If black holes are singularities, what do people mean when they refer to a small, medium, or large black hole?

Asked by anonymous


 A singularity is a rip in space-time, and this “rip” is the very center point of a black hole. This point is so small that it actually has no volume. If we define size in terms of volume, or the amount of space occupied, singularities don’t vary in size, because all of them contain no volume.

Although singularities don’t contain any volume, they contain all the mass of the black hole. If we do a quick math calculation, in which we use the fact that density = (mass) / (volume), we find that black holes have infinite density.

If someone were to throw you into a black hole, you would first pass the event horizon: the point of no return, the point where the gravity of the singularity doesn’t allow anything to escape. Assuming you’re going in feet-first, your feet and legs, hitting the horizon first and being pulled into the singularity, would be stretched and stretched. The rest of your body would follow suit, and you would turn into something akin to spaghetti. The official term for this process is “spaghettification”— we kid you not. You would probably already be dead at this point, but if you weren’t, you would get to experience falling into the singularity and getting all your mass squeezed into a space with no volume. The singularity would assimilate your mass and you would become part of it.

Now, the size of the black hole itself is quantified by the radius, or distance from the singularity to the event horizon. This distance can range from a tenth of a millimeter to more than 400 times the distance between the Earth and the Sun. Keep in mind that light, the fastest thing in the universe, takes 8 minutes to make that journey from the Sun to the Earth.

Generally speaking, the radius of the black hole is correlated to the mass of its singularity. The mass of a singularity can vary from the mass of the moon to the mass of our sun, multiplied by 10 to the 10th power. We refer to the black holes at the top of this range as supermassive black holes, and their mass usually lies between 198,900,000,000,000,000,000,000,000,000,000,000 and 19,890,000,000,000,000,000,000,000,000,000,000,000,000 kilogrammes. In fact, these numbers are so large that there is actually no standard unit prefix for this amount of mass.

To read more, check out this article about the different sizes and types of black holes.

Answered by Deiter H., Expert Leader.

Edited by Jamie V.

anonymous said:

When has Harry ever smoked? (I know he was in on the weedgate video and whatnot) But I've never seen anything that suggests he smokes/has smoked. I thought he had asthma, or something along those lines cause of the inhaler. Can you smoke with asthma? Is asthma genetic or just something that occurs when you are born? How does it feel to have an asthma attack? (Sorry got off topic)

There are some pictures from the Miami pot yacht where it looks like he’s holding a joint (and also inhaling). I think it depends on the severity of the condition, I have a friend who only needs his inhaler at nighttime and on rare occasions because of exercise. He smokes from time to time, usually through a vaporizer (I think that’s the word?) but also occasionally in joint form and he claims that it even relieves the asthma. I’m not saying Harry is self-medicating, I don’t think he smokes much if at all, but he probably could. I’m no doctor but judging by how incredibly energetic he is on stage and how rarely he uses his inhaler my guess would be that his condition isn’t all that severe. I don’t know, I could be wrong, the point is that even though he’s got asthma he could technically still smoke a joint and be perfectly fine.


In QED (Quantum Electrodynamics), when the repulsion of two negatively charged particles is described as the exchange of virtual photons, is a wavelength/energy imputed to those virtual photons?

Asked by tomcatpurrs


Excellent question.  Before we address it though, let’s reverse a bit and provide some background— just to make sure we’re all on the same page, so we can all participate in the discussion.

Photons are the quantum of light— we can think of them as particles that carry light over space. The history of “real” photons began in 1900 with Planck’s proposal of how the electromagnetic radiation emitted by an object (black body radiation) is related to its temperature.  Planck’s findings indicated that these energies must be quantized, meaning that they can only have specific values.

Building on Planck’s ideas, Lenard (1902) determined that the photoelectric effect depends on wavelength and not the intensity of the light, and Einstein (1905) concluded that Lenard’s discovery indicated that light itself must be localized in specific packets, rather than distributed in space uniformly.

While the idea of a particle of light might be ascribed to Einstein, conclusive evidence was not available until 1923, when Compton conducted his scattering experiments, and the term photon was supposedly first used by G.N. Lewis in a paper in 1926. (Check out the Wikipedia Page: “History of Quantum Mechanics” for more info and helpful links to some of the terms we’ve been referencing.)

Also during the 1920s, the fields of Quantum Electrodynamics (QED) and Quantum Field Theory (QFT) were developed in an attempt to unify relativity, electromagnetism, and quantum mechanics – primarily emerging from the ideas and mathematical proposals from scientists like Dirac.

One of the goals of these new fields was to explain how forces, like the electromagnetic repulsion between electrons mentioned in the posted question, can act over distances.  Eventually, scientists were able to come up with an explanation that these forces are exerted by the transfer of virtual particles.

 A useful analogy to envision how the transfer of a virtual particle would explain force at a distance is to imagine two people standing opposite of each other on a sheet of ice.  If one of them throws a ball to the other, the person receiving the ball will be pushed and slide backwards— in other words, he experiences repulsion.  In the case of electron-electron repulsion, the “ball” is a virtual photon.

Virtual particles are actually predicted by several important equations and concepts in quantum mechanics (including Feynman’s famous diagrams).  They are considered virtual because they only exist for a very brief period of time – too brief to ever be observed directly.


Above: example of Feynman diagrams. Click on the diagram to read more about what Feynman diagrams are. 

While virtual particles only exist for extremely small amounts of energy, there are many experiments and observations that indirectly support their existence.  So even though virtual photons aren’t directly observable and permanent like “real” photons, there is ample support for the belief that virtual photons are indeed actual entities.  The Heisenberg Uncertainty Principle, which you may have heard about before, predicts their existence.


Above: Heisenberg Uncertainty Principle

 Virtual photos (and other virtual particles) follow the uncertainty principle, which leads us back to the original question, finally!.  Since Heisenberg’s principle can be arranged to accommodate for uncertainty of time:

ΔEΔt = h/2π,

where ΔE is the uncertainty in energy, Δt equals the uncertainty of time, and h is Planck’s constant,

we can see that the right side of the equation is always a constant number. As such, it follows that the smaller the uncertainty in time, the greater the uncertainty in energy has to be in order to maintain the equality.

Since the fleeting existences of the virtual photons make the uncertainty in time extremely small, the uncertainty in their energies is necessarily large. There is no one set value for energy or wavelength, and there are a wide range of possible values. Even when we do the math, the uncertainty level is high, but that’s just the nature of the quantum world!

Lastly, we’d like to add that these ideas about virtual particles are theories. With the knowledge that we currently have, there seems to be ample evidence supporting these theories, but that doesn’t mean they’re unequivocally true. In fact, these ideas are still a matter of heated discussion and debate among many scientists, and as we discover more information about our universe, we may come to expand on these theories or perhaps prove them wrong. In the meantime, we encourage you to contribute your own thoughts to the ongoing discussion about the world of virtual particles!

Further Reading:

If you’re interested in learning more about virtual particles, try reading the blog post “Virtual Particles:  What Are They” posted by Harvard University Theoretical Physicist Matt Strassler. Professor Strassler makes a valiant effort to explain the complicated physics in not-so-complicated terms, and we definitely think it’s worth a read!


Jones, Goronwy Tudor. “The Uncertainty Principle, Virtual Particles and Real Forces.” Physics Education 37.3 (2002): 223-33.

 Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton, NJ: Princeton UP, 1985. Print.

Answered by Brian C., Expert Leader

Edited by Peggy K. 

"Kuroo Tetsurou is a jerk/jackass/asshole/mean"

yes you’re right he absolutely is


such a jerk


wow look at that jackass


god look at this asshole


he’s so mean


he’s just the absolute worst person


there is not one nice bone in his body


in summary:


ding dong you are so very fucking wrong



How does paint dry?

Asked by klingersdressshop 


Awesome question! We’ve all heard the expression that something’s “like watching paint dry”… but what exactly are we watching?

First, we must understand how paints are made.  Most paints will consist of three basic things:

  • Pigments: the compounds that give paint its color.


Pigments for sale at a market stall in GoaIndia.

  • A binding medium: usually some kind of polymer that carries the pigments, sticks to surfaces, and hardens to form a continuous layer once the paint has dried.
  • Solvent: because the other components tend to be very thick on their own, a solvent is used to dissolve the binding medium and lower the viscosity of the mixture. 


Composition of watercolor paint

Sometimes, there are also additional additives that modify the properties of the paint for certain purposes, such as better durability, consistency, or drying speed.

Paints are a type of mixture called a colloid.  A colloid is a mixture containing tiny particles suspended (but not dissolved!) in another substance. Other examples of man made colloids include mayonnaise, body lotion, detergent, and whipped cream.

In the case of paints, the pigment is the tiny particle that is suspended in the solvent-binding medium solution.  When paint dries, the solvent portion of the mixture evaporates away, leaving the pigment suspended in the binding medium in a hard layer.  These evaporating solvents are also largely responsible for the “smell” of the paint you are using.


Diagram of paint drying

So why doesn’t paint dry out in the can?  Well, the speed at which the paint dries depends on how much surface area is exposed to the air.  When you paint something—for example, a wall—the surface area of the paint increases dramatically relative to the amount of paint present. Accordingly, the rate of solvent evaporation also increases dramatically.

In a tube, can, or other container, the portion of paint that is exposed to air is very small, so the solvent will only be able to evaporate very slowly.  This means that it is unlikely that the paint will dry out in the time that you are using it.  If you were to leave a can of paint open for a long period of time, you might notice a sticky or hard film forming on the top—this is the result of the solvent evaporating from this top layer.  However, if you mix the top layer that is exposed to the air back in with the rest of the paint every once in a while, the overall change in the amount of solvent in the system is small, so you will not notice any difference in the consistency of the paint. 

Additionally, since evaporation requires energy, paint that is heated or in a warm environment will usually dry faster than paint in a cooler environment

Lastly, there are many different kinds of paints, and although most follow this basic procedure of drying via evaporation, each type of paint works a little differently. To read more about the differences between types of paints, please click on the link below.

Answered by Rachel R, Expert Leader.

Edited by Peggy K. 

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Looking at each other and smile like a married couple is MakoHaru’s habit.

On a side note, it’s really rare to see Haru’s loving smile to anyone besides Makoto.

May the fluff save your broken hearts. Remember people, even the most loving and gentle couples have their hard time sometimes. But it’s always after the storm that you see rainbow, so obviously this fight is just a catalyst for their married life to be even more married, okay?

AU in which after escaping the Rook Islands, Jason leaves for the Himalayas to find inner peace/get away from it all/learn how to meditate from buddhist monks etc., only to end up in Kyrat




As far as teaching science goes, I would put evolution high up on the list. Only because its the most misunderstood science theory, while also being the most attempted to be understood. Time and time again people will say to me “So you really think we came from monkeys?” which tells me they are not aware of mutation at all. In those times I think, “Man, I’d really like to show them an animation of the process.” Can we find an way to present evolution that will be easy to swallow?

Asked by benfalkenstein


Evolution is one of the most misunderstood topics in science, and yet it is also one of the most controversial, subject to much debate. Luckily, there are lots of resources to help explain.

Below is a great infographic from sixpenceee that concisely explains that humans did not come from apes. Rather, both evolved from a common ancestor.







As an additional resource, this video (although it’s a bit long) provides a good summary of evolution as a process.

Congratulations on your scientific curiosity and thank you for your question! 

Answered by Claire R., Expert Leader

Edited by: Dylan S.

anonymous said:

Imagine Natasha as asexual.

it’s something she mostly keeps to herself, and that works fine until things start getting heated with sam. his hands are lingering near the hem of her shirt, toying with it, and he looks for permission and she gives the tiniest shake of her head, and immediately his hands are gone.

"you okay?" he asks, leaning back against the wall and settling his hands in his pockets.

natasha shrugs. “i’m just not really into sex,” she says bluntly, because in her mind, that’s about all there is to say about it.

"oh," sam says, looking surprised a moment. "okay. do you like kissing?" he looks worried, because they’ve been making out like teenagers for the past fifteen minutes, and she suddenly feels very fond of him.

she stands on tiptoe and leans in to kiss him, just a light peck. “yes,” she murmurs, smiling.

you know in my fics i’ve always written sherlock as a sad baby who uses sarcasm to cover up the fact that he’s just so damn lonely and heartbroken and afraid to let anyone in and people were like “yes i like your story but sherlock is a bit ooc” and i was like yeah thanks but deep inside i didn’t agree with them and still wrote sherlock as a sad gay baby and then series 3 happened and proved i’ve been right all along isn’t life beautiful



Why is it that the further away an object is, the smaller it appears? Obviously the simple answer is perspective; What causes perspective to be something that we perceive in the way that we do? Why is it that there being three spatial dimensions causes us to perceive objects including ourselves in the way that we do? In a universe wherein four spatial dimensions are at play, does perspective cease to exist? In 4D, do objects still appear smaller the farther away they are from the viewer?

Asked by terraf0rm


Fascinating question! In fact, your question was so awesome that we had two teams of Experts— biology and physics— collaborate to answer.

As you alluded to, your question really gets to the heart of what perception is. When we focus both of our eyes on an object, we receive two-dimensional representations of this object on the center each of our retinas. The images are sent through the optic nerve to the occipital lobe, where our genius of a brain calculates the difference in angle between the two images. This difference, or binocular disparity, is responsible for generating our sense of immersion in our three-dimensional environments.

When viewing an object that is further away, our angle of vision of that object is much more acute than it would be if it were closer. As Stephen Pinker explains in Mental Imagery and the Third Dimension, published in the Journal of Experimental Psychology, ”an object will subtend a smaller visual angle when it recedes from the viewer”. This smaller visual angle makes the distant object seem smaller in our field of view than a closer object with a larger angle would appear. 


Hypothetically, four-dimensional human beings would view their world in the same way— by receiving two slightly different images on their three-dimensional retinas. These retinas would grant four-dimensional humans a sense of depth that isn’t accessible to us, with our mere two-dimensional retinas. Their perceptions of the four-dimensional world of space-time would seem as navigable to them as three-dimensional space is to us.


Just as a two-dimensional being would not be able to see inside a square that they’re next to, three-dimensional humans cannot see inside a cube that is next to them. Four-dimensional humans would have no such limitation, being able to view all sides and inside of the cube simultaneously. Utilizing the logic that perspective exists in a two-dimensional world just as it does in the three-dimensional one, we think a visual system adapted to four-dimensional vision would also be subject to the same perspective constraints, and four-dimensional objects would also look smaller to an observer with increasing distance.

But of course, we mere 3-D humans can’t know for sure. We suppose you’ll have to ask the first 4-dimensional human you meet.


Pinker, Steven. “Mental Imagery and the Third Dimension.” Journal of Experimental Psychology: General 109.3 (1980): 354-71. 

Answered by John M., Expert Leader.

Edited by Dylan S.