what we see

[michel123456]

I can plot random "events" in the sphere around me:

D1,T1

D2,T1

D3,T1

D4,T1

D5,T1

etc.

 

And

D1,T2

D1,T3

D1,T4

 

In this list the only event that is visible is the first one (D1,T1). We dismiss all the others because we can't see them. But if something is mathematically (geometrically) possible, why do we dismiss it?

Edited November 23, 2011 by michel123456

[JustinW]

Maybe because 2 D's might cross at the same T disrupting the outcome of the D you're looking for at the T you're looking for it. Unless I misunderstood.

 

 

[Iggy]

In this list the only event that is visible is the first one (D1,T1). We dismiss all the others because we can't see them. But if something is mathematically (geometrically) possible, why do we dismiss it?

I don't know what that means. We don't dismiss events we can't see. We don't dismiss the moon landing just because we can't see it at the moment. Past events don't need to be on the surface of the light cone to cause present events.

[michel123456]

I don't know what that means. We don't dismiss events we can't see. We don't dismiss the moon landing just because we can't see it at the moment. Past events don't need to be on the surface of the light cone to cause present events.

 

Bad wording of me. I'll try otherwise.

The Moon landing belongs to the past.

The past (our past) is also this sphere that expands around us. Looking around us we don't see the Moon landing. We see the Moon and Neil is not there. We don't see the Moon landing because the factor D/T is too small: the time that separates us from the Neil on the Moon is huge compared to the distance to the Moon. IOW the Moon landing is inside our light-cone. That's why it is invisible to us. The same goes for events outside the light cone.

We can see the event only when the factor D/T is equal to the Speed Of light.

Which means that the huge majority of events is invisible to us.

At astronomical scale, we are not able to see them, we cannot measure them.

But even with the full comprehension of the total physical impossibilty to see them, we have the conviction that we know something about them: they are events linked to the same physical bodies than the one we see, at another coordinate in time.

My question is: where does this conviction come from?

Do we have a way to get any indication about what is not upon the surface of our light-cone?

Edited November 28, 2011 by michel123456

[Iggy]

...even with the full comprehension of the total physical impossibilty to see them, we have the conviction that we know something about them... where does this conviction come from?

You seem to be asking how we can know about the moon landing if it isn't currently visible.

 

Events inside your past light cone affect current events. Information, in other words, can travel less than c. Information about the moon landing can get from the moon 42 years ago to here and now by paths other than direct line of sight.

[michel123456]

You seem to be asking how we can know about the moon landing if it isn't currently visible.

 

Events inside your past light cone affect current events. Information, in other words, can travel less than c. Information about the moon landing can get from the moon 42 years ago to here and now by paths other than direct line of sight.

 

No that was not the meaning of my question.

they are events linked to the same physical bodies than the one we see, at another coordinate in time.

My question is: since the Moon changed coordinates in spacetime, if there were an hypothetical other physical body at the old coordinate, a co-moving body in time, could we see it? Or else, by what other means can we dismiss this possibility? What are our instruments to observe outside the surface of our light-cone?

[Iggy]

My question is: since the Moon changed coordinates in spacetime, if there were an hypothetical other physical body at the old coordinate, a co-moving body in time, could we see it?

A l'impossible, nul n'est tenu. Nothing else can occupy that space-time coordinate.

 

A coordinate in space-time is a specific spatial location at a specific time. "At the moon in 1969" is the space-time coordinate. "The moon landing" is the event. That is the event that goes with that coordinate. What "other physical body" would be there?

[michel123456]

A l'impossible, nul n'est tenu.

Very impressive!

 

 

Nothing else can occupy that space-time coordinate.

 

A coordinate in space-time is a specific spatial location at a specific time. "At the moon in 1969" is the space-time coordinate. "The moon landing" is the event. That is the event that goes with that coordinate. What "other physical body" would be there?

I don't know.

We simply can make the statement that the Moon is not in 1969 any more, so I guess the coordinate is empty. But it is simply a guess. To be sure, I'd like to have a look there to see if the Moon is in 1969, but I cannot. A fellow scientist in present time 42 Light Years away from us can have a look there. The problem is that I cannot have any communication with that fellow scientist (even if he existed). If I go and travel 42 LY away, I will still be upon the surface of Earth's light cone, so I will never be able to see again the Moon in 1969. I am a prisoner of SOL. How can I get away and be sure that nothing else occupies Moon's ancient coordinates?

[Iggy]

To be sure, I'd like to have a look there to see if the Moon is in 1969, but I cannot. A fellow scientist in present time 42 Light Years away from us can have a look there. The problem is that I cannot have any communication with that fellow scientist (even if he existed).

So a person can know that an object is at a time in the past (t) [at that space-time coordinate] if it is at a distance of c times t where c is the speed of light?

Edited November 28, 2011 by Iggy

[michel123456]

So a person can know that an object is at a time in the past (t) [at that space-time coordinate] if it is at a distance of c times t where c is the speed of light?

That is my understanding, yes.

[md65536]

We simply can make the statement that the Moon is not in 1969 any more, so I guess the coordinate is empty. But it is simply a guess. To be sure, I'd like to have a look there to see if the Moon is in 1969, but I cannot. A fellow scientist in present time 42 Light Years away from us can have a look there. The problem is that I cannot have any communication with that fellow scientist (even if he existed). If I go and travel 42 LY away, I will still be upon the surface of Earth's light cone, so I will never be able to see again the Moon in 1969. I am a prisoner of SOL. How can I get away and be sure that nothing else occupies Moon's ancient coordinates?

 

No, it doesn't make sense to say "the coordinate is empty". An event happens at a particular place and time (a 4d point on the spacetime manifold) and that point is fixed in 4d. It doesn't change. See http://www.scienceforums.net/topic/59951-inflation-and-causality/page__view__findpost__p__627339

 

The spatial coordinate can become "empty" as the object that used to be there moves away (so XYZ="where the moon was in 1969", T="2011" could be "empty"), but the spacetime coordinates of an event such as those involving the moon landing, will always be associated with those events.

 

 

 

As I've already mentioned (#39), we know things about the past and the future, to varying degrees of certainty, due to physical laws, and we also know things about the past to varying degrees of certainty, due to memory (which also involves physical laws, allowing information to propagate through confined locations or something). Our conviction comes from our level of certainty of available information and of the physical laws.

 

 

I'm not sure if the question is "Since we can't directly see the spatially near, temporally distant past, ...

... how do we know it hasn't changed?" -- The answer is that the spacetime manifold is fixed. Or,

... how are we certain that it happened as we say it did?" -- We are very very certain of the laws that connect those past events to current events, and of the directly visible information that is causally connected to those events (photographs, millions of human memories, etc).

 

 

 

We're not that certain of everything. We know the solar system formed, and it contains clues about how it formed (ie "memories"), and we can observe other systems forming (visible accretion discs) and consider it likely that ours formed the same way that others did (due to "physical laws"). So we think specific events happened in the past, many of which we're not that certain of.

[michel123456]

I am not discussing if an event in the past happened or not. Of course it happened.

 

You wrote

The spatial coordinate can become "empty" as the object that used to be there moves away

That is what we call motion. The object was here, now he is there: he moved.

 

And for time,

paraphrasing: "the time coordinate can become "empty" as the object that used to be there moves away".

My question resumes to verify if this statement is correct or wrong for time.

How can we verify that?

Edited November 28, 2011 by michel123456

[md65536]

And for time,

paraphrasing: "the time coordinate can become "empty" as the object that used to be there moves away".

I would paraphrase it thusly: Given that the moon "occupies" some given 4d spacetime coordinates...

For space: At another time, the moon might not occupy that same space.

For time: At another location, the moon does not occupy that other location at the same time.

 

 

 

When you're talking about the moon moving away, you're talking about another time.

If you're talking about one specific value for time, then the moon has a fixed spatial location for that fixed time.

 

Sorry, I don't seem to be understanding what you're saying.

[Iggy]

That is my understanding, yes.

If you would say this is true:

 

A person can know that an object is at a time in the past (t) [at that space-time coordinate] if it is at a distance of c times t where c is the speed of light?

 

then what about this:

 

A person can know that an object is at a time in the past (t) [at that space-time coordinate] if it is at a distance of s times t where s is the speed of sound?

 

The person could be in a submarine for example

[michel123456]

(...)For space: At another time, the moon might not occupy that same space.

For time: At another location, the moon does not occupy that other location at the same time.

 

(...)

Sorry, I don't seem to be understanding what you're saying.

Now I am more confused than you are...

One by one.

 

At another time, the moon might not occupy that same space.

a Moon that occupies the same space at 2 different times is a Moon at rest. For an observer on the Moon, that is possible, but that does not help the discussion.

 

For time: At another location, the moon does not occupy that other location at the same time.

That is time by time, I don't follow your phrasing.

 

Under standard concept, and on all known spacetime diagrams, the Moon in the past is a point on a line that extends along time. The Moon through time is not a point, but a line.

 

Which means that contrary to space, where an object change coordinates, in time an object does not change coordinates but "develop" in the form of a line. The object is presumed to be remaining at all its past coordinates. (or i have a bad understanding)

My question is thus, how can we be sure?

Edited November 28, 2011 by michel123456

[md65536]

Under standard concept, and on all known spacetime diagrams, the Moon in the past is a point on a line that extends along time. The Moon through time is not a point, but a line.

The line is a world line. It's a line through two dimensions (on a 2d diagram at least). One represents time, the other represents... space! It is not a line through time, it is a line through spacetime.

 

 

Which means that contrary to space, where an object change coordinates, in time an object does not change coordinates but "develop" in the form of a line. The object is presumed to be remaining at all its past coordinates. (or i have a bad understanding)

No, the object doesn't remain at its past coordinates, it moves through both space and time.

What remains at past spacetime coordinates is "events", including if you will the event of "being" at a certain location at a certain time. The moon of some moment in 1969 was at some location and that event is fixed there and then.

 

We are certain of this to the degree that we are certain of our understanding of applicable aspects of spacetime and reality etc.

 

 

 

To say that the world line "develops" along one dimension and "changes" along another dimension, makes me think that you may have some additional assumptions that are getting in the way of understanding this. I don't think I understand it enough myself, or can explain it any better than has already been explained in this thread, but perhaps someone else can. I think the confusion comes from assuming something more than what was said, so there may be some important bit of information that is missing in these explanations which you are filling in with incorrect assumptions??

 

 

 

The moon, an object, usually refers to a ball of mass as it exists in one specific place and time. The moon described by its world line, something that "is" in all the places its ever been, is not an "object" that has an existence in reality as we know it. The world line is fixed in the 4d spacetime manifold. It only "develops" from the perspective of one who sweeps a plane along the time dimension... which essentially corresponds to our perception of the 4d spacetime manifold. When you view a spacetime diagram, you can look at the whole thing at once. But the way you would experience it in reality is not as a 2d diagram (experiencing it all at once) but as a line that sweeps along the time dimension. If this is confusing it's probably because I don't know the right words or even the right understanding!