Universe Age and Size: A Matter of Perspective?

[PeterDonis]

I was referring to a hypothetical clock that could function in a black hole.

There is no such thing in the sense you mean. If the clock is inside the hole's horizon, as Ibix says, it will be destroyed in the singularity, and anyway there is no way for an observer outside the hole to compare the clock's reading with the reading of any clock outside the hole. If the clock is outside the hole's horizon, it can't stay in a stable orbit close enough to the hole for the time dilation factor to be very significant.

 

[PeterDonis]

Seconds?

According to the clock falling into the hole, the time it takes to fall from the horizon to the singularity is approximately $2M$ in geometric units, where $M$ is the mass of the hole. If you convert this to SI units, you will find that for a black hole of one solar mass, it takes 10 microseconds ($10^{-5}$ seconds) to fall from the horizon to the singularity. The time scales linearly with the mass of the hole (as the formula I just gave makes clear), so you can calculate for yourself what the time would be for other masses.

However, none of this really matters for the question you are asking, because, as I said in my previous post, once a clock is below the horizon, there is no way to compare its reading with that of a clock outside the horizon. So the concept of "time dilation" has no meaning for a clock inside a black hole's horizon anyway.

 

[Ibix]

...and there are no black holes in the early universe anyway.

 

[Chris Miller]

...and there are no black holes in the early universe anyway.

"a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate." Sounds like the original universe to me.

 

[jbriggs444]

"a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate." Sounds like the original universe to me.

But you have quoted out of context, destroying the meaning of the passage you cite.

 

[PeterDonis]

a black hole is a gravitational singularity...

What is the source of this quote?

 

[Ibix]

"a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate." Sounds like the original universe to me.

A very dense point surrounded by vacuum sounds like an entire universe filled with hot dense matter to you?

 

[jbriggs444]

What is the source of this quote?

Chris said in #30 that he had located that quote on Google. Indeed, it can be found in. http://www.physicsoftheuniverse.com/topics_blackholes_singularities.html

Chris quoted with context in #30:

"In the centre of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate."

And without context in #34:

"a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time curves infinitely, and where the laws of physics as we know them cease to operate."

Note how the lack of four words completely obliterates the intended meaning of the passage in question.

 

[Chris Miller]

But you have quoted out of context, destroying the meaning of the passage you cite.

Apologies, all. And thank you, yes, I see what you mean. Wasn't trying to distort, only pare down, the description. Let me change my post to read, The centre of a black hole sounds like the original universe to me.

 

[jbriggs444]

Apologies, all. And thank you, yes, I see what you mean. Wasn't trying to distort, only pare down, the description. Let me change my post to read, The centre of a black hole sounds like the original universe to me.

The universe shares one of the attributes of a black hole -- that it has a singularity. However there are a number of posts on these forums in which members try to ask things like "is the universe a black hole?". The answer is no.

 

[Chris Miller]

It shares one of the attributes -- that it has a singularity. However there are a number of posts on these forums in which members try to ask things like "is the universe a black hole?". The answer is no.

It's difficult to make assertions about something for which so much is unknown/inexplicable. But no, of course not, which is why I used a simile. Scrolling further down that page I see there is a "naked singularity" hypothesis for the Big Bang.

 

[PeterDonis]

Let me change my post to read, The centre of a black hole sounds like the original universe to me.

Any such apparent resemblance is illusory. One big difference is that there is nothing corresponding to the black hole's event horizon in the FRW spacetime we use to describe the universe. Another is that a black hole is a vacuum solution; FRW spacetime is not.