Ask Me Anything Dr Joe AMA

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Hello suneritz, Thanks! For any star, no; only a black hole. Or more precisely, the singularity, because its gravitational field is so extreme, would create a situation where the escape velocity is the speed of light (and not allow light to escape).

The jets aren't emitted by the black hole (nothing is coming OUT of the black hole, if that's what you were thinking), but by reactions in the region near the black hole. The gravity of the black hole is providing the energy that drives the jet, but the jet isn't emanating from the black hole (just near it).

Give me a little more info about what you are thinking is an active star and I can maybe refine my answer!
Thank you Joe. This clarifies a lot. One thing I don’t understand is if both a star and a black hole would have the same mass, why black hole is able to fold (or warp) the space to prevent light from escaping but the star can’t?
Thanks
 
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What do you think about latest season of Star Trek Discovery? Are DMA's sort of exciting to you? Could something like that happen where there is a roaming black hole or Dark Matter Anomaly wandering around eating galaxies and solar systems etc? Also, I think Captain Michael needs to display some more intellect in terms of her approach to the complexities of the cosmos...I'd like to see her geek out a bit more as opposed to always rushing through the plot with a charge first and ask questions later approach...how about you? These are more complex nuanced issues they're dealing with than what Captain Kirk did imho.
 
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DrJoePesce

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Many thanks for your quick reply. But Special relativity tells us that simultaneous events observed in one inertial reference frame must not be simultaneous observed in another inertial reference frame, i.e., clocks synchronized relative to the ground frame must not be synchronized relative to the satellite frames no matter how you correct them, which has been denied by the fact that all the clocks on the GPS satellites and on the ground are synchronized relative to all satellite frames and the ground frame.

Thanks for the follow-up XinhangShen. My understanding (maybe incorrect) is that the GPS satellites' clocks are synchronized. I don't think they are synchronized with the ground clocks. I'll see if I can get someone better versed in this subject to weigh in.
 
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DrJoePesce

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Thank you Joe. This clarifies a lot. One thing I don’t understand is if both a star and a black hole would have the same mass, why black hole is able to fold (or warp) the space to prevent light from escaping but the star can’t?
Thanks

Great question suneritz! (and one I've never had before - kudos!): The difference is in the distance over which the mass is spread out (basically): Gravity is related to mass, but also how distributed that mass is.

The singularity has zero size (radius = 0, diameter = 0), but all the mass (let's say 10 times the mass of the Sun for this discussion). The result of this is an extreme gravitational field, one so extreme that the escape velocity is the speed of light. This is why nothing, not even light, can escape. (By the way, surrounding the singularity is the event horizon, of course. In this case - a black hole with 10x the mass of the Sun - the event horizon is some 36 miles (~60km) in diameter.)

A star with 10 solar masses exerts a gravitational pull stronger than our Sun of course (it is 10x more massive), but that mass is spread throughout the large star (maybe 4 or 5 times the radius of the Sun). Therefore, the gravitational field of the star - even though its mass is the same as the black hole - is nowhere near as extreme as that of a similarly massed black hole.

Does that help?

I point everyone interested in black holes to this site: www.nsf.gov/blackholes densely packed (lol - pun intended) with neat info.
 

DrJoePesce

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What do you think about latest season of Star Trek Discovery? Are DMA's sort of exciting to you? Could something like that happen where there is a roaming black hole or Dark Matter Anomaly wandering around eating galaxies and solar systems etc? Also, I think Captain Michael needs to display some more intellect in terms of her approach to the complexities of the cosmos...I'd like to see her geek out a bit more as opposed to always rushing through the plot with a charge first and ask questions later approach...how about you? These are more complex nuanced issues they're dealing with than what Captain Kirk did imho.
Thanks for the science fiction question lfr2! Yes, I love all Star Trek (but love TOS orders of magnitude more than the others!), so I thought it was good. Characters are all different and unique, and they have their positives and negatives, but I've generally liked everyone in the ST universe.

As for the science related to this: Of course, almost anything could be possible (especially for things we don't yet know about - like DMAs), but while clouds of dark matter probably exist (our galaxy is surrounded by one, essentially), they are on larger scales than we might encounter if we were zipping around the galaxy. Dark matter - even though it isn't detected/detectable electromagnetically (that is it's not emitting or reflecting photons), and even though it weakly interacts non-gravitationally with other matter - is still a gravitating mass. So we would experience its gravity - and maybe be attracted to it. But other than that I think it wouldn't cause any problems.

Roaming black holes, on the other hand: There are probably lots (many billions?) of the stellar sized ones roaming our galaxy. If they are gravitationally bound with a companion star, for example, or they are "feeding" (matter is falling into them), we would probably detect them (there might be a few more ways we could detect them as well). If they are alone, and not feeding, they would be difficult to detect, so might pose a problem. But space is big. Just think about this: There are hundreds of billions of stars in our galaxy, the Milky Way. The Andromeda galaxy (another spiral galaxy like ours, ~2 million light years away ) is similarly large (larger, in fact, with 1 trillion stars). When these two galaxies collide in 5-ish billion years, it is unlikely any stars will collide - they will just pass by each other - there is SO much space between them. (The gas clouds will collide, and that's another topic.)
 
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Great question suneritz! (and one I've never had before - kudos!): The difference is in the distance over which the mass is spread out (basically): Gravity is related to mass, but also how distributed that mass is.

The singularity has zero size (radius = 0, diameter = 0), but all the mass (let's say 10 times the mass of the Sun for this discussion). The result of this is an extreme gravitational field, one so extreme that the escape velocity is the speed of light. This is why nothing, not even light, can escape. (By the way, surrounding the singularity is the event horizon, of course. In this case - a black hole with 10x the mass of the Sun - the event horizon is some 36 miles (~60km) in diameter.)

A star with 10 solar masses exerts a gravitational pull stronger than our Sun of course (it is 10x more massive), but that mass is spread throughout the large star (maybe 4 or 5 times the radius of the Sun). Therefore, the gravitational field of the star - even though its mass is the same as the black hole - is nowhere near as extreme as that of a similarly massed black hole.

Does that help?

I point everyone interested in black holes to this site: www.nsf.gov/blackholes densely packed (lol - pun intended) with neat info.
Thanks you Joe for explaining it so clearly even I may understand. You’ve mentioned the light is bent not by singularity (if you have time could you explain it please) but by event horizon of Black Hole which has a diameter as large as Neutron Star (which is a star but not active?). Does this mean if you could have a Neutron Star as heavy it would also have zero outflows of any kind?
Thanks
 
Nov 18, 2019
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Thanks for the follow-up XinhangShen. My understanding (maybe incorrect) is that the GPS satellites' clocks are synchronized. I don't think they are synchronized with the ground clocks. I'll see if I can get someone better versed in this subject to weigh in.
Thanks, Dr. Joe. I think that all the clocks including the ground clocks should be synchronized so that they all have the same time that can be used to determine the position. Some people argue that the clock on a satellite and the clock on the ground are synchronized only relative to the ground reference frame and not synchronized relative to the satellite reference frame. If it was true, then the difference between the clocks observed from the satellite would monotonically grow but can't be corrected because the clocks are still synchronized observed from the ground frame, which is obviously not the case on the clocks of the GPS.

I think, Lorentz Transformation is a redefinition of space and time (called relativistic time in the following) which is no longer the clock time, but a fake time without physical meaning.

In fact, Lorentz Transformation is mathematically equivalent to the following definitions:

t' = (1/γ)T' - (γv/c^2)X'
x' = γX'
y' = Y'
z' = Z'

where γ = 1/sqrt(1 - v^2/c^2), (X', Y', Z', T') is the Galilean spacetime of the inertial reference frame moving at speed v in the X-direction relative to aether, (x', y', z', t') is the relativistic spacetime of the same inertial reference frame. When v = 0, relativistic spacetime (x, y, z, t) becomes the same as Galilean spacetime (X, Y, Z, T).

Galilean spacetime follows Galilean Transformation:

T' = T
X' = X - vT
Y' = Y
Z' = Z

We can verify that the speed of light defined by Galilean spacetime follows Newton's velocity addition law, while the speed of light defined by relativistic spacetime is constant relative to all inertial reference frames:

C = X/T

C' = X'/T' = (X - vT)/T = X/T - v = C - v

c = x/t = X/T = C

c' = x'/t'
= (γX')/[(1/γ)T' - (γv/c^2)X']
= (X'/T')/[(1/γ^2) - (v/c^2)(X'/T')]
= C'/[(1 - v^2/c^2) - (v/c^2)C']
= (C - v)/[1 - v^2/c^2 - (v/c^2)C + v^2/c^2]
= (c - v)/(1 - v/c)
= c

That means, all what Lorentz Transformation does is to redefine spacetime.

Since the clock time Tc is defined by the number N of cycles of a physical periodical process:

Tc = N/k

where k is a calibration constant and equals 9,192,631,770 for a cesium atomic clock.

The clock times Tc and Tc' of two clocks attached to the inertial reference frames defined by Galilean spacetime (X, Y, Z, T) and (X', Y', Z', T') are

Tc' = N'/k = (T'/Τ' )/k = (T/Τ)/k = N/k = Tc

where the Galilean periods Τ and Τ' of the two clocks are the same because Galilean time is absolute.

This equation tells us that clock time is also absolute same as Galilean time.

In special relativity, the clock times Tc and Tc' of two clocks attached to the inertial reference frames defined by relativistic spacetime (x, y, z, t) and (x', y', z', t') can be calculated by:

N' = t'/𝜏'
N = t/𝜏
t' = t/γ
𝜏' = 𝜏/γ

Tc' = N'/k = (t'/𝜏')/k = (t/𝜏)/k = N/k = Tc

where N', t' and 𝜏' are the number of cycles, elapsed relativistic time and the relativistic period of the moving clock, respectively, N, t and 𝜏 are the number of cycles, elapsed relativistic time and the relativistic period of the stationary clock respectively.

This equation tells us that clock time is Lorentz invariant and thus is still absolute and independent of the reference frame, which confirms that the property of the clock time (i.e. the absoluteness of the clock time) won't change with the change of the definition of time. Thus, the relativistic time is no longer the clock time but a meaningless mathematical variable.

Therefore, based on such a fake time, special relativity is wrong and so are all relativistic spacetime based theories including the Big Bang theory.
 
Oct 23, 2019
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Thanks for the science fiction question lfr2! Yes, I love all Star Trek (but love TOS orders of magnitude more than the others!), so I thought it was good. Characters are all different and unique, and they have their positives and negatives, but I've generally liked everyone in the ST universe.

As for the science related to this: Of course, almost anything could be possible (especially for things we don't yet know about - like DMAs), but while clouds of dark matter probably exist (our galaxy is surrounded by one, essentially), they are on larger scales than we might encounter if we were zipping around the galaxy. Dark matter - even though it isn't detected/detectable electromagnetically (that is it's not emitting or reflecting photons), and even though it weakly interacts non-gravitationally with other matter - is still a gravitating mass. So we would experience its gravity - and maybe be attracted to it. But other than that I think it wouldn't cause any problems.

Roaming black holes, on the other hand: There are probably lots (many billions?) of the stellar sized ones roaming our galaxy. If they are gravitationally bound with a companion star, for example, or they are "feeding" (matter is falling into them), we would probably detect them (there might be a few more ways we could detect them as well). If they are alone, and not feeding, they would be difficult to detect, so might pose a problem. But space is big. Just think about this: There are hundreds of billions of stars in our galaxy, the Milky Way. The Andromeda galaxy (another spiral galaxy like ours, ~2 million light years away ) is similarly large (larger, in fact, with 1 trillion stars). When these two galaxies collide in 5-ish billion years, it is unlikely any stars will collide - they will just pass by each other - there is SO much space between them. (The gas clouds will collide, and that's another topic.)
Thank you Joe for your brilliant answer. I've absolutely loved the original star trek as well. Except now that Captain Michael is so far in the future I think in a world where the Federation is battling a DMA that she needs to like come up with a better plan. Perhaps there could be some crystal or something to trap the DMA. I mean in Star Trek dilithium crysyals seemed to have been just imagined to solve the warp speed travel issue why not lean heavily into crystal mythology or something? Just a thought. All the best! Thanks again!
 

DrJoePesce

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Thanks you Joe for explaining it so clearly even I may understand. You’ve mentioned the light is bent not by singularity (if you have time could you explain it please) but by event horizon of Black Hole which has a diameter as large as Neutron Star (which is a star but not active?). Does this mean if you could have a Neutron Star as heavy it would also have zero outflows of any kind?
Thanks
Thank YOU suneritz. The singularity IS what is causing the extreme gravity. The event horizon can be thought of as the last orbit within which you can't escape and outside of which you can.

The size of the event horizon varies by mass of the black hole (getting bigger with increasing mass), but it is small, as the examples showed. The examples are similar in size to a neutron star, but otherwise have nothing to do with neutron stars.

A neutron star is compressed matter—also caused by an exploding, high mass, star—and it is one possible outcome of an explosion. In the case of a neutron star, the core of the high mass star doesn't collapse into nothing (that is, a black hole), but instead compresses the matter that was there with such intensity that it all is transformed into neutrons, and the physical size becomes very small (tens of kilometers in diameter). Neutron stars have a maximum mass of about three times the mass of our Sun. The gravitational field of a neutron star can be intense, but nowhere near as extreme as a black hole.
 
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DrJoePesce

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Hello Dr. Joe.
Does anything that has fallen into a black hole ever reach the singularity?
Hi Star gazer! Well, we can expect that the mass falling into the black hole might fall into the singularity, but we don't know (and can't know for certain) because we don't know what happens within the event horizon. Remember, no information can escape the black hole, meaning we can't know what's going on inside.
 

DrJoePesce

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Thank you Joe for your brilliant answer. I've absolutely loved the original star trek as well. Except now that Captain Michael is so far in the future I think in a world where the Federation is battling a DMA that she needs to like come up with a better plan. Perhaps there could be some crystal or something to trap the DMA. I mean in Star Trek dilithium crysyals seemed to have been just imagined to solve the warp speed travel issue why not lean heavily into crystal mythology or something? Just a thought. All the best! Thanks again!
Thank you lfr2 - and thanks for the brilliant question!
 
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Hi Star gazer! Well, we can expect that the mass falling into the black hole might fall into the singularity, but we don't know (and can't know for certain) because we don't know what happens within the event horizon. Remember, no information can escape the black hole, meaning we can't know what's going on inside.
Thank you.
 

Catastrophe

"There never was a good war, or a bad peace."
Hi Dr Joe!

I have a post on Dan Hooper's article in Astronomy, in the Forum Feedback section. What is your view on our current status in understanding the <5% of the observable universe available to our observations?

Cat :)
 
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Hey Astronomy Fans!

Happy 2022! Our previous AMA sessions have been a blast, and I hope you agree. Lots of things going on in the universe and I'm back to share them with you.

To recap my background: 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 Officer 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!!

My knowledge base is broad, but I know most about the areas I mentioned above so might have to do some research to find the answer to areas outside my day-to-day experience. Please be patient with me - the universe is enormous!

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. I will try to answer as many questions as possible.

Not a day goes by that something amazing is revealed to us about astronomy. Thank you for letting me share it with all of you!

Dr Joe
Hello, Dr. Joe. We have been told over the years that the explosion of stars is what causes black holes to form. But recently I read something that said black holes might also actually be capable of FORMING stars. I had never heard that before. What can you share about that? Logicalgal175*
 

DrJoePesce

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Hi Dr Joe!

I have a post on Dan Hooper's article in Astronomy, in the Forum Feedback section. What is your view on our current status in understanding the <5% of the observable universe available to our observations?

Cat :)
Hi Cat - thanks for the post. I can't access the original article for some reason, but I think I get the gist of your question.

I'm not sure I agree that we have access to <5% of the observable universe. It is true we haven't yet accessed all we could observe (for various reasons, including technology limitations).

Our understanding is based on what we can see and what we see is "uniform" in space and time - more or less - even if not the complete picture. For example, the cosmic microwave background. And large samples of Type I supernovae at ~1 billion light years from us.

We use models and simulations based on the laws of nature (determined from astrophysics and other realms - like particle physics) as we understand them (and we attempt to refine those, if necessary, all the time). And those models and simulations better match what we observe. If they don't, they need to be refined. If they don't in the future, because we see more and better of the universe, they will be refined.

And even if we are only observing a small fraction of the universe, we better be able to explain that with what we think we know.

There are parts of the universe we will never be able to observe (because they are beyond the light horizon). Are there funky things there that would change our understanding if we COULD observe them? Maybe. There are certainly things that would cause us to refine our views (just like in the observable parts of the universe). But things that are completely different compared to the observable universe? Probably not - I don't think nature is that perverse.
 

DrJoePesce

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Hello, Dr. Joe. We have been told over the years that the explosion of stars is what causes black holes to form. But recently I read something that said black holes might also actually be capable of FORMING stars. I had never heard that before. What can you share about that? Logicalgal175*

Hi Logicalgal175!

Black holes, with their strong gravity, are a significant energy source: The accretion disk surrounding many supermassive black holes is rotating at large percentages of the speed of light, all powered by the intense gravity of the black hole. These accretion disks, and associated jets, can be brighter than trillions of stars in the surrounding galaxy.

So, this power source can produce other phenomena as well: Winds of material flowing off the accretion disk (NOT the jet) can remove gas from the surrounding galaxy, but it can also plow into cold gas clouds that are spread throughout the galaxy. When this happens, the collision can trigger the cold gas clouds to collapse and form stars.

Note, it's not the black hole - directly - producing the stars, but rather the phenomena powered by the black holes' gravity that are doing this! There are many interesting feedback mechanisms between the black hole and its surrounding environment.
 
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DrJoePesce

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Well, Friday snuck up on us!

I've great enjoyed my week here with you, and your terrific, well-reasoned, and thought-provoking questions!

I look forward to being with you all again soon.

In the meantime, keep looking up!

Dr Joe
 
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Catastrophe

"There never was a good war, or a bad peace."
Thank you for your reply, Dr. Joe! I am a scientist B.Sc. (Hons) in Chemical Engineering, so have some understanding of the ways of science. I have also an ongoing 70 year (since age 12) interest in Cosmology/Astronomy and Astrophysics/Planetary Science/Geology. In other words, from "before" the Big Bang to the ground under our feet. Surprising how weak the link seems to be between nucleosynthesis and the amounts of silicon and oxygen beneath our feet (I mean in the view of the average beholder), for whom I add:

"Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. Wikipedia"

There are the age old aphorisms about being made of stardust, but still many of "our" precious belongings are also "on loan" from the cosmic cycle - a fact not generally appreciated by the uninitiated. Glittering things from mammoth explosions grow.

On the universality of the laws of physics, Dan Hooper may appear to be casting some question. We know, for example, that they break down at t=0.
Here is a recent reference:
which includes: "A team of astrophysicists based in Australia and England has uncovered evidence that the laws of physics are different in different parts of the universe."

By the <5%, I was, of course, referring to total less dark (matter + energy).

I do not know how the system works, so understand that this post may not be open for answer. I have emailed you a copy of the post which I am sorry you said (iirc) you had difficulty accessing. If you meant the original published piece, please let me know, and I will send you a copy.

P.S. I have also an ongoing interest in General Semantics, the work of Korzybski, https://en.wikipedia.org/wiki/Alfred_Korzybski which I believe might advantageously be taught in school.

Anyway, hope this clarifies matters. May I also please add my appreciation to the universal acclaim your presence here receives. Please return soon!

Cat :)
 
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