Celebrating 50 Publications & On the 100th anniversary of Albert Einstein’s general theory of relativity!.
Why can’t anything travel faster than the speed of light? Do black holes really suck? And why is the universe expanding?
These are just a few of the questions author and astrophysicist Jeffrey Bennett answers in his new book, “What is Relativity” (Columbia University Press, 2014).
On the 100th anniversary of Albert Einstein’s general theory of relativity, Bennett’s book seeks to explain both special and general relativity in a form that a non expert could understand. [“Book Excerpt: “What is Relativity“]
[Book Excerpt: “What is Relativity”]
Part 1: Introduction:
Chapter 1: Voyage to a Black Hole;
Imagine that the sun magically collapsed, retaining the same mass but shrinking in size so much that it became a black hole.
What would happen to Earth and the other planets? Ask almost anyone, including elementary school kids, and they’ll tell you confidently that the planets “would be sucked in.”
Now imagine that you’re a future interstellar traveler.
Suddenly, you discover that a black hole lurks off to your left. What should you do? Again, ask around, and you’ll probably be told to fire up your engines to try to get away, and that you’ll be lucky to avoid being “sucked into oblivion.”
But I’ll let you in on a little secret that’s actually important to understanding relativity: Black holes don’t suck.
If the sun suddenly became a black hole, Earth would become very cold and dark.
However, since we’ve assumed that the black hole will have the same mass as the sun, Earth’s orbit would hardly be affected at all.
As for your future as an interstellar traveler… First of all, you wouldn’t “suddenly” discover a black hole off to your left.
We have ways to detect many black holes even from Earth, and if we are someday able to embark on interstellar trips we’ll surely have maps that would alert you to the locations of any black holes along your route.
Even in the unlikely event that one wasn’t on your map, the black hole’s gravitational effect on your spacecraft would build gradually as you approached, so there’d still be nothing sudden about it.
Second, unless you happened to be aimed almost directly at the black hole, its gravity would simply cause you to swing around it in much the same way that we’ve sent (such as the Voyager and New Horizons spacecraft) swinging past Jupiter on trips to the outer solar system.
“There are a lot of good books about relativity, but most have been targeted at a high level of understanding,” Bennett said. He wanted to provide an introduction to the ideas that even a smart middle schooler could understand.
Nothing faster than light;
Bennett defines relativity as a modern scientific explanation of space, time, gravity and the universe.
Albert Einstein discovered two things: The laws of nature are the same for everyone, and the speed of light is the same for everyone.
Einstein discovered special relativity first in 1905, but special relativity is just a special case of general relativity that doesn’t involve gravity.
Einstein realized that the speed of light is a constant 186,000 miles/second (300,000 kilometers/second), and that nothing could exceed it.
Imagine a person traveling on a spaceship with headlights.
The light from the headlights would travel at the same speed whether the ship were stopped or moving, so to an observer, the ship would have to be traveling more slowly than light, Bennett explained.
Journey to a black hole;
In his book, Bennett takes readers on a journey to a black hole.
Common sense holds that if the sun were to suddenly be replaced by a black hole, it would suck up the Earth and the other planets. Not so, Bennett said.
Because the nearest black hole is so far away, a person would have to travel at close to the speed of light to get there within his or her lifetime.
– “At that speed, all sorts of weird effects come into play.”
For example, people on Earth would experience more time passing during the journey than the traveler would, a phenomenon called “time dilation”.
Special relativity shows that everyone on Earth would be much older than the traveler when he or she returned.
Once the traveler reaches the black hole, rather than being sucked in, the traveler would simply go into orbit around it (unless he or she were aimed almost directly at it). A black hole has gravity just like any other object.
But very close to the black hole, gravity gets really bizarre.
If you dropped a clock into a black hole, for instance, it would show time passing more slowly, and light from the clock would appear more reddish.
Light is a wave that vibrates at a certain frequency, and red light has a lower frequency than blue light.
Since the clock’s time appears to run slow, its light also has a lower frequency.
Now imagine there’s a second traveler who jumps out of the ship and falls into the black hole.
From his perspective, he would fall faster and faster until he reached oblivion, but from the perspective of the other traveler, the falling traveler would take forever to reach the black hole’s event horizon, the point at which nothing (not even light) can escape.
These effects, known as gravitational time dilation, can be explained by Einstein’s theory of general relativity.
Relativity in daily life;
Relativity seems strange, because it goes against common sense, but in fact, the effects of relativity are always present; “they just aren’t noticeable except at extreme speeds and extreme gravity”, Bennett said. Relativity is critical to everything from how the sun shines to how GPS works. “Relativity is in everything we do in our lives,” he said.
Einstein’s theories yielded the famous equation E=mc2, which shows how mass (m) can be converted into energy (E). For example, nuclear fusion in the sun’s core converts hydrogen into helium, giving off sunlight.
GPS satellites orbit the Earth fast enough and far enough away for time dilation due to both special and general relativity to come into play.
In order to work accurately, GPS must correct for these effects.
Bennett hopes his book will convince readers that anyone can understand relativity, even without a background in physics.
– “Relativity has a reputation for being hard, but I don’t think this book is hard,” he said.
Information Abstract; Einstein’s Theory of General Relativity:
In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers.
This was the theory of special relativity.
It introduced a new framework for all of physics and proposed new concepts of space and time.
Einstein then spent ten years trying to include acceleration in the theory and published his theory of general relativity in 1915. In it, he determined that massive objects cause a distortion in space-time, which is felt as gravity.
The tug of gravity:
Two objects exert a force of attraction on one another known as “gravity.”
Even as the center of the Earth is pulling you toward it (keeping you firmly lodged on the ground), your center of mass is pulling back at the Earth, albeit with much less force.
Sir Isaac Newton quantified the gravity between two objects when he formulated his three laws of motion.
Yet Newton’s laws assume that gravity is an innate force of an object that can act over a distance.
Albert Einstein, in his theory of special relativity, determined that the laws of physics are the same for all non-accelerating observers, and he showed that the speed of light within a vacuum is the same no matter the speed at which an observer travels.
As a result, he found that space and time were interwoven into a single continuum known as space-time.
Events that occur at the same time for one observer could occur at different times for another.
As he worked out the equations for his general theory of relativity, Einstein realized that massive objects caused a distortion in space-time.
Imagine setting a large body in the center of a trampoline.
The body would press down into the fabric, causing it to dimple.
– “A marble rolled around the edge would spiral inward toward the body, pulled in much the same way that the gravity of a planet pulls at rocks in space.”
Although instruments can neither see nor measure space-time, several of the phenomena predicted by its warping have been confirmed.
Gravitational lensing: Light around a massive object, such as a black hole, is bent, causing it to act as a lens for the things that lay behind it.
Astronomers routinely use this method to study stars and galaxies behind massive objects.
Einstein’s Cross, a quasar in the Pegasus constellation, is an excellent example of gravitational lensing.
The quasar is about 8 billion light-years from Earth, and sits behind a galaxy that is 400 million light-years away.
Four images of the quasar appear around the galaxy because the intense gravity of the galaxy bends the light coming from the quasar.
Changes in the orbit of Mercury: The orbit of Mercury is shifting very gradually over time, due to the curvature of spacetime around the massive sun.
In a few billion years, it could even collide with the Earth.
Frame-dragging of space-time around rotating bodies: The spin of a heavy object, such as Earth, should twist and distort the space-time around it.
In 2004, NASA launched the Gravity Probe B.
The precisely calibrated satellite caused the axes of gyroscopes inside to drift very slightly over time, a result that coincided with Einstein’s theory.
Gravitational redshift: The electromagnetic radiation of an object is stretched out slightly inside a gravitational field.
Think of the sound waves that emanate from a siren on an emergency vehicle; as the vehicle moves toward an observer, sound waves are compressed, but as it moves away, they are stretched out, or redshifted.
Known as the Doppler Effect, the same phenomena occurs with waves of light at all frequencies.
In 1959, two physicists, Robert Pound and Glen Rebka, shot gamma rays of radioactive iron up the side of a tower at Harvard University and found them to be minutely less than their natural frequency due to distortions caused by gravity.
Gravitational waves: Violent events, such as the collision of two black holes, are thought to be able to create ripples in spacetime known as gravitational waves.
The Laser Interferometer Gravitational Wave Observatory is presently searching for the first signs of these tell-tale indicators.