reference to discussions on relativity, I would treat the term "event" as a construct.
Then you are not doing relativity. You are doing some other theory, or some speculative hypothesis such as the ones described above.

.Scott said:
Those discussions sometimes involve QM/HUP effects
Which are outside the scope of mainstream relativity, i.e., GR, which is not a quantum theory.

.Scott said:
In this discussion, I think this intrusion of HUP into relativity is warranted. Time is not just philosophical and relativistic, it is also very QM and thermodynamic.
This is just more personal speculation on your part, at least as far as this thread and this forum is concerned. If you can find a valid reference and you want to discuss this kind of thing, please start a new thread in the BTSM forum. It is off topic here.
 
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  • #57
PeterDonis said:
It is in this forum. See below.
Sorry. I didn't notice which forum topic (Special and General Relativity) we were in.
 
  • #58
In classical relativity we very often treat a clock or a spaceship as a point particle. I think this is “much ado about nothing”. It is not relevant to the OP or the topic of the thread
 
  • #59
Closing this thread as the OP's question has been debated and answered.

Thank you for contributing here.

Jedi
 

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