If light can't escape a black hole, how does a graviton escape?

If the intense gravity field of a black hole establishes an escape velocity at or above the speed of light, and if gravitons exist, how do they escape a black hole to establish the gravity field?

One of the most important things to keep in mind when thinking about the next Big Theory that people are developing is that physical theories are like nesting dolls. As we progress, we go to a bigger and bigger nesting doll, which is able to explain more and more things, but at its core, each theory has to explain the same set of phenomena.

So let’s take gravity - our first theory of gravity came from Newton. Newton’s laws are very basic, but they did an excellent job of explaining the way that things fall to the ground, and the way that the planets orbit the sun. These laws pretty much explain most everything we encounter in our every day lives.

However, Einstein’s theory of general relativity went a bit further, and explained that Newton’s equations could be understood as a simple way of looking at a much more complex system. Einstein’s new equations could be simplified to obtain Newton’s laws, in the small size, not very massive limit of General Relativity, but General relativity’s way of thinking about the universe provided explanations for a much wider range of objects. Many of these objects had not yet been observationally discovered, but the math Einstein put forward allows for their existence. Black holes are one of these objects. General relativity permitted objects to exist which are so dense that even light cannot escape, and many years later we found quite a bit of evidence showing that they’re real.

Gravitons come into this equation as an attempt to understand how gravity works. Gravity is kind of a mess, theoretically speaking- it just doesn’t play well with the other fundamental forces - it’s far too weak relative to the rest. We know that gravity can be explained as a curvature of space as a result of the presence of mass, but that doesn’t explain how gravity propagates. Most other forces of nature have what are called “force carriers” - little particles (or packets of energy) that obey quantum rules (in that they can pop into and out of existence) and go between two different particles as the particles interact. The force carrier particles are also called ‘virtual particles’ and, because they obey quantum rules instead of classical ones, can serve as a handy “get out of jail free” card when the classical rules seem a little restrictive. But these force carriers are predicted as part of what’s called the Standard Model.

Gravity is not part of the Standard Model. We can’t figure out how to get it in there. (We’re trying - this is how we wound up with string theory.) The quantum world and the general relativity world seem to be fundamentally incompatible. But if we’re trying to come up with a quantum-world version of gravity, then the assumption becomes that since every other force has a carrier quantum particle, gravity should have one too. And so we’ve called it the graviton, even though there’s no evidence so far that it exists, and very little information on how we think it ought to behave. If it does exist, it should at least play a role in how gravitational waves propagate.

Two more things before we can answer this question: force carriers are active at a quantum scale. This means that they will exist for extremely short periods of time, and the uncertainty principles will play a role in their behavior. Second: force carriers are only needed when there is a transfer of energy. So if a black hole were losing energy somehow (perhaps by colliding with another black hole), then the gravity waves that would be given off would involve the graviton.

But if you just had a black hole that was not losing energy, then the black hole would not be producing gravitational waves. It doesn’t change the fact that the mass of the black hole is enormous. At distances large enough to be outside of the event horizon of the black hole and scales above the quantum level, any graviton-based theory of gravity should simplify to the general relativity situation, where we can understand the mass of the black hole by the curvature of space. This curvature appears like a force because external particles respond to the local geometry of the universe. As far as gravitons go, it’s hard to understand precisely how a theoretical, highly chaotic quantum particle would behave in a region of space that has always been understood in the framework of general relativity, but however it behaves, it must result in the same solutions as general relativity.

Something here unclear, or have your own question? Feel free to ask!

Why are NASA's space suits so clunky?

Why are NASA’s space suits so much clunkier than the ones in science fiction or video games?

NASA has a real problem with the space suits that they stick their astronauts in to perform space walks. They’re massive, hard to get into and out of, and phenomenally unwieldy. The fingers on an astronaut’s gloves are so hard to manage that NASA has run competitions trying to get a better design that’s easier to work in. Ideally, we want to be able to stick people in suits that are easy to move around in, while still providing all the protection they would need. This is really, really hard to do.

If your suit is designed for the vacuum of space, you need to have a pressurized suit. Right now, this is done by inflating the suit with air, to compress the body to a point where the astronaut is comfortable, but not so much that it’s so tightly inflated that the astronaut couldn’t bend any of their joints. (This actually was a pretty severe problem for the first Russian spacewalker; his suit was so pressurized he couldn’t get back into his ship without letting some air out. He had to loosen a gasket on his glove to let the air escape, and then he got decompressed so rapidly he got a pretty nasty case of the bends, which is the same problem scuba divers run into if they surface too quickly.) You also need a suit that will provide some radiation protection, protection from tiny pieces of space junk, and on top of all that, you need your astronaut to be comfortable inside it and able to get in and out of it relatively easily.

On top of that, you have additional challenges if you want to land on a planet. Generally speaking, you don’t want to track dirt in from outside, if you’re on the moon or Mars. The dust on the moon (and we suspect on Mars as well) is such a fine powder that it can become embedded in your lungs and do quite a bit of damage. Mars dust might be even worse for you, since a lot of Mars’s surface material is so chemically toxic that it would burn you like bleach - not something you want in your lungs.

Physics is really not on our side for this venture. We’re asking for a pressurized suit that’s still easily bendable, which is also radiation-resistant, and easy to get into and out of, and durable. If you want to go out on the surface, you need to be able to decontaminate it completely. This space suit has to be a pretty impressive piece of technology.

The solution so far for spacewalks has been the kind of inflatable suit we’re used to seeing our astronauts in. Science fiction films and video games tend to prefer suits that are at least partially skin-tight. These aren’t completely impossible, and at least one person at MIT has been working on trying to design a suit that pressurizes the astronaut through mechanical pressure of the suit on the body, rather than the balloon method of air pressure. The mechanical skin-tight suit is really hard to make, because you have to get even pressure over the entire suit, and have it bendy enough to not restrict motion, and be durable enough to not break any wires if you fall on a rock or from bending the suit repeatedly. These skin-tight suits are also a lot harder to decontaminate, so getting all the dust off of them after a trip outside would be really hard to guarantee. NASA has also been testing a suit that you can crawl into through the back. This would be handy, because it means you can leave the suit attached to the outside of the base, and you won’t have to worry so much about getting the dust off. On the other hand, it’s still pretty clunky.

So unlike the science fiction films and video games, which can invent new materials to evenly pressurize an astronaut’s body, and new ways to decontaminate the suit so no one gets chemical burns from the surface dust once they come inside, while still protecting from radiation and puncture damage, NASA is stuck with the materials and methods that we have right now. We’re working on new methods and new technologies, but for the moment we don’t have anything quite as stylish as science-fiction can manage.

Have your own question, or something here unclear? Feel free to ask!

What would happen if the sun split in half?

This is not possible due to the force of inertia and gravitational force, along with nuclear fusion, but: What would happen if the sun split into 2 ½ – Sized suns?
DG CVn (right) is about one-third the size of our sun (left).  Image credit NASA

DG CVn (right) is about one-third the size of our sun (left). Image credit NASA

Let’s assume that we’re just replacing our sun with a pair of stars which are in a stable orbit around each other, each of which is half the size of the sun, because if our replacement stars are in unstable orbits, this will get super complicated really fast. Even with this simplifying assumption, our solar system would be a very different place. 

A star’s energy output is strongly related to how much mass it has, but it’s not a one to one relation. If you cut the sun’s mass in half, you reduce its brightness by 90%. Gravity doesn’t have as much mass to work with, so the force of gravity can’t crush the star down as much, and the star can’t reach the same pressures and temperatures in its center as a more massive star can. These half-stars are at the upper edge of what’s considered a red dwarf star; they can still burn hydrogen in their cores, but at a much slower rate than our sun. Slower energy production gives you a dimmer star, and a dimmer star means a cooler, redder star.

This means, therefore, that if you cut the sun’s mass in half, you go from a star that looks yellow-white, and has a surface temperature of 5800 Kelvin (9980 F, or 5527 C) to something that will look distinctly orange, tipping towards the red end of orange.  Each of these half-suns would have a surface temperature of only 3700K (6200 F or 3426 C), a drop in surface temperature of 40%.

Our replacement set of stars has a combined energy output of 20% what our sun’s brightness is. As a result of this drop, the other thing that will definitely change about our solar system is the distance from the stars where liquid water can pool on the surface of the planet. With 20% the output of our current sun, Earth would be way too far away from these stars - all the water on our planet would freeze. In fact, the habitable zone would be almost exactly where Mercury’s orbit is now. Mercury’s orbit goes between 0.3 and 0.46 AU (AU being the distance between the earth and the sun). The habitable zone would range between almost these same numbers: 0.32 AU, if the planet was getting light from only one star, up to 0.44 AU, if it was getting sunlight from both of them.

Mercury is really close to the sun. So if we put our habitable planet in the right spot to have life, it will also have to be really close to the two half-suns. But then you start getting into really hairy territory, because any time you have three objects interacting with each other, as you would with a close pair of stars and a nearby, light planet, the orbits can get pretty funky - this is called the 3 body problem. It’s not nearly as nice as having one massive object in the centre for things to calmly orbit around. There are windows of stability, where the stars and the planet can orbit relatively calmly for a long enough time to develop life. The positions of these windows depends entirely on the exact configurations of the orbit of the two stars in question. If the planet is not in a window of stability, one of your three objects can wind up getting tossed out of the solar system, which would be particularly bad for the planet you’re trying to grow things on.

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Why are stars different colors?

Comparison of sizes of Gamma Orionis, Algol B, and the Sun, to scale.  Image credit:  wikimedia user 84user & Paul Stansifer.

Comparison of sizes of Gamma Orionis, Algol B, and the Sun, to scale. Image credit: wikimedia user 84user & Paul Stansifer.

A star’s color is entirely governed by its temperature at the surface. Very similar to our idea of “red hot” and “white hot” giving an idea of the temperature of heated metal, the color of the star gives an idea of the temperature of the star.

The relationship between temperature and color is given by the black-body relation, which applies to any object which gives off light and doesn’t absorb light (like stars and heated metal). So objects like these which glow due to heat change colors as they heat up. The hotter an object is, the shorter the wavelength of the majority of the light it gives off, so as you heat an object, it will transition through the rainbow of colors from red to blue. However, iron really only goes from red to white, whereas stars can have colors anywhere from the extremely hot blue to the cooler deep red stars. 

So the real question is, why are stars different temperatures?

Stars are a delicate balance between gravity crushing a star inwards, and the pressure from the radiation the star generates in its core. The more massive a star is, the more gravitational pressure can be exerted, so the more energy needs to be generated in the centre of the star. Fortunately, the extra gravity of the star can produce extra dense and hot regions near the centre, which allows the nuclear reaction in the centre to kick into high gear. This then produces the extra energy the star needs to balance out its extra weight.

So the heavier the star is, the more energy the star can produce at its core, which in turn means the star burns hotter and faster, and the hotter the star, the bluer it gets. By contrast, the lighter a star is, the less energy is formed at the core, so the surface of the star stays quite cool (relatively speaking) and red.

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