The status of Minkowski spacetime/block universe

Teuton · **#81** by **Teuton** » Oct 24, 2012 8:19 pm

twistor59 wrote:
But what does spatiotemporal existence mean?

"Spatiotemporal existence" refers either to spatially and temporally located existence, i.e. existence in space and time, or to spatially and temporally extended existence, i.e. existence as a four-dimensional entity, an entity which has both spatial and temporal parts.

My point is that a spatiotemporally located entity exists absolutely, i.e. nonrelatively to a reference frame, in spacetime.

iamthereforeithink · **#82** by **iamthereforeithink** » Oct 24, 2012 8:21 pm

twistor59 wrote: 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.

Ah, but would he have a memory of the event P?

Don't mind me, just bookmarking. :coffee:

Reeve · **#83** by **Reeve** » Oct 24, 2012 9:23 pm

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.

I think that physicists and mathematicians have already beaten the philosophers to it: We already know what reality is because we can create it via simulation.

The universe is quite obviously computational - if it was not then computers would not be able to work. I don't think the "models" are models of reality, they are reality; or at least once we have the ideal, accurate model. It'll be maths all the way down!

So, I agree with Seth Lloyd and Max Tegmark that existence is quantum computation.

iamthereforeithink · **#84** by **iamthereforeithink** » Oct 24, 2012 9:31 pm

Niels Bohr wrote:There is no quantum world. There is only an abstract quantum description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Reeve · **#85** by **Reeve** » Oct 24, 2012 10:59 pm

iamthereforeithink wrote:
Niels Bohr wrote:There is no quantum world. There is only an abstract quantum description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Bohr was wrong. Of course there is a quantum world; there is proof of it. :coffee:

mangaroosh · **#86** by **mangaroosh** » Oct 25, 2012 1:43 am

DavidMcC wrote: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.

But I can be described in terms of spacetime co-ordinates; that is, the number of events which make up "me" can be described in terms of spacetime co-ordinates, and we can speak about the simultaneity of those events using the shorthand "me".

mangaroosh · **#87** by **mangaroosh** » Oct 25, 2012 1:53 am

Teuton wrote:
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)

Cheers Teuton, that is a very comprehensive reply.

The question of direct realism isn't necessarily what is in question here though; it's only really the second premise which is important. Ultimately what it reduces to is, is the light reflecting from the object we observe the object itself; that is, if light from a distant object hits my retina now, can I say that the object which reflected the light still exists. The answer is I can't; I can only say that the light hitting my retina, which "carries" the image exists, and it exists in my present. So, I am not seeing the past, I am seeing light that was reflected at a time in the past, which is hitting my retina in the present; whether the image that that light creates is an accurate representation of the past state is not in question here; the only question is whether the object which reflected it still exists, or more pointedly, still exists in the state as represented by the image.

A light signal reaching a distant observer is just an image of an event; indeed, the light hitting the retina of the observer is a separate event from the reflection event. It's like looking at a photograph and concluding that the scene, as represented in the photograph is still physically real, in a manner other than in the form of a photograph.

mangaroosh · **#88** by **mangaroosh** » Oct 25, 2012 2:22 am

twistor59 wrote: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.

Essentially we're looking at relativity of simultaneity, which is a question of physics, isn't it; and asking some basic questions;

Is the question, "what is relativity of simultaneity?", a question which can be answered by physics?
Is the answer, "relativity of simultaneity refers to how objects which are simultaneous in one reference frame can be non-simultaneous in a relatively moving reference frame" an answer provided by physics?

Is the question, "does RoS occur due to an optical illusion?", a question which can be answered by phyiscs?

If not, does that mean that physics is meaningless without an interpretation of the philosophical concepts which underlie it?

twistor59 wrote:But how do you ever know that other things (than yourself) exist, other than by observing them.

That is an interesting philosophical question, which isn't necessarily the purview of this thread, but to answer it in context of the point about the "you in the video", it becomes a question of the nature of existence; that is, we can't say that the past continues to exist, or that our past self still exists, just because we have a video of ourselves when we were younger; instead what exists is the image, or recording of ourselves, which is now playing in the present. It is the reflected light, which imprinted the image on the film or disc which exists, not the past state represented by the image in the video.

Apologies, that was a bit longer than it needed to be; in short, the image of the past state exists, but the past state does not; because the image of the past state is not the past state itself.

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 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.
Mink2.jpg

We can avoid words like "exist" if we want, but we must bear in mind that the fact that there is existence is beyond doubt, the only question is what the nature of that existence is. An accurate model of reality is one that accurately models existence, because reality is the state of things as they actually exist. As mentioned, however, we can avoid words such as "exist", in favour of words such as "physical" or "equally valid".

I would also use words like "past" and "future" in a restricted sense, by qualifying the terms as "what you refer to as, the past" and "what you refer to as, the future"; these terms are fairly well beyond miscomprehension; you can remember what you did yesterday, this is what you consider to be the past; even if someone else considers it to be the present, it is still what you consider to be the past.

I'm just wondering though, how can an event be neither in an observers past, present, or future? If the event is a physical event, and exists in the universe, then it must fit into what any particular observer considers to be the past, present, or future. It might fit differently into those categories for different observers, but every event must fit into one of those categories for every observer - surely.

twistor59 · **#89** by **twistor59** » Oct 25, 2012 6:26 am

Teuton wrote:
twistor59 wrote:
But what does spatiotemporal existence mean?

"Spatiotemporal existence" refers either to spatially and temporally located existence, i.e. existence in space and time, or to spatially and temporally extended existence, i.e. existence as a four-dimensional entity, an entity which has both spatial and temporal parts.

My point is that a spatiotemporally located entity exists absolutely, i.e. nonrelatively to a reference frame, in spacetime.

But then what about the thermal particle spectrum present in Unruh radiation? Those particles are spatiotemporally located entities, and "real" in the sense that they will register on a detector. But in another reference frame they're not there. Their "existence" is reference frame dependent.

twistor59 · **#90** by **twistor59** » Oct 25, 2012 6:46 am

Reeve wrote:
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.

I think that physicists and mathematicians have already beaten the philosophers to it: We already know what reality is because we can create it via simulation.

The universe is quite obviously computational - if it was not then computers would not be able to work. I don't think the "models" are models of reality, they are reality; or at least once we have the ideal, accurate model. It'll be maths all the way down!

So, I agree with Seth Lloyd and Max Tegmark that existence is quantum computation.

Well, I try to stick to simpler questions - just modelling a small bit of the universe is enough to keep me happy :lol:

. However I've always suspected that we should really think of the universe contents as just "information". That information may be representable in completely different ways - as a bulk spacetime, or on a holographic boundary for example, but it's the same information and *how* the model is represented doesn't really matter as long as the representation is faithful.

<speculation>
One motivation for this line of thinking is that the common element in what we think of as "actual stuff" is energy. Particles and fields all seem to have energy. But, there is a connection between energy and information, namely Landauer's Principle. Maybe this is a hint of a much deeper connection between these two entities.</speculation>

twistor59 · **#91** by **twistor59** » Oct 25, 2012 7:03 am

mangaroosh wrote:
twistor59 wrote: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.

Essentially we're looking at relativity of simultaneity, which is a question of physics, isn't it; and asking some basic questions;

Is the question, "what is relativity of simultaneity?", a question which can be answered by physics?
Is the answer, "relativity of simultaneity refers to how objects which are simultaneous in one reference frame can be non-simultaneous in a relatively moving reference frame" an answer provided by physics?

Is the question, "does RoS occur due to an optical illusion?", a question which can be answered by phyiscs?

Are you asking for an exposition of the RoS? I've kind of been proceeding as if people here knew what it was and how it worked, but if that's not the case, it can be described. Sorry if that's insulting your intelligence BTW, but I don't know what sort of reading you've done or training you've had on this.

mangaroosh wrote:
I'm just wondering though, how can an event be neither in an observers past, present, or future? If the event is a physical event, and exists in the universe, then it must fit into what any particular observer considers to be the past, present, or future. It might fit differently into those categories for different observers, but every event must fit into one of those categories for every observer - surely.

Well in my picture, P is neither in O's past nor O's future. As I said, P will eventually enter O's past as O's past cone sweeps forward in time. To be in O's past means that information from P must have been able to reach O. Unless information from an event can reach you that event cannot be said to lie in your past! In fact as far as you're concerned at that time, that event might as well not exist (to use that awful "e" word again).

mangaroosh · **#92** by **mangaroosh** » Oct 25, 2012 8:26 am

twistor59 wrote:Are you asking for an exposition of the RoS? I've kind of been proceeding as if people here knew what it was and how it worked, but if that's not the case, it can be described. Sorry if that's insulting your intelligence BTW, but I don't know what sort of reading you've done or training you've had on this.

No, sorry. I was just trying to highlight the fine line that exists between what might be considered philosophy and physics; where explaining what RoS means might be considered a question suitable for the physics forum, such explanations usually carry implicit philosophical assumptions which also need to be exposed, to make the explanation meaningful.

twistor59 wrote:Well in my picture, P is neither in O's past nor O's future. As I said, P will eventually enter O's past as O's past cone sweeps forward in time. To be in O's past means that information from P must have been able to reach O. Unless information from an event can reach you that event cannot be said to lie in your past! In fact as far as you're concerned at that time, that event might as well not exist (to use that awful "e" word again).

This idea, that in order for an event to lie in your past, that information must be able to reach you, is somewhat puzzling. I get the idea that, for an event to have a causal relatonship with your past, information from the event must have been able to reach you in the past; but that deosn't seem to be a necessary condition for the event to be considered to lie in your past.

Again, if we consider what you refer to as "now", I'm sure you can imagine that there are events occurring, simultaneosuly, in distant parts of the galaxy, light from which won't reach you until some time in your future. Now, just because light from these events doesn't travel instantaneously to you, it doesn't mean that these events are not in your present, it simply means that they won't form part of what your observe in the present moment.

Indeed, we can imagine a number of events occuring simultaneously in the universe, all at different distances from you. Light from these events will reach you at different times; let's take event A which happens one light year away; event B, which happens 5 light years away; and event C, which happens 10 light years away; all three events happen simultnaeously.

Now, the light from A will reach you before B and C, such that, you will make an observation of A in the present moment i.e. light hits your retina and you see an image, but not B and C; this could be termed as a separate event, D. This doesn't mean that events A and D are simultaneous, and that, only a fraction of a second after D, does A move into your past. A moves into your past a fraction of a second after A, B, and C occurring. The same applies to B and C. And just becuase information from B and C hasn't reached you, it doesn't mean that they are not in your past; just that light from those events hasn't hit your retina, to give you the image of B and C. Bearing in mind that images of B and C are not B and C themselves.

DavidMcC · **#93** by **DavidMcC** » Oct 25, 2012 9:02 am

mangaroosh wrote:But I can be described in terms of spacetime co-ordinates; that is, the number of events which make up "me" can be described in terms of spacetime co-ordinates, and we can speak about the simultaneity of those events using the shorthand "me".

... And that is why observers can see in a misleading light - they can get your simultaneity wrong.

twistor59 · **#94** by **twistor59** » Oct 25, 2012 9:06 am

mangaroosh wrote:
twistor59 wrote:Are you asking for an exposition of the RoS? I've kind of been proceeding as if people here knew what it was and how it worked, but if that's not the case, it can be described. Sorry if that's insulting your intelligence BTW, but I don't know what sort of reading you've done or training you've had on this.

No, sorry. I was just trying to highlight the fine line that exists between what might be considered philosophy and physics; where explaining what RoS means might be considered a question suitable for the physics forum, such explanations usually carry implicit philosophical assumptions which also need to be exposed, to make the explanation meaningful.

twistor59 wrote:Well in my picture, P is neither in O's past nor O's future. As I said, P will eventually enter O's past as O's past cone sweeps forward in time. To be in O's past means that information from P must have been able to reach O. Unless information from an event can reach you that event cannot be said to lie in your past! In fact as far as you're concerned at that time, that event might as well not exist (to use that awful "e" word again).

This idea, that in order for an event to lie in your past, that information must be able to reach you, is somewhat puzzling. I get the idea that, for an event to have a causal relatonship with your past, information from the event must have been able to reach you in the past; but that deosn't seem to be a necessary condition for the event to be considered to lie in your past.

The key point is your use of the highlighted phrase. I think you're using it to mean "at a time less than the current time on my watch". The implication is that the time on your watch has a meaning across the entire spatial extent of the universe. Well indeed it can do, you are perfectly entitled to set up a time coordinate system to do this. But it is entirely subjective, it is tied to you. Another observer who's set up another such coordinate system may disagree that the event is in your past. The only objective observer-independent notion of a past is the one I've given, namely that events lie in your past if they lie on or inside your past light cone.

mangaroosh wrote:
Again, if we consider what you refer to as "now", I'm sure you can imagine that there are events occurring, simultaneosuly, in distant parts of the galaxy, light from which won't reach you until some time in your future. Now, just because light from these events doesn't travel instantaneously to you, it doesn't mean that these events are not in your present, it simply means that they won't form part of what your observe in the present moment.

Indeed, we can imagine a number of events occuring simultaneously in the universe, all at different distances from you. Light from these events will reach you at different times; let's take event A which happens one light year away; event B, which happens 5 light years away; and event C, which happens 10 light years away; all three events happen simultnaeously.

Now, the light from A will reach you before B and C, such that, you will make an observation of A in the present moment i.e. light hits your retina and you see an image, but not B and C; this could be termed as a separate event, D. This doesn't mean that events A and D are simultaneous, and that, only a fraction of a second after D, does A move into your past. A moves into your past a fraction of a second after A, B, and C occurring. The same applies to B and C. And just becuase information from B and C hasn't reached you, it doesn't mean that they are not in your past; just that light from those events hasn't hit your retina, to give you the image of B and C. Bearing in mind that images of B and C are not B and C themselves.

Same comment, the highlighted phrases mean that you're using your own timeslicing. This is tied to you. Other observers will have a different view. The physics definition of the past is constructed with the objective of removing this observer dependence. See the definition of chronological past here.

mangaroosh · **#95** by **mangaroosh** » Oct 25, 2012 10:44 am

DavidMcC wrote:
mangaroosh wrote:But I can be described in terms of spacetime co-ordinates; that is, the number of events which make up "me" can be described in terms of spacetime co-ordinates, and we can speak about the simultaneity of those events using the shorthand "me".

... And that is why observers can see in a misleading light - they can get your simultaneity wrong.

but we can calculate the co-ordinates for all of the events and arrive at the conclusion that 50yr old me is simultaneous with the flashes in my rest frame, but not in the relatively moving frame, and hence I should have conflicting memories of the lightning events; unless the events in one of the reference frames are not valid. We don't need to rely on the questionable perception of any observer.

mangaroosh · **#96** by **mangaroosh** » Oct 25, 2012 12:07 pm

twistor59 wrote:The key point is your use of the highlighted phrase. I think you're using it to mean "at a time less than the current time on my watch". The implication is that the time on your watch has a meaning across the entire spatial extent of the universe. Well indeed it can do, you are perfectly entitled to set up a time coordinate system to do this. But it is entirely subjective, it is tied to you. Another observer who's set up another such coordinate system may disagree that the event is in your past. The only objective observer-independent notion of a past is the one I've given, namely that events lie in your past if they lie on or inside your past light cone.

twistor59 wrote:Same comment, the highlighted phrases mean that you're using your own timeslicing. This is tied to you. Other observers will have a different view. The physics definition of the past is constructed with the objective of removing this observer dependence. See the definition of chronological past here.

We don't have to deal with timing devices though; "right now" is a purely experiential thing; I can say "right now" without knowing the time on my clock and understand what it is that I'm referring to; try saying "right now" to yourself without looking at a clock and see do you intuitively understand what you mean? "Right now" is an empirical observation; I don't think the physical experience of "now" can be broken down any further; that is, the term "now" is the last stop before the empirical experience of "now".

Specific events
The issue of timepieces arises when we start to try to determine what events, exactly, are simultaneous with "right now"; if we make assumptions about what events form part of our now, we can run into trouble, and time pieces are necessitated; but we don't have to nake such assumptions. We don't need to assume that the content of our vision i.e. what we observe, is an accurate representation of the events happening right now; given the finite speed of light, if we start making such assumptions then we run into trouble.

If we want to determine exactly, what those events are, it is reasonable to say that we need to set up synchronous clocks; but if we're not specificall interested in what the events are, we can use abstract reasoning to arrive at conclusions which must necessarily be true in the physical world.

the Universe
If we start with a basic premise, that there is existence; that is something which cannot be disputed. What the precise nature of that existence is is certainly up for debate - not necessarily here though. Essentially, "existence" is simply the label we apply to the fact that there is experience. Instead of "I think therfore I am", we could say "there is experience, therefore there is existence".

Now, I don't think we need to discuss the nature of existence of the universe to reasonably say that there is an experience which we label "the universe"; this allows us to say "the universe exists". Again, what the nature of that existence is, isn't necessarily important at this juncture. We are simply looking to explain our experience of what we refer to as "the physical universe", or "physical world".

So, given that the universe exists, and our experience of it is that there are regions of the universe which are a long distance from us, we can reasonably say that distant parts of the unviverse exist. If distant parts of the universe exist, then we can reasonably say that there are events happening in distant parts of the universe

So, if distant parts of the universe exist right now and there are events happening in distant parts of the universe, there are events happening in distant parts of the universe right now; those events are simultaneous with right now and form part of our present.

Finite speed of light
So, we can say that there are 4 events A, B, C, and D which happen right now; let's say that A is us saying right now, and B, C, and D happen in distant parts of the universe; the light from B, C and D, will not reach us right now, because of the finite speed of light. The light leaving those events must travel a long distance at a finite speed. When the light from those events reaches us and hits our retina, it can be considered a separate event, which we label E (corresponding to B), F (corresponding to C) and G (corresponding to D).

Event E is not simultaneous with A, B, C, or D; due to the finite speed of light and the distance it had to travel, when event E happens, A, B, C, and D are, by necessity in the past. Here E represents information which reaches us from the event B. That doesn't mean that event B forms part of our present when the light hits our retina; event B is necessarily in our past; the light hitting our retina is just an image of B; it's information about B, not B itself; just as a photo of an apple isn't the apple itself.

Event B can have a causal influence on us, until the information from it reaches us, but this is different to saying that B isn't in our present or past, until the light from B has reached us.

twistor59 · **#97** by **twistor59** » Oct 25, 2012 12:40 pm

mangaroosh wrote:

Indeed, we can imagine a number of events occuring simultaneously in the universe, all at different distances from you. Light from these events will reach you at different times; let's take event A which happens one light year away; event B, which happens 5 light years away; and event C, which happens 10 light years away; all three events happen simultnaeously.

Now, the light from A will reach you before B and C, such that, you will make an observation of A in the present moment i.e. light hits your retina and you see an image, but not B and C; this could be termed as a separate event, D. This doesn't mean that events A and D are simultaneous, and that, only a fraction of a second after D, does A move into your past. A moves into your past a fraction of a second after A, B, and C occurring. The same applies to B and C. And just becuase information from B and C hasn't reached you, it doesn't mean that they are not in your past; just that light from those events hasn't hit your retina, to give you the image of B and C. Bearing in mind that images of B and C are not B and C themselves.

OK here's a diagram of your scenario:

: Flashes.jpg (18.25 KiB) Viewed 1580 times

I claim that the event A enters the observer's (who is sitting at the origin drifting upwards in time) past at time tA, B at tB etc. This is according to the physics definition of past, i.e. the past is his past light cone.

You, however, would like to say that, actually, at, say, time t1 the three events are in his past. At time t1 I've drawn a horizontal blue line marking all the events at time t1. That's fine, the events are simultaneous and prior to t1 in his frame. But this information is unavailable to and can be of no consequence to him until a light signal reaches him. Causal influences must travel slower than light.

He has no way even of knowing, that those events are simultaneous in his frame, until the signals reach him. That's the first time he could ever possibly determine that they are simultaneous in his frame. (He might, for example, make this determination that they are simultaneous in his frame, because encoded in the light flash is a little signal giving the time, when the flash happened, of the clock which he had set up at that point in space, synchronized with his "at home" clock). But that's precisely the point at which they enter his past according to the physics definition of past.

So only the omniscient flying spaghetti monster could immediately know that those events are simultaneous in the observer's frame. Everybody else has to wait.

VazScep · **#98** by **VazScep** » Oct 25, 2012 1:24 pm

twistor59 wrote:Well, I try to stick to simpler questions - just modelling a small bit of the universe is enough to keep me happy . However I've always suspected that we should really think of the universe contents as just "information". That information may be representable in completely different ways - as a bulk spacetime, or on a holographic boundary for example, but it's the same information and *how* the model is represented doesn't really matter as long as the representation is faithful.

I'm sure I've read similar stuff somewhere. What do the scare-quotes around "information" mean? Do you just mean it informally and as an analogy, or can this be cashed out in whatever technical way information theory uses the term?

At great risk of missing the point, I'm wondering how much of this can be tied in to modern ideas in the philosophy of maths, which likes to think of, say, the natural numbers as just a structure, which could have (infinitely) many different possible representations. If you're interested more in structure, then I think you're supposed to talk more about morphisms between structures. I then always thought of isomorphisms as being ways of translating between different representations, which I tend to think is what it means for two things to represent the same thing: you should know how to translate between them. Any of this workable?

<speculation>
One motivation for this line of thinking is that the common element in what we think of as "actual stuff" is energy. Particles and fields all seem to have energy. But, there is a connection between energy and information, namely Landauer's Principle. Maybe this is a hint of a much deeper connection between these two entities.</speculation>

More speculation is welcome from me. I like the tags, too, so I know which bits are which.

twistor59 · **#99** by **twistor59** » Oct 25, 2012 1:45 pm

VazScep wrote:
twistor59 wrote:Well, I try to stick to simpler questions - just modelling a small bit of the universe is enough to keep me happy . However I've always suspected that we should really think of the universe contents as just "information". That information may be representable in completely different ways - as a bulk spacetime, or on a holographic boundary for example, but it's the same information and *how* the model is represented doesn't really matter as long as the representation is faithful.
I'm sure I've read similar stuff somewhere. What do the scare-quotes around "information" mean? Do you just mean it informally and as an analogy, or can this be cashed out in whatever technical way information theory uses the term?

At great risk of missing the point, I'm wondering how much of this can be tied in to modern ideas in the philosophy of maths, which likes to think of, say, the natural numbers as just a structure, which could have (infinitely) many different possible representations. If you're interested more in structure, then I think you're supposed to talk more about morphisms between structures. I then always thought of isomorphisms as being ways of translating between different representations, which I tend to think is what it means for two things to represent the same thing: you should know how to translate between them. Any of this workable?

Well I was just idly wondering whether the universe "is" just information - there's no substrate if you like, just the information itself. AdS/CFT gives us the first hint that it may be possible to represent the whole damn thing in entirely different ways. And yes, there is a dictionary which allows you to translate between these two representations. Information is conserved under time evolution (whatever that is), this would somehow represent the integrity/stability/consistency of the universe. Dunno maybe just wibble.

By the way I can't do category theory!

BTW I'm not the only one thinking thoughts like this http://arxiv.org/abs/1011.1657

Teuton · **#100** by **Teuton** » Oct 25, 2012 1:49 pm

twistor59 wrote:
Teuton wrote:
My point is that a spatiotemporally located entity exists absolutely, i.e. nonrelatively to a reference frame, in spacetime.

But then what about the thermal particle spectrum present in Unruh radiation? Those particles are spatiotemporally located entities, and "real" in the sense that they will register on a detector. But in another reference frame they're not there. Their "existence" is reference frame dependent.

No. But we may be talking past each other, because I use "to exist" in the untensed sense here, in which sense it doesn't mean "to exist now/presently". In special relavitity, sets of present existents (simultaneously coexisting events/things) are indeed relative to reference frames, but the set of existents which are past, present, or future, i.e. the set of all spatiotemporal entities, is not. That is, if a spatiotemporally located event or thing exists in the untensed sense, meaning that it has existed, exists, or will exist, then this is true independently of any reference frames.

The status of Minkowski spacetime/block universe

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