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

• I
• oxbaker
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?
oxbaker
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?
This is a tough question to phrase but I haven’t been able to find an answer yet, although I’m sure one exists. I’ll try expand what it is I’m asking. I’m no physicist but I think I have a general understanding of what they are saying when they describe a block space-time universe where the past of one physical point exists in the “now” of a separate observer at a different physical point in the universe. So if I were to put an apple down on a table in my “now”, that would become a past event that would exist in the “now” of a separate observer at another point in the universe when the light cone of that event reaches them.

That all makes sense so far. Me putting down the apple still exists for a separate observer at some future point when the light cone of it reaches them. But what about a past event of mine in relation to me at this point in time and space and not to a distant observer? 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 point in space time?

That’s where this whole idea of a block space-time gets fuzzy for me. I know that the past of one observer continues to exist in the future of a separate observer in another location but does my past event (apple on the table) somehow still continue to exists as a real event in relation to me here the next day? Is it permanently written in the block of space-time for everyone everywhere at every time, including future me? Like as some frozen slice behind a vail that I can just no longer see but still exists? Or in relation to me, does that past event cease to exist after I’ve done it?

Hope that makes sense. Because if that past event only continues to exist for another observer at another point then it seems like more of an illusion. To a distant observer a past event at another location may continue to exist for a time until it reaches them but actual events themselves just come and go, ceasing to exist a split second after they occur in relation to the place they occurred at.

Last edited:
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?

Like as some frozen slice behind a vail that I can just no longer see but still exists? Or in relation to me, does that past event cease to exist after I’ve done it?
In the case of difficult to phrase questions, one thing that is useful is to consider what experiment could hypothetically be performed to answer the question.

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?

PeroK
Sorry, I didn’t see any hypothetical experiment there. An experiment is something like “We setup measuring devices <describe measuring devices> and some things to measure <describe what things you are going to measure>. A measured result of X means that it still exists and a measured result of Y means it does not.”

LOL, That's a result of my layman's limited understanding of the scientific method. Ok. I'll give it another shot trying to use your framework. We set up a hypothetical device that detects a past event in space time that occurred at the same location as the device but happened at a previous time. The device detects the presence of apples at it's location in the past. And I don't meant it detects residue of the apple in the present but rather the actual apple that existed there in the past.

I put an apple down where the detector will be at 12pm and remove the apple from that spot at 12:01pm. The the following day I put the detector down at that spot to detect whether an apple existed at that spot at 12pm the previous day. A measurement result of positive means it detects an apple at it's location in the past, meaning that past event is somehow still existing at the location for the future detector to read. A negative result means there is no past event at that location to detect because the event only actually exists for other observers at distant locations and no longer for the detector at that location. Does that at all make sense? Again, please forgive my lack of formal education in this.

Last edited:
All consequences of the apple having been there exist later. And, if we know enough, we might be able to prove that it was there. IMO, anything other than that is a philosophical question. But in philosophy, they would be questioning if it was there when it was there. ;-)

Perfect. I think that's what I wanted to clarify. The apple may have existed in the past at that location in some philosophical sense but in terms of the math that describes what block space-time actually is and how it actually works, it cannot currently address if that past event should still be existing in some real way at that physical point in space that could be tested by scientific method.

Peter Strohmayer
oxbaker said:
Perfect. I think that's what I wanted to clarify. The apple may have existed in the past at that location in some philosophical sense but in terms of the math that describes what block space-time actually is and how it actually works, it cannot currently address if that past event should still be existing in some real way at that physical point in space that could be tested by scientific method.
I'm not sure that I agree. You must distinguish between its existence in space-time versus just saying it still exists there in space. There is no single "now" in space-time because the space-time coordinate system represents all of time, not just "now". So any position in space-time remains there. Other times have other time coordinates. All coordinates exist.

oxbaker said:
it cannot currently address if that past event should still be existing in some real way at that physical point in space
"Exist" is a fuzzy term.

An "event" is conventionally defined using four coordinates: three spatial and one temporal. You placed the apple at a location we will call [x, y, z]. The event's coordinates are [x, y, z, t]. That will always be the coordinate of that event.

Today, you come back to the spatial location but at a new time coordinate, so: [x, y, z, t']. We currently have no way to travel to [x,y,z,t] from [x,y,z,t'].

FactChecker
The block universe is, in my opinion, the most natural interpretation of relativity, but other interpretations are possible and you know what they say about opinions. We can't actually go back to the apple in the past and find out, so we don't strictly know if some flavour of eternaism is true or not.

A lot of stuff written about relativity is written as if the block universe is real: stuff like "spacetime is a 4d manifold". Formally that is wrong, but almost everybody uses that interpretation and it's much shorter than writing "spacetime is apparently accurately described using the same maths as the geometry of a 4d manifold, but other interpretations are possible". So it is very common (and defensible on practical grounds) to skip all the caveats.

Good sources will often note them at least once, though.

syfry and FactChecker
But does the actual mathematical theories that describe what space-time is address in any way whether a coordinate in the past should still exist in some real way in that at space-time block so that it can be detected in a real sense from the same location at a future date?

So if something occurs at a specific spatial location in the past does the math at all describe that time location in the same way as the spatial location? You can move back and forward between point A and B in space and even if we can't currently move back and forth along the time dimension, does the math describe that time dimension existing the same way as the space one where in theory you could move back and forth to different times at the same physical spot?

I mean because as a layman I read about all sorts of aspects of reality that are first described to be a certain way mathematically and then sometimes decades later the math is proven true by experiment. So does the math address anything about whether points in the past continue to exist in the same way that different spatial points continue to exist simultaneously?

Last edited:
Perhaps we can rephrase the question. Is a particular universe described by General Relativity past-deterministic? Is it future-deterministic? We define "past-deterministic" and "future-determistic" as follows.

Pick an orientable space time. Find a space-like hypersurface (or maybe a collection thereof) which bisects the space-time into two halves. One of them we will arbitrarily call the "future" and the other we will call the "past".

A space-time is past-deterministic with respect to a hypersurface if, given complete information on the state (position, velocity, mass, charge, etc) of everything at the hypersurface then there is only one possible configuration for everything in the "past" of that hypersurface.

Similarly for future-deterministic.

The spectre of non-existence of a past occurrence is the case where multiple histories are consistent with everything in the present. At least one history where the occurrence took place and at least one where it did not.

I'll take your word for it that that is a better rephrasing. I accept my current limits to describe or understand these types of issues but look forward to reading any of the responses to it.

oxbaker said:
I don't meant it detects residue of the apple in the present but rather the actual apple that existed there in the past.
Then I think the answer to your question is “no” because to my knowledge no such device exists.

Sorry, thought you asked for a "hypothetical" experiment, as in "supposed but not necessarily real or true".

oxbaker said:
Sorry, thought you asked for a "hypothetical" experiment, as in "supposed but not necessarily real or true".
Sure, but even hypothetical experiments have to follow the laws of nature. Otherwise they don’t tell you anything about physics. You can assume devices that break economic or engineering limits, as long as they respect the laws of nature.

There is no law of nature that I know of which could produce a device like you describe. So again I think the answer to your question is no.

oxbaker said:
Do past events of an observer still continue to exist for them and not just for an observer at another point in space time?

PeterDonis
Dale said:
There is no law of nature that I know of which could produce a device like you describe. So again I think the answer to your question is no.
Ok, now I'm starting to think you are just messing with a new poster. You're saying because I, a self proclaimed layman, can't come up with a scientific experiment that follows known laws of nature that could detect events in the past at a given location, that proves that past events of an observer do not continue to exist in a block spacetime universe?

Last edited:
oxbaker said:
You're saying because I, a self proclaimed layman, can't come up with a scientific experiment
I am saying that because I, as an expert in physics, cannot think of a law of physics that would permit such a hypothetical device (regardless of engineering or economic limits) that the answer to your question is no.

That is actually a very good outcome. Because you specified the type of experiment you had in mind we could come up with a concrete answer.

Dale is not messing with you.

PhDeezNutz
Ok, if you are really being serious then I apologies for my remark but I'm still not following your reasoning in that case. Why would the answer to my question be "no" rather than "we can't prove anything one way or the other". How can us not currently having a means of experimentally testing a theory mean that the theory is definitively false?

I assume you mean that the answer to my question is "no" because you know of some theoretical or mathematical evidence that points towards it not being the case? Or maybe you know of some experiments that have shown it is not true? I mean, I'd even accept that it's not true because I have a misunderstanding of what the "time" in "spacetime" actually is according to the theory. That's what I'm here trying to figure out :)

Last edited:
I am reminded of the old Maine joke that involves a farmer and a moneyed flatlander asking for directions. Punch line:

"I guess you can't get there from here"

/

oxbaker said:
How can us not currently having a means of experimentally testing a theory mean that the theory is definitively false?
This is not an issue of technology. It isn’t that we don’t currently have a means of testing it. It is that the known laws of physics don’t permit such a device. Such a device is not possible based on our current understanding of the universe

Let me recap, and see if that helps. Your original question was:
oxbaker said:
Do past events of an observer still continue to exist for them

To clarify the concept of what it would mean for an event to still exist, you did a good job describing the type of experiment you had in mind:
oxbaker said:
The device detects the presence of apples at it's location in the past. And I don't meant it detects residue of the apple in the present but rather the actual apple that existed there in the past.

All devices which detect an object rely on some sort of residue, electromagnetic, acoustic, chemical, thermal, whatever. But we infer the past existence of something based on a present residue of some sort.

So past events do not continue to exist in the way you are thinking of it.

oxbaker said:
we can't prove anything one way or the other
This isn’t an ambiguous issue. This is an affirmative statement that the laws of physics don’t allow such measuring devices. Since such a device embodies your concept of past events still continuing to exist, it is clear that past events do not continue to exist in that manner.

I don’t know why you are upset at this answer. You asked a question and received a good solid answer. Did you just want someone to tell you a specific answer? If so, you probably should have mentioned which answer you wanted. Frankly, I thought “no” was what you were already leaning towards. So I thought you were going to be satisfied.

Dale said:
Such a device is not possible based on our current understanding of the universe
Which, more immediately to you oxbaker, means the question itself is unfortunately nonsensical. As you are finding, it can't even be formulated using concepts based on our current understanding of the universe.

PhDeezNutz and Dale
I'm starting to feel like that girl on the big bang theory when they are messing with her. I don't know if I'm the farmer or the flatlander in this scenario. I honestly don't mean to sound upset but I'm just trying to follow what the possible existence of this device has to do with my question? Is this some kind of round about way of telling me I didn't phrase my question logically?

Because you can draw a line on a graph does not make it real.
You understand I was not intending to be mean. I think the joke is relevant!

oxbaker said:
Is this some kind of round about way of telling me I didn't phrase my question logically?
I think you phrased it well. I am trying to tell you the answer to your original question.

Why do you think that someone answering your question is messing with you? I don’t know what you want here, but apparently it isn’t an answer.

hutchphd said:
Because you can draw a line on a graph does not make it real.
You understand I was not intending to be mean. I think the joke is relevant!
No worries. I only semi got the joke in the first place so I wasn't offended ;). But the answers I'm getting here seem to have gotten off track enough that I don't think they are addressing my original question anymore anyway. I'm sure it has to do with me not phrasing the question right or logically somehow but I don't think it's worth going in further circles to clarify so I'll just say g'night to you all and thanks for the responses!

oxbaker said:
I don't think they are addressing my original question
For clarity, the question I answered was:
oxbaker said:
Do past events of an observer still continue to exist for them
To which the answer is no.

oxbaker said:
a block space-time universe where the past of one physical point exists in the “now” of a separate observer at a different physical point in the universe
No, the "block universe" concept is much simpler. It says that all events exist. Every event in the entire 4-dimensional spacetime continuum exists. That's all it says.

The reasoning that is used to justify this claim is more complicated and not everyone agrees with it (at least one common argument is refuted in this Insights article). But the claim itself is very simple.

Dale said:
I think you phrased it well. I am trying to tell you the answer to your original question.

Why do you think that someone answering your question is messing with you? I don’t know what you want here, but apparently it isn’t an answer.
Ok, I'll try summarize how this went from my point of view and maybe you'll see where I'm coming from because I do think you were trying to be helpful but it resulted in some kind of disconnect with my original question.

1) I tried to ask a question about whether someone who understood the math behind the theories could help clarify for me in laymans terms whether the past was a real dimension that existed like the spatial dimensions do; where in the block of spacetime that past event continued to exist alongside the present, in the same way that two spatial points can simultaneously exist. I tried to clarify as well as I knew how that I meant in a real sense and not in a logical one like the light of the event traveling to a distant observer or that information cannot be destroyed so we can calculate the exact state of a past event.

2) You asked me to frame what I was asking in the form of an experiment to help clarify what I was after.

3) There's no actual way to currently run a real experiment that validates something is still existing in the past in a real local sense so I made up a hypothetical experiment trying to provide what you were looking for.

4) You said since the experiment I came up with was not currently possible following known laws of physics, that meant that the answer to my question was "No, past events of an observer still do not continue to exist for them".

5) I did not understand how an experiment that cannot currently be performed on a theory meant that the theory was false.

6) You clarified that "Since such a device embodies your concept of past events still continuing to exist, it is clear that past events do not continue to exist in that manner." and you clarified that that was the answer to my question based on the imaginary device I came up with to your original counter question.

Look, I believe you are being genuine when you say you think I phrased it well and you were really trying to answer my question but I also think the disconnect is you answered some kind of expanded question that included that imaginary experiment. I didn't meant to ask if there was an experiment that actually proved what I was asking in an empirical sense. I meant to ask what the theory and the math behind it currently indicated indirectly about it. Hope that helps clarify my confusion but I do appreciate you trying to help.

PeterDonis said:
No, the "block universe" concept is much simpler. It says that all events exist. Every event in the entire 4-dimensional spacetime continuum exists. That's all it says.

The reasoning that is used to justify this claim is more complicated and not everyone agrees with it (at least one common argument is refuted in this Insights article). But the claim itself is very simple.
Thanks! That seems to be addressing what I was actually trying to ask. That's what wasn't clear to me from what I've read. I couldn't tell if they were saying all events actually exist across some whole 4 dimensional spacetime block or if past events only exist in the sense that they are in some distant observers future light cone. Much appreciated!

Edit: Sorry, 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? Meaning that I'm not just existing in the present but every event of my past is actually spread out across some kind of real 4D spacetime canvas that the universe is written across? Hope that makes sense.

Last edited:
oxbaker said:
I didn't meant to ask if there was an experiment that actually proved what I was asking in an empirical sense. I meant to ask what the theory and the math behind it currently indicated indirectly about it.
The theory predicts the results of experiments. Without specifiing an experiment the theory tells you nothing.

oxbaker said:
... do you know if it means the past events still exist in some real sense ...
Since "some real sense" can mean anything, the answer is "sure, whatever".

PhDeezNutz
oxbaker said:
I meant to ask what the theory and the math behind it currently indicated
The purpose of theory and math in physics is to explain experiments. Theory divorced from experiment is just philosophy and not science, since experiment is the heart of the scientific method.

oxbaker said:
no actual way to currently run a real experiment … the experiment I came up with was not currently possible … an experiment that cannot currently be performed
You keep on using the word “currently”. As I mentioned several times, this is neither a technological nor an economic limitation. This is a limitation from the laws of physics. The only “currently” involved is our current understanding understanding of the laws of physics. So rather than “no actual way to currently run a real experiment” it is more like “fundamentally no possibility to run such an experiment based on our current knowledge”.

Last edited:

• Special and General Relativity
Replies
32
Views
867
• Special and General Relativity
Replies
17
Views
749
• Special and General Relativity
Replies
29
Views
746
• Special and General Relativity
Replies
27
Views
2K
• Special and General Relativity
Replies
20
Views
989
• Special and General Relativity
Replies
35
Views
2K
• Special and General Relativity
Replies
16
Views
853
• Special and General Relativity
Replies
21
Views
1K
• Special and General Relativity
Replies
9
Views
2K
• Special and General Relativity
Replies
9
Views
415