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Perception and inertial frames

  1. Feb 27, 2014 #1
    I was wondering how is our perception created according to relativity, since what we perceive depends on the perspective we have on the space-time around us. If two events are simultaneous in our frame by deduction from our perception we will realize that they are, and that the causal process and the propagation of light led to our inital perception.

    But what happens when the same variables are 'viewed' from another frame. For instance two events previously menitoned aren't simultaneous in some other frame, so basically the causal process that leads to a different description where space like separated events don't happen at the same time. For example, if two events are simultaneous w r to one observer and he perceives them like that, how would some other observer in a different frame describe the process of those events (which are not simultaneous) propagating by light to his eye, than to his nerves and brain and finally creating qualia which corresponds to the view from the initial frame?

    I hope you understand my question.
    Regards, Johnny.
  2. jcsd
  3. Feb 27, 2014 #2


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    Strictly speaking, relativity doesn't address the philosophical issues. But I can give a personal answer to some of the philosophical elements (mixed in with some elements that testable rather than philosophical).

    The direct perceptions are the arrival of the light signals. From this, we construct a mental model or map of the world, which defines a "now" (and this mental map also places where objects are "now").

    This mental map is always out-of-date, but usually it's not badly so, the mental processing times to construct the map out of the received signals are more significant than the light speed delays as far as its out-of-dateness goes.

    The construction of "now" was assumed to be universal in Newtonian mechanics, in relativity it's an observer dependent concept.

    In relativity, there is not necessarily a unique time-ordering of events, but there is a notion of cause and effect based on light cones.

    Pre-relativity, there was the universal past, and the universal future, the boundary of which was "now". Post-relativity, cause and effect are determined by light cones on an individual basis. If we have an event, only events in the past light cone of that event can act as a cause. Events in the future light cone of the event can be effects of the event. Events that are not in either the past or future light cone are space-like separated, cannot be a cause or an effect of the event, and do not have a unique causal ordering.
    Last edited: Feb 27, 2014
  4. Feb 27, 2014 #3


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    Perceptions, if by that, you mean what you actually observe, measure, see, etc, as opposed to what you calculate, are not in any way influenced by the space-time around you. Simultaneity is not something that you "perceive". It is something that you calculate based on your definition of the propagation of light which otherwise is outside the realm of your perception. You cannot even measure or calculate simultaneity in your frame simply by perceptions, that is, by passively monitoring what is going on around you. You have to proactively send out signals such as radar signals which reflect off of distant objects and wait for the return signals to get to you. You can measure the round-trip time it takes for this process with your clock. Then you apply Einstein's simultaneity convention which is that the time the reflection occurred is the midpoint of the round-trip time interval.

    If you apply a different simultaneity convention, then you get a different set of events that are simultaneous. Or if a different observer in motion with respect to you applies the same convention based on his clock and his radar signals, then he gets a different set of simultaneous events than you do.

    Simultaneity is not intrinsic to nature. It is not something that nature defines or presents to us. It is something that we apply to nature. Simultaneity is not perceived differently by two observers simply by light propagating from two events entering their eyes, going through their nerves and into their brains. They both have to do more work than that to establish which events are simultaneous. And it is work. Hard work.
  5. Feb 27, 2014 #4


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    Simultaneity is not a physical observable. It requires conventional information about the space-time outside of your light cone. Only the local light cones are intrinsic to space-time-we call this the conformal structure of space-time.

    In principle we can do many of our calculations without ever introducing simultaneity conventions by elegantly using the abstract index calculus (the Ricci calculus) particularly if we care about geometric properties of space-time, of fields on space-time, and frame-invariant quantities of relativistic kinematics.

    We introduce simultaneity mainly because we want to construct coordinate charts relative to specific observers, or families of observers, in order to describe various physics relative to this observer that doesn't necessarily take place in the vicinity of the observer.
    Last edited: Feb 27, 2014
  6. Mar 1, 2014 #5
    What I really wanted to ask was this. For instance, in our frame of reference two events are simultaneous and the light 'from them' reaches our eyes so that the closer one appears first and the more distant one second. Now from a point of view of a different frame, where two events aren't simultaneous, how would this process look like? By this I mean the causal process of those events emitting light which is traveling to the observer previously mentioned, which defines them as simultaneous?
  7. Mar 1, 2014 #6
    just imagine yourself at one of the events......what would you observe?
    Then imagine yourself at the other event...what would you observe?

    All three observations 'look' different....but what is actually simultaneous and what isn't is a different issue already explained in prior posts.
  8. Mar 1, 2014 #7


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    I already answered your question. You cannot establish simultaneity just by observation. You have to send a signal to the event and wait for a response or echo. You average the sent time with the received time. You do this separately for each event. Those that calculate to the same average time are simultaneous in your rest frame.

    In the example you gave, the two events could be simultaneous if the sent time was later for the event for which you received the light from the event first. You just have to do the calculation to see which events are simultaneous.
    Last edited: Mar 1, 2014
  9. Mar 1, 2014 #8


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    Let's go back to the railroad tracks of Einstein's thought experiments. You are standing next to the tracks, and five light-seconds down the tracks there is a bomb. There's a second bomb ten light-seconds down the tracks. The first bomb explodes (call that event A) and light from the explosion reaches your eyes at five seconds past noon. The second bomb explodes (call that event B) and the light from that explosion reaches your eyes at ten seconds past noon. You correctly conclude that A and B both happened at the same time, noon exactly, so they're simultaneous.

    But if you consider the situation from the view of someone on a train moving down the tracks, you'll find that there's no combination of speed and position that will work out so that the moving observer finds the two events to be simultaneous. You can find a way of getting the light to reach the train guy's eyes at the same time, but only if the two bomb sites are at different distances from him, and you can find ways of getting the light to reach his eyes at different times, but not in ways that will exactly balance the light travel time.
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