Time itself was different closer in time to the Big Bang, right? (According to current most accepted thinking.) If the Big Bang was the start, then before it was no time … and after it time existed but behaved differently in those first few ?picoseconds? …how can you measure time at that level? Someone please explain in what way time is thought of differently theoretically at the very start. . Can one, in a thought experiement, travelling backwards in time towards the Big Bang, ever reach it or does time itself morph into something that no longer progresses?
I’m no mathematician or physicist, but from Stephen Hawking’s Brief History I gather that, if you picture a cube, with length being the vertical, height being front-to-back, we’re just gonna ignore width for the moment, and time is left-to-right, then not only does the physical matter we’re graphing in the cube converge towards the left, but also as our eyes track leftwards we find the “cube”'s axes converging, with the line forming the top bending into a parabola and coming to meet the bottom as it bends up to become the same parabola.
Or you could visualize the universe on film, being projected from a movie projector; since the movie started without you, you need to hit the reverse-button to get back to the beginning, but as it reverses closer to the beginning of the tape, the speed of the reverse-motor slows down; the events you’re seeing in fast-reverse on-screen continue to reverse but your ear and a quick peek at the projector shows it’s rate of reverse is slowing exponentially, never getting to the beginning and never grinding to a halt and always reversing the film a little more slowly than it did a moment before… onscreen, meanwhile, the objects being shown continue to get closer and closer to singularity but as the speed of reverse-motion slows the convergence slows and we never get back to the singularity.
Silly metaphors, but I think silly metaphors is the best we’ve got.
Time is just another direction. Almost all the way back it behaved exactly the same. The problem is when you get to the kind of time scales where quantum effects come in. We don’t know how to make quantum theory work with general relativity yet.
To be specific, this happens at the Planck time scale, where Planck time is 5.39121*10<sup>-44</sup> seconds. In case you’re not used to exponential notation:
0.0000000000000000000000000000000000000000000539121 seconds
Beyond that, we dunno.
Is it the Planck Time scale or the Planck Size scale? This has always confused me.
You’re in luck: it’s both. Also mass.
You see, the “speed of light” that relativity people are always on about is really just a conversion factor. There are 12 inches in a foot, three feet in a yard, and about 299792458 meters in a second. We can write this as
299792458 m = 1 s
or
299792458 m s[sup]-1[/sup] = 1
In gravity, Einstein tells us that curvature of spacetime and energy density are really the same thing. Of course, they’ve been measured in different units, so we need another conversion factor in there. Turns out that it’s pretty much just Newton’s gravitational constant, so we find
6.6742*10[sup]-11[/sup] m[sup]3[/sup] s[sup]-2[/sup] kg[sup]-1[/sup] = 1
This converts from cubic meters per square second to kilograms.
Finally, quantum mechanics tells us that energy and frequencies are just the same thing. Since angular frequencies are measured in inverse seconds we find
1.05457168*10[sup]-34[/sup] kg m[sup]2[/sup] s[sup]-1[/sup] = 1
So now we juggle these conversion factors. Multiplying them together in the right way to cancel off all but one basic unit gives our Planck scales. Planck length = Planck time = Planck mass = 1.
Well it is certainly not “just another direction”; it behaves very differently than other directions. In other directions we go either way; in this direction we can only go in one way. In other directions I could theoretically not move; in this direction I must move. MaNishtanah, bubbelah? 
It may be we dunno. But I am going to a New Year’s Day Party today at the house of an Argonne physicist freind. He isn’t a theoretician and is more a particle man anyway, but maybe some of his freinds will have a speculation.
Why do I have the feeling you asked the question because your physicist friends are having a New Year’s Day party instead of a New Year’s Eve party?
Maybe you think they think time can also go backwards, so eventually at the party you’ll all be wearing funny hats and counting the seconds until the Big Ball goes back up?
I’m sure your friend himself has speculations. The whole problem is that we don’t understand enough about particle physics to understand the conditions that held at those length and time scales.
As for it being just another direction, I mean as to how it physically behaves. As to how we percieve it is another question entirely.
If you get a nicely coherent explanation that doesn’t involve equations, could you post it for our edification?
(Actually it’s FermiLab he works at, my mistake. And two sets of freinds have parties at the same time so I’d really like that quantum splitting to work well right about now.)
Mathocist Well, it is just that I hear about “x amount after the Big Bang” and so on and I wonder if any of our Newtonian measurements of spacetime have any meaning at that level. AHunter probably understands what inspired this thought, a GD thread about free will in which posters are going on about predetermination ever since first “cause” and my thinking that to presume a first cause, to presume a beginning, may be a bit, well, presumptuous. It is a bit of a “linear”, or at least “Newtonian” mindset. But I would like to know how informed speculation plays it out. If I can get a coherent (to me) response I’ll post it, AHunter. Thanks for the reduction of my ignorance and explainig the feid’s ignorance, Mathocist
That’s close enough to zero for government work.
Regardless of how you conceptualize/comprehend what “happens” to time as you look back Big-Bangwards, there’s no model by which the Big Bang can be attributed to prior conditions which caused it.
Nor, for that matter, is the universe as it exists right now intrinsically a “different thing” or “different event” than the Big Bang; we just divide things up in our mind because it’s useful and convenient to do so for us. There’s no dividing line on the “before” side of which is The Big Bang and on the “after” side of it is an enormous series of events that can be said to have been “caused by the Big Bang”, right?
I hate it when people say that there was “no time” before the Big Bang. Like I’m not insecure enough already.