Looking Back In Time ?

[modeller]

We are told that we can look back in time, but can we really ? e.g. looking at a star that is say 2 light years away today, but if I look back next month. I would not be 'seeing' the same thing. It may still be two light years away but both we and it are both one month older in real time.

The light from our own sun takes 8 minutes to reach us but wa can't see what it was like say 20 minutes ago , so we are not really looking back in time . The same with the moon we can't see what is was like even a minute ago.

Yesterday it was said we can now see what it was like a fraction of a second after the big bang but that image has long gone past us and can never be seen again. What have I got wrong ?

[Zacs-Master]

Jom, time travels at the speed of light.....Mmm. Have a read of this regarding time as an abstract concept.
http://scienceforums.com/topic/19369-is-time-an-abstract-idea/

[jim360]

Time arguably doesn't travel at all, and it is just another way of labelling your position in "spacetime". Or, arguably, it is massively important since unlike space time moves in only one direction.

It's weird, and confusing, and I'm glad I don't do much general relativity where I'd have to worry about this.

[mibn2cweus]

The light that reaches us in any given moment from our unique perspective is merely a very small fraction of the light which is spread throughout and permeates the universe and depends on the distance to that point in space from which the light originated. The light from the very first instant of the initial event was repeatedly emitted and reabsorbed in the density of the medium which immediately followed the event. It was only after many hundreds of thousands of years after the initial expansion that the universe expanded to a size sufficient for matter to be transparent to light. What the universe was like prior to that age in the evolution of the universe can only be inferred from our understanding of physics and the nature of matter and light.

[jake-the-peg]

Modeler - there is an important difference between looking at a star 20 light years away and looking back at the conditions of the big bang

The early Universe occurred everywhere - it was not some giant explosion into a space that already existed - physical space and time came into being at that point

In one sense you are sitting at the 'point' the big bang happened

That means the background radiation, the 'echo' of the big bang is everywhere.

The telescopes investigating this scan the whole sky and map that signature echo to see what the Universe was like when it was young and very small.

There are some great puzzles

Why wasn't it totally uniform? - why is there more matter than anti-matter?

Some people even hope to see the 'mark of other Universes colliding with ours (although personally I have to say I think that's a bit speculative!)

[modeller]

Many thanks for all your posts it has certainly helped me see through terminology of the media. It's a question of concept.
The words : light, see, image, picture, all have different meanings depending on what we are using to get them.
This raises the point ' W

[jomifl]

Zacs, it could again be argued that everything is an abstract concept as modelled in the human brain, as long as the model works it is 'real'. Things happening far away cannot affect us sooner than the time taken for light to reach us from those events. What happens or happened light years away is just a step further away from our perceived 'reality'. I'm not trying to explain anything, rather demonstrate that 'time' has many meanings both practical and philosophical. As Mrs. O Says 'I'll get me coat' :-)

[jake-the-peg]

It's not so much that time has different meanings, rather it is much more complex that we experience in our daily lives

However it's daily form is embedded into natural language which is what we think in. That makes it real tough to work with when our every day concepts stop working

[modeller]

cont This raises the point when we talk about the speed of light what part/s
of the spectrum are we talking off. When we talk about all the components involved , all the different size particles etc would they all travel out at the speed of light. How far do they go ? Do some run out of energy ? If we took a whole series of images going back say one every billion light years how would they compare ?

Leaving aside the big bang for the moment if I took 'images' of similar stars at different distances from us say 1 billion 2 billion 3 billion etc.
How would they compare ?

[jomifl]

modeller, it seems that all wavelengths from X rays to very low frequency radio waves travel at the same speed although it is possible that they don't but there seems to be no evidence of this. As for running out of energy..that lost energy could only be converted into another form and there is no evidence as far as I know of this happening in a complete vacuum. But interstellar space is not a complete vacuum.

[modeller]

I have now read up the two sites concerning the Planck Telescope and there is one point in particular that is not explained.
It says the image is of the universe 380,000 years ( our years 'not even light years ) after the big bang .
Now we know that all the cosmic particles in that image are travelling at the same speed away from the source , so how can they still be there 14 billion light years later. You could walk further.
http://www.guardian.co.uk/science/across-the-universe/2013/mar/21/planck-unveils-earliest-map-of-universe

[modeller]

jomif l// But interstellar space is not a complete vacuum//

At that point in time there woudn't be any interstellar space.

[jomifl]

Modeller I wasn't refering to the 'time' of the big bang, I was referring to 'now'. The big bang happened in all the existing(then) universe. So all the microwave radiation is still rattling around in the 'universe'. It can't escape the 'universe' because there is nowhere else for it to go. At the time of the big bang I imagine space and time hadn't quite sorted themselves out. Can anybody recommend a bookjthat explains this (genesis excepted)?

[jake-the-peg]

Modeller

It's as I said in my first post

The big bang was not an explosion at a point into an existing space

It was the creation of space and time itself

The big bang happened everywhere - including where you are sitting now

The Universe is expanding and the radio echo of the BB fills all of it - the telescope looks at all parts of the sky to map out the variations in the background radiation

[modeller]

jake I understand , I think, what you are saying.
//The big bang was not an explosion at a point into an existing space//

But I assume it did start as a point albeit not a specific point.

I know we can scan all parts of the sky for cosmic variations but how can we date it ? See my post.
//It says the image is of the universe 380,000 years ( our years 'not even light years ) after the big bang .//

[modeller]

cont Further to that millions of stars have come and gone all adding to the background radiation so how can we say the Planck image is an image of the universe before any stars were even formed, at 380,000 thousand years after the BB.

[jomifl]

I'm not sure that such a thing as a point could have existed when the big bang ocurred, I don't think dimensions as we understand them existed.

[jim360]

When we talk about all the components involved , all the different size particles etc would they all travel out at the speed of light. How far do they go? Do some run out of energy ?[i

Light and all electromagnetic radiation travels at the speed of light. That's effectively by definition. Radiation does lose energy - when the CMB first started it was very energetic, and over time that's been lost from interactions with matter and other light particles, and the vacuum itself. But when light loses energy it doesn't slow down - it changes frequency. The CMB that is now microwaves used to be X-rays, or light, or gamma rays (I'm not sure which, actually, but probably a mixture). As time passes and they lose energy the light turned into microwave radiation.

[i]Now we know that all the cosmic particles in that image are travelling at the same speed away from the source , so how can they still be there 14 billion light years later.]

It was emitted all over the Universe, and because spacetime is curved that means that the light is trapped and can go round and round in circles. There's not going to be a time when we stop seeing this, at least not for a very long time. The CMB isn't a "blink and you missed it" phenomenon, because of the fact that it was emitted all over space itself in all directions.

[i]interstellar space is not a complete vacuum//

At that point in time there woudn't be any interstellar space. [i]

True in the strictest sense, but the space is still there independent of the stars. There is no such thing as a perfect vacuum, just to the complicated laws of Quantum Field Theory. There is always stuff there, popping in and out of existence, and even more happened if you have curved space on top of that.