_heretic Posted August 27, 2012 Posted August 27, 2012 I apologise if similar topics have like this have already been posted on this forum before, but since I've started reading more in depth about Relativity, I am now confused about what the "present" is supposed to be!! Is there really a present? "When" is the present?! What determines what/when the present is?!! How "long" is the present?!!! If there isn't a present, what is the "past" and "future"?!!!! Could someone be kind enough to explain all this to me...
iNow Posted August 27, 2012 Posted August 27, 2012 It's not an easy topic. The concept of a "present" or "now" is really more metaphysical than physical. It's also not in one place, but is relative to the observer or person. Your present and my present are not equivalent. This effect is magnified as you look across great distances. More here: http://www.scienceforums.net/topic/67948-why-is-now-the-same-for-everybody/page__view__findpost__p__693437
elfmotat Posted August 27, 2012 Posted August 27, 2012 I think most people would agree that the "present" is infinitesimally short. It's like a point on a number line: it has no duration, but it exists nonetheless. The present is also relative: my present is not the same as someone who's moving relative to me.
_heretic Posted August 28, 2012 Author Posted August 28, 2012 I think most people would agree that the "present" is infinitesimally short. It's like a point on a number line: it has no duration, but it exists nonetheless. The present is also relative: my present is not the same as someone who's moving relative to me. Does this not imply that while events happen, there's no true "when" they happen?
swansont Posted August 28, 2012 Posted August 28, 2012 Past, present and future are only peripherally involved with relativity. You'll see them on a light cone diagram, but in any more substantive issues, we tend to quantify things. When things happen is relative to your frame of reference; simultaneity is not absolute.
_heretic Posted August 28, 2012 Author Posted August 28, 2012 Past, present and future are only peripherally involved with relativity. You'll see them on a light cone diagram, but in any more substantive issues, we tend to quantify things. When things happen is relative to your frame of reference; simultaneity is not absolute. So if we were to consider a single event on its own, when it happens is relative but we can be sure that: It happens. What does "happening" even mean then?
iNow Posted August 28, 2012 Posted August 28, 2012 Perhaps it just means a change of state of some sort.
swansont Posted August 28, 2012 Posted August 28, 2012 So if we were to consider a single event on its own, when it happens is relative but we can be sure that: It happens. What does "happening" even mean then? Yes. An event that occurs has to happen in all frames.
_heretic Posted August 28, 2012 Author Posted August 28, 2012 Yes. An event that occurs has to happen in all frames. So even events we would consider to be in the past then?
md65536 Posted August 28, 2012 Posted August 28, 2012 (edited) Does this not imply that while events happen, there's no true "when" they happen? I wouldn't say so. Yes, it's true that there's no absolute simultaneity of distant events, so different observers can disagree on exactly when an event occurs relative to themselves (ie. local events). However, events have proper times (the time of the event according to a clock at the event's location) which is invariant. An event's proper time is a true "when" at which the event happens. No observer will disagree. In other words the disagreement is between the event's local clock and the observer's local clock. It's impossible to make all clocks agree. --- Also note that the meaning of clock is representative of time itself... I think often people assume it refers to a particular device but it refers to all measurements of time, ie. to the behavior of time itself. Also, in terms of causality, an event truly occurs after any other events that can influence it, and before any events that it can influence. There is no room for disagreement there either. It is only remote, disconnected events (with a space-like interval) that have no true ordering. Edited August 28, 2012 by md65536
swansont Posted August 28, 2012 Posted August 28, 2012 So even events we would consider to be in the past then? If an event has happened, it's in the past. That's implied by the past tense of the verb.
_heretic Posted August 29, 2012 Author Posted August 29, 2012 If an event has happened, it's in the past. That's implied by the past tense of the verb. In the past of what? It can be in the future of something else. I think what I'm really asking is: To what extent do events exist? And can they said to have stopped existing? (You might dismiss this as metaphysics I suppose but that doesn't really make the question go away or resolve it - we are after all talking about reality here and I think it's important to know what is really going on.)
swansont Posted August 29, 2012 Posted August 29, 2012 In the past of what? It can be in the future of something else. I think what I'm really asking is: To what extent do events exist? And can they said to have stopped existing? (You might dismiss this as metaphysics I suppose but that doesn't really make the question go away or resolve it - we are after all talking about reality here and I think it's important to know what is really going on.) If you've observed an event, you can't communicate with someone about it if it hasn't happened in their frame. By the time your message to ask or tell arrives, the event will have happened.
_heretic Posted August 29, 2012 Author Posted August 29, 2012 If you've observed an event, you can't communicate with someone about it if it hasn't happened in their frame. By the time your message to ask or tell arrives, the event will have happened. Well what about events that won't/aren't/haven't been observed, to what extent are those events extant?
swansont Posted August 29, 2012 Posted August 29, 2012 Well what about events that won't/aren't/haven't been observed, to what extent are those events extant? I don't understand the question. What are the conditions where you won't observe an event? What do you mean by aren't observing an event? The don't make sense to me in the context of a physics/relativity discussion.
_heretic Posted August 30, 2012 Author Posted August 30, 2012 I don't understand the question. What are the conditions where you won't observe an event? What do you mean by aren't observing an event? The don't make sense to me in the context of a physics/relativity discussion. Okay, perhaps this is just an issue of semantics. What constitutes an observer in relativity? That's probably where I'm getting confused.
swansont Posted August 30, 2012 Posted August 30, 2012 Okay, perhaps this is just an issue of semantics. What constitutes an observer in relativity? That's probably where I'm getting confused. Usually a person but really it's just the reference frame they are in.
_heretic Posted September 1, 2012 Author Posted September 1, 2012 Usually a person but really it's just the reference frame they are in. So what is a reference frame?
swansont Posted September 2, 2012 Posted September 2, 2012 So what is a reference frame? A coordinate system.
_heretic Posted September 2, 2012 Author Posted September 2, 2012 A coordinate system. So, does this mean that everything (i.e. every particle) has a reference frame?
md65536 Posted September 2, 2012 Posted September 2, 2012 So, does this mean that everything (i.e. every particle) has a reference frame? No, but not all particles can be observers, either. Photons for example have no frame of reference and do not observe anything.
Iggy Posted September 3, 2012 Posted September 3, 2012 In relativity the present is not well defined. Not only is it not an invariant (already taking away some of its significance in relativity), the concept of non-local simultaneity breaks down and becomes something less meaningful in the presence of mass. Covering the cosmos with a coordinate system on which a line of simultaneity could be drawn is not possible so it loses its ability to be a well-defined thing.
michel123456 Posted September 3, 2012 Posted September 3, 2012 (edited) I apologise if similar topics have like this have already been posted on this forum before, but since I've started reading more in depth about Relativity, I am now confused about what the "present" is supposed to be!! Is there really a present? "When" is the present?! What determines what/when the present is?!! How "long" is the present?!!! If there isn't a present, what is the "past" and "future"?!!!! Could someone be kind enough to explain all this to me... IMHO the right question is: Where is the present ? Edited September 3, 2012 by michel123456
_heretic Posted September 3, 2012 Author Posted September 3, 2012 IMHO the right question is: Where is the present ? Indeed!
Dekan Posted September 3, 2012 Posted September 3, 2012 Indeed! I don't understand why the speed of light is involved in all this. If light travels at a certain fixed speed, why should that affect the time when something happens. For example, suppose at this present moment, a star 10,000 light-years away is exploding in a supernova. We on Earth in 2012, won't see it for another 10,000 years, as it will take light that long to reach the Earth. But surely that doesn't mean the event hasn't happened. Or are we supposed to think - no event happens until it's been seen by us on Earth. Isn't that a bit Geocentric?
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