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Comment by irjustin | Hacker News Reader

To be specific it's the "average time across all space" ran more slowly, which is sort of a weird thing to say, but I'll go with it.

Time runs more slower near massive objects like black holes. Since the average density of the universe when it was young was way higher, then yes on average time ran more slowly.

But it's also a little weird to say because you go back far enough there's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?

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> To be specific it's the "average time across all space" ran more slowly,

No, that is not what happened. The article's description is seriously misleading. The effect it is describing is only an appearance due to the redshift. it does not mean time "actually ran slower" back then.

> here's no "low density" patch so you're actually comparing it to something that doesn't exist quite yet, hence a bit weird?

Not just "a bit weird"--wrong. What you are saying here is a crucial difference between the universe as a whle and an isolated gravitating mass like a black hole, and that difference means that the notion of "gravitational time dilation" that works for an isolated gravitating mass is not valid for the universe as a whole. The "time dilation" described in the article is not like the gravitational time dilation around a massive object. It is like the "time dilation" due to relative motion in special relativity, which is only apparent and does not reflect any actual difference in clock rates.

Why would there not be gravitational time dilation in a denser early universe as the article describes?

The reason the GP gives is a step in the right direction: the density is constant everywhere in space at a given point in time, and variations in "gravitational potential" (which are necessary for gravitational time dilation) can only be present if there are differences in density.

A more technically correct reason is that the concept of "gravitational potential" (and hence gravitational time dilation) is not even well-defined in an expanding universe. "Gravitational potential" is only well-defined in a stationary spacetime, which means one in which the geometry does not change with time. An expanding universe is not a stationary spacetime. (But even in a stationary spacetime, variations in gravitational potential, which require variations in density, are necessary for gravitational time dilation to exist. That is why the GP's answer described above is a step in the right direction.)

Could it be said that entanglement was originally near infinite, as it might now be in black holes? Perhaps now that things are spread out, time appears to move faster because there is less stickiness of entangled particles depending on others in order to proceed, whereas mass entanglement acts like anti-time due to the need of so many webs of chains of entanglement to be notified back to the original particle so they can act in reverse order (as entanglement appears to work). Could a massive density of anti-time entanglement help explain event horizons more completely than gravitational effects alone?

Could the big bang-like event have been entanglement flipping like a magnetic pole and generating a diaspora where things began happening causally -- which is slowly re-condensing when things group densely with the help of gravity as it catches up?

I get the feeling that there is a lot more entanglement than we give credit to, with time-altering effects as complexity grows, maybe leading to brains with consciousness rather than classically predictable chemical networks, and...perhaps mild cell division-sourced innate entanglement? Look forward to finding out.

> Could the big bang-like event have been entanglement flipping like a magnetic pole

No. Nothing like this appears in any of our scientific models of the Big Bang.

And how is density measured? By the amount of energy per cubic meter, yeah? So how is a meter defined? By the distance light travels in 1/299792458 of a second. I get the feeling that it's all homomorphic.

> Time runs more slower near massive objects like black holes.

Do you mean proper time or time as seen by an outside observer?

Originally, I meant it as - as seen from an outside observer.

I'm talking out of my pay grade now, but it also could be for proper time if you started away from the black hole and fell towards it then compared it from your start and end? Probably unnecessarily complex for this discussion though.

Also punching above my weight class here: isn't observation a function of time as well though, so a local observer would notice no difference in their perception of the passage of time? It's only across distance that the perception of time would change per relativity, so obseevation of self would remain the same while observation of distant objects would appear to experience time faster, while an outside observer would perceive their own time constant and that of the infalling observer to slow through time.

Proper time always runs at 1 second per second. It’s only when you compare it to someone else's proper time that you can get a difference.

Yes, I know.

Why does density affect time? How are the two related?

The stress-energy (of which matter density is a component) of a region of spacetime determines its curvature.

In empty space the metric, which tells us how to measure distances between events, is `ds^2 = dt^2 - (dx^2 + dy^2 + dz^2)`. Integrating this along a trajectory tells you the (square of the) proper time experienced by an observer traveling along it. As you can see, the further you move in space in a given amount of coordinate-time, the less proper time you experience: this is the source of time dilation in special relativity.

Around a spherical body of mass M, the metric is (in spherical coordinates, and ignoring some scale factors for convenience):

`ds^2 = (1-M/r)dt^2 - (1-M/r)^-1 (dr^2) - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`

Compare to the flat space metric from before, now in spherical coordinates as well:

`ds^2 = dt^2 - dr^2 - r^2 (dtheta^2 + sin(theta)^2 dphi^2)`

So we pick up some additional time dilation as long as `M < r` (and it always is, unless you're in the interior of a black hole)

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