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Does time exist?
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Were you not saying that an event is only physically in the past the moment a hypothetical particle arrives at the observer? Presumably it must be, because there can be an infinite number of reference frames approaching the speed of light who say that the explosion occurs at 12:07:59.99999999999 recurring, and presumably the explosion can't be in the past at the moment it occurs, or while it is occuring.Yes. That is what I have been saying. All observers agree that the explosion is in the past of 12:08.0 -
Were you not saying that an event is only physically in the past the moment a hypothetical particle arrives at the observer? Presumably it must be, because there can be an infinite number of reference frames approaching the speed of light who say that the explosion occurs at 12:07:59.99999999999 recurring, and presumably the explosion can't be in the past at the moment it occurs, or while it is occuring.
Let be more explicit: The explosion is in the past of the earth clock reading 12:08. This is true for all frames of reference. However, the explosion is not always in the past of any earth clock reading before 12:08. For example "The explosion occurred after the earth clock reads 12:07" is true for some frames of reference.0 -
This means that the explosion event is in the past, for one reference frame, the moment it is occurring; that is, it is simultaneously in the past and the present.Let be more explicit: The explosion is in the past of the earth clock reading 12:08. This is true for all frames of reference. However, the explosion is not always in the past of any earth clock reading before 12:08. For example "The explosion occurred after the earth clock reads 12:07" is true for some frames of reference.
If the arrival of the particle at the earthbound clock, at 12:08, isn't the moment that the object is physically in the past, then it means that at some moment before the earthbound clock strikes 12:08 is when it is physically in the past; however, for any moment before the clock strikes 12:08 there is [hypothetically] a reference frame which says that the explosion occurs at that time reading on the earthbound clock; hence, it says that the event is in the present and the past.0 -
This means that the explosion event is in the past, for one reference frame, the moment it is occurring; that is, it is simultaneously in the past and the present.
If the arrival of the particle at the earthbound clock, at 12:08, isn't the moment that the object is physically in the past, then it means that at some moment before the earthbound clock strikes 12:08 is when it is physically in the past; however, for any moment before the clock strikes 12:08 there is [hypothetically] a reference frame which says that the explosion occurs at that time reading on the earthbound clock; hence, it says that the event is in the present and the past.
You're not using terms consistently. I do not know what you mean when you say "the moment the object is physically in the past".
All observers will agree that the explosion happened before the clock on earth struck 12:08.
Not all observers will agree that the explosion happened before the clock on earth struck 12:07.0 -
You talked about the "physical past, present, and future", so there is a point where an event is physically in the past, present, or future, with regard to another event. In this case the events we are talking about are the explosion of the sun and the arrival of the hypothetical particle at the observer. You mentioned that the explosion event is only in the physical past of the observer at the moment the hypothetical particle arrives at the earth clock and observer, at the time of 12:08.You're not using terms consistently. I do not know what you mean when you say "the moment the object is physically in the past".
The point being made is that if the earth clock/observer and the sun are spatially separated, then the explosion must be in the physical past of the observer prior to 12:08, becuase the hypothetical particle only arrives at 12:08.All observers will agree that the explosion happened before the clock on earth struck 12:08.
Not all observers will agree that the explosion happened before the clock on earth struck 12:07.
The issue is that, for all the times prior to 12:08, there is a hypothetical reference frame which says that the explosion happens at that time; that is, for every temporal value prior to 12:08, as read by the earth clock, there is a corresponding reference frame which notes that as the time of the explosion. This means that there will be a reference frame which says that the explosion occurs at some timme prior to 12:08 which corresponds to the point at which it is in the observers past; meaning that it simultaneously exists in the observers physical past and present.0 -
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You talked about the "physical past, present, and future", so there is a point where an event is physically in the past, present, or future, with regard to another event. In this case the events we are talking about are the explosion of the sun and the arrival of the hypothetical particle at the observer. You mentioned that the explosion event is only in the physical past of the observer at the moment the hypothetical particle arrives at the earth clock and observer, at the time of 12:08.
Yes.The point being made is that if the earth clock/observer and the sun are spatially separated, then the explosion must be in the physical past of the observer prior to 12:08, becuase the hypothetical particle only arrives at 12:08.
The issue is that, for all the times prior to 12:08, there is a hypothetical reference frame which says that the explosion happens at that time; that is, for every temporal value prior to 12:08, as read by the earth clock, there is a corresponding reference frame which notes that as the time of the explosion. This means that there will be a reference frame which says that the explosion occurs at some timme prior to 12:08 which corresponds to the point at which it is in the observers past; meaning that it simultaneously exists in the observers physical past and present.
This is where your phrasing breaks down. What do you mean by observer here? Do you mean the observer at 12:08? 12:07? At any time prior to 12:08? If you mean the observer at any time before 12:08, then yes, there are observers who would not agree that the explosion is in the past. It is only at the 12:08 mark that all observers agree that the explosion is in the past.
From what I can gather, you're simply highlighting the fact that the explosion can be simultaneous with an observer's clock strike, according to one frame of reference, and in the past according to another. That is not a problem for the same reason all of your other thought experiments aren't problems. The hyperbolic spacetime geometry, as expressed in the Lorentz transformations, preserves the causal structure of the universe, leading to no paradoxes.0 -
What was meant by observer was the same as what was meant by observer above, to which you replied yes; although, I would agree that it isn't very clear.Yes.
This is where your phrasing breaks down. What do you mean by observer here? Do you mean the observer at 12:08? 12:07? At any time prior to 12:08? If you mean the observer at any time before 12:08, then yes, there are observers who would not agree that the explosion is in the past. It is only at the 12:08 mark that all observers agree that the explosion is in the past.
From what I can gather, you're simply highlighting the fact that the explosion can be simultaneous with an observer's clock strike, according to one frame of reference, and in the past according to another. That is not a problem for the same reason all of your other thought experiments aren't problems. The hyperbolic spacetime geometry, as expressed in the Lorentz transformations, preserves the causal structure of the universe, leading to no paradoxes.
It seems that the idea is that, when any observer shares their present moment with the earthbound clock at 12:08, the event is in their past, such that the event is in the physical past by the time the hypothetical particle arrives at the clock at 12:08.
As was mentioned, however, the event must be in the physical past at some point prior to 12:08, because the clock and the sun are spatially separated; by the time the particle has traveled half the distance the explosion is in the past - this is true for every reference frame - and the clock has not yet struck 12:08; this is also true for when the particle has traveled 99.999% of the distance. So, every observer would have to agree that the explosion is in the past at some point before the clock strikes 12:08.
There will be one hypothetical reference frame which says that the explosion occurs when the earth clock reads 12:07:59.99999999 (recurring). When the particle has traveled half the distance, the clock still will not have struck 12:08, it will still be on 12:07:59.99999999; the same is true when the particle has traveled 99.99999% of the distance. That reference frame will have to agree that the explosion is in the past and the present at the same moment.
How does the concept of the physical present work?0 -
What was meant by observer was the same as what was meant by observer above, to which you replied yes; although, I would agree that it isn't very clear.
It seems that the idea is that, when any observer shares their present moment with the earthbound clock at 12:08, the event is in their past, such that the event is in the physical past by the time the hypothetical particle arrives at the clock at 12:08.
Yes.As was mentioned, however, the event must be in the physical past at some point prior to 12:08, because the clock and the sun are spatially separated; by the time the particle has traveled half the distance the explosion is in the past - this is true for every reference frame - and the clock has not yet struck 12:08; this is also true for when the particle has traveled 99.999% of the distance. So, every observer would have to agree that the explosion is in the past at some point before the clock strikes 12:08.
Yes.There will be one hypothetical reference frame which says that the explosion occurs when the earth clock reads 12:07:59.99999999 (recurring). When the particle has traveled half the distance, the clock still will not have struck 12:08, it will still be on 12:07:59.99999999; the same is true when the particle has traveled 99.99999% of the distance. That reference frame will have to agree that the explosion is in the past and the present at the same moment.
No. According to this reference frame, it will not still be on 12:07.99999999 when the particle reaches half way. It will instead be on 12:07.999999995. If by 12:07.99999999, you meant 9 recurring, then that is simply 12:08. There is no frame of reference (unless you jerry-rig one for the photon, which doesn't work) which says the explosion happened at 12:07.9999999... i.e. 12:08.0 -
Is there a limit to the number of decimal places?Yes.
Yes.
No. According to this reference frame, it will not still be on 12:07.99999999 when the particle reaches half way. It will instead be on 12:07.999999995. If by 12:07.99999999, you meant 9 recurring, then that is simply 12:08. There is no frame of reference (unless you jerry-rig one for the photon, which doesn't work) which says the explosion happened at 12:07.9999999... i.e. 12:08.
Also, how does the concept of the physical present work?0 -
Is there a limit to the number of decimal places?
No.
http://en.wikipedia.org/wiki/0.999...Also, how does the concept of the physical present work?
I don't understand the question. Two events are labelled simultaneous if they have the same coordinate time. This is frame-dependent and not physical.0 -
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cool! I hadn't come across that before.
I'm not sure it would work for a clock though, assuming we can postulate an infinitely precise clock, because a counter at 12:07:59.(9) would not be the same as a counter at 12:08; would that bring us to the idea of the duration of a moment I wonder?
Does this mean we have a physical past and a physical future, but no physical present?I don't understand the question. Two events are labelled simultaneous if they have the same coordinate time. This is frame-dependent and not physical.0 -
cool! I hadn't come across that before.
I'm not sure it would work for a clock though, assuming we can postulate an infinitely precise clock, because a counter at 12:07:59.(9) would not be the same as a counter at 12:08; would that bring us to the idea of the duration of a moment I wonder?
That's an engineering matter, and not at all relevant to the thread.Does this mean we have a physical past and a physical future, but no physical present?
In the block universe you have a 4D structure of all events. There is no holistic past present and future. Instead, we same some events are physically in the past or future of other events. The present, physically speaking, is local to single events. I.e. An event is in the present of itself. But there is no meaningful physical present between events that are causally unconnected. In relativity, the fundamental entities are events and their causal structure.0 -
OK, there's no point in labouring it any further so.That's an engineering matter, and not at all relevant to the thread.
If you don't mind I'd like to explore this a little further; I don't have a conclusion in mind, I just want to see where the reasoning leads.In the block universe you have a 4D structure of all events. There is no holistic past present and future. Instead, we same some events are physically in the past or future of other events. The present, physically speaking, is local to single events. I.e. An event is in the present of itself. But there is no meaningful physical present between events that are causally unconnected. In relativity, the fundamental entities are events and their causal structure.
I suppose an issue I have trouble with is the idea that an event can physically be in the past of another event, or it can physically be in the future of another event, but it can never physically be in the present of it; also that it is possible for an event to be in neither of those "locations" with respect to another event.
If we look at it in terms of the present being local to single events, and that an event is in the present of itself:
If an event is in the present of itself, physically speaking, and there are physically separated regions of the universe, where other events occur, then those events must be in the present of each other, such that the present isn't simply local to single events; that is, an event is not only in the present of itself. Looking at it in the negative sense seems somewhat questionable; physically speaking, no event is in the present of any other event.
If we have 2 observers at rest relative to each other, in regions of the universe very distant from each other, will those observers be in the present of each other, physically speaking?0 -
If you don't mind I'd like to explore this a little further; I don't have a conclusion in mind, I just want to see where the reasoning leads.
Ok, but forgive me if I am a little suspicious of the above claim. In previous threads, I have repeatedly watched open and honest questions slowly morph into incorrect assertions.I suppose an issue I have trouble with is the idea that an event can physically be in the past of another event, or it can physically be in the future of another event, but it can never physically be in the present of it; also that it is possible for an event to be in neither of those "locations" with respect to another event.
If we look at it in terms of the present being local to single events, and that an event is in the present of itself:
If an event is in the present of itself, physically speaking, and there are physically separated regions of the universe, where other events occur, then those events must be in the present of each other, such that the present isn't simply local to single events; that is, an event is not only in the present of itself. Looking at it in the negative sense seems somewhat questionable; physically speaking, no event is in the present of any other event.
The statement in blue is the problematic one.
Scour your brain of all preconceptions, and try build your image of the block universe from scratch. You can reject the metaphysics later if you wish, but for the time being, let's start from the beginning.
Let's start with events. These events currently have no causal relation defined because we have not given them one yet, and hence we have no past present or future, so let's do that now. First, let's give them a Newtonian causal relation. I.e. Let us suppose there is no speed limit to the propagation of signals through which one event can be said to affect another. We have "instantaneous effects" at a distance. From this, we can build a picture of relation between all events where every event has a "past" consisting of all events that can affect it, a "future" consisting of all events it can affect, and a "present" consisting that all events that would be connected by an "instantaneous" signal.
But now let's give events a relativistic causal structure. Now, signals can't travel faster than a given speed, so there are now events separated in space that cannot be causally connected because no signal can pass between them. Now, what physical meaning is there to saying an event is in the present of another? Since there can be no physical connection, no physical causal relation between the events, there is no compulsion to order the events, or to label them as simultaneous or non-simultaneous.If we have 2 observers at rest relative to each other, in regions of the universe very distant from each other, will those observers be in the present of each other, physically speaking?
Not necessarily. For example, you can have a case where, according to one observer, both observers are alive, and according to the other observer, only he/she is alive.0 -
Understandable, although I would disagree with the idea that all of the assertions are incorrect. Those other threads usually start out from a "suspicion" and develop; in this case, there is no "suspicion". That's not to say that, as the picuture builds there won't be questions or challenges along the way.Ok, but forgive me if I am a little suspicious of the above claim. In previous threads, I have repeatedly watched open and honest questions slowly morph into incorrect assertions.
Part of the issue I have is with the idea of the present consisting of events which have a causal influence on each other; obviously, for each individual, their present moment, or now slice, is made up of events which don't have a causal influence on them.The statement in blue is the problematic one.
Scour your brain of all preconceptions, and try build your image of the block universe from scratch. You can reject the metaphysics later if you wish, but for the time being, let's start from the beginning.
Let's start with events. These events currently have no causal relation defined because we have not given them one yet, and hence we have no past present or future, so let's do that now. First, let's give them a Newtonian causal relation. I.e. Let us suppose there is no speed limit to the propagation of signals through which one event can be said to affect another. We have "instantaneous effects" at a distance. From this, we can build a picture of relation between all events where every event has a "past" consisting of all events that can affect it, a "future" consisting of all events it can affect, and a "present" consisting that all events that would be connected by an "instantaneous" signal.
But now let's give events a relativistic causal structure. Now, signals can't travel faster than a given speed, so there are now events separated in space that cannot be causally connected because no signal can pass between them. Now, what physical meaning is there to saying an event is in the present of another? Since there can be no physical connection, no physical causal relation between the events, there is no compulsion to order the events, or to label them as simultaneous or non-simultaneous.
It might be helpful to look at this from both the perspectives of Minkowskian spacetime and presentism, given that relativity is compatible with both. We have the Minkowskian interprtetation which says that there is no physical meaning of one event being in the present of another. But if we look at things from the perspective of presentism, and through the lens of a finite speed for the propagation of signals, then we have events which don't have a causal influence on each other but which are in the present of each other.
We wouldn't say that they are in the "physical present" because there is no physical thing called "the present", but events would be in the present of each other.
Would gravitational time dilation be the explanation in that case?Not necessarily. For example, you can have a case where, according to one observer, both observers are alive, and according to the other observer, only he/she is alive.0 -
Part of the issue I have is with the idea of the present consisting of events which have a causal influence on each other; obviously, for each individual, their present moment, or now slice, is made up of events which don't have a causal influence on them.
It might be helpful to look at this from both the perspectives of Minkowskian spacetime and presentism, given that relativity is compatible with both. We have the Minkowskian interprtetation which says that there is no physical meaning of one event being in the present of another. But if we look at things from the perspective of presentism, and through the lens of a finite speed for the propagation of signals, then we have events which don't have a causal influence on each other but which are in the present of each other.
We wouldn't say that they are in the "physical present" because there is no physical thing called "the present", but events would be in the present of each other.
This is why I said scour your brain. You are introducing your intuitive notion of present for which there is no physical compulsion to adopt. The physical, fundamental stuff of the universe would be events and their causal structure.Would gravitational time dilation be the explanation in that case?
No. You don't need to suppose any curvature.0 -
As part of the explanation you posed the questionThis is why I said scour your brain. You are introducing your intuitive notion of present for which there is no physical compulsion to adopt. The physical, fundamental stuff of the universe would be events and their causal structure.
I'm just using that example of the present moment to address that question, and to explore the concept of events physically being in the present of each other.Now, what physical meaning is there to saying an event is in the present of another? Since there can be no physical connection, no physical causal relation between the events
On the one hand you are saying that because events don't have causal influence on each other we can't really say that they are physically in the present of each other, or that there is no compulsion to; but if we look at it through the lens of presentism, would we say that events, which are spatially separated and have no causal influence on each other, are physically in the present of each other?
Oh, OK. How does it work?No. You don't need to suppose any curvature.0 -
As part of the explanation you posed the question
I'm just using that example of the present moment to address that question, and to explore the concept of events physically being in the present of each other.
On the one hand you are saying that because events don't have causal influence on each other we can't really say that they are physically in the present of each other, or that there is no compulsion to; but if we look at it through the lens of presentism, would we say that events, which are spatially separated and have no causal influence on each other, are physically in the present of each other?
This brings me back to an earlier point I made. Such a definition of the present would be wholly at odds with presentism. A distant alien's entire life history could be causally unconnected to a moment in your lifetime. Therefore, by the definition above, the history of the alien would all be in your "present".Oh, OK. How does it work?
http://imgur.com/qv29or2
In the Newtonian picture, the cones are flat, since there is no limit to the speed of light. All coordinate systems therefore obey galilean transformation rules, and hence agree on simultaneity. In the Einsteinian picture, the cones are not flat, so there is no agreement between coordinate frames. Even the way I have drawn it tacitly assumes a coordinate system. I could have drawn it another, equally valid way.0 -
The present wouldn't simply be defined as events which have no causal influence on each other are in the present of each other, it would just be possible that events which are causally unconnected to be in the present of each other.This brings me back to an earlier point I made. Such a definition of the present would be wholly at odds with presentism. A distant alien's entire life history could be causally unconnected to a moment in your lifetime. Therefore, by the definition above, the history of the alien would all be in your "present".
If you take your particular now slice, then, according to you, there are events which are spatially separated and therefore causally unconnected, which are in your present. Other spatially separated events which immediately preceded those events, would be in your past.
How is it though, that for two observers at rest relative to each other, one would say that both are alive, while the other says that only they are alive?http://imgur.com/qv29or2
In the Newtonian picture, the cones are flat, since there is no limit to the speed of light. All coordinate systems therefore obey galilean transformation rules, and hence agree on simultaneity. In the Einsteinian picture, the cones are not flat, so there is no agreement between coordinate frames. Even the way I have drawn it tacitly assumes a coordinate system. I could have drawn it another, equally valid way.0 -
The present wouldn't simply be defined as events which have no causal influence on each other are in the present of each other, it would just be possible that events which are causally unconnected to be in the present of each other.
If you take your particular now slice, then, according to you, there are events which are spatially separated and therefore causally unconnected, which are in your present. Other spatially separated events which immediately preceded those events, would be in your past.
Yes, they would be in the past according to your coordinate definition of "present". But that is a different definition from "Not in your future or past light cone".How is it though, that for two observers at rest relative to each other, one would say that both are alive, while the other says that only they are alive?
The dotted lines are the coordinate presents of the two hypothetical observers. Note that one line crosses the history of both observers, while the other only crosses one. The entire worldline of the observer on the right is "underneath" the coordinate present of (i.e. In the coordinate past of) the observer on the left.0 -
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OK, but just looking at the idea of being "physically in the present of other events"; if presentism were indeed the correct state of the universe, would event A, which is spatially separated from event B, be physically in the present of event B?Yes, they would be in the past according to your coordinate definition of "present". But that is a different definition from "Not in your future or past light cone".
Would two observers at rest relative to each other not have the same co-ordinate present, or now slice?The dotted lines are the coordinate presents of the two hypothetical observers. Note that one line crosses the history of both observers, while the other only crosses one. The entire worldline of the observer on the right is "underneath" the coordinate present of (i.e. In the coordinate past of) the observer on the left.0 -
OK, but just looking at the idea of being "physically in the present of other events"; if presentism were indeed the correct state of the universe, would event A, which is spatially separated from event B, be physically in the present of event B?
I don't know which events you are talking about. If presentism is the correct state of the universe, then we have Newtonian and Galilean relations between space and time. All observers would agree on what events are simultaneous, which would permit a global definition of "the present" where arbitrarily separated events A and B could be in the present.Would two observers at rest relative to each other not have the same co-ordinate present, or now slice?
Yes. These two observers would be moving, relative to one another.0 -
To clarify what events I mean; take your now slice; if presentism is correct, then there are events happening in a distant galaxy which are spatially separated from you, which have no causal influence on you, but which are in your present. Would these events be considered to be physically in your present?I don't know which events you are talking about. If presentism is the correct state of the universe, then we have Newtonian and Galilean relations between space and time. All observers would agree on what events are simultaneous, which would permit a global definition of "the present" where arbitrarily separated events A and B could be in the present.
With regard to agreeing on the simultaneity of events, we would still have the disagreement over the time co-ordinates of events, and perhaps a disagreement over whether or not two events were simultaneous, except with the idea that one observer could be wrong.
Sorry, I'm not clear on that. Would two observers at rest relative to each other have the same co-ordinate present; that is, their dotted lines would co-incide?Yes. These two observers would be moving, relative to one another.
Is the picture you posted of two relatively moving observers?0 -
To clarify what events I mean; take your now slice; if presentism is correct, then there are events happening in a distant galaxy which are spatially separated from you, which have no causal influence on you, but which are in your present. Would these events be considered to be physically in your present?
Yes.Sorry, I'm not clear on that. Would two observers at rest relative to each other have the same co-ordinate present; that is, their dotted lines would co-incide?
Is the picture you posted of two relatively moving observers?
Yes to both questions.0 -
Why would the same not be true, then, for the block universe?Yes.
Ah, OK, I think we were at cross purposes there; I was wondering about two observers at rest relative to each other; but the point was geared towards the same answer as above i.e. determining if events can be physically in the present of each other.Yes to both questions.0 -
Why would the same not be true, then, for the block universe?
The issue isn't the block universe per se. You could have a block universe with a Newtonian null cone structure, where transformations between reference frames are galilean, and a global present exists. But if you do this, then the speed of light would no longer be the same for all observers unless you postulate some special dynamics that work to bend and stretch apparatus so that all observers mistakenly measure the speed of light to be c.0 -
Sorry, I was referring more to the Minkowskian version of the block universe.The issue isn't the block universe per se. You could have a block universe with a Newtonian null cone structure, where transformations between reference frames are galilean, and a global present exists. But if you do this, then the speed of light would no longer be the same for all observers unless you postulate some special dynamics that work to bend and stretch apparatus so that all observers mistakenly measure the speed of light to be c.0 -
Sorry, I was referring more to the Minkowskian version of the block universe.
Unlike a Newtonian spacetime, coordinate systems on Minkowski space do not agree on simultaneity. There is nothing in the structure that would allow you to declare one coordinate present as more physical than another.0 -
Clocks are controlled by their individual mechanisms. Time is a construct of man. No other creature0
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