The Future of GR: Is Time Already Set?

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Discussion Overview

The discussion centers on the nature of time in General Relativity (GR) and whether all of time is already contained within spacetime (ST), exploring implications for different observers experiencing time differently. The conversation includes theoretical considerations and potential experimental tests, as well as philosophical implications.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that all of time is contained in spacetime and that this explains why different observers experience time at different rates.
  • Others question the necessity of this view, suggesting that different experiences of time can be understood without assuming all time is contained in ST.
  • A participant argues that the analogy of cars and odometers illustrates that different observers can have different experiences of time without implying a need for all events to be contained in ST.
  • Another participant raises the issue of how events can be perceived differently by observers at the same location, suggesting this supports the idea that all events are contained in ST.
  • Some participants discuss the relevance of experimental tests or thought experiments to validate claims about time in GR.
  • A later reply mentions that time behaves differently in GR compared to Special Relativity (SR), specifically in relation to gravitational effects on time passage.

Areas of Agreement / Disagreement

Participants express differing views on whether all time is contained in spacetime, with no consensus reached. Some agree with the notion while others challenge it, indicating a contested discussion.

Contextual Notes

Participants highlight the complexity of the relationship between time, observation, and spacetime, with unresolved questions about definitions and implications of time in GR versus SR.

daniel_i_l
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the "future" in GR

In GR, is all of time already contained in ST and we're just moving through it? this seems to be the only way to explain the fact that for different observers time passes at different speeds. is this correct? (this isn't a philisofic question)
 
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How would you test this experimentally? (If it isn't to be a philosophical question, there must be an experimental test, at the very least a thought experiment, one which should be possible in princple even if it is currently utterly impractical).
 
daniel_i_l said:
In GR, is all of time already contained in ST and we're just moving through it? this seems to be the only way to explain the fact that for different observers time passes at different speeds. is this correct? (this isn't a philisofic question)
I would say it is correct, but not everybody will agree. I would say that those who do not agree do not take ST literally or sufficiently seriously, i.e., they think that there is more about time than said by ST.
 
pervect said:
How would you test this experimentally? (If it isn't to be a philosophical question, there must be an experimental test, at the very least a thought experiment, one which should be possible in princple even if it is currently utterly impractical).
i'm not sure if experiment is that relevant as we're asking if all time is within GR, a model, not whether GR's view of time (whether all or not) models reality correctly.
 
daniel_i_l said:
In GR, is all of time already contained in ST and we're just moving through it? this seems to be the only way to explain the fact that for different observers time passes at different speeds. is this correct? (this isn't a philisofic question)
Regardless if "all of time is already contained", for which there is no shred of evidence, I disagree with your statement that that seems to be the only way to explain different passages of time for different observers.

There is nothing strange about different passages of time.

Think of cars, do all odometers show the same mileage? Of course not. Are we surprised by that? I think not. Some cars travel more and on longer paths than others.
In space-time wordlines cross both space and time. Some worldlines will be longer than others and basically the longer the worldline between two events the less time will be elapsed. So different observers show different clock values.
Ask yourself why you conclude it it is normal for distance but not for time.
 
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MeJennifer: When i talked about different passages of time i wasn't talking about the "speed" of a clock. rather i was talking about the fact that two people could look at the same place in space but see different events - in other words, they could both be looking at point X in space but one of them would see event A happening and the other event B (which according to the first observer happened after event A). doesn't this prove that all the events are "contained" in ST already?
and if all events aren't "contained", then let's say that the universe isn't expanding (because i don't think that it's relevant here) then how would something that wasn't "contained" before suddenly become "contained"? wouldn't that require some artificial "expansion of time"? and since ST itself is the same for all observers that this "rate of expansion" wouldn't even be defined? so how is it possible to say that not everything is contained?
 
Suggested reading

Hi, Daniel, I can't understand what you are trying to get at, but you might try the very readable book Space, Time, and Spacetime, by Lawrence Sklar, which offers quite a bit of discussion of relativity. (There are of course many other books on the philosophy of spacetime; I did see that you insist that your question is not philosophical but perhaps a philosophy book might help us help you to figure out what your question is?)
 
daniel_i_l said:
When i talked about different passages of time i wasn't talking about the "speed" of a clock.
Neither was I. :smile:

daniel_i_l said:
rather i was talking about the fact that two people could look at the same place in space but see different events - in other words, they could both be looking at point X in space but one of them would see event A happening and the other event B (which according to the first observer happened after event A). doesn't this prove that all the events are "contained" in ST already?
They could both try to look at say a particular atom but the lightpaths from that atom to both observers are obviously different. Relative speed, acceleration and curvature determine what an observer actually sees.

Think of the way we are getting information, say right now you are watching the news and get a live report on something in Pittsburgh PA while I am reading last year's Pittsburgh's Quarterly. Different paths of information both in space and in time about the same object.
 
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One result of GR that is not true of SR is that time will move more slowly for an observer in a large gravity well. (in fact, I believe it has been verified, through spectrum studies, that electron "oscillate" more slowly on the sun than on earth.) Does that qualify as what you mean by "time in GR" being different from time in "SR"?
 

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