Albert Einstein 1879-1955
courtesy http://imagine.gsfc.nasa.gov/Images/people/Einstein.gif
As a student I came to appreciate the works of Einstein. About a hundred years ago, there were several dilemmas in physics which Einstein played a role in resolving. The thing that impressed me the most were the range of disciplines Einstein was adept at, namely thermodynamics, atomic physics, Newtonian mechanics, and optics. In each field, Einstein made major contributions. He won the Nobel Prize in 1921 for explaining the photoelectric effect ( I generally offer my in-class students a buck if they can even tell me what that is ... sorry on-line students ... it would be too easy with a computer).
Almost everyone can recognize an image of Einstein, and most everyone knows that he developed the Theory of Relativity. But most people don't have a clue what that means or can describe the general concepts central to this theory. Let me give you the short version.
Einstein realized there was a problem with classic Newtonian mechanics under certain extreme conditions. Rather than throw out over 200 years of established physics, he "tweaked" the model so it would work under "normal" conditions (described by Newton) as well as these extreme conditions (things that moved very fast).
Not only does time get distorted with speed, so too, are our concepts of distance (length) as well as mass. That is, a meter stick will seem shorter to you if it zooms past you at nearly light speeds ... and it will seem to have more mass! You may be wondering what led Einstein to come up with these ideas. It all came from an earlier observation that the speed of light coming from all stars measured the same ... regardless how the stars were moving. This "conflict" was resolved by Einstein because he understood that speed involves both distance and time. If both are distorted (in the way stated above), he was able to show that the measured speed of light will always be constant (no matter how the source is moving). Maybe the oddest part of this whole story is that he did all this during his breaks while working at a Swiss patent office.
One of the predictions to come out of this theory is that energy and matter are interchangeable. Einstein is telling us that you can change one for the other and this is the conversion factor - E = Mc2 . This equation has been applied in atomic and hydrogen bombs as well as nuclear power plants. You will see later that it also applies to our sun, all the stars and also plays a critical role at the beginning of the universe.
Einstein's insight led him to think about gravity. In his general theory of relativity, published in 1915, Einstein compares the effects of gravity with a person riding in an accelerating spaceship (in empty space). That is, if you stand on a scale at the surface of Earth, the reading indicates the pull of gravity between you and Earth. You could get the same scale reading if the person were standing on a scale inside a spaceship which is accelerating at 9.8 m/s2. In his "principle of equivalence", Einstein states that the effects of acceleration are indistinguishable from the effects of gravity. For example, if you stand on the earth's surface and drop a ball, you observe the ball accelerate toward the ground at 9.8 m/s2 .... but if you release the same ball from your hand within your accelerating spaceship (which is accelerating at 9.8 m/s2), it will appear to accelerate toward your feet as if it were falling in the earth's gravity.
Einstein's principle of equivalence (animation)
This comparison led Einstein to some pretty bizarre realizations about the nature of gravity ... namely:
These concepts are certainly mind boggling and can be summarized in one simple statement ... matter "curves" space, and curved space "tells" matter how to move. In the Newtonian view, there are invisible forces between any two masses ... which allows one object to influence the motion of the other object. In the world of Einstein, each mass "warps" the space around itself ... so when one object gets close to another, it moves according to the rules of the "warped" space it encounters.
About the only analogy I can offer which might give you a clue how this works can be found at almost any shopping mall. Kids are fascinated by the device which lets you roll a coin down a ramp and into a curved surface. The coin rolls in an elliptical path which gradually gets smaller and smaller until it falls into a "hole". The coin has no "free will" to choose a path to travel. It is dictated entirely by the surface which is laid out in advance. Each coin rolled behaves the same way because the "warped" surface doesn't change. If the surface changed, so would the path followed by the coin. In the Einstein model, that can be accomplished by changing the mass of the central object.
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It is fun to watch your coins roll into the "black hole" at the mall .... |
... until all your money is gone! |
Of course, no theory can be proven ... you can only find more and more evidence to support it. Within Einstein's lifetime, two strong predictions of this theory were tested. The first was in 1919 when a solar eclipse made stars visible during the daytime.
In the diagram, consider two stars, labeled "A " and "B" which are both located near the limb of the sun during the eclipse. If a telescope were pointed near the limb of the sun you would expect to see star "A" if space was not warped. But Einstein predicted you would see star "B" through the telescope because a beam of light would be deflected slightly as it moves though the curvature of space (produced by the mass of the sun). When his prediction was realized, Einstein became an instant celebrity.
Another triumph of Einstein's theory came when he was able to apply it to the orbit of Mercury. Astronomers had noticed that the perihelion point was shifting slightly over time but could not explain why. It was believed that some unknown planet was producing these perturbations. Einstein applied the concept of "warped" space and was able to account for the observed motion. Einstein's theory has passed all tests as of this writing.
The idea that a beam of light bends in the presence of a strong gravitational field (or where space is highly "warped") becomes important when we discuss the concept of a black hole.
I made a short video of a PowerPoint presentation about this subject - click here
The idea that light is deflected in the presence of massive objects (strong gravitational field) has proven to be a useful tool for astronomers. It allows astronomers to detect objects which give off little or no radiation on their own. Rather, the mass of the object acts like a lens to amplify the light of a bright object (quasar or bright star) that just happens to lie directly behind it.
Light from a distant, bright, background source (like a quasar)
Just as a lens can refract light to a central point, unseen massive objects can "warp" beams of light in the same way toward the Earth. This effect has been observed in several instances. Please click here for a beautiful example. This phenomena has given astronomers clues that there is much more mass in this universe than we presently see. We will discuss dark matter later in this project.
Hubble took this "smiley face" picture which is a great example of the gravitational lens effect.
Microlensing is a relatively new term which uses this principle to detect objects with much smaller masses, such as planets, brown dwarfs, and white dwarfs. A few dozen extrasolar (exoplanets) planets have already been detected using this technique. Incredibly, astronomers in 2018 announced the discovery of planets found in distant galaxies using this technique (something I thought I would never see in my lifetime).
This is how "gravitational microlensing" can discover if something (labeled closer object) is out there.
One Crazy Idea
Our own sun's gravity is currently acting like a giant lens and focusing light from extremely distant objects. It turns out, the focal point is about 550 AU from the sun (to put this in perspective, Pluto is about 40 AU from the sun). If we could place a spacecraft at the SGL (Solar Gravitational Lens), we could obtain highly magnified images of the source. Imagine being able to view planets around other stars in such detail that you can make out surface features!!! We are decades away from implementing this crazy idea but I'm sure this will be done before we actually travel to these distant worlds (which would take lifetimes to travel to). Read more here.
Allow me to pose a question. What would happen if the sun were to suddenly disappear (don't worry ... it can't happen)? Light takes about 500 seconds to travel a distance of 1 AU ... so we would not go "dark" for another 8+ minutes. But what about the gravitational effects of the sun? Would we leave our orbit the moment the sun disappeared or would we fly off our orbit 500 seconds later? Newton would say instantly ... but Einstein would predict we would have 8+ minutes before Earth moves away from our orbit. Einstein says the effects of gravity move through space at the same speed as light (c = 186,282 miles/second). In other words. the effects of gravity would move like a wave through the fabric of space-time at the speed of light. Do gravitational waves exist? Since there is no way to make the sun (or any other huge mass) magically disappear to observe this effect, it becomes difficult to observe the possible existence of gravitational waves (and thus, further verification of Einstein's Theory of Relativity). But it is not for a lack of trying. Several instruments (including LIGO) have been searching for ripples of gravitational waves that pass by ... with no success .... until 2015. A small compression wave of gravity was detected in 2015 (and announced in 2016) from a pretty spectacular astronomical event, the merger of two black holes in a very distant galaxy. Gravitational waves exist! Good news, the number of confirmed gravitational waves keeps going up. I will stop counting now at three confirmed events. This is another feather in Einstein's cap.
ŠJim Mihal 2004, 2014, 2018, 2023 - all rights reserved
