# What Is Relativity? **Covers**:: **Source**:: [[What Is Relativity? by Jeffrey Bennett]] **Creator**:: [[Jeffrey Bennett]] # Highlights ##### ^299918287 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=99 ###### ^299918287q But I’ll let you in on a little secret that’s actually important to understanding relativity: Black holes don’t suck. ^299918287 ##### ^299918288 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=112 ###### ^299918288q at a distance the gravity of a black hole is no different than the gravity of any other object. ^299918288 ##### ^300340340 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=137 ###### ^300340340q a black hole’s presence may be revealed through the light emitted by gas that surrounds it. ... Any gas that happens to be near a black hole will end up orbiting around it, and because a black hole is both very small in size and very large in mass, the gas that is closest to it must orbit at very high speed. Gas moving at high speed tends to have a very high temperature, and high-temperature gases emit high-energy light, such as ultraviolet and X-ray light. Therefore, if you see X-ray emission coming from the region surrounding a very compact object, there’s a chance that the object is a black hole. ^300340340 ##### ^300340320 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=150 ###### ^300340320q before we conclude that the unseen object is a black hole, we must rule out the possibility that it might be some other type of small but massive object. ^300340320 ##### ^300340321 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=153 ###### ^300340321q black holes are the remains of high-mass stars (stars at least 10 or so times as massive as the Sun) that have died, meaning that they have exhausted the fuel that keeps them shining during the time when they are “living” stars. ^300340321 ##### ^300340341 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=161 ###### ^300340341q supermassive black holes that reside in the centers of galaxies (or, in some cases, in the centers of dense clusters of stars). ... Most other galaxies also appear to have supermassive black holes in their centers. In the most extreme cases, these black holes have masses that are billions of times the Sun’s mass. ^300340341 ##### ^300340323 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=162 ###### ^300340323q In the center of our own Milky Way Galaxy, for example, we observe stars orbiting a central object at such high speed that the object must have a mass about 4 million times that of the Sun, yet its diameter is not much larger than the diameter of our solar system. ^300340323 ##### ^300340325 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=181 ###### ^300340325q light could circle Earth nearly eight times in a single second. ^300340325 ##### ^300340326 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=183 ###### ^300340326q a light-year is just a little less than 10 trillion kilometers (6 trillion miles), ^300340326 ##### ^300340342 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=213 A light year is not a lightyear when you are travelling near the speed of light ###### ^300340342q because light makes the trip in 25 years in each direction, or 50 years for the round-trip, your round-trip at 0.99c will take 50 years and 6 months (which is 50 years divided by 0.99). If you leave Earth early in the year 2040, and allow for 6 months of experiments at the black hole, you’ll return home early in the year 2091. By our ordinary intuition, we’d expect the trip to seem much the same for you, taking 51 years, including your time at the black hole. But it wouldn’t. Here’s what would really happen. To keep things simple, let’s suppose that you make the entire trip at the speed of 0.99c. ... Stars in the vicinity of the black hole will suddenly be much brighter than they were previously,1 as if they were suddenly much closer to you. Indeed, if you could measure it, you’d find that the distance to the black hole was no longer the 25 light-years that you had measured while on Earth, but instead had shrunk to about 3½ light-years. As a result, your speed of 0.99c would allow you to reach the black hole in only a little more than 3½ years. The return trip would take the same, ... Your calendar would say that you were gone for only 7½ years, to the year 2047. You would have needed only 7½ years’ worth of supplies for the trip, and you would be only 7½ years older than you were when you left. But the calendars of everyone who had remained on Earth would say that it is now the year 2091. ^300340342 ##### ^300340330 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=241 ###### ^300340330q When we are speaking of space, however, an orbit is any path that is governed solely by gravity, and it does not matter whether the source of the gravity is a planet, a star, a black hole, or anything else. ^300340330 ##### ^300340331 highlight_tags:: [[gravity]], [[orbit]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=252 ###### ^300340331q ellipses are bound orbits because objects on them remain bonded to the central object by gravity. Parabolas and hyperbolas are unbound because objects following them come in and swing past the central object just once, never to return again, meaning that the central object’s gravity does not have a permanent hold on them. ^300340331 ##### ^300340332 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=315 ###### ^300340332q Light with frequency lower than that of visible light is what we call infrared light, and light of even lower frequency makes what we call radio waves. ^300340332 ##### ^300340333 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=339 ###### ^300340333q Tides on Earth arise primarily from the Moon’s gravitational influence and the fact that our planet is about 13,000 kilometers in diameter, which means that whatever side is facing the Moon at a particular moment is about 13,000 kilometers closer to the Moon than the other side. Because the strength of gravity depends on distance, the Moon exerts a stronger gravitational pull on the parts of Earth that are nearer to it. This difference in the Moon’s gravitational pull on different parts of our planet effectively creates a “stretching force” that makes our planet slightly stretched out along the line of sight to the Moon and slightly compressed along a line perpendicular to that. ^300340333 ##### ^300340334 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=349 ###### ^300340334q The stretching also explains why there are generally two high tides (and two low tides) each day: Because Earth is stretched much like a rubber band, the oceans bulge out both on the side facing toward the Moon and on the side facing away from the Moon. As Earth rotates, we are carried through both of these tidal bulges each day, so we have high tide when we are in each of the two bulges and low tide at the midpoints in between. ^300340334 ##### ^300340335 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=352 ###### ^300340335q tidal force is simply the difference between the tug of gravity on one side of an object and the tug on the other side. The strength of the tidal force therefore depends on two things: (1) the distance across the object and (2) the strength of the gravity acting on it. ^300340335 ##### ^300340336 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=363 ###### ^300340336q the larger size of a supermassive black hole would make its tidal force at the event horizon much weaker. You could therefore live, at least for a little while, to see the inside of the black hole. ^300340336 ##### ^300340337 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=372 ###### ^300340337q The event horizon is the point at which the speed required to leave the vicinity of the black hole (the escape velocity) is the speed of light, and because Einstein’s theory tells us that no material object can reach that speed, nothing can escape from within the event horizon. ^300340337 ##### ^300340338 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=410 ###### ^300340338q A parabola is curved everywhere along it, while at large distances a hyperbola is indistinguishable from a straight line. Mathematically, there is a much wider range of possibilities for hyperbolas than parabolas (which are the borderline case between hyperbolas and ellipses), which is why most unbound orbits are hyperbolic. ^300340338 ### Part 2 EINSTEIN’S SPECIAL THEORY OF RELATIVITY ##### ^300574933 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=435 ###### ^300574933q the special theory applies only to the special case in which we ignore any effects of gravity, while the general theory includes gravity. ^300574933 ##### ^300574934 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=456 ###### ^300574934q Relativity tells us that everyone always measures the speed of light to be the same, and this agreement about the speed of light leads inevitably to the fact that you cannot outrace your own light. ^300574934 ##### ^300574935 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=464 ###### ^300574935q 300,000 kilometers per second, is actually the maximum speed of light; light travels slower when it passes through materials such as water, air, or glass, and in recent years scientists have found ways to slow light down to pedestrian speeds in the laboratory. ^300574935 ##### ^300574936 highlight_tags:: [[c1]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=474 ###### ^300574936q the prohibition on faster-than-light travel applies only to the ability to transmit matter or information from one place to another, or to say that nothing can travel through space at a speed faster than light. ^300574936 ##### ^300574937 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=476 ###### ^300574937q nothing can outrace light. ^300574937 ##### ^300574938 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=479 ###### ^300574938q the special theory of relativity takes its name from the idea that motion is always relative. ^300574938 ##### ^300574939 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=550 ###### ^300574939q The concepts of up and down have meaning only when we can describe them relative to a planet’s (or other object’s) surface. ^300574939 ##### ^300574940 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=733 ###### ^300574940q This effect, in which time runs slower in reference frames that are moving relative to you, is called time dilation; ^300574940 ##### ^300574941 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=784 ###### ^300574941q the effects on distance and length in relativity are usually called length contraction. ^300574941 ##### ^300574942 Goto: https://readwise.io/to_kindle?action=open&asin=B00I2G6XNM&location=785 ###### ^300574942q The factor by which length contracts for a moving object is the same as the factor by which we see its time dilated, ^300574942