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Author Topic:   How is the Universe here?
onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 16 of 131 (487078)
10-27-2008 1:18 PM
Reply to: Message 12 by Straggler
10-27-2008 7:36 AM


Re: Entropy
Hi Straggler,

Interesting point,

Straggler writes:

Is our view of the total entropy of the universe also individual if our view of time is individual? Or can the concept of entropy be used to relight the idea of universal time existing in some sense after all?

Im sure cavediver will straighten out our understanding of it but, just to add to your point, isn't the entropy of the universe only measured as a whole because the entire universe is expanding pushing us further away from equilibrium?


"All great truths begin as blasphemies"

"I smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your mouth."--Bill Hicks

"I never knew there was another option other than to question everything"--Noam Chomsky


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onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


(1)
Message 17 of 131 (487082)
10-27-2008 1:30 PM
Reply to: Message 14 by johnfolton
10-27-2008 11:46 AM


Hi John'

John writes:

The earth is it not moving thru space and time so for an happening to happen before another shared the same happening how is this not placing the cart ahead of the horse?

An easy example would be photons leaving the Sun. It takes 8 minutes for light to reach our planet, if you were 1000ft away from the Sun(hypothetically of course), the light hits you in your present, while it would reach me in your future. 8 minutes later.

For distances in the billions of light years, the future could be 100's of years away. So a Supernova explosion, billions light years away, would be seen by Aliens near it long before us here on Earth see it. Making their present experience, of seeing the Supernova explode, an experience that we'll see 100's of years later.

Relative to the observer present events can be another observes future event.

I hope this helped.

--Oni


"All great truths begin as blasphemies"

"I smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your mouth."--Bill Hicks

"I never knew there was another option other than to question everything"--Noam Chomsky


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Huntard
Member (Idle past 584 days)
Posts: 2870
From: Limburg, The Netherlands
Joined: 09-02-2008


Message 18 of 131 (487087)
10-27-2008 3:30 PM
Reply to: Message 17 by onifre
10-27-2008 1:30 PM


onifre writes:

For distances in the billions of light years, the future could be 100's of years away. So a Supernova explosion, billions light years away, would be seen by Aliens near it long before us here on Earth see it. Making their present experience, of seeing the Supernova explode, an experience that we'll see 100's of years later.


Wouldn't a supernova happening billions of light years away actually take billions of years to be seen here on earth? Since the light that that explosion produces would take billions of years to get here? Or am I missing something?


I hunt for the truth

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Replies to this message:
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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 19 of 131 (487089)
10-27-2008 4:11 PM
Reply to: Message 13 by Thugpreacha
10-27-2008 10:21 AM


Sorry, been a very long day!!!

Are you saying that an event in my past may be an event in your future?

Yes :) But not all events. In Newtonian space-time, there is a universally agreed now (as we all set our watches by Greenwich) and all events occur either now (on our current sheet of paper), in our past, or in our future. We could be opposite sides of the universe, but we'd still agree on this three-way split of events. And, thinking ahead a bit, our relative motion would be irrelevant.

But in Special Relativity, we have the finite speed of light... Things in our past have to be able to influence us now - that's what 'in our past' means. So it must be possible to send a signal from a past event to us now. This signal can only travel at the speed of light, max. What about events that are sufficiently far away from us that they could not affect us *now*? For example, in takes 8 minutes for light to reach us from the Sun. So what about an eruption on the Sun's surface 4 minutes ago? That cannot affect us *now*. So it is not in our past :) But nor is it in our future. We say that it is 'elsewhere', because us phsyicists think that sounds cool :cool:

Now, no-one else will ever see our *past* events as anything but in our past - otherwise, they would see me influenced by something yet to happen! And that breaks causality. But *elsewhere* events cannot influence me *now*, so even though I think the explosion occured 4 minutes before I clap my hands, there's nothing wrong with someone else thinking that it happened four minutes AFTER I clapped my hands. And that may well be what they see if they are travelling at an appropriate speed and direction relative to me and the Sun.

Diagram time:


\ /
\ Future /
\ (Light / \
\ cone) / \
\ / \
\ / \ ^ Time
\ / Elsewhere |
Elsewhere \ / \ |
X------Me *now* \ |___ Space
/ \ \
/ \ \
/ \ \
/ Past \ \
/ (Light \ *
/ Cone) \ Sun goes bang
/ \

Edited by cavediver, : No reason given.


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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 20 of 131 (487091)
10-27-2008 4:35 PM
Reply to: Message 12 by Straggler
10-27-2008 7:36 AM


Re: Entropy
How does the idea of ever increasing entropy fit into individual, as opposed to universal, time?

If we consider just our standard Universe, with the Big bang, then all possible paths through space-time begin in the Big Bang (all of the matter in your body for example began here) and then wind off into the future. All start in a region of low entropy and entropy increases as time increases. Some of those paths may be extremely relativistic, such that a particle my arrive here on Earth that has only seen 48 hours since the Big Bang! But it still has travelled from a low entropy time to a higher entropy time. Entropy merely reveals the flow direction of these paths, but not the rate of passage of time along these paths. Analogy - all possible paths lead down from the summit of a mountain (by definition of 'summit') but not all paths have the same gradient.

Is our view of the total entropy of the universe also individual if our view of time is individual?

It shouldn't be. Our different views cannot change 'reality'. If I see A happen before B, and you see B happen before A, that simply means that A and B are not within each other's light cones, and thus cannot affect each other. We say they are space-like separated. And there is no such thing as a time-ordering for A and B, despite our (contradictory) observations to the contrary.

But is it thought that time moves forward in discrete "sheets"

Good question - we don't know. Despite claims to the contrary, quantisation does not necessarily imply discreteness.


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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


(1)
Message 21 of 131 (487092)
10-27-2008 4:48 PM
Reply to: Message 9 by Agobot
10-26-2008 1:55 PM


Cavediver writes:


SR showed this to be incorrect. There is no universal cross-section of the Universe, no universal now. Each of us has our own path through space-time, and we each define our own "now", which will not necessarily agree with someone else's "now". My "now" is no longer the piece of paper I'm sat on, but rather a a slice through the stack of paper at an angle, and your "now" might be a different slice, so that some things I regard as happening in my past, you may regard as yet to occur.

I am not sure most folks would understand this quoted paragraph

I thihk you may be right - it's a bit crap on second reading :)

Perhaps some diagrams would help?


Newtonian Picture:

Future

^ Time
|
----Now------------Me-------You--------Now--- |
|_____ Space

Past

Special Relativity:

\ /
\ Future /
\ (Light /
\ cone) /
\ /
\ / ^ Time
\ / Elsewhere |
Elsewhere \ / |
X------Me *now* |___ Space
/ \
/ \
/ \
/ Past \
/ (Light \
/ Cone) \
/ \

If we happen to meet, but are travelling at speed relative to each other:
^ Time
|
|
|___ Space

----____ Your now
----____
----____
---my now-----------------Us---------------my now------
----____
----____
----____ Your now
----


Any better?

Edited by cavediver, : No reason given.


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onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 22 of 131 (487096)
10-27-2008 5:00 PM
Reply to: Message 18 by Huntard
10-27-2008 3:30 PM


Hi Huntard,

Wouldn't a supernova happening billions of light years away actually take billions of years to be seen here on earth? Since the light that that explosion produces would take billions of years to get here?

I meant to say millions since I said 100's of years to reach us. But, even an event at 1Bly away does not take 1 billion years to reach us since light travels at (c). I believe, if im not mistaken, that 1Bly away would take light 1000 years to reach us.


"All great truths begin as blasphemies"

"I smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your mouth."--Bill Hicks

"I never knew there was another option other than to question everything"--Noam Chomsky


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Replies to this message:
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onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 23 of 131 (487097)
10-27-2008 5:02 PM
Reply to: Message 20 by cavediver
10-27-2008 4:35 PM


Re: Entropy
Hi cavediver,

All start in a region of low entropy and entropy increases as time increases.

Is it assumed that it was at low entropy? I believe I've read that the original entropy is not known but assumed to be low. Or did I confuse what I read?


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Huntard
Member (Idle past 584 days)
Posts: 2870
From: Limburg, The Netherlands
Joined: 09-02-2008


Message 24 of 131 (487098)
10-27-2008 5:03 PM
Reply to: Message 22 by onifre
10-27-2008 5:00 PM


But if a light year is the distance travelled by light in one year, then a supernova 1 billion light years away would mean that the light emitted from that explosion takes 1 billion years to get here, right?

{ABE} I feel so smart now, thanks Cavediver! :P :D

Edited by Huntard, : edited in little text patting myself on the back


I hunt for the truth

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cavediver
Member (Idle past 1932 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 25 of 131 (487099)
10-27-2008 5:04 PM
Reply to: Message 22 by onifre
10-27-2008 5:00 PM


But, even an event at 1Bly away does not take 1 billion years to reach us since light travels at (c). I believe, if im not mistaken, that 1Bly away would take light 1000 years to reach us.

No, the light year is defined as the distance light travels in one year. So one billion light years is traversed by light in one billion years (ignoring the slight cosmological considerations.) the value of c is usually measured in metres per second, ~ 300,000,000 ms-1


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onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 26 of 131 (487100)
10-27-2008 5:11 PM
Reply to: Message 24 by Huntard
10-27-2008 5:03 PM


But if a light year is the distance travelled by light in one year, then a supernova 1 billion light years away would mean that the light emitted from that explosion takes 1 billion years to get here, right?

Remember its a measure of distance, not time. One light-year is equal to 9,500,000,000,000 kilometers. For example the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km(or 2.3Mly).


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Huntard
Member (Idle past 584 days)
Posts: 2870
From: Limburg, The Netherlands
Joined: 09-02-2008


Message 27 of 131 (487102)
10-27-2008 5:16 PM
Reply to: Message 26 by onifre
10-27-2008 5:11 PM


Yes, I know it's a measure of distance. The distance light travels in one year. As to what the distance to Andromeda has to with this I'm not sure. But if there would be a supernova in Andromeda, it would take us 2.3 million years to notice, since that's the first time the light gets here.


I hunt for the truth

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onifre
Member (Idle past 1240 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 28 of 131 (487103)
10-27-2008 5:18 PM
Reply to: Message 27 by Huntard
10-27-2008 5:16 PM


But if there would be a supernova in Andromeda, it would take us 2.3 million years to notice, since that's the first time the light gets here.

Yeah I was confusing myself with my original example to John. Thanks.


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Cold Foreign Object 
Suspended Member (Idle past 1337 days)
Posts: 3417
Joined: 11-21-2003


Message 29 of 131 (487124)
10-27-2008 7:02 PM
Reply to: Message 3 by cavediver
10-26-2008 11:44 AM


Why don't you just answer the question that you have posed?

Cavediver: "How is the Universe here?"

You need to make your point or points or otherwise you are teaching an online class in an out of place debate forum.

Ray


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New Cat's Eye
Inactive Member


Message 30 of 131 (487170)
10-28-2008 11:11 AM
Reply to: Message 29 by Cold Foreign Object
10-27-2008 7:02 PM


Hey, you STFU and GTFO.

I like reading this stuff.


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