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dmvprof
2005-May-13, 02:35 PM
One thing that I&#39;ve read briefly about in a Brief History of Time is the ability to apply our physics in a way that allows us to wind back the universe to a point shortly after the big bang.

From what I read, we do reach a point somewhere near the singularity in which our laws don&#39;t work anymore.

I can&#39;t imagine the depth of the mathematics here, it seems truly incredible that we are able to do this.

But how accurate is it? Do the models decrease in accuracy as they wind back?

Can anyone give a low level example of how this is accomplished?

Nyrath
2005-May-13, 06:33 PM
Originally posted by dmvprof@May 13 2005, 02:35 PM
But how accurate is it? Do the models decrease in accuracy as they wind back?

Can anyone give a low level example of how this is accomplished?
Divide 1 by 0.00000000001
Divide 1 by 0.000000000000001
Divide 1 by 0.0000000000000000001
Divide 1 by 0.00000000000000000000001

so far, so good.

Divide 1 by 0

Oops. Singularity.

antoniseb
2005-May-13, 07:13 PM
Originally posted by Nyrath@May 13 2005, 06:33 PM
Divide 1 by 0
Oops. Singularity.
Nyrath expresses it well. I&#39;l like to point out that you don&#39;t need to go back to zero for things to be in a state where none of our laws have been tested.

When the universe was so small that the matter/energy in it far exceded the density of an atomic nucleus, how would you describe the forces acting within it? You can&#39;t really because we can&#39;t test it anymore.

There is some practical point beyond which you might assume that the universe was tiny but expanding, yet we can&#39;t yet know the physics of that time. As we advance our knowledge, that time will roll back some.

dmvprof
2005-May-13, 07:45 PM
I don&#39;t think my question is coming across.

The calculations I&#39;m thinking of actually wind back the positions of galaxies all the way to the big bang.

antoniseb
2005-May-13, 08:42 PM
Originally posted by dmvprof@May 13 2005, 07:45 PM
The calculations I&#39;m thinking of actually wind back the positions of galaxies all the way to the big bang.
Really, do they all end up at the same geometric point at the same time, within a few femtoseconds of each other? That&#39;s some high precision cosmology you&#39;re doing.

I think there&#39;s some uncertainty in the measurement, and though we all think it is safe to assume that everything started in one place at one time, observation allows for a little wiggle room there.

dmvprof
2005-May-16, 12:06 PM
Originally posted by antoniseb+May 13 2005, 08:42 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb @ May 13 2005, 08:42 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-dmvprof@May 13 2005, 07:45 PM
The calculations I&#39;m thinking of actually wind back the positions of galaxies all the way to the big bang.
Really, do they all end up at the same geometric point at the same time, within a few femtoseconds of each other? That&#39;s some high precision cosmology you&#39;re doing.

I think there&#39;s some uncertainty in the measurement, and though we all think it is safe to assume that everything started in one place at one time, observation allows for a little wiggle room there. [/b][/quote]
It&#39;s eithor that, or Stephen Hawking is just making up a lot of science fiction.

I&#39;ll try to get a reference.

2005-May-16, 04:23 PM
Divide 1 by 0* * Oops. Singularity. I have been thinking otherwise; as the denominator approaches zero the quotient approaches infinity which seems to be going in the opposite direction from approaching a singularity defined as:
a point* or region of infinite mass density at which space and time are infinitely distorted by gravitational forces and which is held to be the final state of matter falling into a black hole.

Although I still struggle to comprehend what could have fluctuated before "before" had meaning and spacetime existed, the quantum fluctuations prancing around since the BB will surely render running the clock backwards near impossible to return to whatever the starting point of spacetime may have been. The forward pointing vector of time renders it extremely unlikely, if not impossible, to assign meaning to (even the theoretical) rewinding of the BB (spacetime) much less composing a meaningful model of the process due to the randomness of quantum fluctuations. I still cling to the fantasy that the universe abhors infinite density as much as it abhors a vacuum.

Rewinding from different points in spacetime is likely to result in approaching "the origin" with a proximity and configuration distortion peculiar to each rewind starting point, no one of which lends itself to that level of predictability that allows meaningful (theoretical) modeling.

antoniseb
2005-May-16, 04:45 PM
Originally posted by GOURDHEAD@May 16 2005, 04:23 PM
I still cling to the fantasy that the universe abhors infinite density as much as it abhors a vacuum.
I think the idea that "Nature abhors a vacuum" was first expressed more than 2000 years ago, by a very bright fellow (Aristotle, or perhaps someone before him that he quoted). But this man said it without understanding our idea of what a vacuum is. He was talking about how pumps work.

Today, you cannot really apply the anthropomorphic concept of abhoring to Nature. It is true that statistically, there is likely to be SOMETHING, in any cubic centimeter of the universe you care to look in, even if it is just a photon or neutrino, but that is not because Nature recoils in horror whenever some small space is empty.