Is There A Universal Now?

Discussion in 'Physics & Math' started by Cyperium, Jun 14, 2022.

  1. Cyperium I'm always me Valued Senior Member

    I was watching Kyle Hill's video of The Andromeda Paradox on YouTube, I had been thinking about the present time and if there was a universal instant for some time before that and I feel that he didn't really answer the question, because he said that "so what is now? It depends on who you ask." in reference to the implications of instantaneous events in relativity.

    That we can disagree on which events happens in Andromeda at which time doesn't seem to correlate to the experience of now. These events happened millions of years ago, so the actual now for them wouldn't be affected by how I move at earth, just as the now for me is exactly now completely irrespective of how someone on Andromeda moves relative to me.

    The intuitive answer would be that the present time for me, would also be the present time for Andromeda, but that poses problems as well, it would mean that the entire Universe is updated all at once. Maybe not that problematic, but it would seem (to my feeble mind) that such a feat would require infinite effort.

    It does seem to be the case though because:
    What if instead the Universe is updated at the speed of light. That the now someone on Andromeda is experiencing is actually the time that light from Andromeda reaches us?

    That would mean that if you had a gigantic telescope pointed to Andromeda and see a "person" there, you would actually be experiencing the same now, synchronized by the speed of light. The problem with that is obviously that the "person" would be millions of years in the past and events have definitely happened there up until the present time (so was he not aware of them then?), but also that if he would take a telescope 2,5 millions of years ago, he would see earth as it was 2,5 millions of years from that which would be (to us) 5 million years ago, and that was definitely not the present time for us 2,5 millions of years ago.

    Ergo, the universe updates universally all at the same time and there is a universal now irrespective of how we move relative to eachother.

    You could say that, if I take off at near the speed of light, time moves slower for me and thus faster for someone on earth relative to me. Sure, but it is the same *now* that is moving along slower for you, and it is the same *now* that is moving faster for them, no one notices this of course, but we are still bound by the same present *now*.
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  3. Ssssssss Registered Senior Member

    No there isn't a universal now. Before relativity you can divide everything up into the past where events can affect you now and the future where there are events you can effect and there's a dividing line called now and we don't ask too many questions about whether events happening now can affect us now and vice versa or only once they've moved into the past. Relativity instead divides everything into your personal causal past (not the same as my causal past) and your causal future (not the same as my causal future) and a whole bunch of events that have left your causal future so you can't affect them but haven't entered your causal past so they can't affect you until later. The Andromeda Paradox isn't a paradox unless you try to pretend that relativity has the same causal structure as pre-relativistic physics. Then you're fooling yourself and the resolution is to accept that what you mean by "now" anywhere distant from you is a choice so whether something is happening now in Andromeda or not is just a choice and not something that physics defines for you.

    Yeah you can say that what's happening in Andromeda now is what you see but there's a shitpot of complications if you do. For one you've defined light speed to be infinite in the direction from Andromeda to here which means that it has to be \(c/2\) in the other direction to preserve the two way speed of light being \(c\) which we know it always is so looking at stuff in the other direction you are seeing even further into the past. And it means that if you look at Andromeda you see "now" but the person looking back at you sees you as you were 2.5 million years ago which he calls 5 million years in the past because the time he calls now is 2.5 million years in your future. Are you having fun yet? The next thing you screw up is that you've picked a completely arbitrary direction and decided to make it important to your way of doing physics which means you always have to be able to keep track of the direction to Andromeda whenever you're measuring anything because you will need to adjust clock synchronisations depending on that direction so you get consistent speed measurements. And the maths gets nasty unless you do everything in full blown covariant tensor formulations. All pointless complexity 99% of the time. You are usually best to choose to define "now" so that the one way speed of light is the same in all directions for you because it makes the maths much easier and there are no arbitrary directions that everyone has to agree to do physics. But that means that your mate who is walking past you will make a different choice which does make a difference to interpretation of events (not to the events but only to how you interpret them) at very long distances.
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  5. Seattle Valued Senior Member

    My response would be that the question doesn't make sense in terms of relativity. Relativity refers to spacetime and not just time (or space). "Now" is just the present in your local frame of reference.

    It's not about a "universal update" or about what you could see with a "giant telescope".

    You remember the time dilation thought experiment using the clock, moving train with observers both on the train and on the train platform. There is no common "now" in that scenario. Time is relative to the local reference frame.

    It's the same problem with explaining where you are located in space. You need a reference point as there is no natural reference point, up, down, etc. has no meaning.

    Relativity means just that, everything other than the speed of light is relative. Everyone in their local reference frame understands what "now" or the present means but if you try to relate it to other reference frame in some way it loses its meaning.
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  7. C C Consular Corps - "the backbone of diplomacy" Valued Senior Member

    As global now would have to accommodate subatomic events transpiring within, say, a yoctosecond. If each particular state of the world only existed for a measurable interval like that before changing to the next one, then the specious "now" of perception or consciousness couldn't "fit" into it. The latter abides in the vastly "longer" range of milliseconds.[1] (Actually supervening over a sequence of scattered neural processes unfolding over time.)

    So something would have to give. The universal "now" of presentism believers either ironically slash bio-centrically corresponds to the one that brains output as a unit of change (with the "faster" changes of particle interactions being demoted to illusionary yardsticks)... Or else a frame or increment of consciousness extends over countless co-existing yoctosecond states of the world and microscopic modifications of the applicable brain (or whatever human-independent, elemental durations).

    If the latter, then presentism and its "global now" are demolished as a consequence. So it's not really an option for believers.

    - - - footnote - - -

    [1] "The orthodox view in the cognitive neurosciences is that time consciousness extends over a duration between a few hundred milliseconds and a few seconds over what is otherwise called the 'experienced moment' or 'subjective present'."

    Orders of magnitude (time)

  8. LaurieAG Registered Senior Member

    You could also say that using any timing system that is 'relative to the speed of light' just means that you will have an incredible amount of 'measurable' lag.
  9. James R Just this guy, you know? Staff Member

    The opening post is a bit muddled.

    Special relativity assigns coordinates to events in spacetime. For instance, we could have one event (A) here on Earth (say, you reading this post) with coordinates (x,y,z,t)=(0,0,0,0) and another event (B) happening at the same time at Alpha Centauri with coordinates (x,y,z,t)=(4 light years, 0,0,0).

    If "now" means time coordinate t=0, then both of these two events are happening "now". Of course, here on Earth we can't know that event B has happened before light can get to Earth from Alpha Centauri. The arrival of that light will be a third event (C), with coordinates (x,y,z,t)=(0,0,0, 4 years), let's say.

    Consider an observer in a spaceship moving towards Earth from the direction of Alpha Centauri. That observer is free to assign has own coordinates to any ONE event in spacetime. For instance, he might assign event A coordinates (x',y',z',t')=(0,0,0,0). But he will then find that the time coordinate of event B is not equal to zero in his frame of reference. In other words, while events A and B are simultaneous for observers on Earth, they are NOT simultaneous for the observer in the spaceship.

    In fact, for this particular pair of events, relativity shows us that we can find certain observers who will observe that event A happens before event B, and other observers (with different relative motion) who will observe that B happens before A.

    The idea of what is happening "now" depends on the observer's state of motion. Therefore, relativity shows us that the idea of a "universal now" is false.
  10. Cyperium I'm always me Valued Senior Member

    Yeah, I don't really care about the paradox, I don't see that there would be any paradox, in my reference frame I could see one event happen in Andromeda and another person could see a different event happening because he is moving relative to Andromeda. What I am suggesting though is that relativity doesn't actually say anything about a universal 'now'. Irrespective of what events happen in Andromeda in which order for us on earth looking at it, there is a definite 'now' in Andromeda which happens with a definite order to them. I suggest that irrespective of relativity we still all share the same now.

    That is not what I'm saying though. I'm saying that there is a 'now' in Andromeda, and that it is the same 'now' that we have and that relativistic effects only come about with our observations of events. If we move in respect to Andromeda time will go slower for us, which will be observed as time going faster in Andromeda (as we always experience normal time in our own perspective), even if time goes faster or slower though, there is still the same now.

    It is not what we observe, I'm saying that even if our observations are limited by the speed of light, the progression of 'now' isn't. I gave an example of what would happen *if* the progression of 'now' was limited by the speed of light. As such 'now' "progresses" as if it was instant throughout the universe. That's the only sensible conclusion.
  11. Cyperium I'm always me Valued Senior Member

    What does this mean for someone on Andromeda, if time goes faster or slower in relation to someone on earth? I still would think that 'now' in Andromeda is exactly 'now' whenever it is on earth. On the same note, whereever we are in the universe, we are always where we are, even if I travel fast or slow. There is no definite position, but it's always where we are. For the same reason there should also be the same 'now' wherever we are.

    If we look at time as a series of snapshots of the universe, and we are in the middle of that snapshot as our own frame of reference, how far does that snapshot extend? How far from us would it extend until we can't call it the same 'now'?
  12. Cyperium I'm always me Valued Senior Member

    Why would a "global now" be demolished just because our conscious experience of now extends (or is built) over a duration of time? In a way we consciously perceive the past because time is always ahead of us due to the processing required to consciously perceive, but does that show that a global now doesn't exist slightly ahead of our conscious experience?
  13. Cyperium I'm always me Valued Senior Member

    Yeah, something like that.
  14. Cyperium I'm always me Valued Senior Member



    Yes, this is exactly what the paradox states. But this is observations. There is a definite order to the events on Alpha Centauri. So even if we observe that the events happen in the opposite order than what is the truth on Alpha Centauri what we observe isn't what is real.

    Just as A happened before B in Alpha Centuari, there is a definite 'now' on Alpha Centauri, irrespective of my observations affected by my movement relative to the 'now'. I don't see how relativity explains that away, I don't see that relativity says anything at all about a universal now. Sure the idea or observation of what is happening now depends on the observer's state of motion, just as the idea or observation of which events come in which order, depends on the observer's state of motion, but the truth is that the order of events happened in one order and one order only in Alpha Centauri and if we apply the necessary mathematics we could reconstruct what order they actually happened in if we take account of our motion relative to them. For the same reason there is an actual now that joins us irrespective of our movement and we could also calculate what that would be.
  15. Ssssssss Registered Senior Member

    Yes there is an ordering along anybody's worldline.
    No the whole point is that there is nothing in physics that defines a universal now.
    No time dilation is symmetric so if we see Andromeda going slow they see us going slow and this is a consequence of no universal now.
    Nah mate it's the kind of conclusion you come to if you don't really study any relativity but just read a few half arsed popular articles on it.
  16. C C Consular Corps - "the backbone of diplomacy" Valued Senior Member

    Notice I said "presentism" is demolished. And there is no co-existing past moment or future moment in presentism, with respect to its "now", to make either "ahead of" or "behind" possible. (Presentism is a kind of temporal solipsism, in contrast to its rival opposite: eternalism).

    The "growing block universe" model would allow one's experienced version of "now" to superimpose over or through those ludicrously "slim", accumulating, multiple past states or "pages" of the world (and the concomitant brain) that are retained in GBU rather than eliminated/replaced (as in presentism).

    However, GBU's universal now is rendered superfluous if people in the past (or past neural states) are conscious. IOW, GBU potentially collapses into eternalism. That's one of the criticisms against GBU:

    "Recently several philosophers [...] have noted that if the growing block view is correct then we have to conclude that we do not know whether now is now."

    And the rejoinder or reciprocal revisionary view that the "now" of GBU is actually special (i.e., past neural states are non-conscious or people of the past are akin to p-zombies) renders GBU trivial in the other direction: it potentially collapses into presentism due to similar "what good is it, then?" assessment.

    But the bottom line is still that a yoctosecond or any other measurement of changes occurring at the atomic and subatomic level (in terms of those being the most elemental changes that could qualify for the interval of a "global now" -- that the latter belief has to accommodate) cannot contain my "elephant-sized", experienced version of "now" that is milliseconds in duration, that transpires over a chunk-sequence of "faster" brain process modifications. A chunk sequence that doesn't co-exist at all in presentism, and exists but is inert in [revised] GBU.

    The end result is that presentists (the primary believers in a universal now) have to reify their own experienced "now" as the objective now of the cosmos, with the fundamental increments of subatomic events thereby rendered illusionary, in terms of that status. Insanely, this is reflexively what 99.999% of humans do anyway, including probably most experts. It's that much of a zoom over our heads in terms of any iota of applied critical attention or recognition of inconsistency occurring within the topic matter.
    Last edited: Jun 18, 2022
  17. Dicart Registered Senior Member


    Its simple.
    You can choose one or an other conception, depending if you believe that events are causaly connected or not.

    Einstein : Take two dices and i bet 1$ the sum of the result will be a 6.
    Dicart : Ok lets try, look like a fair play.
    Einstein : Oh, we have a 5 and 4, the sum is 6
    Dicart : Thats amazing ! How did you ?
    Einstein : The events of both dices are causaly connected, therefore you can only have a 6.
    Dicart : But 5 and 4 do they not have any real meaning ?
    Einstein : You fool ! 5 and 4 depend on frame reference, there is not such reality here.
  18. Ssssssss Registered Senior Member

    Tell me that you didn't understand a Wikipedia article without telling me.
  19. Dicart Registered Senior Member

    Tell me you dont understand the two meanings of the "now" concept without telling me.

    I will help (not you, because it looks like you already know all...) Cyperium.
    There is two ways you can understant the "now" concept.

    "Now", is like you have understand : There is a now occuring simultaneously everywhere.
    If the time flow faster or slower here or there, it doesent matter, "now" exists (you can always divide an infinite line).
    There is for this reason only 1 reality, and other people who are not close, if they compare their "point of view" to the phenomenon can have the illusion that there are many realities.
    This "now", same as with 2. doesent say that the clock here or there would tell the same time.
    If you see a mirage in the desert, you know that this is not the reality, there is no water in the desert : it is an illusion.

    "Now", is not like you have understand : There is no now occuring simultaneously everywhere.
    There is for this reason as many realities as observer.
    This is due to Einstein, who think that, scientificaly, the reality IS linked with the interaction (all the SR theory behave on that).
    The only reality that exists, is what a scientist can "observ" (interact with at speed of light or at some speed)
    If you cant observ it, it doesent exists !
    This is the minimalist version of what we can tell reality (using science and observation, nobody can ever contradict this principia...).
    Last edited: Jun 22, 2022
  20. Ssssssss Registered Senior Member

    Some rubbish you made up? Of course I don't understand it because it has no connection to anything.
    This might just possibly be a bad description of the ether frame in Lorentz's interpretation of relativity except that the rest of the paragraph is garbage.
    This might just possibly be a bad description of the past light cone except that past light cones aren't reality and they definitely aren't now because there is a one way causal connection along them.
    No it isn't a minimal description would say that if you cannot observe it you do not know if it exists or not but stating that something you cannot see definitely doesn't exist is an unverifiable claim that you've added to that.
  21. Dicart Registered Senior Member

    If you are specialist OK.
    Now, tell us clearly what "now" mean and why we can only apply this definition.
  22. Ssssssss Registered Senior Member

    Now means the set of events that have the same time coordinate as the event where you are when you use the word but you can pick from a lot of different definitions of the time coordinate. If you actually mean the Lorentz ether frame in your definition 1 then that's an acceptable definition of now but only one of many and a lot of the stuff you wrote about it is wrong but your definition 2 has nothing to do with now. And all of this is you trying to distract from your post about results of die rolls being different depending on frame of reference which is beyond dumb.
  23. Dicart Registered Senior Member

    What events ?
    You need to have acces to those events to give them some coordinate.
    Perhapse you are some almighty God capable to see the whole univers in a blink.
    Or you meant the event, when AFTER the NOW you FINALY known you could have given it some coordinate ?

    The example is very accurate and it is intended to make yourself thinking.
    But you dont need if you dont need.

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