0

Am sorry if this is a silly question.

I have heard something like this from cosmologists:

"There is no intrinsic arrow of time. Today, we believe that the direction of this arrow is due to our specific initial condition. For reasons unknown, we had a low-entropy Big Bang."

The words that I have in bold assume the Big Bang is in the past, it seems to me. Obviously, the physicists who said it did not mean what they said. They must have meant something else, but thought that too much rigor was unnecessary. So, can somebody explain to me what they mean when they say initial conditions and Big Bang causing an arrow of time?

EDIT: Sorry for not making sense. I will try to make my reasoning clear. I think that time-reversible laws means no time and here's why:

Time reversible laws means you can find out past states just as you can predict future states. But then doesn't it mean that there is no intrinsic difference between past and future. Nothing about the laws and their deductions will change if you swap their meanings. But then we have past and future as distinct ideas in our heads and from our experiences. So, shouldn't there be some time assymetry that can explain this.

Please correct me if my reasoning is fallacious.

PhyEnthusiast
  • 2,908
  • Trying to parse your reasoning here: are you saying that because the laws of physics are essentially time-reversible, physicists can't refer to events as being in the past, rather than in the future? – probably_someone Mar 07 '18 at 15:07
  • If the laws are reversible which it seems they assume, then physicists can't refer to events as being in the past, or so it would seem – PhyEnthusiast Mar 07 '18 at 15:08
  • Possible duplicates: https://physics.stackexchange.com/q/4201/2451 , https://physics.stackexchange.com/q/14004/2451 , https://physics.stackexchange.com/q/18702/2451 and links therein. – Qmechanic Mar 07 '18 at 15:11
  • I don't see how that follows. Physicists (indeed, all of us) are currently occupying a particular moment in time. The past can be defined as the set of all points in spacetime about which we could conceivably have observational information in this present moment. In this present moment, we have information about direct signals (the abundance of lithium, for instance) generated by the Big Bang, which means it happened in the past. Just because the laws of physics are time-reversible does not mean that physicists exist outside of time. – probably_someone Mar 07 '18 at 15:20
  • @probably_someone But then they assume some arrow of time. Then there is one more arrow of time whose existence must be explained using time-reversible laws? – PhyEnthusiast Mar 07 '18 at 15:27
  • The existence of "past" and "present" is a fact. The mystery is, why can we only go from "past" to "present" and not from "present" to "past"? – valerio Mar 07 '18 at 15:30
  • @valerio The existence of "past" and "present" is a fact. The mystery is why. Given time-reversible laws of physics. – PhyEnthusiast Mar 07 '18 at 15:31
  • 1
    Time-reversible laws do not imply that there is no time (why should it be so?) The mystery is not the existence of time, but its unidirectionality. – valerio Mar 07 '18 at 15:32
  • @valerio But, shouldn't time-reversible laws imply no time. Either that or the laws are incomplete? – PhyEnthusiast Mar 07 '18 at 15:34
  • @valerio Is there any difference between time and arrow of time. The thing that differentiates time from spatial dimensions for example, is its unidirectionality, is it not? – PhyEnthusiast Mar 07 '18 at 15:40

0 Answers0