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In this video, Dan Burns, a physics teacher, shows other teachers how to use a large lycra sheet to demonstrate how a mass produces a gravitational field around it, and how objects move in this field.
If you want to build one of these space-time sheets, learn how here: https://youtu.be/2JOf1ub9US0
Be the first kid on your block to warp space-time in your basement.
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What is a field?
The most successful theory of gravity describes it as a field, not like a corn field, but like a magnetic or electric field. These fields are called vector fields. What is a vector? What is a field?
Mr. Anderson can help.
(Believe me, you need to get the gist of this to appreciate some cool stuff that is coming below on this page.)
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The presence of a field means the possibility of waves. Einstein's general theory of relativity predicts gravitational waves, traveling ripples in the fabric of space-time. It took more than one hundred years for scientists to find a way to detect them, and thus to confirm yet another prediction of relativity theory.
What !? No light at the detector?
Hey, wait a minute! Waves are energy, right? Energy is conserved, right? How can two waves combine to produce no wave at all !? How can two bundles of energy meet and then there is no energy at all?
Truth is, photons of light are quanta, and quanta are wierd. They are neither wave nor particle, although they have properties of both (momentum, interference), plus some strange properties of their own (entanglement). The notion of a wave split in two and recombined is a gross oversimplification of what is going on in an interferometer or any other event involving paths of quanta (which don't have paths anyway, but don't get me started on that).
One way to calculate the outcome of events involving quanta is to assume that quanta travel every possible path, but interference among paths cancels most of them out, leaving only one or a few where quanta could be detected. In the interferometer, we are looking at one path only, and that's the path that will tell us if the instrument is being distorted by a passing gravitational wave. If path lengths are adjusted so that interference is causing no energy to arrive at the detector (as it is when we are waiting for a wave), then it is merely causing the energy to go somewhere else, where we don't happen to be looking (because it wouldn't tell us anything that its non-arrival at the detector doesn't already say).
Quanta are weird (is there an echo in here?); there's no way around it. But they obey conservation of energy, just like everything else in the universe. Wave interference makes it impossible for the detector to catch that energy, because the laser's energy is going somewhere else, until that gravity wave comes along.
Here's another way to look at waves and interference, and it comes closer to showing you what I mean when I say that quanta take all possible paths, but some cancel out and no energy is detected along those paths.
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What can gravitational waves tell us?
We answer this question for electromagnetic radiation (EMR) by the familiar annotated chart of the electromagnetic spectrum:
The electromagnetic spectrum
Can scientists do the same for the gravitational waves?
Yes, they can. See what information you can glean from the following chart, which is the gravitational-wave spectrum. The top rows show you sources of gravitational waves; that is, is shows what kinds of events produce the waves. Above and below the (false-colored) spectrum, are the wavelengths (in years, a time axis, not a length axis), and frequencies (in cycles per second). The bottom row shows the kinds of detectors needed to measure each frequency. All these types of detectors are in currently in operation or development. The LIGOs in the states of Louisiana and Washington and Virgo in Italy are the current terrestrial interferometers.
The gravitational-wave spectrum
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Relativity theory is poorly named. Uninformed people think that the theory simply says that everything is relative. It's a lot more specific than that, and says much more surprising things than simply that things look different from different points of view, and that motion must be described relative to the specified position and motion (called a frame of reference) of an observer. Copernicus knew that. Galileo knew that. Newton knew that. What Einstein added is that time looks different from different frames of reference. What is startling about Einsteins theories of relativity (called the special theory and the general theory) is that he was saying time is relative. The theories of relativity are about the relativity of time.
Nobody knew that until Einstein came along.
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How do we know that a large mass like the sun changes the shape of space around it?
Einstein's general theory of relativity suggests that light is subject to gravity (what!?), and that clocks run slower in stronger gravitational fields.
Of the two star images in each pair, which one is the apparent position during the eclipse, and which one was taken at night when the sun was not along the telescope's line of sight? Explain your choice.
Two Strange Questions About Relativity
These questions have to do with Einstein's special (not general) theory of relativity, which suggests that clocks do not run at the same speed for all observers.
1) How fast do we travel in time? Ordinary objects, like ourselves, cannot travel through space faster than the speed of light (c, or 3.00x10^8 m/s). If we live in a 4-dimensional (4D) space-time, with three spatial dimensions (x, y, z) and one time dimension (t), how fast do we travel in time when we are at rest (with respect to some observer) in space?
provocative answer coming
2) How fast does light travel through time? It always travels at c in space, but what about its velocity in space-time?
provocative answer HERE.
3) Why do clocks on a moving object run slower when seen by a stationary observer?
provocative answer coming
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Finally, this video will either help or just completely mess up your mind. Either way -- have fun.
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