Ask Me Anything AMA with Dr. Joe Pesce

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DrJoePesce

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Mar 31, 2020
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Hey Everyone!

I had such a great time with the last AMA that I decided to come back to answer some more astronomy questions!

If you're new here, allow me to introduce myself:

I'm an astrophysicist primarily interested in the environments of the galaxies hosting supermassive black holes (also known as Active Galactic Nuclei). I've worked with clusters of galaxies, and the atmospheres of giant and supergiant stars. Currently I'm a Program Director at the National Science Foundation (NSF), Division of Astronomical Sciences, responsible for the National Radio Astronomy Observatory (NRAO); a part-time Professor at George Mason University in Fairfax, Virginia; and a Visiting Professor at the University of Colorado, Boulder, Colorado. And I'm a Fellow of the Royal Astronomical Society. Oh - and I LOVE all things Star Trek!!

I have a broad understanding of the field, but it’s vast (one could even say “astronomically” large), so I might need some time to find the answer.

Also, there may be lots of questions I can’t answer, because I don’t know – and maybe the answer isn’t yet known (that’s a fun part about astronomy – lots of unknowns still). Please keep this in mind.

As always, I will do my best, but I may not be able to answer all of the questions.

And, any opinions, findings, conclusions, or recommendations presented in my answers are only mine and do not necessarily reflect the views of the National Science Foundation.

Let's have some (more) fun!

Dr Joe

P.S. If you like the virtual background I've got in my photo below, liven up your next Zoom call with this and more incredible imagery from the NSF!

 
Apr 5, 2020
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Well, I had some questions left last time. Like:

1. You told me last me last time that blackholes are an example of strange Physics where the gravity is so strong that the matter is almost sucked out of existence. My question is that, how can things that don't exist can have an effect on existence?

2. Are Dark Energy and Dark Matter constants or variables?
 
Mar 27, 2020
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I've got one...Since time slows near the event Horizon, and basically stops at the singularity (correct?), then when we see objects fall towards a black hole from our position on Earth, what is the relative state of time -- to us -- for that object we are observing?

In other words, when NSF's Next-Generation EHT is complete and it watches a black hole as a movie, we may see debris get consumed "in real time". Accounting for the time it took for the photons to get to Earth , what exactly is the context for time as we watch the matter get pulled apart? Time is variable across that swath of destruction, correct?
 
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DrJoePesce

Verified Expert
Mar 31, 2020
88
229
410
Well, I had some questions left last time. Like:

1. You told me last me last time that blackholes are an example of strange Physics where the gravity is so strong that the matter is almost sucked out of existence. My question is that, how can things that don't exist can have an effect on existence?

2. Are Dark Energy and Dark Matter constants or variables?
Hello IG2007! Good to see you again.

A1. Certainly, the singularity has no size, which is what we mean by crushed out of existence. BUT, and this is the important bit, the MASS remains (remember Einstein’s famous equation, E=mc^2, which means we can convert mass into energy and vice versa, but we can’t destroy either – conservation of mass/energy). This is an example of our language failing us: In this case, “crushed out of existence” doesn’t necessarily mean “gone”. We don’t know what happens to matter that falls in through the event horizon, but it’s there in some way, because its mass remains.

A2. Well, they are “stuff”, so those concepts don’t really apply. What I think you mean is can they vary? Dark matter is probably just a different type of matter from what we normally experience (for example, some type of subatomic particle), so it’s probably been here all along (just like hydrogen has been here all along). The force of Dark Energy *could* be variable, over time. But it could also be that the amount or force of Dark Energy is relative to the density of matter in the universe (which changes given that the amount of matter is finite, but the volume of the universe is changing with expansion). In which case it could seem to vary, but that is just an effect of the varying density of matter.
 
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Apr 5, 2020
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Thank you, @DrJoePesce . I have some more questions.

The second law of thermodynamics says that disorder in a closed system will always increase over time. Does that mean that we won't have blackholes after billions and billions of years? And also, is the universe a closed system?
 
Aug 3, 2020
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Dear Dr Joe Pesce,

If we had to draw a line to define the universe, in two dimensions, will it be more or less a circle?

Kind regards

Akli Tabti

Penn

Buckinghamshire

UK
 
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Aug 3, 2020
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I have a question...... When stars start to shrink under their own gravitational pull, form black holes upon shrinking beyond their schwarzschild radius, given that their mass is greater than 1.44 Solar masses. But what would have happened when our universe would have crossed it's schwarzschild radius? It's current radius in nearly 46.5 billion light years, whereas it's schwarzschild radius is just 13.7 billion light years. Objects can have a schwarzschild radius greater than its current radius. It depends upon its mass as per the formula-

R= 2GM/c²


.... And a huge amount of mass was formed during the big bang. Can't it be like what we are claiming to be 'The Big Bang' is actually the event when the universe crossed it's schwarzschild radius? But since there in nothing outside the universe (if multiverse doesn't exist), it didn't turn into a black hole.
 
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Aug 4, 2020
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I've developed a bit of an interest with Black Holes since the first picture of M87*. They are incredible!

My question is, if black holes really do rip the fabric of space-time and if black holes move through space(quite fast), wouldn't that mean the black hole would be constantly ripping space-time as it moved? I'm sorry that I can't really word my question better!
 
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Jun 1, 2020
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I am puzzled what might best describe a very long steel rod falling into a SMBH? If the escape velocity is c at the EH, does it fall into the EH at c, similar to objects falling to Earth have at least our escape velocity?

At c, a massive rod would need infinite energy, which is a shade beyond what is given it. Since the gradient field of a SMBH doesn’t spaghettify the rod, and it remains in tack, and it’s < c in velocity, would anything allow transmission from just inside the EH to outside of it? No doubt, the answer is no, but is that my weak understanding of GR and inertial frame differences or does the rod inside the EH have v=c?
 
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DrJoePesce

Verified Expert
Mar 31, 2020
88
229
410
I've got one...Since time slows near the event Horizon, and basically stops at the singularity (correct?), then when we see objects fall towards a black hole from our position on Earth, what is the relative state of time -- to us -- for that object we are observing?

In other words, when NSF's Next-Generation EHT is complete and it watches a black hole as a movie, we may see debris get consumed "in real time". Accounting for the time it took for the photons to get to Earth , what exactly is the context for time as we watch the matter get pulled apart? Time is variable across that swath of destruction, correct?
Thanks for the question!

According to the theory of general relativity, space/time is distorted near the black hole, such that – to a distant observer – a clock falling into the black hole appears to slow down (time dilation). Photons moving away from the clock are also affected and experience gravitational redshift – they get redder and dimmer – and eventually they fade away and you can’t see the clock. Of course to the clock falling in experiences none of this – and just falls through the event horizon.

By the way, gravitational redshift has been observed. For example, see the work by one of NSF’s funded researchers, Andrea Ghez, observations of a star orbiting very close to the black hole in the center of our Galaxy:

https://newsroom.ucla.edu/releases/einstein-general-relativity-theory-questioned-ghez
 
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DrJoePesce

Verified Expert
Mar 31, 2020
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410
Thank you, @DrJoePesce . I have some more questions.

The second law of thermodynamics says that disorder in a closed system will always increase over time. Does that mean that we won't have blackholes after billions and billions of years? And also, is the universe a closed system?

The entropy – disorder – of the ENTIRE universe will increase, because it is a closed system. In local regions, however, there can be violations of the 2nd law, as long as overall the entropy increases.

I’m not sure what that means for black holes in the future. The expansion of the universe, in particular in the distant future, will play a bigger role in that “stuff” will eventually be too far apart to fall into black holes.
 
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DrJoePesce

Verified Expert
Mar 31, 2020
88
229
410
I have a question...... When stars start to shrink under their own gravitational pull, form black holes upon shrinking beyond their schwarzschild radius, given that their mass is greater than 1.44 Solar masses. But what would have happened when our universe would have crossed it's schwarzschild radius? It's current radius in nearly 46.5 billion light years, whereas it's schwarzschild radius is just 13.7 billion light years. Objects can have a schwarzschild radius greater than its current radius. It depends upon its mass as per the formula-

R= 2GM/c²


.... And a huge amount of mass was formed during the big bang. Can't it be like what we are claiming to be 'The Big Bang' is actually the event when the universe crossed it's schwarzschild radius? But since there in nothing outside the universe (if multiverse doesn't exist), it didn't turn into a black hole.

When massive (greater than about 5 times the mass of the Sun) star collapses (and produces a gigantic explosion called a supernova), it might form a black hole; but it also might form a neutron star or destroy itself completely. The outcome is based on mass, explosion particulars, and other factors and is quite complicated and not completely understood.

The schwarzchild radius is indeed related to mass of the object and is quite small, astronomically. For example, if we create a one-solar mass black hole, it would have a schwarzschild radius of about 6 km.

Remember that in the Big Bang, the universe starts to EXPAND immediately: At the beginning it was probably a singularity with zero radius and infinite density, but then immediately broke this condition.
 
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Aug 4, 2020
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Hi Dr. Joe

If photons are massless particles, why are they affected by the extreme gravity of Black Holes. If they have no mass then they should be able to escape the gravity of a Black Hole or is it the extreme curvature of Spacetime in Black Holes that trap photons and not actually gravity itself?
 
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DrJoePesce

Verified Expert
Mar 31, 2020
88
229
410
I've developed a bit of an interest with Black Holes since the first picture of M87*. They are incredible!

My question is, if black holes really do rip the fabric of space-time and if black holes move through space(quite fast), wouldn't that mean the black hole would be constantly ripping space-time as it moved? I'm sorry that I can't really word my question better!
That’s fantastic! I’m glad astronomy has been able to reach you! And I’m particularly happy about your curiosity.

No worries! Human language is not good at describing astronomical things in general, and certainly for the extreme cases.

I’m not sure about “ripping”, but they are certainly deforming space-time. And this deformation moves as the black hole moves. Think about a heavy weight on a sheet of rubber – the deformation of the sheet moves as the weight moves.
 
Jun 1, 2020
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I believe I have read descriptions that space (spacetime) flows into blackholes. Is this a legit hypothesis and testable? Or is it a way to better map events around it? Or is spacetime just warped, not dissimilar than my memory of that flowing account, perhaps. ;)
 
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Mar 27, 2020
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Thanks for the question!

According to the theory of general relativity, space/time is distorted near the black hole, such that – to a distant observer – a clock falling into the black hole appears to slow down (time dilation). Photons moving away from the clock are also affected and experience gravitational redshift – they get redder and dimmer – and eventually they fade away and you can’t see the clock. Of course to the clock falling in experiences none of this – and just falls through the event horizon.

By the way, gravitational redshift has been observed. For example, see the work by one of NSF’s funded researchers, Andrea Ghez, observations of a star orbiting very close to the black hole in the center of our Galaxy:

https://newsroom.ucla.edu/releases/einstein-general-relativity-theory-questioned-ghez
The clock is exactly the analogy I needed, I've been wondering about this for a year......I'd never made the connection to Andrea's discovery, but of course this now makes sense. I love Nicolle's image on that release...Thanks, Joe!
 
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