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Does time exist?

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  • Registered Users Posts: 966 ✭✭✭GO_Bear


    aidan24326 wrote: »
    Personally I think time is more of a pyschological construct than a real thing in and of itself.

    Like an abstract , like love ? While we cannot hold it in our hands we see its effects .
    What about space ? is it a pyschological construct we designed to determine positions of objects in a system in relation to each other ?


  • Closed Accounts Posts: 26 h4ck573r


    Joe1919 wrote: »
    Yes, you are correct here. A clock does not measure time but counts the swings of a pendlum or the vibration of a electronic crystal etc. and time is always measured or compared 'relative' to this. Similarly, the calender and larger quantities of time are 'relative' to the perceived movements of the sun and moon 'relative' to the earth.

    IMO, you cannot attribute any reality to time but you can to the change that takes place.

    Also, I am in agreement with Augustine that the past and future only exist in the mind (as memories or anticipations) and only the 'now' or present moment exists. Hence time travel will never be possible. (imo)

    Besides memory, what does exist is 'traces' of the past but these are really only the effects of change.

    I agree with slight change...We do not see present , because of the delay we see near past.
    The time that is required for our senses to perceive makes the perceived reality already past, because it is not instant.


  • Closed Accounts Posts: 26 h4ck573r


    About the existence of time...
    It is relative but does exist, time is not an illusion, without it you would not be able to post this question at all, or perhaps you never did.
    Time is an effect in which events take place, it is measured by observing change to a referent object.
    This is just my opinion, I am not that arrogant to assume that I am right.


  • Closed Accounts Posts: 96 ✭✭CuriousOne


    HydeRoad wrote: »
    The subject of time fascinates me. The human describes time for the most part in manmade measurements of years, days, hours, seconds. The fact that these are linked to the rotation of the Earth is immaterial. That is a measurement of convenience, nothing more. However, our perception of time would seem to be related to the heartbeat.

    If you look at small animals with a short lifespan in human terms, take a mouse for example, they have a rapid heartbeat, and seem to scuttle along and perform their daily tasks with lightning speed. When a mouse runs across your path, his legs seem to work at an impossibly fast rate. To a mouse, the world probably appears much as it does to a human, and humans probably appear as huge, very slow, lumbering beasts.

    Conversely, long lived animals with slow heartbeats, such as elephants and tortoises, probably perceive humans as scuttling around too fast, much as we perceive mice. Picture an old Buster Keaton movie reel played at fast speed, with the actors scuttling about everywhere much too fast.

    Taking this into account, while it is very easy to think back to what the world was like a couple of years ago, and what we were doing at any given time in our lives, it is very hard for a human to perceive the vast timescales of prehistory. The dinosaurs died out 65 million years ago. It is impossible for the human brain to quantify that in relation to our own puny lifespan.

    On a universal scale, if there were a greater consciousness capable of it's consideration, then the billions of years of the Earth's formation and the evolution of life, would probably appear to happen unimaginably quicker than the human brain is capable of doing. A single lifespan, or even the whole history of humanity, would be no more than a bubble appearing in a test tube and popping almost immediately out of existence.

    Conversely again, on the atomic level, there could be a whole universe of life and interaction happening in the time it takes a human to blink an eyelid.

    We are constrained by an unwritten law, which would seem to allow us only to perceive whatever is within our heartbeat's ability to perceive, on a human scale. That which is eons greater, or ions smaller, is outside of that, and can only be related in mathematical equations, but not in terms of human perception or experience.


    Now that's really good. I believe in a 'Now' that contains past, present and future, which only exists/existed for an instant and which we are able to perceive in a super-slow motion, a kind of suspended animation. The Universe has been and gone and we exist in its echo.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Came across this a while ago. It's an interesting read, even if you don't get the maths (which I don't) - the Nature of Time - Julian Barbour
    Abstract: A review of some basic facts of classical dynamics shows that time, or precisely duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change.

    A further quote from the article
    time is an abstraction at which we arrive by means of the changes of things....
    - Ernst Mach


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  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    Came across this a while ago. It's an interesting read, even if you don't get the maths (which I don't) - the Nature of Time - Julian Barbour

    Do you believe, as the physics in the above piece implies, that the past and the future are just as real as the present? I had a previous discussion with you where I inferred you didn't think so.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    Do you believe, as the physics in the above piece implies, that the past and the future are just as real as the present? I had a previous discussion with you where I inferred you didn't think so.

    I wouldn't be inclined to accept all of the theory, or at least as I understand it from this:



    I do think it is a step in the right direction, however. If it is acknowledged that time is abstracted from change, then the notion of moment to moment snapshots existing can be examined.


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    I wouldn't be inclined to accept all of the theory, or at least as I understand it from this:



    I do think it is a step in the right direction, however. If it is acknowledged that time is abstracted from change, then the notion of moment to moment snapshots existing can be examined.

    The document presents the picture where all the snapshots or "configurations" exist. The set of all these configurations is called a configuration space. Each location in this space corresponds to a different snapshot. Time, as well as "change", is derived from the curves in this space that minimises the action. So even change would not be fundamental, but a derivation of the structure of all the snapshots. Neither time nor change is fundamental.

    You seem to be suppose something different. You are supposing that only one snapshot represents the "present" and all other snapshots of the past and future don't exist any more, or haven't existed yet. But "present" is relative to different frames of reference. What is the present for me, can encompass the past, present, and future for you. In other words, there are many different ways to take a snapshot, and there is no snapshot that is more real than any other. The notion of change is no more fundamental than the notion of time.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    The document presents the picture where all the snapshots or "configurations" exist. The set of all these configurations is called a configuration space. Each location in this space corresponds to a different snapshot. Time, as well as "change", is derived from the curves in this space that minimises the action. So even change would not be fundamental, but a derivation of the structure of all the snapshots. Neither time nor change is fundamental.

    You seem to be suppose something different. You are supposing that only one snapshot represents the "present" and all other snapshots of the past and future don't exist any more, or haven't existed yet. But "present" is relative to different frames of reference. What is the present for me, can encompass the past, present, and future for you. In other words, there are many different ways to take a snapshot, and there is no snapshot that is more real than any other. The notion of change is no more fundamental than the notion of time.

    I think we can drop the idea of snapshots altogether, they are superfluous and probably just a hangover from the subconscious attachment to the notion of time being existential.

    If you imagine the movement of the planets, in our solar system, around the sun - and take it as a microscosmic model of the universe - where do the snapshots come into play? Is what we see as the movement of the planets just a series of snapshots on a roll of film? What is the process of transition from one snapshot to another? Why is that even necessary, why can't the planets themselves exist and move around the sun, without the need for there to be an infinite number of snapshots of them.


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    I think we can drop the idea of snapshots altogether, they are superfluous and probably just a hangover from the subconscious attachment to the notion of time being existential.

    If you imagine the movement of the planets, in our solar system, around the sun - and take it as a microscosmic model of the universe - where do the snapshots come into play? Is what we see as the movement of the planets just a series of snapshots on a roll of film? What is the process of transition from one snapshot to another? Why is that even necessary, why can't the planets themselves exist and move around the sun, without the need for there to be an infinite number of snapshots of them.

    There is no true snapshot of the "present". Say you are sitting on a park bench, and I am walking down the street at a couple of kilometres per hour. From my perspective, a star in Andromeda might be about to go supernova. From your perspective, the star may have already gone supernova. Your present contains my some of my past, present and future, and vice versa. I must stress that this is not due to light reaching you before or after me. Your snapshot is intrinsically different to mine. So there cannot be only a single snapshot.


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  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    There is no true snapshot of the "present". Say you are sitting on a park bench, and I am walking down the street at a couple of kilometres per hour. From my perspective, a star in Andromeda might be about to go supernova. From your perspective, the star may have already gone supernova. Your present contains my some of my past, present and future, and vice versa. I must stress that this is not due to light reaching you before or after me. Your snapshot is intrinsically different to mine. So there cannot be only a single snapshot.

    Although I am arguing against the notion of snapshots, your use of it here is very helpful as an explanatory aid.

    I think we can differntiate between what people perceive in the present and the present itself. On that basis I think it might be more accurate to say that no individual's perspective is a true snapshot of the present, as opposed to their not being one.

    If we imagine the universe going about it's usual course of events and then suddenly pausing, granted, each individual may have a different perception of things, but that just means they have a different perception in that moment. The actual state of the universe would represent the "true snapshot".


    The issue of why people perceive things differently is incidental, but I still have trouble seeing why it is that the distance light has to travel doesn't have a material effect. Taking the example of the Andromedan star, we know that the light is travelling outwards. If I am closer to the star then the light reaches me first, and you second, meaning that each light quantum (or photon, or whatever the correct term is) will pass me first, before it passes you. That way, I will "see" each quantum of light first.

    If the star goes supernova, then the resultant light will pass me before it passes you, meaning that I will see it first. No?


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    Although I am arguing against the notion of snapshots, your use of it here is very helpful as an explanatory aid.

    I think we can differntiate between what people perceive in the present and the present itself. On that basis I think it might be more accurate to say that no individual's perspective is a true snapshot of the present, as opposed to their not being one.

    If we imagine the universe going about it's usual course of events and then suddenly pausing, granted, each individual may have a different perception of things, but that just means they have a different perception in that moment. The actual state of the universe would represent the "true snapshot".

    The issue of why people perceive things differently is incidental, but I still have trouble seeing why it is that the distance light has to travel doesn't have a material effect. Taking the example of the Andromedan star, we know that the light is travelling outwards. If I am closer to the star then the light reaches me first, and you second, meaning that each light quantum (or photon, or whatever the correct term is) will pass me first, before it passes you. That way, I will "see" each quantum of light first.

    If the star goes supernova, then the resultant light will pass me before it passes you, meaning that I will see it first. No?

    If you and I were sitting on park benches, with you closer to the star, the light from the supernova would reach you before me. We would do a quick calculation, taking into account the finite speed of light and our respective proximities to the star, and conclude that the star went supernova at some time we both agree on. But if I was walking, and let's say I was still further from the star, and we took into account the same considerations, we would both disagree on when the star went supernova. Your "present" would be different to mine. Similarly, let's say I was passing you at the exact moment the light reached us, so that we both detected the supernova at the same time. We would still disagree on when it occurred.

    I cannot stress enough that it is not an aberration due to light. The absence of a single, correct present is evident even after we take into account the finite speed of light.



  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    If you and I were sitting on park benches, with you closer to the star, the light from the supernova would reach you before me. We would do a quick calculation, taking into account the finite speed of light and our respective proximities to the star, and conclude that the star went supernova at some time we both agree on. But if I was walking, and let's say I was still further from the star, and we took into account the same considerations, we would both disagree on when the star went supernova. Your "present" would be different to mine. Similarly, let's say I was passing you at the exact moment the light reached us, so that we both detected the supernova at the same time. We would still disagree on when it occurred.

    I cannot stress enough that it is not an aberration due to light. The absence of a single, correct present is evident even after we take into account the finite speed of light.


    Assuming that the above is true, it still only demonstrates the difference between individuals perception of the present and the present itself.

    I watched the video you posted - cheers, I'm always interested in things like that that offer clear explanations of scientific ideas - but if you play it to the end, the selection of related videos at the end contains one which, to the lay persons mind, is pretty interesting. It attempts to highlight the flaws in the theoretical model that Einsteing used (the flashes of lightning), which potentially address the issues of simultaneity. It's worth a watch - although, unfortunately, I am not in a position to really critique it; rather ask if certain elements are correct or not.

    youtube slide-show

    It tried embedding the video but couldn't get it to work, if someone can manage to do it, that would be great. If they could also post the unparsed link as well, just to show what part of the url goes into the youtube tags (bcos I tried every possible way, I think) that would be sweet, cheeers.

    The related website, absolute-relativity, is probably worth a read also, because he goes into more depth, which will probably be easier interpreted by yourself and others.


  • Registered Users Posts: 3,809 ✭✭✭CerebralCortex


    roosh wrote: »
    Assuming that the above is true, it still only demonstrates the difference between individuals perception of the present and the present itself.

    I watched the video you posted - cheers, I'm always interested in things like that that offer clear explanations of scientific ideas - but if you play it to the end, the selection of related videos at the end contains one which, to the lay persons mind, is pretty interesting. It attempts to highlight the flaws in the theoretical model that Einsteing used (the flashes of lightning), which potentially address the issues of simultaneity. It's worth a watch - although, unfortunately, I am not in a position to really critique it; rather ask if certain elements are correct or not.

    youtube slide-show

    It tried embedding the video but couldn't get it to work, if someone can manage to do it, that would be great. If they could also post the unparsed link as well, just to show what part of the url goes into the youtube tags (bcos I tried every possible way, I think) that would be sweet, cheeers.

    The related website, absolute-relativity, is probably worth a read also, because he goes into more depth, which will probably be easier interpreted by yourself and others.




  • Registered Users Posts: 2,552 ✭✭✭roosh



    cheers for that CC.

    I thought I had tried pretty much every possible permutation of the url (I was sure I had tried that one too)


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    Assuming that the above is true, it still only demonstrates the difference between individuals perception of the present and the present itself.

    But what is the real "present"? You do not like the concept of time because it is a derivative quality, not directly observed, but rather derived from other observations. Yet the "true present" is not only unobservable, but also cannot even be derived from observation. It is even less real than the notion of time. Any snapshot is just as physically meaningful as any other, and it makes little sense to suppose an extra-meaningful snapshot that is undetectable.
    I watched the video you posted - cheers, I'm always interested in things like that that offer clear explanations of scientific ideas - but if you play it to the end, the selection of related videos at the end contains one which, to the lay persons mind, is pretty interesting. It attempts to highlight the flaws in the theoretical model that Einsteing used (the flashes of lightning), which potentially address the issues of simultaneity. It's worth a watch - although, unfortunately, I am not in a position to really critique it; rather ask if certain elements are correct or not.

    youtube slide-show

    It tried embedding the video but couldn't get it to work, if someone can manage to do it, that would be great. If they could also post the unparsed link as well, just to show what part of the url goes into the youtube tags (bcos I tried every possible way, I think) that would be sweet, cheeers.

    The related website, absolute-relativity, is probably worth a read also, because he goes into more depth, which will probably be easier interpreted by yourself and others.

    The video is a mishmash that seems to be ignoring the fact that the speed of light is the same for all observers. Observer2 and Observer1 would agree on the speed of light, and hence disagree on the time of the flashes.

    The website is bizarre, but it seems to be insisting on a "real space" (and a strange notion of photons without momentum, a demonstrably false notion). This "real space" is undetectable and arbitrary.

    Also, these type of videos and websites tend to employ tunnel vision. They never address the fact that a) The experimental tests of relativity have gone far beyond the Michelson-Morley experiments of old, and if "real space" was physically meaningful, it would have been detected in modern experiments. And b) Quantum field theory is the quantum-mechanical extension of special relativity, has made predictions accurate to one part in a trillion.

    To embed youtube videos, just enclose the last string of letters from the url in youtube tags. I.e. Your video would be (youtube)Lvx945SP(/youtube) only with square brackets instead of round.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    But what is the real "present"? You do not like the concept of time because it is a derivative quality, not directly observed, but rather derived from other observations. Yet the "true present" is not only unobservable, but also cannot even be derived from observation. It is even less real than the notion of time. Any snapshot is just as physically meaningful as any other, and it makes little sense to suppose an extra-meaningful snapshot that is undetectable.



    The video is a mishmash that seems to be ignoring the fact that the speed of light is the same for all observers. Observer2 and Observer1 would agree on the speed of light, and hence disagree on the time of the flashes.

    The website is bizarre, but it seems to be insisting on a "real space" (and a strange notion of photons without momentum, a demonstrably false notion). This "real space" is undetectable and arbitrary.

    Also, these type of videos and websites tend to employ tunnel vision. They never address the fact that a) The experimental tests of relativity have gone far beyond the Michelson-Morley experiments of old, and if "real space" was physically meaningful, it would have been detected in modern experiments. And b) Quantum field theory is the quantum-mechanical extension of special relativity, has made predictions accurate to one part in a trillion.

    To embed youtube videos, just enclose the last string of letters from the url in youtube tags. I.e. Your video would be (youtube)Lvx945SP(/youtube) only with square brackets instead of round.

    I don't think it ignores the fact that the speed of light is constant in every reference frame, at all; what it says is that the model used to represent the reality of the observed phenomena is flawed, that it doesn't take into account certain relevant information.

    What it says, with regard to the original thought experiment Einstein used, is that there are four "time points" which are relevant, and which, when taken into consideration, would lead to both observers calculating that the flashes of lightning occured simultaneously.


    What it says is:
    • The flashes of lightning occur at rods A (behind train) & B (in front of train) at time=t1 - neither observer sees the flashes in that instant.
    • The light from the flashes travels towards the midpoint (M) where Observer 2 is standing.
    • As the train is in motion, Observer 1 will meet the light from rod B at a point beyond M (call it X) at time=t2
    • Observer 2 will see light from rods A & B at the same time, at time interval t=3 (Observer 2 will have travelled a further distance along the track to Y)
    • The light from Rod A will reach Observer 1 at time=t4 when Observer 1 is at location Z along the track

    According to the author, once all these times and co-ordinates are taken into account then both observers would calculate that the flashes occurred at A & B simultaneously.

    The assumption as far as I am aware, is the speed of light is constant for both observers.


    Real Space
    With regard to the "real space" the author is insisting on I think the distinciton is made between
    real space, where planets or space crafts are travelling in, corresponds to Newton's absolute space as the absolute reference at absolute rest
    the mathematical artificial two-dimensional "space" as defined by that reference frame of Obs2 is thus virtual in the mind of Obs2 which does not exist in reality outside the mind of Obs2 as a real space.
    I think the distinction is more between conceptual space, as defined mathematically, and real, non-conceptual space.

    Momentum of photons
    Again, I don't think he/she is using the strange notion of photons without momentum, what is said is that the use of geometrical "zig-zeg" lines, to represent the beam of light, is an incorrect model of the phenomena of light, which doesn't take into account the fact that light isn't necessarily a single "ray" but rather is made up of quanta. They say that these quanta don't travel in the "zig zag" fashion as depicted in the models, but follows the trajectory as outlined in figure 9 on this page.
    Each trajectory is perfectly perpendicular to the x-axis since the laser pulse is NOT inheriting the velocity component of the laser in the x-axis direction.

    The individual photon trajectories in Figure 9 are parallel to one another. It should be remarked in that respect that Figure 5, which is based on classic optics, indeed suggests that the laser pulse is perfectly following the laser for whatever velocity of the laser ; thus that the laser pulse "inherits" in the x-axis direction the velocity of the laser itself. That is already a contradiction within classic physics where it is stated that the speed of light is not influenced by the mechanical velocity component of the light source itself

    From what I can gather, it doesn't suggest the notion of photons without momentum, rather the idea that the photons will not inherit the velocity of the laser, that they will travel perpecdicular to the X-axis (along which the laser moves) as opposed to at an angle as suggested by the "zig-zag"! model.

    Tunnel vision
    Unfortunately I can't engage in any meaningful critique of that part of your post, other than to assert that the observations from experiments applying GR won't actually change, but the manner in which they are interpreted might. Perhaps a change in model could lead to progress in the quest to unify GR and QM.


    Objections to the notion of time
    The challenge to the notion of time isn't necessarily on the basis that it is unobservable, or a derivitave quality, the objection is that it isn't even necessarily derived from other observations, it is assumed to exist a priori. The subsequent attempt to derive its existence simply isn't logical, and again relies on an unjistified assumption.

    The assumption is that certain, either specific or non-specific, physical phenomena can be used to measure this thing called time, when, in fact, those physical phenomena are simply used as standard units of comparison - they [may indeed] exist, and the things which are measured against them [may indeed] exist, but the thing called time is not measured against them.

    Time is* the name given to the system of measurement, it is not the thing which is measured.

    *or at least it should be recognised for what it is.


    "True present"
    The issue of "the derivative nature of time" does not necessarily apply to the notion of the "true present". If we all live in the same universe (as opposed to occupying our own separate but interacting universes) then the notion of a "true present" is a necessary consequence, even if we cannot observe it directly - much as we cannot observe sub-atomic particles directly but can create a model to represent reality.

    If we all live in the one universe, then we can imagine the entire universe being paused, or a snapshot of the entire universe; while each observer might have a different perspective of the present, we can still deduce that there would be a "true present" or snapshot of the universe. We could theoretically put an infinite number of observers in an infinite number of reference frames (or the corresponding number of observers in the corresponding number of reference frames) and work out what the "true present" would be. It's simply a lack of information that prevents us from doing it, as opposed it not being possible.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    If you and I were sitting on park benches, with you closer to the star, the light from the supernova would reach you before me. We would do a quick calculation, taking into account the finite speed of light and our respective proximities to the star, and conclude that the star went supernova at some time we both agree on. But if I was walking, and let's say I was still further from the star, and we took into account the same considerations, we would both disagree on when the star went supernova. Your "present" would be different to mine. Similarly, let's say I was passing you at the exact moment the light reached us, so that we both detected the supernova at the same time. We would still disagree on when it occurred.

    I cannot stress enough that it is not an aberration due to light. The absence of a single, correct present is evident even after we take into account the finite speed of light.



    just to relate it back to the video explanation of time dilation & simultaneity. It is the model of the path of the light, as observed by Albert, - "the essence of his [Lorentz's] reasoning" - and the basis for the derivation of gamma (in the video), which is inaccurate.


  • Closed Accounts Posts: 2,491 ✭✭✭Yahew


    The fact that most people cannot agree on the exact nature of the present does not make time disappear. It flows in one direction, but it is very really there.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Yahew wrote: »
    The fact that most people cannot agree on the exact nature of the present does not make time disappear. It flows in one direction, but it is very really there.

    The fact that most people cannot agree on the exact nature of the present does not mean that time exists - which is partly the point being made.

    It doesn't flow at all, because it doesn't actually exist. It is an abstraction; a concept; a system of measurement, not something to be measured.

    How do you know that it is there?


    EDIT: just to clarify, I'm not saying that time does not exist because people cannot agree on the exact nature of the present, I'm saying that the fact, that they cannot agree, is not evidence that time exists.


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  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    I don't think it ignores the fact that the speed of light is constant in every reference frame, at all; what it says is that the model used to represent the reality of the observed phenomena is flawed, that it doesn't take into account certain relevant information.

    What it says, with regard to the original thought experiment Einstein used, is that there are four "time points" which are relevant, and which, when taken into consideration, would lead to both observers calculating that the flashes of lightning occured simultaneously.


    What it says is:
    • The flashes of lightning occur at rods A (behind train) & B (in front of train) at time=t1 - neither observer sees the flashes in that instant.
    • The light from the flashes travels towards the midpoint (M) where Observer 2 is standing.
    • As the train is in motion, Observer 1 will meet the light from rod B at a point beyond M (call it X) at time=t2
    • Observer 2 will see light from rods A & B at the same time, at time interval t=3 (Observer 2 will have travelled a further distance along the track to Y)
    • The light from Rod A will reach Observer 1 at time=t4 when Observer 1 is at location Z along the track

    According to the author, once all these times and co-ordinates are taken into account then both observers would calculate that the flashes occurred at A & B simultaneously.

    The assumption as far as I am aware, is the speed of light is constant for both observers.

    Let L be the length of the train, and c be the speed of light. Observer 2 would calculate that the flashes occurred at A & B simultaneously, at time t3 - L/2c. Observer 1 would calculate that flash A occurred at time t4 - L/2c, and that flash B occurred at time t2 - L/2c. We can see that, since the speed of light is the same for both observers (i.e. c is the same for observer 1 and 2), flash B happened before flash A for observer 1, and both flashes happened at the same time for observer 2. The only way they could both conclude that the flashes happened simultaneously is if they disagreed on the speed of light.

    The time interval L/2c comes from the fact that time = distance / speed. So the time it takes for the flash of light to reach an observer is distance (L/2) / speed (c).
    Real Space
    With regard to the "real space" the author is insisting on I think the distinciton is made between
    I think the distinction is more between conceptual space, as defined mathematically, and real, non-conceptual space.

    Conceptual space is the co-ordinate system we apply to the universe. It defines distances, as well as present, past, and future. When the author describes "real space", they are defining an absolute, "true" co-ordinate system. There is no evidence for this "real space", as the co-ordinate system of any reference frame is no more or less meaningful than any other. Neither is more "true" than any other. What is real is the Light cone structure, or "causal structure" of the universe. This is the same for all observers.
    Momentum of photons
    Again, I don't think he/she is using the strange notion of photons without momentum, what is said is that the use of geometrical "zig-zeg" lines, to represent the beam of light, is an incorrect model of the phenomena of light, which doesn't take into account the fact that light isn't necessarily a single "ray" but rather is made up of quanta. They say that these quanta don't travel in the "zig zag" fashion as depicted in the models, but follows the trajectory as outlined in figure 9 on this page.

    From what I can gather, it doesn't suggest the notion of photons without momentum, rather the idea that the photons will not inherit the velocity of the laser, that they will travel perpecdicular to the X-axis (along which the laser moves) as opposed to at an angle as suggested by the "zig-zag"! model.

    Firstly, it sounds like you are assuming the website is authoritative on what is and is not a correct model. Why?

    Secondly, and more importantly, it is the website that is presenting an incorrect model. The photon zigzags because it has momentum. It "inherits" the velocity of the emitter in the same way an egg thrown from a moving car, at a perpendicular angle, "inherits" the velocity of the car (until wind resistance slows it down.)
    Tunnel vision
    Unfortunately I can't engage in any meaningful critique of that part of your post, other than to assert that the observations from experiments applying GR won't actually change, but the manner in which they are interpreted might. Perhaps a change in model could lead to progress in the quest to unify GR and QM.

    How would they be interpreted? How would gravitational time-dilation be interpreted? How would high-energy particle collisions, or slow muon decay be interpreted?

    Julian Barbour is exploring the unification of GR and QM. Special relativity and quantum mechanics has already been successfully unified by abandoning the Galilean notion of an absolute present.

    Objections to the notion of time
    The challenge to the notion of time isn't necessarily on the basis that it is unobservable, or a derivitave quality, the objection is that it isn't even necessarily derived from other observations, it is assumed to exist a priori. The subsequent attempt to derive its existence simply isn't logical, and again relies on an unjistified assumption.

    The assumption is that certain, either specific or non-specific, physical phenomena can be used to measure this thing called time, when, in fact, those physical phenomena are simply used as standard units of comparison - they [may indeed] exist, and the things which are measured against them [may indeed] exist, but the thing called time is not measured against them.

    Time is* the name given to the system of measurement, it is not the thing which is measured.

    *or at least it should be recognised for what it is.

    We don't have to assume it exists a priori. Under Julian Barbour's formalism, it is an emergent quality, a shorthand for the structure of trajectories through configuration space, generated by the Hamiltonian.

    You can talk about time in the context of a metric, the structure of events in the universe. Or you can talk about time in the context of configuration space. But however you talk about time. You cannot talk about a true present, because it an arbitrary notion, with no obeservational evidence to infer it from.

    "True present"
    The issue of "the derivative nature of time" does not necessarily apply to the notion of the "true present". If we all live in the same universe (as opposed to occupying our own separate but interacting universes) then the notion of a "true present" is a necessary consequence, even if we cannot observe it directly - much as we cannot observe sub-atomic particles directly but can create a model to represent reality.

    A true present is not a necessary consequence. Let's say there is a radio in the centre of the room. Person A stands near the radio, Person B stands farther away. Person A will register a higher number of decibels than person B, but neither perspective is wrong. We do not need to infer a logically necessary "true" decibel level. Similarly, an event might be in the future for person A, and in the present for person B, but there is no need for a "true" co-ordinate system of events.
    If we all live in the one universe, then we can imagine the entire universe being paused, or a snapshot of the entire universe; while each observer might have a different perspective of the present, we can still deduce that there would be a "true present" or snapshot of the universe. We could theoretically put an infinite number of observers in an infinite number of reference frames (or the corresponding number of observers in the corresponding number of reference frames) and work out what the "true present" would be. It's simply a lack of information that prevents us from doing it, as opposed it not being possible.

    An infinite number of observers would not tell you what the "true" present is. Each reference frame will be just as physically meaningful as the others. An infinite number of people around the radio will not tell you the "true" decibel level of the radio.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    Let L be the length of the train, and c be the speed of light. Observer 2 would calculate that the flashes occurred at A & B simultaneously, at time t3 - L/2c. Observer 1 would calculate that flash A occurred at time t4 - L/2c, and that flash B occurred at time t2 - L/2c. We can see that, since the speed of light is the same for both observers (i.e. c is the same for observer 1 and 2), flash B happened before flash A for observer 1, and both flashes happened at the same time for observer 2. The only way they could both conclude that the flashes happened simultaneously is if they disagreed on the speed of light.

    The time interval L/2c comes from the fact that time = distance / speed. So the time it takes for the flash of light to reach an observer is distance (L/2) / speed (c).

    I'm rushing out so I'll just respond to this first part.

    I'm presuming that the length of the train L corresponds to the distance between rods A and B, such that the distance between A and B = L, is that correct?

    Also, the flash of lightning occurs when both observers are at the midway point between the rods, is that also correct?

    If so, and this is the point that the author makes: neither observer will see the lighning strike in the instant it strikes, as the light must travel from each rod toward the midpoint.

    The lighning strikes both rods at time t1.

    As the light travels from both rods towards the midpoint, observer 1 will have moved away from the midpoint towards rod B - as the train is in motion. That means, the distance the light from rod B travels is not actually L/2 but L/2 - the distance travelled. Observer 1 meets the light from rod B at time t2.

    Observer 2 calculates that the light hits both A and B simultaneouly at time t3 - at that time, observer 1 has moved further along the track, because the train is in motion.

    As the train continues to move, the distance the light from rod A has to travel to meet observer 1 is not L/2, but L/2 + the distance travelled. Obesrver 1 sees the flash of lighning strike rod A at time t4.


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    I'm rushing out so I'll just respond to this first part.

    It is good that you are looking at this in some detail. It am going to take this as an invitation to treat the problem rigorously. Something I probably should have done in the beginning. There will be a bit of maths, but nothing more complicated than simple division, multiplication Etc.
    I'm presuming that the length of the train L corresponds to the distance between rods A and B, such that the distance between A and B = L, is that correct?

    Yes, but perhaps that is only confusing the matter. We can ignore what I said about the train and only consider the lightning rods. But now that we are taking a more rigorous look at the problem, we must take into account that the distance between the rods will be in different in each reference frame. The will be closer together in the reference frame of observer 1. I will go into further detail below.
    Also, the flash of lightning occurs when both observers are at the midway point between the rods, is that also correct?

    That is what observer 2 will have deduced. Observer 1 will disagree. Further detail below.
    If so, and this is the point that the author makes: neither observer will see the lighning strike in the instant it strikes, as the light must travel from each rod toward the midpoint.

    This has never been in dispute.
    The lighning strikes both rods at time t1.

    As the light travels from both rods towards the midpoint, observer 1 will have moved away from the midpoint towards rod B - as the train is in motion. That means, the distance the light from rod B travels is not actually L/2 but L/2 - the distance travelled. Observer 1 meets the light from rod B at time t2.

    Observer 2 calculates that the light hits both A and B simultaneouly at time t3 - at that time, observer 1 has moved further along the track, because the train is in motion.

    As the train continues to move, the distance the light from rod A has to travel to meet observer 1 is not L/2, but L/2 + the distance travelled. Obesrver 1 sees the flash of lighning strike rod A at time t4.

    The short, handwavey answer to this is the author is not treating observer 1 correctly. Observer 1 would not say the distance the light has to travel is L/2 - distance travelled by the train. From observer 1's perspective, he is stationary, and instead the light has to travel a distance of ɣL/2.

    Where does ɣL/2 come from? Let's go into more detail and treat the problem rigorously. When we are referring to times or distances in observer 2's reference frame, we will use the unprimed symbols t and x. When we are referring to observer 1's reference frame, we will use the primed symbols t' and x'. For example, let's say the speed of the train from observer 2's perspective is v = 0.8 c, wherec is the speed of light (I.e. The train is travelling 80% of the speed of light). From observer 1's perspective, the train is travelling at v' = 0. I.e. It is stationary from their perspective, since they are on the train.

    According to observer 2: The rods are a distance L apart (Rod A is at location xA. Rod B is at location xB. Observer 2 is at x = 0). Let's say, for simplicity, that according to observer 2, the strikes both occur at t = 0. I.e. Observer 2 will detect both flashes simultaneously, at t = L/2c (Or t3 to use you name), and deduce that they occurred at t = 0. In this same reference frame, the light from rods A and B will reach observer 1 at times L/2(c+v) and L/2(c-v) respectively (or t2 and t4 to use your names).

    I will introduce a set of relationships that can be used to transform one reference frame into another. By this, I mean you can use these relationships to "transform" the perspective from one observer to the other. The first set are called Galilean transformations. These are the transformations we use if we ignore relativity (I.e. if the speed of light is not the same for all observers). They look like this

    t' = t
    x' = x-vt


    When we include relativity in our calculations, we must use Lorentz transformations. They look like this

    t' = ɣ(t - vx/c^2)
    x' = ɣ(x - vt)


    Now let's look at observer 1's perspective. First let's ignore relativity by using the Galilean transformations. Considering times first, we use the relation t' = t. I.e. The times are all the same! Observer 1 deduces that the flashes occured at

    t' = t = 0

    that the light from rod A reaches him at

    t2' = t2

    and the light from rod B reaches him at

    t4' = t4

    What about locations? Rod A is struck at

    xA' = xA-vt = xA-v(0) = xA

    Rod B is similarly struck at

    xB' = xB-vt = xB-v(0) = xB

    Similarly, observer 1 will conclude that the rods are the same distance L apart. In other words, observer 1 will agree with observer 2 that the lighting was simultaneous, and that the reason he sees flash A before flash B is because of the relative motion between him and the rods.

    So what happens when we take into account the fact that the speed of light is the same for all observers? Let's first look at locations. When rod A is struck by lightning, it is at

    xA' = ɣ(xA - vt) = ɣ(xA - 0) = ɣxA

    Similarly, Rod B is at

    xB' = ɣxB

    Notice that the length between the rods is no longer L, but instead ɣL.

    Now let's look at the times. Lightning strikes Rod A at

    t' = ɣ(t - vxA/c^2) = ɣ(0 - vxA/c^2) = -ɣvxA/c^2

    It strikes Rod B at

    t' = ɣ(t - vxB/c^2) = ɣ(0 - vxB/c^2) = -ɣvXB/c^2.

    I.e. The light from Rod A reaches observer 1 at

    t2' = ɣ(t2 - vx/c^2) = ɣ(t2 - t2v^2/c^2) = ɣt2(1 - v^2/c^2)

    and the light from Rod B reaches the observer at

    t3' = ɣ(t3 - vx/c^2) = ɣ(t3 - t3v^2/c^2) = ɣt3(1 - v^2/c^2)

    Since rod A is at

    xA' = ɣxA

    the lightning must have struck rod A at

    t2' - light travel time = ɣt2(1 - v^2/c^2) - ɣxA/c = -ɣvxA/c^2 = t' (Rod A)

    Similarly, lightning struck rod B at

    t3' - light travel time = ɣt3(1 - v^2/c^2) - ɣxB/c = -ɣvxB/c^2 = t' (Rod B)

    Notice that t' is different for rod A and B. I.e. They didn't strike at the same time! Notice as well that it is not simply that the times t2' and t4' are different, but that t' the actual lightning events, are different.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    before replying to both posts, I just want to say that I appreciate you taking the time to give such detailed replies

    EDIT: apologies, I will reply to the most recent post for now, because there are a few issues I'd like to discuss which are more central I think.

    EDIT 2: plus, I'm not entirely sure how to address some of the issues raised, so I've contacted the author for his opinions.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Just to address one of the things you mentioned in your second last post, with regard to my assuming that the website is authoritative: that probably is the case to a certain extent, but not entirely I would say.

    Confrimation bias
    Obviously there is the potential issue of confirmation bias, - although that doesn't necessarily invalidate the information - where I have come across something (directly from the video you posted) which confirms my own intuition. Unfortunately, I don't really know enough about the ins and outs, of SR or GR, to be able to identify the issues with what the author has posted on his website. The points made, however, do appear, to my mind, to be logical, hence I am tending towards accepting them, without fully accepting them as "gospel"; but the only way I have [at present] of scrutinising them is by presenting them and seeing what the arguments against them are.

    Now, I have a certain level of understanding of SR and GR, and what the author has posted on his website, which means that there are certain issues that I can potentially discuss - specifically in relation to the logic of certain assumptions - which I will try and do in this post.

    Observer 1
    Based on your detailed explanation above, it appears as though there is a fundamental issue with how Observer 1 is treated. It appears either as though Observer 1 is coming to an incorrect conclusion, or that we are projecting an incorrect assumption onto Observer 1; that assumption, or incorrect conclusion, is in relation to the distance the light has to travel to reach Observer 1.


    When the lightning strikes both rods, Observer 1 doesn't see the lightning but sees the rods, and at that point he concludes that the rods are equidistant to him.

    When the light from rod B reaches him, given the same ability to measure distances, he will see that rod B is now closer to him, while rod A is now further away.

    When the light from rod A reaches him, he will observe that rod A is now even further away from him, and that the distance between him and rod B has again changed.

    On his approach to that section of the tracks between the rods, he would also observe the changing distances of rods A and B; on his approach he would pass rod A and on his exist he would pass rod B, so he should be able to deduce something with respect to the changing distances.

    Observer 1(a)
    To solve the problem of our Observer 1 who believes that he is not moving, can we introduce Observer 1 (a) who got on the train at the last stop. In this case Observer 1 (a) would have seen the train pulling into the platform and so could deduce that the train is in motion when the lightning strikes are witnessed. While Observer 1 (a), not knowing he is on a moving train, and failing to take notice of the changing distances of rods A and B, with respect to himself, might calculate that the lightning strikes didn't occur simultaneously, Observer 1 (a), with the additional information, should be able to carry out a more accurate calculation.

    Observer 1 (b)
    If we install another observer, Observer 1 (b), who is the driver of the train, or the co-pilot perhaps, who knows the exact velocity of the train; again, armed with this important information, he might come up with a different calcuation, no?


    Observer 1 (c)
    Alternatively, if we were to install Observer 1 (c), who has a highly-sophisticated, laser menasurement system, which continuously monitors the distance between her and each individual rod. When the light from a rod reaches her, or rather a device on her person, it registers the distance to that rod, giving her an extremely accurate measurement of the distance the light had to travel. Armed with this, she would definitely conclude that the distances, that each flash of light had to travel, were different.


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    Observer 1
    Based on your detailed explanation above, it appears as though there is a fundamental issue with how Observer 1 is treated. It appears either as though Observer 1 is coming to an incorrect conclusion, or that we are projecting an incorrect assumption onto Observer 1; that assumption, or incorrect conclusion, is in relation to the distance the light has to travel to reach Observer 1.


    When the lightning strikes both rods, Observer 1 doesn't see the lightning but sees the rods, and at that point he concludes that the rods are equidistant to him.

    But you are again mixing observer 1 and observer 2's perspective. Observer 1, from the perspective of observer 2, is equidistant from the rods when the lightning strikes, while neither observer sees the lightning strike yet.

    From observer 1's perspective, the lightning events are equidistant to him (The total distance being ɣL), but they occur at different times. When rod B is struck, it is a distance ɣL/2 away from observer 1. This occurs before both rods are equidistant to observer 1. (I probably should have shown in my last post that there is a length contraction between the rods. They are a distance L/ɣ. The ɣL should be called the distance between lightning strikes.) So by the time observer 1 is passing observer 2, there has already been a bolt of lightning.
    When the light from rod B reaches him, given the same ability to measure distances, he will see that rod B is now closer to him, while rod A is now further away.

    There is a very important clarification that needs to be made here. It might be the reason for the author's confusion.

    He won't "see" the rod closer to him. When the lightning struck, rod B was a distance of ɣL/2 away from him. So the flash of light contains information about a rod of distance ɣL/2 away. At the instant he sees the flash, the rod will be closer, but he won't see a closer rod because that information has not reached him yet. So he sees a rod ɣL/2 away get struck. He knows he is seeing an image from the past, and by considering the speed of light and the distance it travelled, he can work how when, in the past, the lightning struck. He will reach a different conclusion to observer 2.

    A common example is the moon. We don't actually see where the moon is. We see where it was about a second ago.
    When the light from rod A reaches him, he will observe that rod A is now even further away from him, and that the distance between him and rod B has again changed.

    The same goes for rod A. Rod A will be further away, but he will not observe rod A to be further away as that information has not reached him. What he sees is how far away rod A was when it was struck. This allows him to calculate the distance travelled by the light, and deduce when rod A was struck.
    On his approach to that section of the tracks between the rods, he would also observe the changing distances of rods A and B; on his approach he would pass rod A and on his exist he would pass rod B, so he should be able to deduce something with respect to the changing distances.

    This would only happen if there was a change in relative velocity between the train and the platform.
    Observer 1(a)
    To solve the problem of our Observer 1 who believes that he is not moving, can we introduce Observer 1 (a) who got on the train at the last stop. In this case Observer 1 (a) would have seen the train pulling into the platform and so could deduce that the train is in motion when the lightning strikes are witnessed. While Observer 1 (a), not knowing he is on a moving train, and failing to take notice of the changing distances of rods A and B, with respect to himself, might calculate that the lightning strikes didn't occur simultaneously, Observer 1 (a), with the additional information, should be able to carry out a more accurate calculation.

    Observer 1 (b)
    If we install another observer, Observer 1 (b), who is the driver of the train, or the co-pilot perhaps, who knows the exact velocity of the train; again, armed with this important information, he might come up with a different calcuation, no?

    Observer 1 (c)
    Alternatively, if we were to install Observer 1 (c), who has a highly-sophisticated, laser menasurement system, which continuously monitors the distance between her and each individual rod. When the light from a rod reaches her, or rather a device on her person, it registers the distance to that rod, giving her an extremely accurate measurement of the distance the light had to travel. Armed with this, she would definitely conclude that the distances, that each flash of light had to travel, were different.

    Now we are getting to the crux of the matter. Observer 1(a) and 1(b) and 1(c) (Assuming 1(c) got on the train with 1(a) ) would intuitively conclude that the train is moving, and that the platform is stationary. But is this true? The earth is rotating, so the ground at the equator is moving at 1000 miles per hour. If the train heads off in the opposite direction, it would be stationary, even though observers would assume the train is moving.

    So the train would be stationary with respect to the centre of the earth. But is the earth stationary? It is orbiting the sun at 30 kilometres per second. But is the sun stationary? Our solar system is orbiting the galactic centre at 250 kilometres per second. Is the galactic centre stationary? Is the centre of mass of all the galaxies in the universe stationary? Maybe everything is drifting.

    This is the core lesson of relativity. No instrument or collection of instruments, no matter how sophisticated, no matter how many frames of reference you consider, can produce an absolute velocity. It must always be measured relative to the velocity of something else. This is similar to the abandonment of absolute time. So you should really be arguing against the author.


  • Registered Users Posts: 3,457 ✭✭✭Morbert


    EDIT 2: plus, I'm not entirely sure how to address some of the issues raised, so I've contacted the author for his opinions.

    Has he gotten back to you? I am interested in hearing his response.


  • Registered Users Posts: 2,552 ✭✭✭roosh


    I think we are getting to the crux of the issue.
    Morbert wrote: »
    But you are again mixing observer 1 and observer 2's perspective. Observer 1, from the perspective of observer 2, is equidistant from the rods when the lightning strikes, while neither observer sees the lightning strike yet.

    From observer 1's perspective, the lightning events are equidistant to him (The total distance being ɣL), but they occur at different times. When rod B is struck, it is a distance ɣL/2 away from observer 1. This occurs before both rods are equidistant to observer 1. (I probably should have shown in my last post that there is a length contraction between the rods. They are a distance L/ɣ. The ɣL should be called the distance between lightning strikes.) So by the time observer 1 is passing observer 2, there has already been a bolt of lightning.

    This is how I understand it: Observer 1 is moving relative to Observer 2; Observer 2 is moving relative to Observer 1; at a velocity of 0.8c in both cases. The crux of the issue, as far as I can see, is that Observer 1 is also moving relative to the rods, while Observer 2 is at rest, relative to the rods.

    So, strictly from the perspective of Observer 1: he is equidistant from rods A and B when the lighning strikes, but he doesn't know that, and he doesn't know that the lightning has struck because he can't see the rods and the the information doesn't reach him instantaneously.

    If Observer 1 moves relative to Observer 2, or vice versa, then Observer 1 and the rods also move relative to each other. Observer 1 doesn't know this, because he can't see the rods, but this is what happens.

    Given the positions of the rods relative to one another, and the fact that he and the rods are moving relative to each other, when the light from rod B reaches him, he will no longer be equidistant from both rods; likewise when the light from rod A reaches him.

    Morbert wrote: »
    There is a very important clarification that needs to be made here. It might be the reason for the author's confusion.

    He won't "see" the rod closer to him. When the lightning struck, rod B was a distance of ɣL/2 away from him. So the flash of light contains information about a rod of distance ɣL/2 away. At the instant he sees the flash, the rod will be closer, but he won't see a closer rod because that information has not reached him yet. So he sees a rod ɣL/2 away get struck. He knows he is seeing an image from the past, and by considering the speed of light and the distance it travelled, he can work how when, in the past, the lightning struck. He will reach a different conclusion to observer 2.

    A common example is the moon. We don't actually see where the moon is. We see where it was about a second ago.


    The same goes for rod A. Rod A will be further away, but he will not observe rod A to be further away as that information has not reached him. What he sees is how far away rod A was when it was struck. This allows him to calculate the distance travelled by the light, and deduce when rod A was struck.

    Here again is another critical issue I think. You mention that when he sees the lighning strike rod B, rob B will actually be closer but the information contained in the flash will be of a rod further away; similiarly, you say the flash from rod A will contain information showing the rod closer, even though it is now further away. I'm not sure this is actually correct however.

    The assumption is that the flash only contains information about the rod, but again, that isn't necessarily true, as it will also contain information about its journey to the observer, and the exact distance it has traversed; in the case of the strike at rod B, the information will show that the distance traversed was shorter compared to the flash form rod A.

    You mention that what he is seeing is an image from the past, much as what we see of the moon is an image from a second ago, but, strictly speaking, this isn't the case. What he is actually seeing is the light hitting his retina, having travelled a certain distance - much as what we see of the moon is the light from the moon hitting our retina in the present.

    Morbert wrote: »
    Now we are getting to the crux of the matter. Observer 1(a) and 1(b) and 1(c) (Assuming 1(c) got on the train with 1(a) ) would intuitively conclude that the train is moving, and that the platform is stationary. But is this true? The earth is rotating, so the ground at the equator is moving at 1000 miles per hour. If the train heads off in the opposite direction, it would be stationary, even though observers would assume the train is moving.

    So the train would be stationary with respect to the centre of the earth. But is the earth stationary? It is orbiting the sun at 30 kilometres per second. But is the sun stationary? Our solar system is orbiting the galactic centre at 250 kilometres per second. Is the galactic centre stationary? Is the centre of mass of all the galaxies in the universe stationary? Maybe everything is drifting.

    The crux of the issue, as far as I can see, isn't the relative motion of either Observer with respect to the earth, the sun, or our solar system; it is that Observer 1 is moving relative to Observer 2, as well as both rods, A and B, while Observer 2 is at rest relative to the rods.
    Morbert wrote: »
    This is the core lesson of relativity. No instrument or collection of instruments, no matter how sophisticated, no matter how many frames of reference you consider, can produce an absolute velocity. It must always be measured relative to the velocity of something else. This is similar to the abandonment of absolute time. So you should really be arguing against the author.

    With respect to the instruments, would some form of sophisticated laser measurement system not provide relevant information? If each rod was fitted with some form of emitter which was capable of registering lightning strikes; if the emitter sent out a constant pulse to a device monitored by Observer 1, and sent out a single, different pulse when strick by lightning, but then reverted back to its usual signal, could some form of triangulation be set up, which would provide more accurate info?


  • Registered Users Posts: 2,552 ✭✭✭roosh


    Morbert wrote: »
    Has he gotten back to you? I am interested in hearing his response.

    he emailed me back, but I must have another read of what he says, to make sure I understand it.

    He asked me not to copy and paste his replies, but if I want, to put it in my own words and reference him - hence the need to re-read.

    I'm not sure me being the middle man would be the ideal way to go about questioning him though, but there is a "contact" button on his website which is how I contacted him.

    If you do decide to email him, would it be cheeky of me to ask to be ccd on it, as I would be very interested in reading and informed discussion.


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  • Registered Users Posts: 3,457 ✭✭✭Morbert


    roosh wrote: »
    This is how I understand it: Observer 1 is moving relative to Observer 2; Observer 2 is moving relative to Observer 1; at a velocity of 0.8c in both cases. The crux of the issue, as far as I can see, is that Observer 1 is also moving relative to the rods, while Observer 2 is at rest, relative to the rods.

    So, strictly from the perspective of Observer 1: he is equidistant from rods A and B when the lighning strikes, but he doesn't know that, and he doesn't know that the lightning has struck because he can't see the rods and the the information doesn't reach him instantaneously.

    If Observer 1 moves relative to Observer 2, or vice versa, then Observer 1 and the rods also move relative to each other. Observer 1 doesn't know this, because he can't see the rods, but this is what happens.

    Given the positions of the rods relative to one another, and the fact that he and the rods are moving relative to each other, when the light from rod B reaches him, he will no longer be equidistant from both rods; likewise when the light from rod A reaches him.

    Again, nobody is disputing that the light will reach him first. The light will reach him at t2 and t4 (from the perspective of oberver 2), and at t2' and t4' from the perspective of observer 1. But since the speed of light is the same for both observers, they must disagree on when the lightning struck. Let my explain why your scenario would mean the speed of light is not the same for all observers:

    According to your scenario, rod B is a distance L/2 away when lightning strikes. Both observer 1 and observer 2 are at location x = 0 when this happens. Observer 1 speeds towards the light wave at some velocity v. Let's say he meets the light wave one second after the lightning strikes. So in that second, the wave of light travels at speed c, so that it is a distance of one light-second away from the rod. Both observer 1 and observer 2 would agree that the light was one light-second away from the rod when it met observer 1. Observer 1 would therefore conclude, in agreement with observer 2, that the the lightning struck one second after observer 1 met the wave. The wave would continue on to observer 2 (say 1.5 seconds after the lightning struck). So far so good.

    But here is the catch. Observer 1 sees the rod travelling towards him at a speed of v. If the light was able to speed ahead of the rod, so that it was one light-second ahead of the rod after one second, then observer 1 would observe the light travelling at a speed of (c+v)! Otherwise how could it be a light-second ahead of the rod? Observer 1 could do a quick calculation and work out that the speed of light, with respect to its source, is c. But the speed of light is c with respect to all observers, not just the rest-frame of the source.

    In other words, if both observers agree about the distance the light travelled before meeting observer 1, and both observers disagree about the motion of the rod, then they cannot agree on the speed of light.
    Here again is another critical issue I think. You mention that when he sees the lighning strike rod B, rob B will actually be closer but the information contained in the flash will be of a rod further away; similiarly, you say the flash from rod A will contain information showing the rod closer, even though it is now further away. I'm not sure this is actually correct however.

    The assumption is that the flash only contains information about the rod, but again, that isn't necessarily true, as it will also contain information about its journey to the observer, and the exact distance it has traversed; in the case of the strike at rod B, the information will show that the distance traversed was shorter compared to the flash form rod A.

    And to reiterate, what is the distance it has traversed? According to observer 2, the rod is stationary, and the light has traversed a distance of one light second to meet observer 1. But according to observer 1, if we follow your train of thought, the rod is observed to be moving towards him at speed v, and the light still traverses a light-second ahead of the rod.
    You mention that what he is seeing is an image from the past, much as what we see of the moon is an image from a second ago, but, strictly speaking, this isn't the case. What he is actually seeing is the light hitting his retina, having travelled a certain distance - much as what we see of the moon is the light from the moon hitting our retina in the present.

    You are reading to much into my use of the word image. I meant it in the same way a photograph is an "image" of the past. We don't directly see the moon, but instead see an "image" of where it was when the light left it. Likewise, observer 1 sees an image of where the rod was when light left it. I cannot stress enough that, when scientists are talking about time dilation, or the lack of an absolute present, they are not merely talking about the different times of detection of light. That is incorporated into relativity.
    The crux of the issue, as far as I can see, isn't the relative motion of either Observer with respect to the earth, the sun, or our solar system; it is that Observer 1 is moving relative to Observer 2, as well as both rods, A and B, while Observer 2 is at rest relative to the rods.

    Then what is the absolute "present"? What must we be stationary with respect to in order to determine the true present?
    With respect to the instruments, would some form of sophisticated laser measurement system not provide relevant information? If each rod was fitted with some form of emitter which was capable of registering lightning strikes; if the emitter sent out a constant pulse to a device monitored by Observer 1, and sent out a single, different pulse when strick by lightning, but then reverted back to its usual signal, could some form of triangulation be set up, which would provide more accurate info?

    We can assume from the start that any postulated observer is using instruments with perfect precision. We can even assume the instruments are better than is physically possible.


This discussion has been closed.
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