Looking at the past

[morgsboi]

The way I see it, all the time, wherever we look, we are looking into the past. So if you look through a telescope and something that was 10 light years away, then exactly what your seeing would have happened 10 years ago. So while your looking at your computer right now, you are actually seeing the past but on a extremely small scale.

Would this be correct?

[Iggy]

That is my understanding as well.

 

Everything that we see happened some time in the past because it takes light time to get from there to here. Like you say, we would just now observe an event that happened 10 years ago and is 10 light-years away.

 

The scientific name for the set of events extending into the past which we currently see is our "past light cone".

[michel123456]

The way I see it, all the time, wherever we look, we are looking into the past. So if you look through a telescope and something that was 10 light years away, then exactly what your seeing would have happened 10 years ago. So while your looking at your computer right now, you are actually seeing the past but on a extremely small scale.

Would this be correct?

Yes.

The present is not observable.

 

That is my understanding as well.

 

Everything that we see happened some time in the past because it takes light time to get from there to here. Like you say, we would just now observe an event that happened 10 years ago and is 10 light-years away.

 

The scientific name for the set of events extending into the past which we currently see is our "past light cone".

 

I was sent once to the trash can for the following so I will reiterate my statement (using Iggy's) with a slight difference: as much as I understand the set of events extending into the past which we currently see is upon the surface of our past light cone.

Edited November 5, 2011 by michel123456

[Ophiolite]

You also have to add in processing time for the brain so that we perceive what we are looking at a moment after we see it.

[morgsboi]

You also have to add in processing time for the brain so that we perceive what we are looking at a moment after we see it.

 

Yes, when your looking from a human point of view, but its not just your brain that might see it. Say it was a lens but nothing was looking through it, you wouldn't have a processing time because what counts is the amount of time it takes the light to travel from A to B.

[Iggy]

as much as I understand the set of events extending into the past which we currently see is upon the surface of our past light cone.

I believe that to be true by definition, absolutely.

[michel123456]

I believe that to be true by definition, absolutely.

What a relief! I am not so crazy after all. Or at least I am not alone. Iggy you made my day (my night actually).

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

So, to resume my point:

1. we can only see what is on the surface of out past light cone.

2. what is outside our past light cone we cannot see

3. what is inside our past light cone we cannot see

 

It is so simple but I can not find that in the existing litterature.

Edited November 5, 2011 by michel123456

[DrRocket]

Yes.

The present is not observable.

 

 

 

I was sent once to the trash can for the following so I will reiterate my statement (using Iggy's) with a slight difference: as much as I understand the set of events extending into the past which we currently see is upon the surface of our past light cone.

 

 

Since "see" implies light reaching us at a point in spacetime, this is indeed true, essentially by definition. Causality rules out the light approaching from the future.

[Iggy]

What a relief! I am not so crazy after all. Or at least I am not alone. Iggy you made my day (my night actually).

Some would question my fitness as the benchmark for sanity

 

So, to resume my point:

1. we can only see what is on the surface of out past light cone.

2. what is outside our past light cone we cannot see

3. what is inside our past light cone we cannot see

 

It is so simple but I can not find that in the existing litterature.

It might be better to say "event" like you did yesterday. "Events outside our past light cone we cannot see".

 

While we don't generally see events inside our past light cone, it isn't an impossibility. Light could, for example, pass through a medium with a high refractive index (slowing the light) between the event and the observer.

 

Events outside are unobservable. Literature to that effect... well, there's wikipedia:

 

Therefore no event outside the light cones can be reached from the origin, even by a light-signal, nor by any object or signal moving with less than the speed of light.

 

http://en.wikipedia.org/wiki/Minkowski_diagram#The_speed_of_light_as_a_limit

[DrRocket]

 

 

It might be better to say "event" like you did yesterday. "Events outside our past light cone we cannot see".

 

...

 

Events outside are unobservable. Literature to that effect... well, there's wikipedia:

 

 

 

This statement relies rather heavily on "see" implying literal detection of an electromagnetic signal, and that being a pure signal -- i.e. not the result of some event subsequent to the one of interest such as passing through a medium as you mentioned.

 

Given those restrictions and idealizations the statement is essentially a trivial application of the definition of a light cone and the rather obvious fact that light travels at the speed of light. Logic trumps Wiki for understanding every time..

 

Note that you can easily be affected by events in the interior of the past light cone. If fact the set of events that can affect you is precisely your past light cone. That is whole reason that the past light cone is of interest.

[michel123456]

This statement relies rather heavily on "see" implying literal detection of an electromagnetic signal, and that being a pure signal -- i.e. not the result of some event subsequent to the one of interest such as passing through a medium as you mentioned.

 

Given those restrictions and idealizations the statement is essentially a trivial application of the definition of a light cone and the rather obvious fact that light travels at the speed of light. Logic trumps Wiki for understanding every time..

 

Note that you can easily be affected by events in the interior of the past light cone. If fact the set of events that can affect you is precisely your past light cone. That is whole reason that the past light cone is of interest.

 

All right.

What is of great interest to me is the part inside the light cone.

It is well understood that the internal part contains the events that are causaly connected to the observator.

But, from the entire set of these events, only a few are directly observable.

 

These are the ones that we can observe through EM radiation (placed upon the surface of the light cone, these are the tremendous majority), these who deflect slightly from Speed Of Light as Iggy pointed out (it is as if the surface of the light cone had a width). But, since signals from the past traveling at speed lower than SOL (IOW the material ones, with mass) have already reach us in the past (for example an asteroid hitting the Earth), in fact the main "volume" of our past line cone is directly unobservable. IMHO.

 

Of course one can observe the remains of past events causaly connected to the observator, like the crater of the asteroid, but one cannot observe today an asteroid hitting the Earth millions of years ago. Similarly, we cannot observe directly our own past because our own past lies inside our past light cone.

Edited November 6, 2011 by michel123456

[Genius13]

i have a question

we say that the universe is infinetly big , but cant we calculate its lenght?

we just calculate its speed and the time it had to expan ( 13.7 bilion years ) and we get it

 

 

and another thing :

what what speed is the universe expanding? relativity thory forbides it to go faster than the speed of light, but then we could see the end of the universe ( because light would have time to reach us if its faster than the expantion speed )

pls answer

[imatfaal]

i have a question

we say that the universe is infinetly big , but cant we calculate its lenght?

some people think it is infinite - others have other ideas infinite/finite bounded/unbounded flat/curved etc...

 

we just calculate its speed and the time it had to expan ( 13.7 bilion years ) and we get it

but we have no real idea how big it was to start with

 

and another thing :

what what speed is the universe expanding? relativity thory forbides it to go faster than the speed of light, but then we could see the end of the universe ( because light would have time to reach us if its faster than the expantion speed )

Nothing can go THROUGH space faster than the speed of light - but if space itself is expanding then it can seem that things are separating at a speed faster than light. there are various analogies to help understanding of this - perhaps this one can help.

 

Imagine two model trains tail to tail and pointing in different directions on a very long straight track (you know the sort that comes in stretches of about 12 inches and you can attach them together)

 

well if the trains move one section per minute then after a hour of moving away from each other then they will be 120 unit sections apart.

 

now imagine that as you set them off again from being together tail-to-tail that every 10 minutes you add a section of track in between each join. It is quite clear that after a hour of each train travelling (one unit section per minute away from each other) that the actual distance will be well in excess of 120 unit sections.

 

So the distance between them has increased at a greater speed than the sum of their speeds over the track. This is because the very background, the track is expanding as well as the trains moving along the track.

 

In simplified terms this is what is happening to galaxies (and the rate of expansion is less hectic). Whilst they may have only been travelling for 13.billion years - they are separated by far more than 13 billion light years because space is expanding and they are travelling through space.

[Genius13]

i know its speeding up but what speed is it going ? and if we know can we calculate its lenght?

[imatfaal]

Obviously my analogy failed - cos if you had read it and understood it you would have realised that there is no single speed. damn.

 

The increase in separation between two galaxies depends on the distance between them. The further galaxies are apart then the quicker they are separating - you might like to look up hubble's law on wikipedia.

 

we can calculate the size of the observable universe - but anything else is guesswork, and it is guesswork based on many assumptions that are not agreed.

[Airbrush]

i know its speeding up but what speed is it going ? and if we know can we calculate its lenght?

 

Correct me if I am wrong, but I believe the way to calculate the average speed the observable universe has been expanding, just take the current distance to the furthest observable object, the cosmic microwave background radiation, currently about 46 Billion light years away, and divide that by the age of the universe, 13.7 Billion years, and you get about 3.3 times the speed of light. That only applies to the average speed the CMB has been moving away from us since the Big Bang. Objects closer to us have been moving away at a slower average speed.

Edited November 10, 2011 by Airbrush

[Genius13]

so u can calculate it by 3.3c times 13.7 bilions years ( if u dont include the speed up and the inflation ) and u get its lenght?

[michel123456]

Correct me if I am wrong, but I believe the way to calculate the average speed the observable universe has been expanding, just take the current distance to the furthest observable object, the cosmic microwave background radiation, currently about 46 Billion light years away, and divide that by the age of the universe, 13.7 Billion years, and you get about 3.3 times the speed of light. That only applies to the average speed the CMB has been moving away from us since the Big Bang. Objects closer to us have been moving away at a slower average speed.

 

The most funny thing is that if what we observe came to us at 3.3 times the SOL, what we see is not the past, it is the future

[Summerwind]

While we share the excitement with others regarding recent discovery that the universe is accelerating in its expansion, the dark energy part is rather spooky. There is another model that gives the same results without such energy and we will greatly appreciate experts who can review the model. The author is a simple engineer, not an astrophysicist. He said he has a headache each time he tries to imagine what the dark energy is like http://www.atlanticd...com/science/109 (A Clumping Universe) Thank you.