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Is the PAST determined?

suncowiam

Well-Known Member
Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.

I would like to address the reverse question: Is the *past* determined?

In other words, is the past fixed once we pass it?

Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?

When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.

So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?

If I had a video of the event with a timestamp, would that count?

Law enforcement would say yes, but I know you're a physicist so * Shrug *.
 

Milton Platt

Well-Known Member
Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.

I would like to address the reverse question: Is the *past* determined?

In other words, is the past fixed once we pass it?

Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?

When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.

So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?

In theory, if we have enough information, we could extrapolate back to a past event.....we do it all the time. Since we know the distance and speed a planet travels around the sun, we could extrapolate it's position at any time in the past. We can and have extrapolated past solar and lunar eclipses.
In your examples, you are simply acknowledging a lack of information to do the extrapolation. There are simply too many variables along the way. Our lack of knowledge or ability to pinpoint virtually all past events does not prevent extrapolation of some events.
 

robocop (actually)

Well-Known Member
Premium Member
Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.

I would like to address the reverse question: Is the *past* determined?

In other words, is the past fixed once we pass it?

Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?

When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.

So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?
For me, the past exists only as a type of thought; hence it is not determined; it's not even existent.

But I picked up a Physics book to challenge my thinking.
 

Hockeycowboy

Witness for Jehovah
Premium Member
Many people like to ask whether the future is determined. This has relevance to issues like free will, for example.

I would like to address the reverse question: Is the *past* determined?

In other words, is the past fixed once we pass it?

Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?

When framed in this way, I have to say that it seems unlikely. For example, it seems quite unlikely that the weight of Casar's last drink is fixed from anything available in the universe today. It seems unlikely that the question 'was there a T-Rex standing in this spot 68 million years ago exactly' actually has an answer that is determined by the state of the universe now.

So, to what extent is the past determined? If it is NOT determined, how does that affect your views of the past? if it *is* determined, in what sense is it so?
Yep, I believe so. I certainly hope so!

If time travel to the past ever becomes possible, people’s very existence could be negated!
 

Hockeycowboy

Witness for Jehovah
Premium Member
Anyone ever see “Final Countdown”, w/ Kirk Douglas and Martin Sheen?

They couldn’t, so that settles it! Lol.

It was a good movie.
 

shmogie

Well-Known Member
Depends on where you travel back. If you travel fast away from Earth and our galaxy and from every light photon so that you're no longer interacting with the same photons, then you can travel to the past somewhere else, theoretically. It will be so far from Earth that you will never be able to interfere with your past here. Its not convenient in any way except that you could start over somewhere else with younger stars.
Just quoting Hawking and his view on time travel.
 

Polymath257

Think & Care
Staff member
Premium Member
In theory, if we have enough information, we could extrapolate back to a past event.....we do it all the time. Since we know the distance and speed a planet travels around the sun, we could extrapolate it's position at any time in the past. We can and have extrapolated past solar and lunar eclipses.
In your examples, you are simply acknowledging a lack of information to do the extrapolation. There are simply too many variables along the way. Our lack of knowledge or ability to pinpoint virtually all past events does not prevent extrapolation of some events.

Ahh...but how do you know all that information is preserved? In fact, don't we expect the information to gradually degrade, meaning more than one past becomes possible?

And I agree, there are a LOT of past events that *are* determined by the present. Anything from which the information hasn't decayed too much will be determined. But it seems quite unlikely that *all* past events are of that sort.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
Ahh...but how do you know all that information is preserved? In fact, don't we expect the information to gradually degrade, meaning more than one past becomes possible?

And I agree, there are a LOT of past events that *are* determined by the present. Anything from which the information hasn't decayed too much will be determined. But it seems quite unlikely that *all* past events are of that sort.
It also appears that as you go forward in time there are fewer possible pasts which can explain it. Example: there are only so many ways that our solar system could be arranged in a way that would allow life to thrive on Earth, so that excludes numerous possible pasts.
 

Polymath257

Think & Care
Staff member
Premium Member
Yep, I believe so. I certainly hope so!

If time travel to the past ever becomes possible, people’s very existence could be negated!


But I'm not even going as far as to allow travel into the past. I am simply saying that certain events in the past are underdetermined and so have no actual truth. Other events certainly *are* determined, at least to some extent.

We always assume there is a 'truth' about the past that is well-determined in some way. But what is that assumption is wrong? That for some events in the past, it is actually the case that they could have been either of a couple (or a few) different ways and still lead to the present that we see today?
 

Polymath257

Think & Care
Staff member
Premium Member
It also appears that as you go forward in time there are fewer possible pasts which can explain it.

I'm not of that. Again, if the information decays, then the number of events in the past consistent with the present would *increase* over time. As an example, I'm sure that quite a few different pasts for 68 million years ago would be consistent with the present. The information determining the past has been degraded to such an extent that saying there is only one past seems to be less clear.

Part of this, of course, is figuring out in what sense 'the past' exists. And, for that matter, to what extent something existed if there is no information that has survived showing its existence.
 

shunyadragon

shunyadragon
Premium Member
But I'm not even going as far as to allow travel into the past. I am simply saying that certain events in the past are underdetermined and so have no actual truth. Other events certainly *are* determined, at least to some extent.

We always assume there is a 'truth' about the past that is well-determined in some way. But what is that assumption is wrong? That for some events in the past, it is actually the case that they could have been either of a couple (or a few) different ways and still lead to the present that we see today?

I believe considering it a 'truth' issue goes too far.
 

Polymath257

Think & Care
Staff member
Premium Member
At relativistic speeds you and I could theoretically travel to the past and change the past of some far away galaxy though the journey would take thousands of years, however we could not travel to the Earth's past. This is not the same as saying our past is missing but the opposite. It evidences that our past is unchangeable by us.

Hmmm....I'm not sure where you get this from, but as stated it seems to be wrong. We are limited to travel in our future light cone. We would only be able to interact with that galaxy in *its* future light cone. So I see no sense in which we can go into the past of even a distant galaxy and change it.

There is another limit to changing the past since we have learned of Entropy. If we assume that the universe is burning out slowly then someday it will no longer be possible for the past to change. That implies an end to change which propagates backwards to our time, a final, settled condition for space-time.

Again, this is certainly not part of any standard cosmological dynamic. Do you have a reference for this?
 

cladking

Well-Known Member
Nothing is determined or determinable.

But still only one single past existed and only one single future will exist. In the future we will be able to determine things about the past that we can't at this time just as we'll get better and better at predicting the future. The difference is that the past is set and immutable but the future is subservient to free will.

The present is also an enigma but we don't really notice this because we live in our models and see what we expect.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
I'm not of that. Again, if the information decays, then the number of events in the past consistent with the present would *increase* over time. As an example, I'm sure that quite a few different pasts for 68 million years ago would be consistent with the present. The information determining the past has been degraded to such an extent that saying there is only one past seems to be less clear.

Part of this, of course, is figuring out in what sense 'the past' exists. And, for that matter, to what extent something existed if there is no information that has survived showing its existence.
What counts as or is an example of degrading of the information?
 

Polymath257

Think & Care
Staff member
Premium Member
What counts as or is an example of degrading of the information?

Well, for example, for Casar's last drink, the size of the cup, the amount of water in the pitcher, etc, would be information on how much he consumed. But the cup is probably long gone and any association with him irreproducible. The pitcher is also long gone and was probably reused so retained very little information about Ceasar's last drink, even before it was destroyed.

Or, we can mention someone's last words that were not recorded. The vibrations in the air went to equilibrium pretty quickly and and anyone with memories would have died, so if there isn't another source of information about those last words, they are simply not determined by the state of the universe now.

Or suppose that there were two people there when those last words were spoken, but one remembers one thing and the other a different thing. Is there any real sense in which one or the other is 'correct'?

Typically, all increases of entropy are equivalent to a decrease of information.
 

Road Less Traveled

Active Member
Ahh...but how do you know all that information is preserved? In fact, don't we expect the information to gradually degrade, meaning more than one past becomes possible?

And I agree, there are a LOT of past events that *are* determined by the present. Anything from which the information hasn't decayed too much will be determined. But it seems quite unlikely that *all* past events are of that sort.

Perhaps it is dependent on if all information is preserved, stored, carrying on in a form of energy/light that can’t be destroyed. And then if that information can separate and branch out. Like the complete set of information regarding something/a particular event being an entire puzzle, and if it can branch out into many many pieces.
Then if that information can be re-written over, slightly altered, etc. Routinely traveling through past, present, future in its whole/pure form or dispersed into many pieces.

I haven’t kept up on information models and what’s been tested, and/or theoretical in awhile. And as always, I’m sure that there is so much still unknown regarding information, what degrades if anything, how it’s preserved, where it’s preserved, what exactly is preserved, where it goes, how and if it can disperse into pieces through ‘time,’ how’s it subjected by ‘time’ if at all.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
Hmmm....I'm not sure where you get this from, but as stated it seems to be wrong. We are limited to travel in our future light cone. We would only be able to interact with that galaxy in *its* future light cone. So I see no sense in which we can go into the past of even a distant galaxy and change it.
It is from an explanation that treats spacetime like a loaf of bread.
youtube go to minute 4:25 Its based on something called the block time universe. Its attributed to Brian Greene, although I don't know where and if he publishes that. He does a talk about string theory (which I have not seen) on Ted talks.

Again, this is certainly not part of any standard cosmological dynamic. Do you have a reference for this?
Entropy is a thermodynamic relation in which order tends towards chaos and heat tends to dissipate into coldness. In some cases it can be quantified, but generally its used only as a relation. It is believed by most Astronomers that the universe is getting colder and that it will eventually reach maximum entropy at which time all light will be gone, and the temperature will be very low or possibly zero Kelvins. Thus most Astronomers believe there is an end to change in this universe, and if that is so then there is a lowest energy state at that end.

I don't have a reference for my comment, but I want to clarify . I wasn't considering the possibility of information degradation only the possibility of time travel causing changes in the past. If time travel was the only way of changing the past then the end of the universe would imply that all time travel eventually ended, that spacetime ceased changing; but that would contradict time travel. It would not be consistent with the ability to time travel, even with such far fetched means as traveling in that cone. A final, uniform, low entropy universe seems inconsistent with time travel.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
...they are simply not determined by the state of the universe now...
Or so you represent to me. They certainly are not visible to us today, but I'm not sure how you insist the information is gone when it may merely be unreachable.

Typically, all increases of entropy are equivalent to a decrease of information.
So this seems to beg the question, since its a thread about whether the past is preserved. I don't think it has to be preserved in movie format or on a stored drive to exist in the past apart from us. We can assume that the past disappears or not. What is the evidence that it is gone? Well we do have the fact that nobody remembers how much Caesar's cup holds; so we can't compare. That's an uncertainty.
 

Brickjectivity

Turned to Stone. Now I stretch daily.
Staff member
Premium Member
Hmmm....I'm not sure where you get this from, but as stated it seems to be wrong. We are limited to travel in our future light cone. We would only be able to interact with that galaxy in *its* future light cone. So I see no sense in which we can go into the past of even a distant galaxy and change it.
Oh..yes I am wrong about that. We can't.
 

Skwim

Veteran Member
I would like to address the reverse question: Is the *past* determined?
Yes.

In other words, is the past fixed once we pass it?
Yes.

Another interpretation of the problem, possibly a different spin: Given the state of the universe *now* (and I am flexible about what this means), is the entirety of the past determined? Can every event of the past be *theoretically* deduced from the information of the present?
Of course not because an event can be caused in more than one way, and without access to the past there's no way of determining which of the "more than one way" was responsible for the event.

.
 
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