david345 Posted May 10, 2015 Posted May 10, 2015 post-107966-0-49029200-1431232059.jpg Is the "space-like hypersurface" the same thing as what has been called a "slice" in this thread? Yes as far as I can tell.
michel123456 Posted May 10, 2015 Posted May 10, 2015 Well, what I say is that we cannot get info from those surfaces, they are not observable. What is observable is what lies on the surface of our past light cone. All the rest is extrapolation. And also that these surfaces, that ressemble some sheet of papers, are NOT structured. There is no force, nothing that can physically directly relate the elements of the sheet of paper. All events that lie on this slice can not have au causal effect on each other. Each event of a slice is related to another event on another slice through the diagonal that is imposed by the constancy of C.
david345 Posted May 10, 2015 Posted May 10, 2015 post-107966-0-49029200-1431232059-b.jpg Well, what I say is that we cannot get info from those surfaces, they are not observable. What is observable is what lies on the surface of our past light cone. All the rest is extrapolation. They are not instantaneously observable. It takes time for the light to reach us.
michel123456 Posted May 10, 2015 Posted May 10, 2015 They are not instantaneously observable. It takes time for the light to reach us. At cosmological scales, you should wait billion of years and gather the sum of info from these billion of years in order to get an image of these slices. What we actually have at disposal are snapshots of intersections. Not to say that are struggling to know where the intersections really are. A misinterpretation of distance will place the event on the wrong slice...
david345 Posted May 10, 2015 Posted May 10, 2015 post-107966-0-49029200-1431232059-b.jpg Well, what I say is that we cannot get info from those surfaces, they are not observable. What is observable is what lies on the surface of our past light cone. All the rest is extrapolation. And also that these surfaces, that ressemble some sheet of papers, are NOT structured. There is no force, nothing that can physically directly relate the elements of the sheet of paper. All events that lie on this slice can not have au causal effect on each other. Each event of a slice is related to another event on another slice through the diagonal that is imposed by the constancy of C. In response to your modified post. You seem to claim that something is only real if it can have an instantaneous (faster then light) effect on something else. Some of us have have our own definitions of reality...
michel123456 Posted May 10, 2015 Posted May 10, 2015 (edited) In response to your modified post. You seem to claim that something is only real if it can have an instantaneous (faster then light) effect on something else. Some of us have have our own definitions of reality... No no, I claim exactly the contrary. The slice is unphysical because exactly as you wrote: nothing can have an instaneous effect. Nothing can go faster than light. Edited May 10, 2015 by michel123456
david345 Posted May 10, 2015 Posted May 10, 2015 At cosmological scales, you should wait billion of years and gather the sum of info from these billion of years in order to get an image of these slices. What we actually have at disposal are snapshots of intersections.Not to say that are struggling to know where the intersections really are. A misinterpretation of distance will place the event on the wrong slice...You're argument is nothing more then the statement " A human error can occur." Please provide something better. No no, I claim exactly the contrary. The slice is unphysical because exactly as you wrote: nothing can have an instaneous effect. Nothing can go faster than light. So you are claiming everything is unphysical because nothing can go faster then light.
michel123456 Posted May 10, 2015 Posted May 10, 2015 It is not a matter of human error. It is matter of physical impossibilty to access instantaneously the infos coming from the universe. The info we gather is delayed, and is only a tiny part of the whole thing. What we may call "Reality", or "what we observe now", is not an image of the slice, it is an image of the surface of our past light cone.
david345 Posted May 10, 2015 Posted May 10, 2015 It is not a matter of human error. It is matter of physical impossibilty to access instantaneously the infos coming from the universe. The info we gather is delayed, and is only a tiny part of the whole thing. What we may call "Reality", or "what we observe now", is not an image of the slice, it is an image of the surface of our past light cone. What causes the past light cone? A past slice. Why am I even responding? Your comment came from a past slice. Obviously I did not receive you're comment instantaneously. Your comment must be unphysical (according to you).
michel123456 Posted May 10, 2015 Posted May 10, 2015 What causes the past light cone? A past slice. Why am I even responding? Your comment came from a past slice. Obviously I did not receive you're comment instantaneously.Yes, that is correct Your comment must be unphysical (according to you).No, that is not what I stated. So you are claiming everything is unphysical because nothing can go faster then light. We are cross posting our comments. No I am not claiming that "everything is unphysical because nothing can go faster then light". I am claiming that the slice on the diagram is unphysical.
david345 Posted May 10, 2015 Posted May 10, 2015 Yes, that is correctNo, that is not what I stated.Yes that is what you stated. You claim it isn't what you stated. I can also make a statement and it's negation: "I said A" "I did not say A" You said: "No no, I claim exactly the contrary. The slice is unphysical because exactly as you wrote: nothing can have an instaneous effect. Nothing can go faster than light." Yes, that is correctNo, that is not what I stated.We are cross posting our comments.No I am not claiming that "everything is unphysical because nothing can go faster then light".I am claiming that the slice on the diagram is unphysical.Was your comment faster then light or was your comment "above the slice"?
michel123456 Posted May 10, 2015 Posted May 10, 2015 In this diagram, The "common moment of NOW throughout the Universe" for observer A is represented by the slice t3. I claim that 1. what observer A observes at time t3 is not the slice 2. the slice is made up of events that are not directly related by cause & effect. For example, gravity does not act directly between the objects that lie upon the slice.
david345 Posted May 10, 2015 Posted May 10, 2015 T3 can be observed at a later time. We just can not observe t3 at t3. You previously said: "Well, what I say is that we cannot get info from those surfaces, they are not observable. What is observable is what lies on the surface of our past light cone. All the rest is extrapolation." Your picture clearly shows A getting info from t1. Your picture clearly demonstrates that the slices are observable.
Strange Posted May 10, 2015 Posted May 10, 2015 I see the thread has moved on a long way but ... That is an interpretation that we get from of a 2D graph on a sheet of paper. No it isn't. I am not the only one who has ever argued that time does not flow but that all objects "move" in time. IOW that Time (exactly as Space) is a receptacle, a container. It is not something that makes us stretch. And we are not 4D objects that extend in a 4D space, we are 3D objects that "move" into a 4D substract. Your personal philosophy (baseless opinion) on the nature of time has nothing to do with the subject of this thread, or how time is described in GR. In the same way that we have extent in the 3 spatial dimensions, we also have extent in the time dimension. It is meaningless (in the context of GR) to talk about either "movement through time" or "time flowing".
michel123456 Posted May 11, 2015 Posted May 11, 2015 Your personal philosophy (baseless opinion) on the nature of time has nothing to do with the subject of this thread, or how time is described in GR. In the same way that we have extent in the 3 spatial dimensions, we also have extent in the time dimension. It is meaningless (in the context of GR) to talk about either "movement through time" or "time flowing". We should begin a new thread about "are we 3D or 4D?"
Spyman Posted May 11, 2015 Posted May 11, 2015 For example: _we suppose that there is a galaxy "NOW" 1 billion LY away from us. We cannot see this galaxy NOW, so it is a supposition. In this galaxy, we suppose there exist an earthlike planet with an alien looking at us (that we cannot observe because it is NOW. We suppose that this alien NOW is observing the Earth as it was a billion years ago. That is also a supposition. All suppositions. There is no physical way to communicate with this earthlike planet, because it is NOW. That makes a lot of suppositions. Because if we are indeed "moving" through time, then this alien, if existing, is not looking at the Earth a billion years ago. He is looking at something else. That we have no clue about. Assuming that we are moving through time and all the suppositions in your example is true, can you explain why this alien are not looking towards where the Earth was one billion years ago and don't see Earth as it was then? Why would we not have a clue about the alien view?
Delta1212 Posted May 11, 2015 Posted May 11, 2015 Assuming that we are moving through time and all the suppositions in your example is true, can you explain why this alien are not looking towards where the Earth was one billion years ago and don't see Earth as it was then? Why would we not have a clue about the alien view? I think michel is saying that the alien isn't looking at a billion-year-old Earth because Earth currently occupies "now" and something else is now located at the place and time a billion years ago that billion-year-old Earth occupied. Which, just...
michel123456 Posted May 11, 2015 Posted May 11, 2015 (edited) is completely crazy. We and the observable universe are the black dots. Click the image if nothing happens, it's a .gif. We are translating into time from bottom up. Edited May 11, 2015 by michel123456
swansont Posted May 11, 2015 Posted May 11, 2015 We should begin a new thread about "are we 3D or 4D?" ! Moderator Note Go right ahead. But please stay on topic in this thread; your personal philosophy on the nature of time does not fall into that category.
Spyman Posted May 12, 2015 Posted May 12, 2015 I think michel is saying that the alien isn't looking at a billion-year-old Earth because Earth currently occupies "now" and something else is now located at the place and time a billion years ago that billion-year-old Earth occupied.But that doesn't make sense, because Michel should now that we have observational evidence from astronauts at the Moon, looking towards the 1.26 light-seconds distant Earth and seeing the Earth as it was 1.26 seconds in their past: Earthrise: Taken by Apollo 8 crewmember Bill Anders on December 24, 1968, at mission time 075:49:07 (16:40 UTC), while in orbit around the Moon, showing the Earth rising for the third time above the lunar horizon. http://en.wikipedia.org/wiki/Earthrise Earth and Moon, showing their sizes and distance to scale. The yellow bar represents a pulse of light traveling from Earth to Moon (approx. 400,000 km or 250,000 mi) in 1.26 seconds. http://en.wikipedia.org/wiki/Moon#Relationship_to_Earth
MigL Posted May 12, 2015 Posted May 12, 2015 Haven't read the whole thread, just skimmed through it, but I believe its been mentioned several times ( by AJB or xyzt ) that 'now' is only defined ( extremely ) locally and is undefined globally. Even over relatively short distances, like Earth to moon, there is a problem with defining 'now'. In GR past present and future are 'fixed', and it is the 'window' of the present or the 'now' that translates forward along the time dimension. That extremely thin window is the only part of the time dimension that we can interact fully with.
md65536 Posted May 12, 2015 Posted May 12, 2015 Haven't read the whole thread, just skimmed through it, but I believe its been mentioned several times ( by AJB or xyzt ) that 'now' is only defined ( extremely ) locally and is undefined globally. Even over relatively short distances, like Earth to moon, there is a problem with defining 'now'. In GR past present and future are 'fixed', and it is the 'window' of the present or the 'now' that translates forward along the time dimension. That extremely thin window is the only part of the time dimension that we can interact fully with. Restating what's been said, so it's clear there are no contradictions: 1. The answer is "no", there is not a common moment throughout the universe that could be considered "now" by all observers who experience it. 2. Our locally defined 'now' could not be used as a global now, because for one thing it is not even a definite moment in some locations. 3. We could arbitrarily define a moment using a Cauchy surface, and that would globally (across the surface) be experienced as a moment and consistently separate a past and future everywhere, but it would not meaningfully represent 'now'. The answer is still no, but if you wanted to invent your own meaningless coordinate system it's possible to define 'now' consistently everywhere, just like you could choose a center of the universe or a middle of the surface of the Earth... meaningless outside of your invented coordinate system.
david345 Posted May 13, 2015 Posted May 13, 2015 (edited) In the RW metric there is a global time (cosmic time) that is the same for all fundamental observers. The fundamental observers can synchronize their clocks to a predetermined time when the universe reaches a certain density. The big bang singularity is often used as the start of cosmic time. If two fundamental observers are using different units of time then they can use time since the big bang to convert one unit to another. Time since the big bang will be the same for all fundamental observers in the universe. It is believed that FLRW closely approximates our universe. There are questions about how well it handles the "lumpiness" of our universe. Edited May 13, 2015 by david345
md65536 Posted May 13, 2015 Posted May 13, 2015 In the RW metric there is a global time (cosmic time) that is the same for all fundamental observers. The fundamental observers can synchronize their clocks to a predetermined time when the universe reaches a certain density. The big bang singularity is often used as the start of cosmic time. Time since the big bang will be the same for all fundamental observers in the universe. It is believed that FLRW closely approximates our universe. There are questions about how well it handles the "lumpiness" of our universe. So that would be... time according to a set of eternally inertial physical clocks that are infinitely far away from mass (or far enough that acceleration is negligible, since the fundamental observers are never accelerated), or time according to a nearby (generally passing by) eternally inertial clock that ticks as if there were no mass in the universe (essentially an abstract clock)? Would those be the fastest possible clocks? Any clock that is accelerated or in a gravity well would tick relatively slower? Very deep in a gravity well, you may have "cosmic time" ticking by very fast. I suspect it might not even be defined, with extreme-enough "lumpiness". If that's right, the "cosmic time" could not be represented with a Cauchy surface; the latter would follow around the curvature of "lumps" while the former would cut right through them. This is another notion of a common "now" in the universe. This would be no different than choosing any other clock and calling it a universal clock and counting all of time in the universe according to the chosen clock while ignoring local clocks, except that by choosing an inertial clock away from matter, you can have "identical" clocks all over, all marking the same time.
Alan McDougall Posted May 13, 2015 Posted May 13, 2015 Time "Moves" at different rates, even from a persons head to that of his feet below?
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