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Thread: Photographing time

  1. #1 Photographing time 
    Forum Radioactive Isotope zinjanthropos's Avatar
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    This summer a buddy and I were talking about time and space. Really we're just a couple of armchair guys with lots on our minds but not knowing where to put it. Talk got around to calculating where a point in time actually is. I suggested that a photograph is a point in time but my friend said that a photograph is many points of time because of the depth dimension. I argued that regardless, the point in time in the photograph is the same for every square inch of it and depth had nothing to do with it. He was convinced that because all photons captured in the photo came from different times in the past that there was no way the point in time was the same for all. Who's right?

    Couple more questions.....First thing, is there a scientific term for a point in time? Instant and moment come to mind but how accurate or credible are they? Does the Uncertainty Principle mean a point in time cannot be precisely calculated, as in if I could measure the time coordinate would I know the space coordinate?


    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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    Moderator Moderator Markus Hanke's Avatar
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    A photograph is a representation of all those photons that hit a surface ( the photographic plate, or digital sensor ) at a given instant in time. It really does not matter when and where those photons originated, you are capturing them all at the same instant on the same surface. This is different from asking how "old" each individual photon is.

    First thing, is there a scientific term for a point in time?
    A hypersurface of simultaneity.

    Does the Uncertainty Principle mean a point in time cannot be precisely calculated
    No, the HUP means that you cannot measure two complementary variables simultaneously with arbitrarily high precision. In your case, the pair of observables would be time and energy, and it specifically refers to the lifetime of quantum states - the shorter the lifetime, the less well defined its energy, and the more accurate you want the energy to be, the longer the lifetime of the quantum state has to be.

    There is a caveat here though in that "time" is not actually an observable in quantum mechanics, in the same manner as position or momentum would be. That complicates the interpretation of this particular uncertainty relation a bit.

    as in if I could measure the time coordinate would I know the space coordinate?
    This would be a different issue - in order to do this you need a classical system that is completely deterministic, and the evolution of which is fully known to you.


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  4. #3  
    Forum Radioactive Isotope zinjanthropos's Avatar
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    A hypersurface of simultaneity
    As usual your answers are great. This hypersurface is a reference to some geometry I suppose, not a space time surface if there is such a thing?
    All that belongs to human understanding, in this deep ignorance and obscurity, is to be skeptical, or at least cautious; and not to admit of any hypothesis, whatsoever; much less, of any which is supported by no appearance of probability...Hume
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  5. #4  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    A hypersurface of simultaneity.
    Maybe that will replace the circumlocution "at the present moment" for "now" in business-speak.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  6. #5  
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    Quote Originally Posted by Markus Hanke View Post

    First thing, is there a scientific term for a point in time?
    A hypersurface of simultaneity.
    I am surprised that a surface can only contain one point (is a hypersurface not a normal surface? I think I get that it is 4D with one dimension negative )

    Will use "hypoface" as per Strange's advice.
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  7. #6  
    Moderator Moderator Markus Hanke's Avatar
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    A hypersurface of simultaneity is a cross-section of spacetime in which all events share the same t-value, in some given coordinate system. So essentially, you are looking at a cross-section of spacetime while holding time constant. This still leaves three spatial dimensions, so such a hypersurface is a 3-dimensional region of space.

    So yes, it is a (3D) surface in spacetime, and as such it contains many points.

    If you think of spacetime as a loaf of bread, then a hypersurface of simultaneity is the surface you get when you cut the loaf in half - it's a cross section at a specific point ( in time ).
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  8. #7  
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    Quote Originally Posted by Markus Hanke View Post
    A hypersurface of simultaneity is a cross-section of spacetime in which all events share the same t-value, in some given coordinate system. So essentially, you are looking at a cross-section of spacetime while holding time constant. This still leaves three spatial dimensions, so such a hypersurface is a 3-dimensional region of space.

    So yes, it is a (3D) surface in spacetime, and as such it contains many points.

    If you think of spacetime as a loaf of bread, then a hypersurface of simultaneity is the surface you get when you cut the loaf in half - it's a cross section at a specific point ( in time ).
    So at our own origin and setting t at 8 mins this surface includes all events simultaneous with the light we see coming from the Sun?
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    Moderator Moderator Markus Hanke's Avatar
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    So at our own origin and setting t at 8 mins this surface includes all events simultaneous with the light we see coming from the Sun?
    It would include all events that are simultaneous ( by your chosen convention ) with your clock at that instant. So effectively, it would be a snapshot of the universe at that moment, hence it’s a 3D construct - all of space “frozen” at a particular moment in time. What exactly this would actually look like is a pretty complex question however, because not all observers in the universe share the same notion of simultaneity. So the hypersurface of simultaneity is not an absolute thing, it explicitly depends on which clock you choose to construct it.

    In your initial example though things are straightforward, because you are not only constraining a moment in time, but also a small region of space ( the camera sensor ), so simultaneity issues don’t really arise - we can, to a very good approximation, say that all points on the camera sensor share the same notion of simultaneity. A photo is a cross section in time, but also a cross section in one spatial direction, so you end up with a 2D representation.
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  10. #9  
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    All "simultaneity" is observer dependent . I wonder if we can sell this idea to the Flat Earthers?
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  11. #10  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by geordief View Post
    All "simultaneity" is observer dependent . I wonder if we can sell this idea to the Flat Earthers?
    Only if they can find a way of twisting it to support their beliefs!

    Photographing time ... you might like this: Photographing Time - Artist Jay Mark Johnson Captures the 4th Dimension | Chase Jarvis Photography

    And more here: https://singularityhub.com/2012/10/2...pace-and-time/
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  12. #11  
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    Quote Originally Posted by Strange View Post
    Only if they can find a way of twisting it to support their beliefs!

    Photographing time ... you might like this: Photographing Time - Artist Jay Mark Johnson Captures the 4th Dimension | Chase Jarvis Photography

    And more here: https://singularityhub.com/2012/10/2...pace-and-time/
    Trying to get my head around that.
    I just found these on the BBC website.

    Your pictures: Camouflage - BBC News

    The 4 th and the 11 th photos in descending order look rather similar to them . Coincidence , I suppose.
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  13. #12  
    KJW
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    It should be pointed out that a photograph isn't a 3D hypersurface of simultaneity, but a past 3D lightcone.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  14. #13  
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    Quote Originally Posted by KJW View Post
    It should be pointed out that a photograph isn't a 3D hypersurface of simultaneity, but a past 3D lightcone.
    Sorry ,what does "past 3D" mean?
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  15. #14  
    KJW
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    Quote Originally Posted by geordief View Post
    Quote Originally Posted by KJW View Post
    It should be pointed out that a photograph isn't a 3D hypersurface of simultaneity, but a past 3D lightcone.
    Sorry ,what does "past 3D" mean?
    A lightcone is a spherical surface that is either outwardly expanding at the speed of light or inwardly contracting at the speed of light. A spherical surface is two-dimensional but sweeps a three-dimensional space as it expands or contracts. A lightcone that is outwardly expanding is a future lightcone because it is expanding toward the future, whereas a lightcone that is inwardly contracting is a past lightcone because it is contracting from the past (a spacetime diagram shows a future lightcone as being in the future, and a past lightcone as being in the past).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  16. #15  
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    A photograph is independent of time, which can only be measured in some way by observers.
    Like any other stuff of matter a photograph is made of atoms which are temporarily held in a transitory state.
    The more composite an object is the more temporary are the atoms in that position.
    So the photograph will fade and disappear with time even if artificially preserved.
    The person who took the image is also made up of atoms which are only held there until the person dies and the atoms recycled.
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  17. #16  
    Brassica oleracea Strange's Avatar
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    Well, that was cheerful.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  18. #17  
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    Quote Originally Posted by KJW View Post

    A lightcone is a spherical surface that is either outwardly expanding at the speed of light or inwardly contracting at the speed of light. A spherical surface is two-dimensional but sweeps a three-dimensional space as it expands or contracts. A lightcone that is outwardly expanding is a future lightcone because it is expanding toward the future, whereas a lightcone that is inwardly contracting is a past lightcone because it is contracting from the past (a spacetime diagram shows a future lightcone as being in the future, and a past lightcone as being in the past).
    Is it possible to see the scenario as the past being projected onto a screen which is at the origin? (the photo)
    Last edited by geordief; October 2nd, 2017 at 04:49 AM.
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