Frequently Asked Questions

This famous formula was Einstein’s most intriguing prediction. Einstein made two assumptions: (1) all motion is relative — you can’t be moving unless you’re moving relative to something else; (2) everyone measures the same speed of light. With these two assumptions, and a little math, Einstein proved that the only way these two assumptions could be consistent with each other is if the energy E of a resting body with mass m is \(mc^2\).

Yes! On September 14, 2015, LIGO directly detected gravitational waves for the first time in history. You can read more about that event here. Since then, there have been several other signals that have been detected — either as confirmed gravitational-wave events or possible events without quite enough evidence to be sure that they weren’t just noise. Astronomers hope to learn more interesting facts about our universe from future direct detections.

Before that, though, gravitational waves had only been detected indirectly. The 1993 Nobel Prize in physics was awarded to Russell A. Hulse and Joseph H. Taylor, Jr., for their observation of a binary pulsar, which lost energy in exactly the way that we expect gravitational waves would carry energy away.

Gravitational waves are predicted by Einstein’s theory — the fundamental theory of gravity that everything else in astronomy relies on — but have not yet been observed. We need to know if Einstein’s theory is really right before we can make strong claims about the nature of the Universe. If the theory is indeed correct, finding gravitational waves will give us new insights into the theory, and will give us powerful new tools observe the Universe.

“White dwarf” is short for “white-dwarf star”, so that “dwarf” is really thought of in its adjective form — it is a “substantive adjective”. Substantive adjectives are pluralized as though they were nouns. Note that “Dwarves” is the plural form for characters from fantasy books, like Tolkien’s The Hobbit. “Dwarfs” is the plural for real people affected by dwarfism. Perhaps the use of “white dwarfs” suggests their existence in the real world, rather than fantasy.

It would be pretty hard to tell for sure. We would see stars or planets being pulled toward a seemingly empty patch of space, which would mean that there is something heavy there. By measuring how hard the stars or planets are being pulled, we could figure out how heavy is the object doing the pulling. If it’s heavy enough, and small enough, we would guess that it must be a black hole. We could be much more sure if we saw the gravitational waves coming from something falling into a black hole. These waves would show us very precisely what spacetime looks like near the place they were produced. Thus, gravitational waves give us a way to really “see” a black hole.

Black Holes can actually evaporate, through a process called Hawking Radiation. A Black Hole actually gives off a steady steam of particles, losing its mass. If nothing falls into the Black Hole to replace this loss, the Black Hole could actually disappear.

We’re almost positive. Astronomers can point to places in space where they know there are heavy objects pulling other things toward them.

There is no lower limit, in principle, but it’d be difficult to form a black hole lighter than a few times the mass of the sun. Similarly, there is no upper limit, in principle. We expect to find black holes that weigh in the range of a few to a few million times as much as our sun.

That depends on how much it weighs. A black hole weighing as much as our Sun could fit through a hula hoop that is just 6 miles around. A black hole twice as heavy would need a hula hoop twice as large.

This isn’t very likely. If astrophysicists understand stars correctly, our Sun will end up as a White Dwarf, several billion years from now.

Not at all. Far outside a Black Hole, gravity feels just the same as it would if the Hole were a regular star. For example, if our Sun were suddenly replaced by a Black Hole with the same mass as the Sun, the Earth would continue orbiting as usual (though things would get pretty cold). The Sun’s replacement would actually trap fewer things floating through space than the Sun would, though they would stay trapped in a black hole.

This was the surprising discovery made by Stephen Hawking. Nothing can escape a Black Hole, only if General Relativity is completely correct. It turns out that GR is not< completely correct, just mostly correct. If we include quantum mechanical effects, we find that some things can escape.

Maybe, but not for humans just yet. Black holes can help give rise to wormholes. A wormhole is a bridge from one part of spacetime to another — a bridge through space, or through time, or through both. However, wormholes are almost certainly unstable — they will collapse very quickly, unless held up by something. It might be possible that some advanced civilization could figure out how to keep a wormhole open, and then use it for traveling through space and time.

Our understanding of physics starts just after the Big Bang. What happened before that is as much a mystery to physicists as to anyone else. Our lack of understanding, however, is no reason to stop trying. Perhaps someday we will understand what came before the Big Bang; perhaps we will never know. It is interesting to search for the answers.

Current models of the Universe’s past, together with observations indicate that the Universe is roughly 14 billion years old. Now, we know that our models aren’t exactly correct, but they are pretty close to the correct answer. That means that there is a little wiggle room in this number, but it’s pretty nearly right. In fact, astronomers have seen stars that are at least 10 billion years old.

We don’t know. We know that it’s been around for at least 10 billion years, and the light in our Universe has been traveling since then. Thus, if the “edge” of the Universe exists, it is at least 10 billion light-years away from us. It’s possible that the Universe is bigger than that, though — maybe even infinitely large!

We can’t quite tell yet. The Universe is right on the edge between expanding forever, and collapsing back to a point. There may be a reason for this that we don’t yet understand, or it could just be a coincidence. Only more astronomical observations, and better theories, will be able to tell us for sure.

Scientists use this notation as an easy way to write really large, or really small numbers. A number like 10⁸ represents 10 multiplied by itself 8 times, which is a 1 with 8 zeroes after it: 100,000,000.

A number like 10^-21 represents 0.1 multiplied by itself 21 times, which is a 1 with 21 zeroes in front of it (including the one before the decimal point): 0.000000000000000000001.

This is frequently a much more efficient way to write numbers. For example, 3.765x10¹⁷ is much easier to write and think about than 376,500,000,000,000,000.