Can we see it?

[michel123456]

(...)

sorry for not giving that simple answer...just imagining what you said got me pondering..lightspeed to me seems to be time (or distance) itself...its the limit BUT what if we found a mirror ball in space and zoomed in on it 1.5 mil LY away??? would we see the future??? past or present of ourselves??

 

We would see in the mirror what the inhabitants of such a planet were looking at: we would see the Earth 1,5 + 1,5 = 3 millions years ago, roughly speaking.

 

--------------------

Hm, editing. We would see a portion of an image of the universe as seen by the inhabitants of this planet at that time of the past. If the mirror is flat & oriented to us, we wouldn't see ourselves 3 millions years ago, but we would see the object that was positionned at our actual coordinates 3 millions years ago, i.e. probably another planet or simply nothing.

Edited March 20, 2010 by michel123456

[Iggy]

All that stuff mentionned by Spyman is too complicated in comparaison with my purpose.

 

Then you could avoid all that by explaining in your thought experiment that by “seeing” you mean only the observation of unimpeded light traveling the shortest available distance between the two. Otherwise, “you cannot see the past event of the pencil falling down.” sounds like you’re saying it is impossible. You’re saying “cannot” rather than “won’t”.

 

Figuring out how long ago an event happened involves the path length of the mediating particles and their speed. Where you say “distance tells exactly what your friend is able to see” I think you would do much better to either say “distance tells exactly what your friend sees” or if you really do want to imply that the observation is impossible then you could say “the distance, path, and speed of the signal tells exactly what your friend is able to see”--if you establish in your thought experiment that you're only talking about one kind of path and one kind of speed then that would be 100% correct.

[michel123456]

Then you could avoid all that by explaining in your thought experiment that by “seeing” you mean only the observation of unimpeded light traveling the shortest available distance between the two. Otherwise, “you cannot see the past event of the pencil falling down.” sounds like you’re saying it is impossible. You’re saying “cannot” rather than “won’t”.

 

Figuring out how long ago an event happened involves the path length of the mediating particles and their speed. Where you say “distance tells exactly what your friend is able to see” I think you would do much better to either say “distance tells exactly what your friend sees” or if you really do want to imply that the observation is impossible then you could say “the distance, path, and speed of the signal tells exactly what your friend is able to see”--if you establish in your thought experiment that you're only talking about one kind of path and one kind of speed then that would be 100% correct.

 

Sure. Your post means you have understood my thoughts. Thank you for your observations, and sorry if I missed the correct words.

 

Now, i think we can all agree that we can see a planet placed in the past 1,5 million years ago, & 1,5 million LY from us. There is no trick, it is a simple standard planet like many others.

 

but whats the point??

says Law.

 

The point is this:

Taking my telescope and looking at that distant planet, 1,5 million LY away (& thus as it was 1,5 million years ago), I am astonished seeing my E.T. friends (the inhabitants) looking in their telescopes too. They don't look at me. They are actually observing an event I cannot see. They are observing an exploding star, positionned relatively to me 2 millions years ago, one million LY from me (the star of my first post).

 

That is more difficult to understand.

I'll try to explain.

[Iggy]

The point is this:

Taking my telescope and looking at that distant planet, 1,5 million LY away (& thus as it was 1,5 million years ago), I am astonished seeing my E.T. friends (the inhabitants) looking in their telescopes too. They don't look at me. They are actually observing an event I cannot see. They are observing an exploding star, positionned relatively to me 2 millions years ago, one million LY from me (the star of my first post).

 

That is more difficult to understand.

I'll try to explain.

 

I'm having some trouble with your wording, but the concept is correct. If you graph space and time it could be easier to understand what is happening.

 

 

The star explodes at A. The light first reaches Planet X at B 1.5 million years ago then reaches Earth at C 1 million years ago. Event B is then seen by Earth at D--the here and now.

 

I am astonished seeing my E.T. friends (the inhabitants) looking in their telescopes too. They don't look at me... They are observing an exploding star

 

Rather than saying "they don't look at me" and "they are observing an exploding star" you would avoid confusion if you said "they weren't looking at me" and "they were observing an exploding star". It would be in the past tense because it happened in the past.

 

They are actually observing an event I cannot see.

 

If there were a mirror on Planet X then you would see the exploding star--the same one they were looking at through their telescope--so it is misleading to say that you cannot see it. It is an event that you do not see when looking directly at the position where the star was. It takes light longer to travel ABD than it does to travel AC.

Edited March 20, 2010 by Iggy

[michel123456]

Iggy, you are not following my thoughts, you are preceding.

Marvelous.

Light does not follow ABD trajectory.

Light coming from star A follows AC. (& AB)

 

From our point of vue, at D, we cannot see the exploding star. Point A is INSIDE THE LIGHT CONE and is not directly observable. that's what Sisypuhus meant when saying in his previous post

"In both cases, you're taking in sensory information, and interpreting it to piece together an event in the past. The only difference is that in the first case you are on the "leading edge" of the information cone (or actually not, because light doesn't travel at C through air, and the biological processing takes time, but close to it), and in the second case you're farther behind the leading edge.

"behind the leading edge" is "inside the light-cone."

 

---

Merged post follows:

Consecutive posts merged

And from point D, we are not able to see point C either. Point C is the Earth's own past and is not currently visible.

 

But that is not the important point.

The important point IMO is that A (the exploding star event) and B (the planet) are related through EM radiation & through gravitation, since it has been demonstated that gravitation is transmitted at Speed Of Light.

In other words, the effect of A upon B can only be interpretated by us through some effects on B, and not by observing directly A, because A is hidden "somewhere" inside the light-cone.

An effect comparable to the definition of dark matter.

Edited March 20, 2010 by michel123456
Consecutive posts merged.

[Hendrix]

since it has been demonstated that gravitation is transmitted at Speed Of Light.

 

Demonstrated how?

[Iggy]

A is hidden "somewhere" inside the light-cone.

 

That's an interesting way of looking at things, I suppose.

 

It would usually be said that any event in A's future light cone can be causally connected to A. That is to say, A could affect any of the events in the graph I posted.

 

But, you are focused narrowly on the idea of "seeing" A--that only an event at distance ct can "directly observe" A. That may be true in your thought experiment, and it's usually true in astronomy, but it is not always true--it doesn't have to be true. We can observe some astronomical bodies like Einstein's cross where the light has not made a straight path to us. The light was bent and slowed even though it didn't technically interact with any other particles.

 

Also, why do you think something being "directly observable" is important? All of the objects in your thought experiment are separated by some distance so none of them can directly interact. In order for one to communicate with another (or to observe another) means that some signal or some particles need to be sent between them. By only considering the one kind of path and one kind of particle (light) you neglect other methods of observation or communication.

 

Also, where are you going with this?

[michel123456]

That's an interesting way of looking at things, I suppose.

 

It would usually be said that any event in A's future light cone can be causally connected to A. That is to say, A could affect any of the events in the graph I posted.

 

But, you are focused narrowly on the idea of "seeing" A--that only an event at distance ct can "directly observe" A. That may be true in your thought experiment, and it's usually true in astronomy, but it is not always true--it doesn't have to be true. We can observe some astronomical bodies like Einstein's cross where the light has not made a straight path to us. The light was bent and slowed even though it didn't technically interact with any other particles.

 

Also, why do you think something being "directly observable" is important? All of the objects in your thought experiment are separated by some distance so none of them can directly interact. In order for one to communicate with another (or to observe another) means that some signal or some particles need to be sent between them. By only considering the one kind of path and one kind of particle (light) you neglect other methods of observation or communication.

 

Also, where are you going with this?

 

What are those "other methods"?

 

_Nothing can travel faster than C. (It concerns what is outside the light-cone)

_Electromagnetical radiation is condamned to travel at speed=C (that is the surface of the light-cone)

_Gravitation travels also at speed = C.

_Only matter can travels at speed less than C. That concerns what is the inside of the light-cone.

 

So, the only method of observation available to investigate the inner part of the light-cone is to collect matter.

The only & most usual matter at our disposition is the Earth itself, and some rare asteroids. Most other material elements ,galaxies, stars, planets, gases & plasma, are detected through some radiation (see above). Neutrinos travel at speed near C.

What else?

 

If you put all the above upon a diagram, you will see that the observable parts are close to the surface of the light-cone, except Earth's life-line (the vertical DC axis in your diagram).

All the rest is not observable.

 

Also, where are you going with this?

 

My point is, the Earth past light-cone is full of regular matter that we cannot observe.

That means, for us, the light-cone looks empty.

 

You can plot into your diagram a lot of other stars like A, inside the light-cone, that are not observable.

All the observable objects placed upon the surface of the light-cone are influenced by those non observable stars, made of regular matter, hidden in the past.

So, maybe that is dark matter. Not hidden in space but hidden in time.

[Iggy]

My point is, the Earth past light-cone is full of regular matter that we cannot observe.

That means, for us, the light-cone looks empty.

 

You can plot into your diagram a lot of other stars like A, inside the light-cone, that are not observable.

All the observable objects placed upon the surface of the light-cone are influenced by those non observable stars, made of regular matter, hidden in the past.

So, maybe that is dark matter. Not hidden in space but hidden in time.

 

I understand completely now.

 

You are confusing events with objects. An event happens at a single place and time. On the previous diagram the events were A, B, C, and D. Objects are represented as world lines. On the diagram they were the lines extended from the bottom to the top (earth, planet X, and star):

 

 

I didn't draw the whole world line for the exploding star because it would have fanned out into a supernova remnant and been complicated to draw.

 

The point is that mass cannot be hidden inside a past light cone the way you are thinking of an event hiding because mass doesn't last for an instant the way an event does. For example, Neil Armstrong walking on the moon was an event. We are currently inside the future light cone of that event so we cannot directly observe it today. In your words, it is hidden inside our past light cone. But, this does not mean the mass of the moon or the mass of Neil Armstrong is missing to us. It is directly observable today, we are just seeing it at a different time.

 

Reading this page might be helpful: http://en.wikipedia.org/wiki/World_line

[michel123456]

I understand completely now.

 

You are confusing events with objects. An event happens at a single place and time. On the previous diagram the events were A, B, C, and D. Objects are represented as world lines. On the diagram they were the lines extended from the bottom to the top (earth, planet X, and star):

 

 

I didn't draw the whole world line for the exploding star because it would have fanned out into a supernova remnant and been complicated to draw.

 

The point is that mass cannot be hidden inside a past light cone the way you are thinking of an event hiding because mass doesn't last for an instant the way an event does. For example, Neil Armstrong walking on the moon was an event. We are currently inside the future light cone of that event so we cannot directly observe it today. In your words, it is hidden inside our past light cone. But, this does not mean the mass of the moon or the mass of Neil Armstrong is missing to us. It is directly observable today, we are just seeing it at a different time.

 

Reading this page might be helpful: http://en.wikipedia.org/wiki/World_line

 

Iggy, it is a pleasure to discuss with you.

 

The world line gives a width to the light-cone. The width is not given by the object, but by the observator. When we see a star, we know that it existed yesterday, and that it will exist tomorrow. We can draw its world-line by some approximation. But we cannot do the same for an object we have never seen.

At the scale of the diagram, the length of humanity's life-line (lets say 10.000 years) is equal to the width of the line you drawed.

From the invention of telescope till today, only 500 years have passed. At this scale, a line of 500 years width is thinner than a hair.

At the scale of the universe (say 12 billions years), the line-life of the Earth (4 billions years) is only a third. So, even if we were able to collect all the information from the birth of our planet till now, we woud collect only a third of the universe's information.

 

Now, if you believe that the only mass is the mass we can see & observe, put yourself at planet B in your diagram, and draw the new light-cone from your new point of vue. You will observe the universe differently, isn'it? You will see other stars at other places. The same that we are observing today? That's the question.

[Spyman]

The important point IMO is that A (the exploding star event) and B (the planet) are related through EM radiation & through gravitation, since it has been demonstated that gravitation is transmitted at Speed Of Light.

In other words, the effect of A upon B can only be interpretated by us through some effects on B, and not by observing directly A, because A is hidden "somewhere" inside the light-cone.

An effect comparable to the definition of dark matter.

The event when the star exploded might be 'hidden' inside our lightcone but the matter and energy that was there during the event did not vanish in a 'puff' - they are still around and are at least theoretically observable.

If an object like B is acting strangely we can calculate where A should be and then with better observation find the remnant of the star, because it is still there, on the edge of our lightcone.

Not at all comparable to Dark Matter.

 

All the observable objects placed upon the surface of the light-cone are influenced by those non observable stars, made of regular matter, hidden in the past.

So, maybe that is dark matter. Not hidden in space but hidden in time.

There might be objects outside of our observable part of the universe which are able to interact strongly with objects on the border of our visible limits, but if the discussion is moving in that direction you really need to include the 'complicated stuff' I mentioned, because at those large distances they will make a very big difference.

 

If you neglect the expansion of space and look at closer objects like the Milky Way then like you said yourself, gravity and electromagnetical radiation travel at lightspeed and matter always travel slower, so if something is able to interact with us gravitationally, then it is also able reach us with its electromagnetical radiation.

 

There is no object made of normal matter inside our galaxy that is outside of our visible sphere, (unless it is moving faster than light), we are able to observe its location, speed and direction, somewhere and sometime in the past and account for its mass and location in our calculations.

 

Sure, some very massive object outside our visible universe might be able to interact with stars on the other side of our galaxy, but at those distances it's gravity will be exceptionally tiny and also directed away from the center of Milky Way.

 

You are not going to be able to explain Dark Matter with object outside of our visible light cone, like with for instance the influence on Galactic rotation curves from Dark Matter.

 

At the scale of the universe (say 12 billions years), the line-life of the Earth (4 billions years) is only a third. So, even if we were able to collect all the information from the birth of our planet till now, we woud collect only a third of the universe's information.

The age of the planet Earth or how long we have been able to observe our sky with telescopes has nothing to do with it, the parts that make up our planet today has a lifeline that is as old as the rest of the Universe, we are today able to observe objects and actions almost all the way back to the Big Bang, like for instance the Cosmic Microwave Background Radiation, thus at the scale of the lifetime of the Universe we are able to observe most of it.

(At the scale of the size of the Universe we are probably only able to observe a tiny part but thats a different story.)

 

Now, if you believe that the only mass is the mass we can see & observe, put yourself at planet B in your diagram, and draw the new light-cone from your new point of vue. You will observe the universe differently, isn'it? You will see other stars at other places. The same that we are observing today? That's the question.

If you place planet B at the edge of our visible universe then they will be able to see a large different part of the Universe and we will be able to see a large different part than them but a large part of our views will still overlap where we will see the same stars, albeit in different stages and positions depending of distance.

 

If planet B is inside the Milky Way the differences in our visible Universes will be small, they might have observed when a star exploded 100 000 years ago, while we still can see it shining bright today, but we can both still observe the star/remnant, it is not 'hidden' in time.

 

It doesn't matter how far away a star is, it can not hide in time anymore than the pen on my desk can. Every time I look it is there, the event when it fell to the floor and was picked up might have passed and are lost in the past, but the pen itself is still here on my desk.

Edited March 22, 2010 by Spyman
Spelling

[michel123456]

Well I have to admit you Iggy & Spyman made some points.

I made some diagrams of your point of vue, and they make sense. I am taking back my proposal for dark matter, it is not compatible with the Big Bang Theory. If all matter-energy that makes up the Universe has been created at a single moment in the past, and if no matter-energy can be created or destroyed since, you are right, and I am wrong.

 

But I still find intriguing that the universe we are looking at in our telescopes is collated upon the surface of the light-cone. We cannot see anything outside neither inside the light-cone.

 

----------------------------------

Here is a draft.

 

 

Looking around us, back in time, upon our light-cone we see ABCDE.

Today, ABCDE are in the present (tautology), they are upon the horizontal line, and not where they are observed.

And we declare that X and Y do not exist, because we cannot see any of them.

Edited March 22, 2010 by michel123456

[Iggy]

But I still find intriguing that the universe we are looking at in our telescopes is collated upon the surface of the light-cone.

 

I'd agree.

 

Here is a draft.

 

 

Looking around us, back in time, upon our light-cone we see ABCDE.

Today, ABCDE are in the present (tautology),

 

That's a good diagram. I would just be careful in that you've labeled A, B, C, D, and E as events but you're talking about them as objects. The way you've drawn them they happened in the past so you would say "A, B, C, D, and E are in the past." But, your point is correct. Whatever matter accompanied those events are currently in the present--definitely a tautology.

 

they are upon the horizontal line, and not where they are observed.

And we declare that X and Y do not exist, because we cannot see any of them.

 

X and Y would go a lot further toward existing if you drew world lines for them. I don't believe cosmologists would say that they don't exist. It's just that their light isn't reaching earth right now. Here is a website: http://www.astro.virginia.edu/class/whittle/astr553/Topic16/t16_light_cones.html with a similar diagram, and I'll quote some.

 

Solid red line shows, on both sides, the trajectory of a light ray which starts at the big bang from our current particle horizon. It is also the event-line which we witness now as light comes to us, ie we see only those events lying on the red line (light rays starting at other space-time events have either already arrived or will arrive in the future) -- the light cone is for light arriving now.

[liarliarpof]

No.

[Spyman]

And we declare that X and Y do not exist, because we cannot see any of them.

I agree with Iggy, you need to separate the events and the objects and make clear distinctions between them.

 

If you draw the world lines for events X and Y, you can see that they also cross our lightcone so the objects involved in both the events X and Y are observable from Earth today.

 

But we see the object in X before the event X and the object in Y after the event Y.

 

It is thus very clear that both the objects in X and Y exists, since we can observe them today.

 

The event Y might also be provable, if we are observing the remnants of a supernova today where the world line from event Y cross our lightcone, we can conclude that it did go supernova in the past and might even from our observations calculate when it did happen.

 

The event X is not a clear cut since we can only predict when that event will happen from the observation we make, when the world line from event X cross our lightcone. In a sence that event is in our future by observational limits, even though the event already has happened, but we are not able to observe it yet. Although we know that it will happen even if we can't pinpoint when, where and how exactly.

Edited March 23, 2010 by Spyman

[michel123456]

At least, 2 other people agree with me that the event Y is not observable.

Not too bad.

Spyman wrote:

you need to separate the events and the objects and make clear distinctions between them.

 

IMO it is difficult to do that. All that we are observing are only events.

Here below is another draft diagram of what we are observing:

 

 

We are observing events ABCDE & abcde. All the rest, the life-lines, the Bang etc, are deductions & theories.

We are not observing the event Y.

Quite an empty diagram, isn't it?

The observable universe.

------------------------------

From the above diagram, we have to explain something that looks like this:

 

 

That is not the observable universe any more. It is the Universe.

Edited March 23, 2010 by michel123456

[Sisyphus]

We are observing events ABCDE & abcde. All the rest, the life-lines, the Bang etc, are deductions & theories.

 

Events ABCDE and abcde are also just deductions. What we "observe" is our retina registering a few photons. From that we deduce that a giant ball of fusing hydrogen exploded millions of years ago. This isn't obvious. An ancient might think it's a hole in the Celestial Sphere. Similarly, you deduce that giant lizard-like creatures once walked the earth, based on curiously shaped rocks.

[Iggy]

Michel, you need to go back and draw world lines for every event on every diagram you've made. If you do, you'll figure out real quick what's wrong with this:

That is unless you're really thinking that stars randomly pop into existence, emit some light, and quickly disappear.

Edited March 23, 2010 by Iggy

[michel123456]

What is wrong with this?

 

You can draw the world lines like in the previous diagram, and it fits.

You can draw other world lines, and it fits too.

The Earth is nowhere, lost in the universe. It is not upon the axis, it is not at the edge. IMO it looks O.K.

The Earth is also nowhere in time, completely nowhere, not at some peculiar moment from the beginning. It makes some sense to me.

From any object (event, point) of the diagram, you can draw a similar light-cone. Every point will have about the same view of the universe. It makes sense to me.

The mass of the universe is much larger than what we can see. That is not bad IMO.

The only catch is that it doesn't fit with the BB theory, I must admit.

But that doesn't mean that "stars randomly pop into existence, emit some light, and quickly disappear." IMO there are other possibilities, but to get into this, we will slide into speculations.

So I prefer agree with you and admit it is all wrong.

I had the chance to discuss seriously about all this, and that's enough to me.

[Spyman]

So I prefer agree with you and admit it is all wrong.

I had the chance to discuss seriously about all this, and that's enough to me.

Hmm, I don't know if there is any point of continuing then...

 

Spyman wrote:

you need to separate the events and the objects and make clear distinctions between them.

IMO it is difficult to do that. All that we are observing are only events.

This seems to be the crux of the matter, you need to think much more about this.

 

I don't have any trouble to differentiate between objects and events.

 

An Event is a phenomenon that happens, an occurrence, that takes place in a certain time and place, it contain actions between objects but it is not something that persists, it's more lika a snapshot of frozen time.

 

An Object is a material thing, composed of matter and energy, it moves through time and space and is involved in many events, although the object itself can be disintegrated, the matter and energy inside can not vanish.

 

As one object travels from the Big Bang to the present it moves along its world line and for each timestamp it passes through a new event. We are observing objects in certain events, but the objects continue towards the future while the events we observed remains fixed in the past.

[michel123456]

Spyman wrote:

An Object is a material thing, composed of matter and energy, it moves through time and space and is involved in many events, although the object itself can be disintegrated, the matter and energy inside can not vanish.

 

As one object travels from the Big Bang to the present it moves along its world line and for each timestamp it passes through a new event. We are observing objects in certain events, but the objects continue towards the future while the events we observed remains fixed in the past.

 

You look in agreement with my pencil universe.

http://www.scienceforums.net/forum/showthread.php?t=47326

 

It has been pushed away in Speculations. Bizarre, because after all it looks like the standard point of vue.

[Spyman]

You look in agreement with my pencil universe.

No, I suggest you read what peoples told you in that thread one more time.

[michel123456]

No, I suggest you read what peoples told you in that thread one more time.

 

 

Hm. i don't get it.

On a space-time diagram, how do you represent an object? (by differenciation with an event)

[Iggy]

On a space-time diagram, how do you represent an object? (by differenciation with an event)

 

A material object is a line. Noether's theorem would demand it. An event is a dot.

[michel123456]

A material object is a line. Noether's theorem would demand it. An event is a dot.

 

If that is your point of vue, you must be confident with this (the lines being the objects & the events being the dots)

 

 

Which is not basically different with the next one (the lines are simply missing, you can draw the objects-lines at will following the one or other theory)

 

 

In both cases, the Spacetimed Universe is full of matter (objects) from which we can see only a tiny part: the dots at the intersection of the light-cone in the first diagram.