Hacker News Reader

Comment by joshuacc | Hacker News Reader

"It so happens that "negative time", or even time t=0, is equally as meaningless as Elephant Time or Verb Time. They are all undefined."

I'm very curious about the assertion that t=0 is meaningless. Is this the conclusion of a line of reasoning, or is it an axiom assumed in order to make the theory work?

It looks like the latter to me, but that may just be because I'm not familiar with the literature and concepts.

View on HN

In the standard theory, t=0 is a "singularity." Energy density becomes infinite, for example, at t=0. What that really means is that t=0 is undefined in the same sense that 1/0 is undefined.

Consider the function 1/x. For all positive x, 1/x is defined. As you approach x=0 from the right, 1/x approaches infinity. Same thing for the density of the universe. But the point 1/0 itself is undefined, even though the limit is well defined from the right. Likewise, t=0 itself is undefined, even though the density is well defined for all times greater than t=0.

I wouldn't really consider this an "assumption." It is actually a consequence of assuming that the universe is expanding in accordance with the law of gravity and that energy is conserved. The only way to do it is to construct a "metric" ( http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre... ) where a bunch of parameters are undefined at t=0, but well-defined thereafter.

Thanks for the explanation. I think part of my confusion arises from the (apparently) mistaken idea that t=0 is equivalent to "the first moment of time." But it looks like t=0 is closer to "before the first moment of time". In a certain sense, that's a contradiction, because you can't have "before" time, but in another sense it's the only way to speak of that which (logically, not necessarily chronologically) precedes time.

Would you say that's a decent summary, given that I'm translating from scientific terms to more philosophical terms?

> Thanks for the explanation. I think part of my confusion arises from the (apparently) mistaken idea that t=0 is equivalent to "the first moment of time." But it looks like t=0 is closer to "before the first moment of time". In a certain sense, that's a contradiction, because you can't have "before" time, but in another sense it's the only way to speak of that which (logically, not necessarily chronologically) precedes time.

Yes, I think that's right. It's more accurate to say that there was no first moment rather than that t=0 was the first moment. Reconciling that with the fact that time is nevertheless finite is an important part of understanding the big bang.

To put it mathematically, the set of points of time is open on the left end. The set of times is this: (0,\infty). And not this: [0,\infty)

Edit: By the way, when I say that time is "finite", I mean finite from now into the past. Not finite from "start" to "finish". The standard cosmological model with dark energy have time being infinite from "start" to "finish"

Yes, exactly, in the standard theory, the Universe is undefined. :-)

Have fun with that and your "dark energy." I will continue to laugh at you. :-D (And will get downvoted as a result, but it's worth it.)

Reply on news.ycombinator.com