DavidMcC wrote:Not to you, though. Perhaps to the remote observer in a different inertial frame.

But if the remote observers reference frame is as equally valid as mine; that all the events are equally physical, must I not concede that my past self physically exists, albeit in my past; must I not concede that, what I consider to be the past, continues to physically exist?

EDIT: that is, if it exists for one observer, then it exists within the universe, and if it exists within the universe, it must exist for all observers, including myself?

mangaroosh wrote:

twistor59 wrote:Suppose you have lunch at 12pm today. Suppose, also that Zog also has lunch at a particular time/day on a planet in Andromeda. Unless you can send a light (or slower) signal between your lunch event and Zog's lunch event, it's meaningless to say whether his lunch event is in the future or past of your lunch event. If you can't connect the events with such a signal, the two events are said to be spacelike separated.

Now, you *could* in principle strew the space between here and Andromeda with clocks, place one every meter say, and synchronize them with your home clock using light pulses. Then when Zog has his lunch, he could stop the clock nearest to him. You could then go there and collect it and compare the recorded time with the time you had lunch. You could then say if his lunch was to the future of your lunch.

But you've only achieved this designation *with respect to the coordinate system (the clocks) you've set up". It has no fundamental "invariant" meaning. It is purely artificial. Someone else zapping around in a rocket might record different times and ordering for the two events.

Cheers twistor, I am familiar with the idea you're talking about and have heard the term "spacelike separation" before, but didn't know it referred to that phenomenon.

Some of that seems somewhat strange to me, for the simple fact that we cannot send an instantaeous light signal to any location that is any distance apart from us, so theoretically we could never say that anything is in our present. The idea of sending a light signal between our lunch times obscures the issue a little bit, because we imagine lunchtime to be of a certain duration; but, if we consider events of sufficiently short duration, such that by the time we send a light pulse from where we are, to where it is, that event would have finished, we could conclude that we can never say anything is in our present, despite knowing that somethings must be.

If we take an event which occurs just one metre away from us, and imagine that event to be of sufficiently short duration, that by the time we send the light pulse to it, it would have finished; while the event might have actually been in our present, it would fail the test above.

Yes, events in this contexts are always instantaneous point events. It was meant to be a very short lunch!
Sending the light signal to the event isn't meant to be thought of as "oh, there's an event, let me send a light signal to it", rather here's an instant of time at one point in space, heres another instant at another point in space. Can I connect the instants with a light signal (or slower)?

mangaroosh wrote:
Zog
I'll try and say it somewhat clearer in terms of Zog, because even the above isn't that clear to me

Let's not take a specific event for Zog or ourselves; let's take "now", an idea we are all familiar with, even if it doesn't lend itself to easily to definition. Assuming the existence of the universe other than where we are, Andromeda for example, then we can reasonably deduce that right now, in other parts of the universe, something must be happening, regardless of what it is. In Andromeda Zog could be just getting up, he could be having his lunch, or he might not even be conceived yet; either way, we can reasonably imagine that right now, other locations in the universe exist and things, or events, are happening, even if we cannot determine, precisely, what those things, or events are. Those events would make up what we consider to be the present. We can also reasonably deduce that events occurred before those events, and will occur after those events; even if we cannot determine what they are, those events would still be categorised as what we consider to be the past, present, and future.

But "now" only has meaning with respect to a given reference frame. To define "now" you have to do something like what I was suggesting in setting a bunch of synchronized clocks up. If you do that, you can talk about now. You cannot ask "what's happening now on the planet Zog?" unless you do this. Of course for events close to us, and where things are moving slowly compared to SOL, then you can talk about now to a very good approximation.

mangaroosh wrote:
Relativity of Simultaneity
Now, taking into account RoS, and as you mention; a relatively moving observer might record the times and ordering of those events differently; that such an observer could make such an observation has physical implications for the structure of the universe; assuming that those events are real, physical and not simply optical illusions. Such a conclusion implies something about the structure of the universe, and events, something which might be deducible. Block universe theorists seem to believe that the implications of RoS imply a block like structure extended through time.

But leaving the block universe aside, can we deduce what it means for us in particular? Some people seem to believe that RoS implies that events which we consider to be the present, as outlined above, could be what another observer considers the future; and events that another observer considers to be the present, could be what we consider to be our past, or indeed, our future.

Would you say that that is a fair deduction, on their behalf?

Re highlighted: Absolutely, these considerations define the causal structure of the universe.

mangaroosh wrote:

DavidMcC wrote:Not to you, though. Perhaps to the remote observer in a different inertial frame.

But if the remote observers reference frame is as equally valid as mine; that all the events are equally physical, must I not concede that my past self physically exists, albeit in my past; must I not concede that, what I consider to be the past, continues to physically exist?

EDIT: that is, if it exists for one observer, then it exists within the universe, and if it exists within the universe, it must exist for all observers, including myself?

Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does. People in a distant star system might now (where we have to strictly define what we mean by your "now" as in the post above) be observing a past "you" because the light signals emitted when you were having a crafty cigarette behind the bikesheds at school have just reached them.

twistor59 wrote:Yes, events in this contexts are always instantaneous point events. It was meant to be a very short lunch!

ah, OK.

twistor59 wrote:Sending the light signal to the event isn't meant to be thought of as "oh, there's an event, let me send a light signal to it", rather here's an instant of time at one point in space, heres another instant at another point in space. Can I connect the instants with a light signal (or slower)?

But given that light travels at a finite speed, that would mean that any event which we can connect with a light signal wouldn't actually be simultaneous with the event of sending the light signal, it would be in the future of the sending the light signal event; meaning that we could never determine any event that happens in our "now". Perhaps you are simply referring to the Einstein synchronisation method, but the point about connecting instants of time with a light signal doesn't seem to be the same; that is, two instants of time which can be connected by a light signal are, necessarily, not simultaneous; due to the finite speed of light.

twistor59 wrote:But "now" only has meaning with respect to a given reference frame. To define "now" you have to do something like what I was suggesting in setting a bunch of synchronized clocks up. If you do that, you can talk about now. You cannot ask "what's happening now on the planet Zog?" unless you do this. Of course for events close to us, and where things are moving slowly compared to SOL, then you can talk about now to a very good approximation.

But we don't necessarily need to determine the contents of our now, or to define what is in our now, to understand that there must be simultaneous events happening elsewhere in the universe, which form part of our present, and other events which preceded those, which form our past, and events which have not yet happened, which form our future - all of this is within our own reference frame, of course; but we could reasonably say that all events fit into one of those three categories - "what we consider to be the past", "what we consider to be the present", or "what we consider to be the future" - could we not?

twistor59 wrote:

mangaroosh wrote:
Relativity of Simultaneity
But leaving the block universe aside, can we deduce what it means for us in particular? Some people seem to believe that RoS implies that events which we consider to be the present, as outlined above, could be what another observer considers the future; and events that another observer considers to be the present, could be what we consider to be our past, or indeed, our future.

Would you say that that is a fair deduction, on their behalf?

Re highlighted: Absolutely, these considerations define the causal structure of the universe.

Cool. The remaining part forms part of the other point we are discussing, I think.

twistor59 wrote:Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does. People in a distant star system might now (where we have to strictly define what we mean by your "now" as in the post above) be observing a past "you" because the light signals emitted when you were having a crafty cigarette behind the bikesheds at school have just reached them.

The idea of a light signal from my past event reaching a distant oberver now, is quite different from that event being simultaneous with the distant observers now; that is, the event is not simultaneous with them, the receipt of the light signal from the event is; it's more like the arrival of a photograph [of the event] is simultaneous with them, not the event which is in the photograph; with the light signal simply representing a form of intergalactic, photograph courier service.

As mentioned in the other post; two instants of time which can be connected by a light signal are necessarily, not simultaneous, given the finite speed at which light propagates.

twistor59 wrote:
Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does.

Events are either objectively real, or they are not. Or are we solipsists now?

mangaroosh wrote:But if the remote observers reference frame is as equally valid as mine; that all the events are equally physical, must I not concede that my past self physically exists, albeit in my past; must I not concede that, what I consider to be the past, continues to physically exist?

Only space-time events physically exist. What the observer makes of them is going to depend on his observations, and these are distorted by being in a different inertial frame, and by being remote. They are not valid for you, and cannot affect you, because of his remoteness.

mangaroosh wrote:

twistor59 wrote:Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does. People in a distant star system might now (where we have to strictly define what we mean by your "now" as in the post above) be observing a past "you" because the light signals emitted when you were having a crafty cigarette behind the bikesheds at school have just reached them.

The idea of a light signal from my past event reaching a distant oberver now, is quite different from that event being simultaneous with the distant observers now; that is, the event is not simultaneous with them, the receipt of the light signal from the event is; it's more like the arrival of a photograph [of the event] is simultaneous with them, not the event which is in the photograph; with the light signal simply representing a form of intergalactic, photograph courier service.

As mentioned in the other post; two instants of time which can be connected by a light signal are necessarily, not simultaneous, given the finite speed at which light propagates.

Well it depends what you mean by "exist". They're gathering information about your past, so in this sense your past exists for them. They can see it happening. If they say, "oh well, we know that light has taken x years to get here because we worked it out from the redshift, so we're actually looking into the past", then they will *infer* that what they're looking at doesn't exist any more. Does what you're looking at out of the window "exist"? Really you're looking at the past.

Reeve wrote:

twistor59 wrote:
Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does.

Events are either objectively real, or they are not. Or are we solipsists now?

In a spacetime context it does exist, yes. It could "exist" for you if you watch an old video of yourself. But this circular discussion is resulting from the use of the term "exist". It doesn't give us anything concrete to work with. Discussion of whether the past or future "exist" really belongs to philosophy.

DavidMcC wrote:Only space-time events physically exist. What the observer makes of them is going to depend on his observations, and these are distorted by being in a different inertial frame, and by being remote. They are not valid for you, and cannot affect you, because of his remoteness.

But I could be considered a spacetime event; that is, at any given time I can be described in terms of spacetime co-ordinates. If the events in the distant observers reference frame are as objectively real as the ones in mine, and what I consider to be "my past self" forms part of his present, then I must surely concede that "my past self" is objectively real.

twistor59 wrote:Well it depends what you mean by "exist". They're gathering information about your past, so in this sense your past exists for them. They can see it happening. If they say, "oh well, we know that light has taken x years to get here because we worked it out from the redshift, so we're actually looking into the past", then they will *infer* that what they're looking at doesn't exist any more. Does what you're looking at out of the window "exist"? Really you're looking at the past.

We're not really looking at the past though, the image that we see is derived from the light that hits our retina in the present. It's difficult to even say that we see an image of the past, because light from different sources, different distances away reach our retina at the same moment. The accuracy of the image, or how the penomenon pertains to the noumenon is philosophical question well discussed.

There is a marked difference between a light signal reaching a distant part of the universe and the cells of a body, which were simultaneous with the sending of that light signal, still existing; the latter would constitute the past still existing and being simultaneous with the distant observer; the former is simply akin to a photograph.

It's a bit like watching a movie and thinking that the actors are just then performing live; they aren't, and their past isn't simultaneous with your present; it's just an image.

twistor59 wrote:In a spacetime context it does exist, yes. It could "exist" for you if you watch an old video of yourself. But this circular discussion is resulting from the use of the term "exist". It doesn't give us anything concrete to work with. Discussion of whether the past or future "exist" really belongs to philosophy.

Part of what we are trying to do is bring meaning to the concept of Relativity of Simultaneity; essentially, the concept of RoS is meaningless without some of the philosophical terms being explained. The philosophical concept of time, being one of them - but that is a discussion we can avoid simply by exploring the logical consequences of RoS and seeing what is implied.

That events which are simultaneous in one reference frame can be non-simultaneous in another has material consequences, and different possible interpretations. For example, if we say that the non-simultaneous events are just optical illusions, while the simultaneous events are not optical illusions, but real physical events - or vice versa - it has very different consequences than if we say that both sets of events are equally physical, equally valid, and equall real i.e. neither of them is just an optical illusion.



In terms of the video of yourself, the "past self" in the video cannot really be said to exist; an image of the past self could be said to exist, but that is materially different to suggesting that the cells which made up your body, at the time of shooting the film, still exist and are manifest in the video.

mangaroosh wrote:

DavidMcC wrote:Only space-time events physically exist. What the observer makes of them is going to depend on his observations, and these are distorted by being in a different inertial frame, and by being remote. They are not valid for you, and cannot affect you, because of his remoteness.

But I could be considered a spacetime event; that is, at any given time I can be described in terms of spacetime co-ordinates. If the events in the distant observers reference frame are as objectively real as the ones in mine, and what I consider to be "my past self" forms part of his present, then I must surely concede that "my past self" is objectively real.

No, you cannot be considered to be "a spacetime event", because that is defined as occupying only one specific set of co-ordinates, x, y, z, t. The two lightning flashes represent two separate such events, which are related in a specific way for you, even though that relationship might seem to be different to an observer.

twistor59 wrote:

Reeve wrote:

twistor59 wrote:
Not really, as far as you're concerned, your past doesn't exist any more. As far as other observers are concerned, it does.

Events are either objectively real, or they are not. Or are we solipsists now?

In a spacetime context it does exist, yes...Discussion of whether the past or future "exist" really belongs to philosophy.

Ah the limits of science, yes?

mangaroosh wrote:
EDIT: that is, if it exists for one observer, then it exists within the universe, and if it exists within the universe, it must exist for all observers, including myself?

You're right, spatiotemporal existence or occurrence is not relative to a reference frame.

mangaroosh wrote:
We're not really looking at the past though, the image that we see is derived from the light that hits our retina in the present. It's difficult to even say that we see an image of the past, because light from different sources, different distances away reach our retina at the same moment. The accuracy of the image, or how the penomenon pertains to the noumenon is philosophical question well discussed.
There is a marked difference between a light signal reaching a distant part of the universe and the cells of a body, which were simultaneous with the sending of that light signal, still existing; the latter would constitute the past still existing and being simultaneous with the distant observer; the former is simply akin to a photograph.
It's a bit like watching a movie and thinking that the actors are just then performing live; they aren't, and their past isn't simultaneous with your present; it's just an image.

No, this is a false analogy, because perception is not a kind of imagination.

"The Time-Lag Argument:

First Premise. We cannot perceive physical objects or events unless light is reflected or emitted from them to our visual system.

Second Premise. Light travels at a finite velocity, and so there is always some time interval between the reflection or emission of light from a physical object or event and the light’s reaching our eyes. In the case of nearby objects or events, the time interval may be minute. But in the case of a distant star, the time interval may be so considerable that, by the time the light reaches our eyes, the star may no longer exist.

Third Premise. If something no longer exists, we cannot now perceive it, let alone directly perceive it. And so, assuming the distant star no longer exists, we cannot directly perceive it when its light reaches our eyes. But since we are perceiving something, the object of (direct) perception must be something other than the distant star.

Fourth Premise. Though time lags are most significant in cases of distant objects such as stars, any time lag, however mminute, between physical objects or events and our perception of them is incompatible with Direct Realism, for given the time lag, we cannot directly perceive physical objects and events as they presently are at the time of our perception. Since we perceive something, the object of (direct) perception must be something other than physical objects or events.

Conclusion: Direct Realism is false. We do not directly perceive physical objects and events.

The proponent of the argument then usually proposes some other candidate as the object of direct visual awareness (e.g., a sense-datum, or sensum, or sensation, or idea, etc.).

How Direct Realists May Counter the Time-Lag Argument:

Direct Realists should concede the truth of the first two premises, and focus on the third and fourth premises which constitute the crux of the argument. The claim 'if something no longer exists, we cannot now perceive it' can be interpreted in at least two distinct ways: (a) if something no longer exists, we cannot now perceive it as it presently is, or (b) if something no longer exists, we cannot now perceive it as it used to be.
When taken in the sense of (a), the claim is obviously true, or so we may suppose. But when taken in the sense of (b), the claim is far from obviously true. For why couldn’t we now be visually aware of something as it was but which no longer exists? Isn’t it precisely because there is a time lag that we now see stars (as they used to be) which no longer exist, and that when we see nearby objects, we now see them as they were (perhaps only a few microseconds ago)? Direct Realists need not deny the existence of time lags in perception, nor need they be committed to 'endowing' human percipients with miraculous perceptual abilities inconsistent with our best physical theories.
In brief, it does not follow from (i) there are time lags in perception, that (ii) we cannot directly perceive external physical objects or events. What does follow from (i) is (iii) that we cannot (given the laws of physics) directly perceive external physical objects or events without a time lag, however minute. And since (iii) is consistent with accepting Direct Realism, the Time-Lag Argument fails to establish (ii), and hence fails to refute Direct Realism."

(Le Morvan, Pierre. "Arguments Against Direct Realism and How to Counter Them." American Philosophical Quarterly 41, no. 3 (July 2004): 221-234. pp. 223-4)

twistor59 wrote:
I think the term "coexist" belongs to philosophy and doesn't really mean anything concrete.

As for the concept of (spatio-)temporal coexistence in the nonrelativistic Newtonian context and in the relativistic Einsteinian context, see:

Yuri Balashov: "Relativity and Persistence" (1998)

By the way, it seems there are three sorts of coexistence:

1. temporal coexistence (simultaneous coexistence in time)

2. transtemporal coexistence (nonsimultaneous coexistence in time)

In this (eternalistic) sense, the dinosaurs and the humans coexist, because they are equally embedded in spacetime.

3. atemporal coexistence

If x and y coexist atemporally, then x doesn't exist in time, y doesn't exist in time, or both x and y don't exist in time. In this sense, abstract objects and concrete objects can coexist. But even concrete objects can coexist atemporally, because if God exists and isn't located anywhere in spacetime, then he and the living theists coexist atemporally.

mangaroosh wrote:
In terms of the video of yourself, the "past self" in the video cannot really be said to exist; an image of the past self could be said to exist, but that is materially different to suggesting that the cells which made up your body, at the time of shooting the film, still exist and are manifest in the video.

But how do you ever know that other things (than yourself) exist, other than by observing them. You can surely see how this is straying into philosophy and is begging the moderators to move it to the philosophy section. I can't see any questions being asked which are addressable by physics at the moment.

I don't see physics as dealing with reality (although some physicsts might disagree!), hence I would try to avoid using words like "exist". I see physics rather as dealing with representations or models of reality. The task is just to make better and better models. Even the words "future" and "past" are used in very restricted senses in physics. For example, in this picture, the observer O is at rest, just drifting upwards in time. His future is the red triangle. His past is the blue triangle. The spacetime event P is neither in his future nor in his past in this snapshot. Although if you looked at it sometime later, O would have drifted upwards in time and P would be included in his past.

Teuton wrote:

mangaroosh wrote:
EDIT: that is, if it exists for one observer, then it exists within the universe, and if it exists within the universe, it must exist for all observers, including myself?

You're right, spatiotemporal existence or occurrence is not relative to a reference frame.

But what does spatiotemporal existence mean?

twistor59 wrote:I don't see physics as dealing with reality (although some physicsts might disagree!), hence I would try to avoid using words like "exist".[ I see physics rather as dealing with representations or models of reality.

The physicists certainly develop and use representations of physical reality, but the subject matter of physics is physical reality. Physics is not the historiography of physical theories.

twistor59 wrote:The task is just to make better and better models.

The best model of reality is the true model of reality.