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Big Bang Now

Dorfl

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Joined
Jun 19, 2005
Messages
523
When an observer moves, that observer's present is tilted with respect to another observer. In all normal situations the effect is negligible, and the observers will agree in which order things happen.

The most obvious exception is when an observer is travelling close to the speed of light. Then you can get situations where observers will disagree as to the order of events (if they are space-likely separated). Another exception is when you look at a part of an observer's present which is really far away. Then the slight tilting can be enough to move the present plane very far in time. Our lecturer did a demonstration of this when he walked back and forth in the classroom, and claimed that the differences in velocity were enough to shift what was present to him in the Andromeda Galaxy back and forth by about a day.

It did not occur to me to ask this to him until after the course was over, so I ask the forum: Does this mean that the parts of the Big Bang that are really far away can be part of my present? or does something, for example some effect of General Relativity, forbid this?

And if my present intersects the Big Band, does it also extend into some sort of pre-Big Bang state, or what?
 
I know modern telescopes have managed to see very far back, to within a few hundred thousand years as I recall. It's difficult, simply because you're trying to capture light that's 13 billion years old.

Of course, the cosmic background radiation is the result of the big bang, and can be measured here on earth.
 
wackyvorlon said:
I know modern telescopes have managed to see very far back, to within a few hundred thousand years as I recall. It's difficult, simply because you're trying to capture light that's 13 billion years old.

Of course, the cosmic background radiation is the result of the big bang, and can be measured here on earth.
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Yes. But my point wasn't about what we can see now. I was wondering if the Big Bang is part of my present, never mind how I would have to go about actually seeing it.

By definition, the present consists of events that I will have to wait to be able to observe. So any discussion about it pretty much has to be purely theoretical if we are talking about things further than 100 light-years away.
 
Dorfl said:
Yes. But my point wasn't about what we can see now. I was wondering if the Big Bang is part of my present, never mind how I would have to go about actually seeing it.
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Yes, in this sense the big bang is part of your "present". Read up on the cosmic microwave background radiation - the "afterglow" of the big bang - to learn why. Anyone with the proper equipment can detect it.

By definition, the present consists of events that I will have to wait to be able to observe. So any discussion about it pretty much has to be purely theoretical if we are talking about things further than 100 light-years away.
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Why 100 light-years? We observe all events in the past because of the finite propogation of light. Your assertion about 100 light-years seems rather arbitrary.
 
MattusMaximus said:
Why 100 light-years? We observe all events in the past because of the finite propogation of light. Your assertion about 100 light-years seems rather arbitrary.
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I would guess because anything presently happening more than 100 lights years away means you'll have to live for more than 100 years to see it. 100 years is arbitrary. 50-80 years seems more reasonable for most people already old enough to be posting here.
 
wackyvorlon said:
I'm afraid I don't believe I understand your question.
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Since I don't know how much relativity you have read, I am not sure exactly how to clarify it. Sorry if the following is just repeating a lot of stuff you know, without actually clarifying a fuzzy question:

Any observer traces out a world-line in spacetime. At any point on that line the observer has a present, which is a volume containing the events which the observer would consider to be occurring at that moment. Note that even though he will have to wait 100 years to actually see an event currently happening 100 ly away, it is still part of his present. If you draw a diagram with the x-axis representing some spatial axis and the y-axis representing the time axis, then the observer, if we consider him stationary, traces a straight line upwards in the diagram and his present at any point on that line is just another perpendicular line. It is usually convenient to scale the diagram so that a light pulse from the observer makes a line with a 45° slope.

However, if we consider him to be moving, then his present will not be perpendicular. Instead, it will be tilted, with the same angle from the horizontal as the observer has from the vertical. This means that what the things he consider to be part of his present, the events he considers to be simultaneous, will vary depending on his velocity. So if he zig-zags through space time (as the lecturer did by walking back and forth), his present will sort of wobble in space time, meaning that an event can be current while his is still, future when he is moving one way and past when he is moving the other. The effect is normally negligible, unless he is moving close to light speed (giving him a very noticeable angle in the diagram) or the event is very far away (putting it far off on the x-axis).

Of course, this just describes what happens in one dimension, but it can easily be generalised to three. It just becomes harder to show on paper. (Speaking of which, this would be much easier if I could scribble on a napkin or something while talking.)

Anyway, his present continues to infinity in both directions, even though he will have to wait longer to find out about them, the farther away events in his present are. To me, it seems reasonable that if you look at his present far enough away from him, it should intersect the Big Bang. I could be completely wrong though, but then I would like to know why.

I'm not sure if that clarifies things or not. Tell me if it was something completely different you wondered about.

ETA: According to wiki–http://en.wikipedia.org/wiki/World_line–it is incorrect to refer to events outside of the observer's light cone as either ''future'' or past'', so I guess it is more correct to say that as the observer zig-zags events end up above, below or intersecting his present.
 
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MattusMaximus said:
Yes, in this sense the big bang is part of your "present". Read up on the cosmic microwave background radiation - the "afterglow" of the big bang - to learn why. Anyone with the proper equipment can detect it.

Why 100 light-years? We observe all events in the past because of the finite propogation of light. Your assertion about 100 light-years seems rather arbitrary.
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Because I'll be dead by then. Bob Klase is right that maybe 60-70 would be a more realistic estimate of my life expectancy, but it was a nice round number.
 
I was going to suggest you study up on the idea of light cones, but I see you already referenced that. I think that may be your best shot at an answer to your question. A light cone model should clarify whether something can or cannot extend into your present.
 
Psi Baba said:
I was going to suggest you study up on the idea of light cones, but I see you already referenced that. I think that may be your best shot at an answer to your question. A light cone model should clarify whether something can or cannot extend into your present.
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Yeah... We did go through light cones pretty thoroughly in Special Relativity, and from what I learnt then it should definitely be possible for my present to intersect the Big Bang. Problem is that it should also intersect the pre-Big Bang, which doesn't seem to make sense. Especially if there is no ''before the Big Bang''. So I wondered if there was any General Relativistic reason why it cannot. Space curvature or something, I don't know.
 
Dorfl said:
When an observer moves, that observer's present is tilted with respect to another observer. In all normal situations the effect is negligible, and the observers will agree in which order things happen.

The most obvious exception is when an observer is travelling close to the speed of light. Then you can get situations where observers will disagree as to the order of events (if they are space-likely separated). Another exception is when you look at a part of an observer's present which is really far away. Then the slight tilting can be enough to move the present plane very far in time. Our lecturer did a demonstration of this when he walked back and forth in the classroom, and claimed that the differences in velocity were enough to shift what was present to him in the Andromeda Galaxy back and forth by about a day.

It did not occur to me to ask this to him until after the course was over, so I ask the forum: Does this mean that the parts of the Big Bang that are really far away can be part of my present? or does something, for example some effect of General Relativity, forbid this?

And if my present intersects the Big Band, does it also extend into some sort of pre-Big Bang state, or what?
Click to expand...

This concept is discussed by Brian Greene in The Fabric of The Cosmos, where he describes all of space and time as a "frozen river" (using only two dimensions of space). The Maximum angle that can be used is 45o because of the limit of light speed. I believe this restriction would not allow our "present" to be at the time of the big bang. This concept this has nothing to do with light cones, which in reality would exclude causality in this definition of "present."
 
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Perpetual Student said:
This concept is discussed by Brian Greene in The Fabric of The Cosmos, where he describes all of space and time as a "frozen river" (using only two dimensions of space). The Maximum angle that can be used is 45o because of the limit of light speed. I believe this restriction would not allow our "present" to be at the time of the big bang. As you point out this has nothing to do with light cones, which in reality would exclude causality in this definition of "present."
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I don't know. I picture the Big Bang as an essentially straight line at some distance back in the time direction. This would imply that the line of the present pretty much has to intersect it at some point. Maybe that picture is too simplistic, though.

Hm, I have a bookstore gift card which I wondered what to do with today. I suppose I might as well see if they have Greene's book. :)
 
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Actually, Greene does say that if space is infinite, any slice of spacetime can be part of your present. So, the 45o restriction wouldn't prohibit the big bang as part of your present. If you can find the book, just read pages 127 through 142 (soft cover edition) for his description of the "frozen river." It's merely an elaboration of the description given by your professor. Again, this does not incorporate the concept of lightcones and the issues of causality.
 
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Perpetual Student said:
Actually, Greene does say that if space is infinite, any slice of spacetime can be part of your present. So, the 45o restriction wouldn't prohibit the big bang as part of your present. If you can find the book, just read pages 127 through 142 (soft cover edition) for his description of the "frozen river." It's merely an elaboration of the description given by your professor. Again, this does not incorporate the concept of lightcones and the issues of causality.
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I suppose that any part of spacetime can be part of my present, at some point in my world line. But that only means that I was simultaneous with the Big Bang when I too was Big Banging, does it. I don't think it necessarily means I can be simultaneous with parts of it now.

Speaking of infinite space: what happens to the present if space is finite and closed? Does it sort of wrap around, or join with itself on both ends, or what? Maybe I should start another thread on that.
 
Sorry, I missed that bit.

I'll see if I can find the book. Or if not that, if wiki-ing will turn up something I can understand.
 
Dorfl said:
It did not occur to me to ask this to him until after the course was over, so I ask the forum: Does this mean that the parts of the Big Bang that are really far away can be part of my present?
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Yes.

And if my present intersects the Big Band, does it also extend into some sort of pre-Big Bang state, or what?
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Well, no one knows. That's equivalent to asking what came before the big bang.
 
If spacetime were infinite, what would it be "in"?
And if not, what would whatever it is in, be "in"?

I feel I need more coffee.
 
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