11 ms·

It seems to me that most people view "time" as it "exists" in the universe as though there infinitely many infinitesimal sequences of "snapshots" of the univers

• RichardHeart 4y ago
  I agree
• shaky-carrousel 4y ago
  Yes, the past as a place to visit doesn't make sense.
  • tsimionescu 4y ago
    Why not? In SR/GR, (this place, yesterday) is just as much a "place" as (that place, tomorrow). Furthermore, even though these two "places" are ordered such that one is in your past light cone and the second is in your future light cone, it doesn't mean that for some other observer they can't be the reverse - for them, the event you consider "tomorrow in that city" may have already happened, and the event that you consider "yesterday in this city" hasn't happened yet - so they are literally visiting your past.
    • the_other 4y ago
      Whilst I find it easy to imagine time as "a place", and movable-to, I find it very hard to imagine how anything could change position on the time-like axis. This uncertainty holds for "teleport" too. From the perspective of the force or system required to produce a discontinuous temporal displacement, what "are" any of the objects being displaced? How could such a system differentiate between "me" and "my chair" and "the ground objecting to my chair's and my motion towards the nearest significant centre of gravity"? Zoom in further... how could the "time travel machine" identify the atoms at the very edges of the dead skin cells resting between the hairs of my arms and the atoms of the myriad gasses and pollutants beside them in order to alter their temporal position. Which bits are "me"? What gets left behind? What is "me" from the perspective of a temporal displacement system? Then zoom out a bit... "here" in space isn't "here" in time, yesterday. "here" in space yesterday was actually "over there" because the object we're riding on has moved around its orbit around ITS nearest centre of gravity since then.
      • tsimionescu 4y ago
        Well, in GR a time machine would be no different from any other propulsion device. To visit your own past, you would need extreme and probably impossible conditions, but to visit somebody else's past simply requires moving fast enough to reach that point before it passes. Of course, if that someone else is very close to you (say, living in the same solar system), you'll reach the same limits as visiting your own past. But visiting the past of someone from another galaxy is simply a matter of getting in a rocket and getting there. If you move very fast, you'll reach their "past", if you move slow you'll reach their "future", taking any arbitrary point as their "present". For example, it's in principle perfectly possible for someone from another galaxy who lives for 100 years from their own point of view to visit the time of Jesus, and then to visit us today and tell us what they saw happen. But it's not possible as far as we know for someone from Earth to do so (or even for someone who lived on Pluto). Also, it's not possible for that same someone to do the opposite - if they visit us today, they can't then visit the time of Jesus and send him a message from us.
        • Guillaume86 4y ago
          Your use of the word "past" is very confusing, yes a very fast spaceship can be used to travel to the future, no you can't travel into the past, that's all there is to say in short.
          • tsimionescu 4y ago
            Well, yes, but that means that my past and yours still exists as a real place that others can visit, in some sense. That's why I gave the example. Of course, that visitation has already happened in our past. But the same person can also know things we havent experienced yet, but whose consequences we will experience in our future.
            • deleted 4y ago
              [deleted]
            • kgwgk 4y ago
    • kgwgk 4y ago
      Two events with « time-like separation » like those have a definite order. If one is in your past light cone and the second is in your future light cone, it does indeed mean that they can't be the reverse for any observer.
      • tsimionescu 4y ago
        Oops, you're right, I thought about that wrong. Still, the example I gave is correct - an alien that visited the earth 2022 years ago could also arrive again today, and from their point of view, the journey could have taken 20 years let's say. And during this journey, they may have seen a star explode whose light will only reach Earth 2000 years from now, right? Edit: the answer to my second question is also no - that would require them obviously moving faster than the light of the star... But, what is possible is that from their point of view the star exploded before my birth, while from my point of view it exploded after my birth. Still, by the time they arrive, we will both agree that the explosion of the star is in the past.
    • shaky-carrousel 4y ago
      Let me put it in another way. A person in a point, cannot visit that same point in the past because that past does not exist anymore. You cannot travel to the past because it doesn't exist.
      • contravariant 4y ago
        By the same token you can't travel into the future because it doesn't exist yet. If you're arguing you can only visit a point in time when some kind of 'clock' is showing the same time then you're begging the question w.r.t to time travel.
        • shaky-carrousel 4y ago
          And you cannot travel to the future either. You cannot enter on some machine and suddenly jump 100 years forward. You have to let time pass. Time can pass slower in your local frame than in another frame, but it must pass anyways.
          • contravariant 4y ago
            By that definition you cannot travel to France.
• wmwragg 4y ago
  Whenever I think along these lines, I can never reconcile it with relativity, and the fact that gravity effects time. If it's just a human construct, how can it be effected by gravity?
  • ilitirit 4y ago
    If I understand correctly, General Relativity doesn't consider this in the same conceptual terms. "Gravity" doesn't affect "time" - mass/energy curves space-time (simplified for the sake of brevity).
    • wmwragg 4y ago
      Sorry, yeah I was also oversimplifying, what I meant was that General Relativity treats time as a real thing, which is effected by some other real thing, which doesn't track with time being just a human concept
  • mjevans 4y ago
    Time can also be viewed as the propagation of information, or the transmission of energy. That probably not even correct either. It reminds me of the Plank (sp) constant, a minimum unit of distance (? unsure if I remember / ever understood this correctly). A pure (parallel) event driven model of time might be seen as the propagation of energy with some form (information). In that model folded dimensions, gravity, even the concept of space or time can distort as relationships strain. That's how space and time could be 'affected by gravity'. Yet even with that framework I can't imagine time travel anywhere remotely like it exists in fiction. I would really like to think that it's just my very non-expert knowledge, and the general immaturity of even our knowledge as a species that keeps the universe such a mundane place. Any form of FTL, time travel, or similar fun scifi concept would keep life interesting.
    • renox 4y ago
      > Time can also be viewed as the propagation of information, or the transmission of energy. ?? Given that photons which transmit energy are 'timeless' that's a strange claim.
• tsimionescu 4y ago
  Well, the nature of time is not really understood so far in physics. It's a very basic empirical observation that time exists, and there are laws of physics that crucially separate the future from the past (e.g. second law of thermodynamics, weak force in QM). In SR/GR we also see time as separate from space (since the distance metric is x²+y²+z²-ct²). Computation is a potential source of a definition of time - a computation naturally introduces an order of successive operations, which happens to match our observation of time very well (you can extend computation to a continuous process if needed, though of course we don't know if time is necessarily continuous). On the other hand, both GR and QM are also consistent with an interpretation where the universe with its entire Time-line is a fixed object, where we can imagine the past and future exist just as much as distant galaxies. In this view, the concept of time travel makes some intuitive sense, as you can imagine closed loops where you can reach (here, yesterday) just like you can reach (other city, tomorrow).
  • simonh 4y ago
    >also consistent with an interpretation where the universe with its entire Time-line is a fixed object... That sounds like block time, but in that case I think the operative part is "fixed object", so there would be no concept of changing the past or having multiple time lines. If there were a loop, it would "always" have been there. One interpretation I quite like is the idea that objects travel through time just as they do through space, and so if you went back to a past time there would be nothing there. All the matter is in the 'now' on a trajectory into the future, and both the past and future are the time equivalent of empty space.
    • tsimionescu 4y ago
      > If there were a loop, it would "always" have been there. Yes, if the universe is fixed, anything in it has always been there. Basically it would be like a recording of a movie - both the intro and the ending exist at the same time, and the film could be circular.
  • _0w8t 4y ago
    All laws of physics are CPT-invariant. Moreover, there is no separation of future and past in thermodynamics. Philosopher Huw Price argued about it in [1] rather conveniently. In general it is a deep unresolved question why our personal perception of time with its direction, flow and existence of "now" has no reflections in physical models where time is static, has no future/past asymmetry and there is no preferred time moment like now. [1] - https://prce.hu/w/TAAPch1.html https://prce.hu/w/TAAPch1.html
• kumarvvr 4y ago
  Interesting, since time itself is measured and linked to motion. Perhaps it is a construct for human consciousness, because time cannot exist without consciousness. There must be a perceptor to preceive the universe and the notion of time. However, how can time dialation be explained? Would we actually age slower when we travel close to speed of light? That throws a wrench into the concept of time being just a construct and not an actual facet of our universe.
  • tsimionescu 4y ago
    > Would we actually age slower when we travel close to speed of light? Technically, from your point of view you will never age slower. Depending on your speed compared to others, those others may age faster or slower than you, but you yourself will never experience time at a different rate. Say you were somehow guaranteed to live 120 years. Then, no matter what you did, whether you stayed perfectly still in empty space or moved at almost the speed of light, or got infinitesimally close to a black hole, you will only ever experience 120 years of life.
    • kumarvvr 4y ago
      So, when time is different for different entities under different conditions, then time is another part of universe, rather than just a mathematical convenience of a human perception. In the thought experiment, in two frames of references moving at different speeds but close to speed of light, how do molecules, cells and enzymes know how fast to age?
      • tsimionescu 4y ago
        Everything has its own frame of reference. Everything is stationary relative to itself, so everything's "clock" is perfectly synchronized with everything else's, relative to themselves. "Time dilation" only comes into play when comparing the clocks of two different things moving at different speeds or accelerations relative to each other, or existing in two different gravitational fields (which is equivalent).
• the_other 4y ago
  Disclaimer: Non-physicist here. And not even that bright on a HN scale, I suspect. > I was always under the impression that time as we know it is merely an idea and variable we add to equations to explain the motion we perceive. This seems non-sensical to me. How can you have motion without time? > We see one thing happen after another thing, and conjure up the idea of time to capture a duration between the two occurrences How can you have duration without time? They seem like the same thing to me.
  • kumarvvr 4y ago
    Another way to think of it is, can you really define time without lenth? So the idea of space is required for the idea of time. It might well be possible that the idea of space is sufficient and time is just a derived perception.
    • tsimionescu 4y ago
      > It might well be possible that the idea of space is sufficient and time is just a derived perception. Not really, since the observed behaviors of physics require an extra dimension that is different in nature to the three spatial dimensions. You can't explain the behavior of light in a universe with 4 space-like dimensions, for example.
    • skissane 4y ago
      > So the idea of space is required for the idea of time I can conceive of the idea of a disembodied mind who exists in time, who experiences auditory qualia without any directional component, but does not experience other kinds of qualia such as visual or tactile qualia. I don't think such a scenario requires any space – some might say that auditory qualia require sound waves which require space, but a disembodied mind by definition does not have physical sense organs and their auditory qualia cannot be directly produced by physical sound waves–otherwise, they would not be disembodied (their auditory qualia would have to be hallucinations, dreams, direct telepathic communication with other minds, divine revelations, etc.) So, if I can conceive of a mind existing in time but not space, who naturally could have an idea of time but would struggle to have any idea of space (since it has never experienced it), and if conceivability is prima facie evidence of possibility, it is reasonable to conclude that it is possible for such a being to exist, and hence possible to have an idea of time without having any idea of space. Therefore, the idea of space is not required for the idea of time.
• hanoz 4y ago
  > I was always under the impression that time as we know it is merely an idea and variable we add to equations to explain the motion we perceive. We see one thing happen after another thing, and conjure up the idea of time to capture a duration between the two occurrences. That makes about as much sense to me as claiming that space is merely an idea we've invented to account for our perception that things are in different places.
• skissane 4y ago
  > It seems to me that most people view "time" as it "exists" in the universe as though there infinitely many infinitesimal sequences of "snapshots" of the universe in a linear line, upon which you could (if time travel was possible) jump between them. I don't think most people view "time" as anything in particular. They just live it. Whether or not time is composed of "infinitely many infinitesimal sequences" is the kind of highly abstract question which most people will never ask. > I was always under the impression that time as we know it is merely an idea and variable we add to equations to explain the motion we perceive. Who can really say what time is – is it a basic category, and as such incapable of being explained in terms of anything more basic? Or is the temporal reducible to or emergent from the non-temporal? Some have opinions – most people don't – but who can really know whether anyone's opinions are right? > We see one thing happen after another thing, and conjure up the idea of time to capture a duration between the two occurrences. Do we "conjure up the idea of time"? Or is it an "innate idea", an "a priori concept", etc? Rather than something we "conjure", I think all thought, having any ideas at all, even being a mind, has time as a necessary precondition. To form the idea of time is simply to recognise something which has been there from the very beginning of one's existence, something which is an essential part of being what one is, being the kind of being which one is. To know that there is time is part of mind's ability to recognise what it is to be a mind.
• mshaler 4y ago
  David Hume would agree