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Thread: Time travel question

  1. #1 Time travel question 
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    Hello, I'm not good at physics at all but I just have a question that will probably seem silly to physicists.


    I've watched a lot of time travel movies, but the time travel movies, but the type of time travel seen a lot is 'do over' time travel. Where time just keeps resetting every day or something. The type of time travel is nicknamed the Time Loop and the most famous movie with it in it is Groundhog Day. I just wanted to know, are time loops possible, even if the chances are small? As they don't make paradoxes like other time travel movies and seem consistent. Or is there some laws of physics that make them impossible?

    Thank you for reading.


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    They are possible in general relativity. But, on the other hand, there is no reason to think they can actually exist.

    Possibly a future theory that combines quantum theory and GR will show that they are not possible. (Or, less plausibly, show us how to build a time machine!)


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    Speaking of time, does it expand as the distances between galaxies increase? Does FTL expansion affect time in some way?
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    The problem with the film "Groundhog Day" is that the lead character is (once he realises) consciously aware of the time loop, i.e his mind is working independent of the time frame he exists in. If time loops were possible then I doubt you would be able to be consciously aware of it anyhow. Which means time loops may well exist but its impossible to independently experience them.
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    Quote Originally Posted by zinjanthropos View Post
    Speaking of time, does it expand as the distances between galaxies increase? Does FTL expansion affect time in some way?
    What does it mean for time to "expand"? (But I think the answer is "no", anyway.)
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    This wouldn't actually be a loop. It can't be. If Bill Murray litreally went back to the same time and place, he would run into himself.

    This is parallel universes. Each time he goes back, he spawns a new universe that starts off identical to the old one, but events immediately diverge in independent directions.

    Presmably, everybody is acting with free will, even as they repeat the same actions in every universe. Murray though, having memory of the previous universe, acts with that knowledge, changing his actions.
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    Quote Originally Posted by DaveC426913 View Post
    This wouldn't actually be a loop. It can't be. If Bill Murray litreally went back to the same time and place, he would run into himself.

    This is parallel universes. Each time he goes back, he spawns a new universe that starts off identical to the old one, but events immediately diverge in independent directions.

    Presmably, everybody is acting with free will, even as they repeat the same actions in every universe. Murray though, having memory of the previous universe, acts with that knowledge, changing his actions.
    If it makes parallel universes, does that mean what Bill Murray went through is possible? Or is there still laws that don't allow it.
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    Quote Originally Posted by PhysicallyStupid View Post

    If it makes parallel universes, does that mean what Bill Murray went through is possible? Or is there still laws that don't allow it.
    Well, parallel universes do avoid that particular paradox.
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by PhysicallyStupid View Post

    If it makes parallel universes, does that mean what Bill Murray went through is possible? Or is there still laws that don't allow it.
    Well, parallel universes do avoid that particular paradox.
    Yeah but it doesn't explain what's causing it, like why is he teleporting into parallel universes? It's still time travel either way as he's going back in time. So, wouldn't it still violate certain laws of physics?
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    Quote Originally Posted by PhysicallyStupid View Post
    Yeah but it doesn't explain what's causing it, like why is he teleporting into parallel universes?
    Implicitly, it is some supernatural intelligence.

    It becomes apparent that it is happening to him for a reason. He will continue to re-experience that day until he improves himself.
    Thus, some thing is making such a judgement and eventually deems him worthy.

    Quote Originally Posted by PhysicallyStupid View Post
    It's still time travel either way as he's going back in time.
    It's not really time travel if a parallel universe is created anew each time.
    Which seems to be the case, since each new universe is not the one he left - the difference being that his original is not in it when he arrives.

    Quote Originally Posted by PhysicallyStupid View Post
    So, wouldn't it still violate certain laws of physics?
    There is no physical law that says time travel is not possible.

    On the other hand there are solutions to some of Einsteins equations that do permit time travel.
    They could, in theory, be exploited with fantastically-advanced technologies.
    Wormholes, toroidal black holes, and nigh-infinitely long cylinders, all make appearances in some of these designs.
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by PhysicallyStupid View Post
    Yeah but it doesn't explain what's causing it, like why is he teleporting into parallel universes?
    Implicitly, it is some supernatural intelligence.

    It becomes apparent that it is happening to him for a reason. He will continue to re-experience that day until he improves himself.
    Thus, some thing is making such a judgement and eventually deems him worthy.

    Quote Originally Posted by PhysicallyStupid View Post
    It's still time travel either way as he's going back in time.
    It's not really time travel if a parallel universe is created anew each time.
    Which seems to be the case, since each new universe is not the one he left - the difference being that his original is not in it when he arrives.

    Quote Originally Posted by PhysicallyStupid View Post
    So, wouldn't it still violate certain laws of physics?
    There is no physical law that says time travel is not possible.

    On the other hand there are solutions to some of Einsteins equations that do permit time travel.
    They could, in theory, be exploited with fantastically-advanced technologies.
    Wormholes, toroidal black holes, and nigh-infinitely long cylinders, all make appearances in some of these designs.
    Yeah, so it's basically out of the realm of science then, if it is supernatural. Still a cool plot though.

    Also, doesn't entropy mean time travel is impossible, that and causality.
    Last edited by PhysicallyStupid; February 11th, 2018 at 07:01 PM. Reason: xd
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    Oops
    Last edited by PhysicallyStupid; February 11th, 2018 at 07:02 PM.
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    Quote Originally Posted by PhysicallyStupid View Post
    Also, doesn't entropy mean time travel is impossible, that and causality.
    Entropy doesn't forbid it.
    You can reduce entropy locally and temporarily. (That's what life does.)
    But a reduction in entropy always increases entropy in the larger, longer context.
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by PhysicallyStupid View Post
    Also, doesn't entropy mean time travel is impossible, that and causality.
    Entropy doesn't forbid it.
    You can reduce entropy locally and temporarily. (That's what life does.)
    But a reduction in entropy always increases entropy in the larger, longer context.
    Ok. But we can agree that the stuff we were talking about (time loops) are impossible, right. They aren't part of science.
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    Quote Originally Posted by PhysicallyStupid View Post
    Ok. But we can agree that the stuff we were talking about (time loops) are impossible, right. They aren't part of science.
    Not?
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    Quote Originally Posted by PhysicallyStupid View Post
    Ok. But we can agree that the stuff we were talking about (time loops) are impossible, right. They aren't part of science.
    I do not agree. Theory supports time travel.

    It does not support Groundhog Day time travel, no.
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  19. #18  
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    My biggest problem with time travel is what if everyone who travels in time (could be millions) by some device wanted to attend the same event? How would a million people fit on the Titanic (unless they brought their floatation gear to brave the North Atlantic)? That bucket of bolts might sink before it reaches the iceberg or slow down so much that it never hits it? How many people would it take to bring down The Spirit of St Louis if tt's decided to ride along with Lindberg? If tt is possible than there's no doubt in my mind that history changes, there are parallel universes or there's something remarkably weird with the universe we're in.
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    Quote Originally Posted by zinjanthropos View Post
    My biggest problem with time travel is what if everyone who travels in time (could be millions) by some device wanted to attend the same event?
    Ah that’s easy: Laws and restrictions on time-travel predate the birth of time-travel technology (much like Asimov’s laws of Robotics before the “singularity”). Boy it must be stressful being a time-travel controller! I imagine they have a lot of paid leave.
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    Quote Originally Posted by Vexspits View Post
    I imagine they have a lot of paid leave.
    How would you even know they went for a coffee break? Maybe returning to their desk looking 10 years older might be a clue.
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    Quote Originally Posted by zinjanthropos View Post
    My biggest problem with time travel is what if everyone who travels in time (could be millions) by some device wanted to attend the same event? How would a million people fit on the Titanic (unless they brought their floatation gear to brave the North Atlantic)?
    Don't envision it as some human-sized door you can step through and close behind yourself. That's TV stuff.

    The examples I listed above (an infinitely long cylinder rotating at near the speed of light, a toroidal black hole, etc.) are (not to put too fine a point on it) rather larger than a cruise ship.

    They require (and thus emit) truly colossal amounts of energy. These things have so much energy, they twist spacetime itself though 90 degree turns. That tends to have some ... collateral influence.

    One should not expect a toroidal black hole to appear in the ship's ballroom without atomizing the ship, the planet, and - indeed - a large chunk of the solar system.
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by zinjanthropos View Post
    My biggest problem with time travel is what if everyone who travels in time (could be millions) by some device wanted to attend the same event? How would a million people fit on the Titanic (unless they brought their floatation gear to brave the North Atlantic)?
    Don't envision it as some human-sized door you can step through and close behind yourself. That's TV stuff.

    The examples I listed above (an infinitely long cylinder rotating at near the speed of light, a toroidal black hole, etc.) are (not to put too fine a point on it) rather larger than a cruise ship.

    They require (and thus emit) truly colossal amounts of energy. These things have so much energy, they twist spacetime itself though 90 degree turns. That tends to have some ... collateral influence.

    One should not expect a toroidal black hole to appear in the ship's ballroom without atomizing the ship, the planet, and - indeed - a large chunk of the solar system.
    Well then there's really no point discussing tt on a human scale. Apologies for going off course.
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    Quote Originally Posted by zinjanthropos View Post
    Well then there's reallysci no point discussing tt on a human scale. Apologies for going off course.
    No, that's cool. I was just injecting some hard science into the discussion for balance.

    After all, it was originally about a movie.
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by zinjanthropos View Post
    Well then there's reallysci no point discussing tt on a human scale. Apologies for going off course.
    No, that's cool. I was just injecting some hard science into the discussion for balance.

    After all, it was originally about a movie.
    Alas, I've never watched Groundhog Day in its entirety. Edge of Tomorrow with Tom Cruise was a time loop movie except his character had to be killed in order to go back and start over again, and again, and again..... Might have been a reincarnation (or multiple universe?) movie too because he always maintained the memory of his past repetitive lives..
    Last edited by zinjanthropos; February 17th, 2018 at 09:30 AM.
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    Quote Originally Posted by zinjanthropos View Post
    Edge of Tomorrow with Tom Cruise
    I've never even heard of that, let alone seen it.
    Was it not popular?
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    Quote Originally Posted by DaveC426913 View Post
    Quote Originally Posted by zinjanthropos View Post
    Edge of Tomorrow with Tom Cruise
    I've never even heard of that, let alone seen it.
    Was it not popular?
    Not sure if it was popular/box office success, but I saw it at the local Cinema. Not too long ago it was repeatedly shown on the Space Channel here.

    From Wiki: Edge of Tomorrow initiated its theatrical run in several territories on May 28, 2014, and rolled out to a total of 28 territories for its opening weekend of May 30 – June 1, 2014.[55][60] It grossed $20.1 million on its opening weekend.[61] For the second weekend of June 6–8, 2014, it was released in 36 additional territories.[62]Edge of Tomorrow grossed $100.2 million in North America and $270.3 million in other territories for a worldwide total of $370.5 million.[3] After the film's theatrical run, Entertainment Weekly said it had a "lukewarm box-office reception" despite praise from critics.[63]
    Last edited by zinjanthropos; February 17th, 2018 at 11:50 AM.
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    Quote Originally Posted by zinjanthropos View Post

    Not sure if it was popular/box office success, but I saw it at the local Cinema. Not too long ago it was repeatedly shown on the Space Channel here.
    Looked it up on imdb. Vague recollections of it.
    Also vague recollections of not getting interested in it because it looked like time travel was just being used as a vehicle to make two straight hours of combat into a plot.
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    OK, just for fun permit me more questions on objects moving through time & space. Here's a few things I've wondered about (space) time itself. Can I go anywhere in this universe and experience what I call real, true or undistorted space time? Since I am composed of particles with rest mass then my presence anywhere would actually distort space time, no? Even if only in a nearly undetectable way, if space time is distorted by my presence in the universe then might that affect tt for a life form such as ours? Same goes for a mechanical device travelling through time. Could we ever be aware of undistorted space time? Would we require undistorted time or the knowledge of it to even attempt tt?
    Last edited by zinjanthropos; February 18th, 2018 at 01:05 PM. Reason: there was a 'not' in the wrong place
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    Quote Originally Posted by zinjanthropos View Post
    OK, just for fun permit me more questions on objects moving through time & space. Here's a few things I've wondered about (space) time itself. Can I go anywhere in this universe and experience what I call real, true or undistorted space time? Since I am composed of particles with rest mass then my presence anywhere would actually distort space time, no? Even if only in a nearly undetectable way, if space time is distorted by my presence in the universe then might that not affect tt for a life form such as ours? Same goes for a mechanical device travelling through time. Could we ever be aware of undistorted space time? Would we require undistorted time or the knowledge of it to even attempt tt?
    That is how I see it.Distortion is the norm and Euclidean geometry is an idealization but one that is essential to understanding ** and measuring how curvature varies fr one location to another

    Despite curvature I think all the mathematics in GR is actually Euclidean that is "shoehorned" into service.

    Very much subject to correction as usual

    ** which of course I haven't
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    Quote Originally Posted by zinjanthropos View Post
    OK, just for fun permit me more questions on objects moving through time & space. Here's a few things I've wondered about (space) time itself. Can I go anywhere in this universe and experience what I call real, true or undistorted space time? Since I am composed of particles with rest mass then my presence anywhere would actually distort space time, no? Even if only in a nearly undetectable way, if space time is distorted by my presence in the universe then might that not affect tt for a life form such as ours? Same goes for a mechanical device travelling through time. Could we ever be aware of undistorted space time? Would we require undistorted time or the knowledge of it to even attempt tt?
    That is how I see it.Distortion is the norm and Euclidean geometry is an idealization but one that is essential to understanding ** and measuring how curvature varies fr one location to another

    Despite curvature I think all the mathematics in GR is actually Euclidean that is "shoehorned" into service.

    Very much subject to correction as usual

    ** which of course I haven't
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    Questions I was afraid to ask in the recently trashed rubber sheet analogy thread.... What of space time in a 2D universe, would it be distorted by a 2D object's mass (is 2D mass even a possibility?) or is that impossible and thus 2D space time is undistorted despite the presence of 2D objects? Is a 3rd dimension the minimal requirement for mass to produce a space time distortion?

    My feeling is that before we go wandering through time that perhaps we'll need to know a hell of a lot more about it than we do now.
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    What are your 2 dimensions? 1 Time and 1 Space or do you just mean 2 spatial dimensions (+1 of Time)?
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    Quote Originally Posted by geordief View Post
    What are your 2 dimensions? 1 Time and 1 Space or do you just mean 2 spatial dimensions (+1 of Time)?
    2 + 1
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    Are you asking whether objects can exist in only 2 spatial dimensions?

    Have you an example of them doing so?

    I have heard of atoms sandwiched in one atom deep layers of material with their freedom of movement so restricted and they were described as effectively 2 dimensional.

    But that did not convince me.
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    Quote Originally Posted by geordief View Post
    Are you asking whether objects can exist in only 2 spatial dimensions?

    Have you an example of them doing so?

    I have heard of atoms sandwiched in one atom deep layers of material with their freedom of movement so restricted and they were described as effectively 2 dimensional.

    But that did not convince me.
    I read this on Reddit:
    However, things that matter is made out of (electrons, quarks, etc) are, as far as we can tell, point particles. These point particles are 0-dimensional objects, as they don't have a size, and they do have mass. So theoretically, you could construct a technically 2-dimensional object out of points, and it would have mass.
    Don't know if science agrees with this. I'm thinking that if 2D objects with mass exist then they must be on a 2D plane where things don't go over or under one another, just go around. I don't see how 2D mass objects could warp time let alone the space they exist in if there's only one plane for the universe. An extra dimension would seem to be necessary to me if space time is to be affected by mass. Remember I'm just a layperson trying to understand so my questions are not worded correctly most times.
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    That is beyond me.I have also heard (and wondered) about "point particles"

    Perhaps dimensions are different in QM?

    If quantum objects move differently then our macro spatial dimensions might not apply.
    Help
    Last edited by geordief; February 18th, 2018 at 09:28 PM.
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    Can I go anywhere in this universe and experience [...] undistorted space time?
    No, not if by „undistorted“ you mean flat, Minkowski spacetime. You can only go to regions very far away from sources of gravity (or restrict your attention to very small, local patches), where spacetime is approximately flat.

    Since I am composed of particles with rest mass then my presence anywhere would actually distort space time, no?
    Yes, that too.

    Despite curvature I think all the mathematics in GR is actually Euclidean that is "shoehorned" into service.
    What makes you think that?

    What of space time in a 2D universe, would it be distorted by a 2D object's mass (is 2D mass even a possibility?)
    General Relativity is independent of the number of dimensions, so you can apply the field equations to a 2D universe too. The result is fairly trivial though, because in 2D the Riemann tensor (the object describing curvature) has only one functionally independent component, so everything reduces down to just a single equation. Essentially you get a surface that is completely described by just a scalar curvature. Higher order effects such as gravitational radiation etc do not exist here. In fact, gravity as we observe it needs at least four dimensions to work the way it does.
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    Quote Originally Posted by Markus Hanke View Post

    Despite curvature I think all the mathematics in GR is actually Euclidean that is "shoehorned" into service.
    What makes you think that?
    https://www.youtube.com/watch?v=UfThVvBWZxM

    I have been told that this is a good ,video and in I see that normal 180 degrees and "normal" operations are employed at the infinitesimal scale (eg at 17.59 minutes)

    It feels like the tangent is "manually"reset as one moves along the curve ("shoehorning" seemed like an analogy ).

    I was surprised to find plain flat geometry being used at the most local level (even though I had of course been told this before)

    Seeing a geometric animation seemed to bring out that aspect for me

    Maybe I have badly misunderstood or ,more likely badly expressed myself.

    Seems like a good vid to me. I have been told elsewhere to stick with it. (I haven't understood any of the intricacies of it yet ,just perhaps the overview)

    Rereading my post should/could I change "all the mathematics in GR " to some of the fundamental mathematics"?
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    [QUOTE=Markus Hanke;613362]
    Can I go anywhere in this universe and experience [...] undistorted space time?
    No, not if by „undistorted“ you mean flat, Minkowski spacetime. You can only go to regions very far away from sources of gravity (or restrict your attention to very small, local patches), where spacetime is approximately flat.

    Since I am composed of particles with rest mass then my presence anywhere would actually distort space time, no?
    Yes, that too.

    Thanks Markus. MY mind is shorting out on this subject so please bear with me and my unprofessional approach....I going to assume that because mass is present in the cosmos that there isn't one part of the universe that doesn't experience time distortion(flattening if you will), even if barely measurable. Does that mean the present, the moment or the instant is not a true time experience but some sort of stretched version of it? If so then wouldn't all observers including mechanical devices experience time differently from one another? What of the universe itself, would it not also experience only a distorted time? If so then it leads me to think of whether or not an undistorted time has ever existed at all but if it has then I'm thinking it had to be before the universe came into being or at least prior to the introduction of mass or an extra dimension suddenly unfolded.

    I'm only farting around with this because if I hopped on a Time Machine tomorrow and if it was distorting time so that it doesn't match mine then how in hell could I calibrate it? One nanosecond difference might see me hurled to the other side of the universe. Once I entered such a machine then my own mass would distort time even more, I think. Is there anyway to make an accurate measurement of undistorted time by accounting for the distortion?
    Last edited by zinjanthropos; February 19th, 2018 at 09:43 AM.
    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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    I was surprised to find plain flat geometry being used at the most local level
    Yes, locally - i.e. in a small enough region - all curvature is negligible. This is why, when you look out the window at your garden, you don’t notice the curvature of the Earth. The patch is small and local.
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    Moderator Moderator Markus Hanke's Avatar
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    If so then wouldn't all observers including mechanical devices experience time differently from one another?
    No, all observers will experience time the same way within their own frames of reference, but if you were to compare the clocks used by different observers after a while, then yes, you might see discrepancies. Remember that time dilation is a relationship between observers, it’s not something that “happens” locally.

    Curvature to some degree or another is present everywhere in the universe, but in some regions it might be so small as to be negligible.
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  43. #42  
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    Quote Originally Posted by Markus Hanke View Post
    I was surprised to find plain flat geometry being used at the most local level
    Yes, locally - i.e. in a small enough region - all curvature is negligible. This is why, when you look out the window at your garden, you don’t notice the curvature of the Earth. The patch is small and local.
    Doesn't seem to apply to your garden . I can plainly see the curvature in them there hills

    Or is that not a spade you have there ?A sleán ?
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