Astrophysicist Joe Pesce answers your questions about the universe (AMA Recap!)

Astrophysicist Joe Pesce.
(Image credit: Joe Pesce)

Hey, space fans! Recently, we were fortunate enough to hold an AMA with the great Dr. Joe Pesce on our forums. An astrophysicist with a focus on the environments of galaxies hosting supermassive black holes, he’s currently the Program Director at the National Science Foundation (NSF), Division of Astronomical Sciences, responsible for the National Radio Astronomy Observatory (NRAO).

He provided such a dense amount of insight in an easily understandable way that anyone reading through his answers found themselves sucked in.

We've collected the first page of Q&A for you below and hope that you have as much of a good time reading it as we did hosting. You can find the complete AMA here on our forums and you should definitely check it out.  

Supermassive thanks to Dr. Joe for taking the time to stop by and to everyone who participated.

Let's talk about space! (Image credit: Future)

Question: If a black hole is basically just a hyper-concentration of gravity, and a star is a concentrated ball of energy, is there any point at which a black hole can get "filled up"? If so, how many stars would it take to fill one? Or, does the act of "filling" a black hole make it even bigger?

Pesce: A black hole is a “singularity” surrounded by an event horizon. The singularity has zero radius, but the event horizon has a finite size (proportional to the mass within it). We do not (cannot?) know what happens to matter and radiation once it is inside the event horizon, so anything about that is pure speculation. Having said that, when matter falls through the event horizon, the mass of the black hole increases (as does the radius of the event horizon). To date, we have observed some very massive black holes: For example, the supermassive black hole in the galaxy M87 is about 6.5 BILLION times the mass of the sun.

Question: Have we, to date, witnessed a perceivable increase in the mass of a black hole due to matter falling into it? And, if this is the case, and black holes simply get larger and larger as they consume matter, doesn't that mean that eventually the universe will simply collapse into one supermassive black hole?

Pesce: Black holes generally "feed" slowly (in a relative sense) such that the added mass is a small percentage of the total mass. This is true for a stellar-sized black hole (say with 5 times the mass of the sun) pulling mass from a nearby companion star, or a supermassive black hole (say with 1 billion times the mass of the sun) consuming a star with 2-3 solar masses. If we had sensitive enough instruments we could measure that change, but I don't believe we are quite there yet.

The universe is expanding, meaning stuff is getting farther away from other stuff, so probably the whole universe collapsing into a black hole is not possible.

As for the really big black holes - say the one in M87 with 6.5 billion solar masses: That's a LOT of mass. But the galaxy in which that big black hole is found has trillions of solar masses in gas, dust, stars, dark matter. So the black hole is big - but it's minuscule compared to the rest of the galaxy.

Question: What happens if the beam from a pulsar comes within the event horizon of a black hole? Would it bend the beam which would then continue off into space, or would it circle the drain like light?

Pesce: If a pulsar beam (which is light) comes within an event horizon, the photons will be absorbed by the black hole. If the beam comes very close to – but outside – of the event horizon , it will indeed bend. The amount of bending depends on how close, but you could get a situation where the photons will go in orbit around the black hole.

Question: So if you had a couple small black holes in orbit around a pulsar that you could fine tune positioning on, could you theoretically use them to "aim" the pulsar beam? Not that any sane person would want to do that, of course.

Pesce: You could, I suppose, but they would be orbiting each other so that would really complicate any aiming. If you had the capability of doing that, frankly, I think it would be easier just to torque the entire pulsar around to whatever direction you had in mind…

Question: Does anything actually cause a black hole to move?

Pesce: Sure, black holes are masses so they are affected by other masses (and hence gravity) around them. Small black holes in the galaxy orbit around the galaxy just like the stars, they orbit companions, they fall into the centers of galaxies, etc.

Question: How come black holes are possible? I mean, how can mass be there where there is no space as there cannot be any particle without space?

Pesce: This is a story about gravity. Gravity is an attractive force that operates on objects with mass. Gravity’s “goal”, if you will, is to crush things out of existence. If there is enough mass, and with a violent, energetic, event (such as the explosion of a massive star), gravity can win and crush the mass of the core of the star “out of existence” – into a black hole. The singularity has zero radius but contains the mass of the object that was crushed. This means the gravitational field is immense, so strong that not even light can escape. Certainly, the laws of physics break down at the singularity (fundamentally because it has zero radius, but many physical laws have a radius term in them. So, when radius is zero, those equations don’t “work” anymore). It’s for this reason (primarily) that we can’t understand what happens inside the event horizon of a black hole.

Question: Should we be worried about black holes here on planet Earth? My daughter is semi-concerned that we'll get sucked up into one eventually, haha.

Pesce: Neither your daughter nor you (nor anyone else!) should worry: Even though black holes can have a lot of mass, their size is (relatively) small. For example, if I could make our sun magically turn into a black hole, the event horizon would have a radius of only about 2 miles. [NOTE: The sun can NOT turn into a black hole. I was using my magical astrophysicist wand (also called imagination) to do this.] Even the supermassive black holes in the centers of galaxies – millions to billions of times the mass of our sun – are only about the size of our solar system (big for humans, tiny for the universe).

By the way, the nearest stellar-sized black hole so far detected is about 1,000 light years away. The nearest supermassive one is in the center of our galaxy, about 26,000 light years away. 

Question: Are there any interstellar objects composed of dark matter?

Pesce: Dark matter is everywhere, and surrounds galaxies (made up of normal matter), for example. There could be clouds composed only of dark matter, but I don’t believe we have detected, definitively, such objects

Question: Do you feel confident that we have a good understanding of dark matter and we're just waiting to fill in some blanks, or that we may come to find that we'll need to start from scratch to better understand the processes driving the universe outward?

Pesce: I feel confident dark matter exists. Do we truly understand it? There’s a lot of understanding left to do, but we are certainly learning more and more every day. I think the area that might cause us to rethink our understanding of laws of physics is when we finally determine what sort of thing dark matter is (for example, what sort of sub-atomic particle dark matter is).

Question: Do the more massive galaxies typically, perhaps always, have a higher ratio of dark matter to normal matter compared to smaller galaxies?

Pesce: I’m not sure about always, but it does appear that more massive galaxies have more dark matter compared to smaller galaxies. Why? Don’t know, but a question is which came first, the dark matter or the galaxy? And more observations will help us understand this fundamental question.

Question: Based on your years of experience in the field, how likely do you think a multiverse is? Is there a mirror version of this universe where we're all there as well, but with goatees and bad intentions?

Pesce: The multiverse is certainly an intriguing idea. And could be a possibility (including all those things you mention). Many aspects of the multiverse can’t yet be tested, so it remains in the realm of science fiction; but that will change with time.

Question: Do active galaxies demonstrate greater metallicity as one gets closer to the "juicy center of the Tootsie Pop" (from Spielberg's Ready Player One)?

Pesce: I like Tootsie Pops – one, two, thareee… (get it!). Ah, sorry, got distracted.

I’m not aware of any studies that show changing metallicity as one gets closer to the nucleus. So just to add some context: Active galaxies (or, Active Galactic Nuclei – AGN, to give their full name) have a supermassive, feeding, black hole in their centers (their nuclei). Metallicity refers to how many heavy elements are present in a gas (for astrophysicists, “heavy element” means elements heavier than helium). All such elements are created in the fusion reactions in the centers of massive stars, or in their explosions as a supernova (there are some other sources…). The first generation of stars was formed out of material from the big bang, so hydrogen (mostly) and helium only. After that first generation, heavier elements started to “pollute” the gas clouds from which later generations of stars formed, and so the metallicity increases. For this question, I suppose there could be unusual star formation near the active nucleus, but that’s probably not a big deal. Rather, studies seem to indicate that there is a connection between metallicity in the galaxy and any active nucleus, maybe indicating that galaxy mass (and star formation) is the important factor in causing the nuclear activity.

Question: Any insight into how you hope gravity wave detections will help you and others?

Pesce: Gravitational waves give us a new “window” on the universe, and as such, allow us to study new objects we didn’t previously know exist, and learn about old friends in a new way. So, yes, gravitational wave detections will be a most useful tool! And, while we are on the topic, here is a cool new result from NSF's LIGO (Laser Interferometer Gravitational-Wave Observatory).

Question:  Do you think a wormhole network is the only way interstellar travel will ever be possible?

Pesce: Wormholes. Predicted by Einstein, but not yet detected (hmmm, is this the only thing predicted by Einstein but not yet proven to exist? Maybe…). Certainly, wormholes are the foundation of lots of Sci-Fi stories, where we can play and speculate. If wormholes work as they are depicted in Sci-Fi, then they are a great way to travel large distances. And traveling large distances is the key, as even for the shortest trips (to the nearest star, say), the distances are astronomical.

Question: What kind of technology would be at the foundation of a wormhole super highway and is there anything we are working on now that will lead to this technology?

Pesce: I would love for wormholes to exist – and to be able to use them as superhighways – but we will have to wait and see. The short answer to your question about anyone working on technology is probably no. But all basic research (for example in physics and astrophysics – the sort of research we fund at the NSF) informs our understanding of the universe around us, advances technology, and makes future technology (that we can’t even imagine today) possible. So, in this sense, the answer is yes!

Question: What do you think about Relativity? Have we got enough evidence to prove it wrong?

Pesce: Relativity is great! Such an insightful mind Einstein had. No evidence has been found to prove it wrong: Every test of relativity to date has failed to invalidate it.

Question: Was there any "eureka" moment for you when you decided that astrophysics was your passion?

Pesce: I’ve been interested in astronomy almost my entire life: I read an astronomy-related book, probably when I was about 5 years old and was hooked. (this is quite common among astronomers, by the way). Star Trek (TOS) played a big influence on me, in general, and becoming an astronomer in particular.

Question:  As a Star Trek fan... who is your favorite Captain? And what do you think of the new Picard series?

Pesce: This is the question that's going to get me into the most trouble! I love all the Captains, because I love the entire Star Trek franchise. BUT it's got to be Kirk for me; followed closely by Picard (I'm loving the new series by the way).

You didn't ask, but to make even more trouble, I will let you know my favorite character (limited to TOS at least). You might have said Spock, since he's a scientist; maybe Kirk, because of leadership. Sulu, because he started out as an astrophysicist (or Physicist in the Astro Science Department - but we know that's an astrophysicist). These are all great suggestions (and great characters). But my favorite is Scotty: He's calm and collected, and is the intellectual equal of Spock. He may not be as quick as Spock (but who is), but he's not far behind and is insightful at the same level. (And he's British, and I'm an anglophile.)

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