# I How did time move forward during the Big Bang?

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1. Dec 14, 2017

### DAirey

As I understand it, time runs backwards inside a black hole. If the universe were far denser than a black hole when the universe was a fraction of a second old, then how did physical processes ever move forward? That is, how can a clock advance in something as dense as the initial universe?

2. Dec 14, 2017

### lekh2003

Here's your problem. This is not true. I think this myth that was regarded as truth was from a sci-fi book. Time moves slower and slower as you approach denser objects and relativistic effects increase. The idea was that as black holes get infinitely dense, time starts going backwards.

This is a misconception. It arises from the fact that nothing becomes infinitely dense and even if it did, time would "stop". I hope you understand a little bit more about time now.

3. Dec 14, 2017

### newjerseyrunner

No, as you approach a black hole, time moves forwards exactly the same way it does if you were somewhere else. Time slows only from the reference frame of an outside observer, so actually the opposite of your statement is true. As you approach a black hole, time does it's normal thing for you and the rest of the universe speeds up.

4. Dec 14, 2017

### lekh2003

Well that's being overly technical. Both of our statements might be opposite but are equally as true depending on the reference frames which are both valid.

5. Dec 14, 2017

### phinds

6. Dec 14, 2017

### lekh2003

Ok, thanks for the clarification if I was wrong.

7. Dec 14, 2017

### phinds

No, I didn't say you were wrong, I said you were misleading. Your statement, to a newbie, implies that time ACTUALLY slows down for an infalling person. You are correct that your statement is true for a distant observer

8. Dec 14, 2017

### lekh2003

Ahhh. I see what you are saying. I'm really sorry for misleading.

I know what you mean since I had the same misconception when I was a newbie that time literally slowed down. It only seems like it slows down to people around you, but it doesn't for you (directly).

9. Dec 14, 2017

### newjerseyrunner

No worries, science people have to chose their wording very carefully. Everyone in here has said ambiguous things before that needed clarification. I know I certainly have.

10. Dec 15, 2017

### nikkkom

It's more complex than that. More like "time runs to the center inside a black hole". After you cross event horizon, all possible future-oriented timelike trajectories for you, all possible futures, go towards and end on the singularity. You can not avoid getting there just like you can not avoid arriving into tomorrow.

This is a good question since it uncovers one important misconception about BB: very early Universe was very dense, yes, but it also was expanding VERY quickly. For example, at the age of 1 second since BB the entire volume of what we today call "Observable Universe" expanded to ~8 light years radius. Expansion rate of 8ly/s! (Light in flat Minkowski spacetime can't even reach the Moon from the Earth in one second.)

Thus, very dense matter/energy at any chosen point did not gravitationally "feel" _all_ the matter around it - it could only interact with fairly small immediate neighboorhood, and most of those surroundings were receding from it at velocities comparable to speed of light. This is quite different situation from, say, the core of a collapsing star.

11. Dec 18, 2017

### DAirey

I'm even more confused now. If all possible futures go towards and end on the singularity, then how did we escape it?

12. Dec 18, 2017

### Bandersnatch

13. Dec 18, 2017

### Staff: Mentor

That's not what he said. He said all possible futures inside the horizon. Big difference.

By not being inside the horizon.

14. Dec 18, 2017

### newjerseyrunner

If you are referring to the Big Bang, it was not a black hole. The energy densities were enormous but the universe itself was flying apart way faster than it could collapse in a process called inflation.