Transcript Measurement , Relativity, Einstein and Everything Else
Measurement, Relativity, Einstein and Everything Else
Michael Bass Professor Emeritus College of Optics and Photonics, CREOL University of Central Florida © M. Bass
The essence of science
Science differs from religion in that science deals with what can be measured/quantified. This means that science requires standards of measurement.
In other words standard units that we can all agree upon.
It means that if two different labs make a measurement of some quantity they can trust that they are using the same yardstick, clock or mass.
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Length, time, and mass
All our units of measurement derive from just these three: Length – the meter Time – the second Mass – the kilogram All others are derived units.
In fact, many theories in physics are set up so that the units are correct – both sides of the equations must have the same units!
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Newton’s Mechanics
You all remember
F
= m
a
(or more correctly
F =
m(d
v
/dt)) Here Newton connected the response of an object having mass, m now measured in kg, to the application of a force,
F
, – it would change its velocity,
v
, and accelerate at
a
measured in meters per second per second.
In fact, what Newton did here was define the unit of force now called the Newton and it is kg m/s 2 .
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Newton at a deeper level
Newton’s concept was that there was some
universal reference frame against which all measurements could be referenced
.
It came to be known as the ether – a massless, frictionless, undistortable coordinate system through which all things moved.
His mechanics and all that followed from them (thermodynamics, the industrial revolution, the ascendance of the west, the modern world and etc.) seemed to work with this assumption.
However,
it proved to be wrong
.
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Electromagnetics
By the end of the 19 th century the work of Michael Faraday, James Clerk Maxwell, Carl Freidrich Gauss, Guglielmo Marconi and many others established that Electromagnetic waves (everything from radio waves to visible light to gamma rays) were transverse waves traveling at the speed of light.
They carried energy and so could be useful.
Could they be used to measure the earth’s motion through the ether?
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Enter Albert Michelson
Michelson was a Professor of Physics at Case University in Cleveland. He invented an optical device (!!) – a Michelson interferometer with which he proposed to determine the earth’s motion through the ether.
mirror Half silvered mirror Monochromatic light source mirror Here there be fringes!!!
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The Michelson-Morely Experiment
Float a Michelson interferometer in a pool of mercury and hope you don’t go mad.
Point one arm in the direction of the earth’s movement around the sun.
While watching the fringes determine any change as the interferometer is rotated so the other arm points in that direction.
The change in the fringes would be a measure of the earth’s speed as it altered the light speed in the arms of the interferometer.
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The result
No change in the fringes.
Either the earth was not moving or the speed of light did not change according to the motion of the light source.
The first could be ruled out so the second had to be true.
But this violated the certainty of Newton’s assumption of a universal reference frame.
Light traveled at a speed independent of motion through the reference frame so why did we need such an all encompassing assumption?
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Attempts to “save the model”
Lorentz in Holland and Fitzgerald in Ireland suggested that perhaps the arm in the direction of motion contracted direction.
– got shorter – so that the light actually only had to travel a shorter distance in this Except in Star Trek and Star Wars this contraction actually does occur when an object’s speed approaches that of light.
Just proposing the Lorentz transformation rules as he did, did nothing to deal with the fundamental problem There seemed to be no universal reference frame!!
Why – what did it mean – was the universe so unlike our expectations?
As speed approaches that of light you get squeezed and become more massive. You also age more slowly.
Slow, fast, faster, very fast © M. Bass
Enter Albert Einstein (1879–1955)
In 1905 in Annalen der Physik there appeared 4 papers by an obscure patent clerk in Bern, Switzerland:
One explained Brownian motion of particles.
One explained the photo-electric effect.
One explained special relativity.
One explained mass-energy equivalence.
For any one Einstein could have won a Nobel Prize The paper on special relativity however was a major turning point in the history of physics and in the story of our understanding of the world and the universe Ever since we have to say
“we live in a Newtonian world embedded in an Einsteinian universe.”
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Albert and Mileva
She was the only female physics student in his class.
She was bright and interesting but Did not pass her Ph. D. exams They married, had two children and drifted apart.
When he went to Berlin he struck up a relationship with Elsa, his cousin, and his marriage got worse.
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The theory
Assumptions:
There is no absolute reference frame.
All observers in their own reference frames moving with respect to each other must arrive at the same laws of physics.
Specifically, each observer must measure 2.997924589 x 10 8 m/s for the speed of light in a vacuum – a universal constant. By assuming the speed of light to be absolute and unvarying Einstein was forced to conclude that
space and time are relative.
Einstein claimed not to know of the Michelson-Morely result and that he just felt the universe must be this way. Since Michelson became the first American to win a Nobel prize in 1907 this claim seems unlikely.
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The results
From these simple assumptions he showed that
length in the direction of motion got shorter
time slowed down as you moved near the speed of light, mass and energy were related by the famous equation, E = mc 2 , Now mass-energy and not just mass or energy had to be conserved (a re-write of the first law of thermodynamics), and
you could never travel faster than c
His predictions have been tested experimentally and they have never failed (except in science fiction) Ever since Einstein all physical theories that would be of any use had to be relativistically invariant. © M. Bass
Einstein’s critical thought experiment
Suppose I were on the rear platform of a train looking at a clock in the train station. While the train was still my watch and the clock would measure the same time intervals .
If my train moved away from the clock at some speed then the light from the clock traveling at speed c would take time to reach me .
But the light from my watch would not take any time so I would think the clock at the station was running slower than my watch .
If my train were moving at speed c was at the moment of departure. the light leaving the clock after I departed would never catch up. I would always see the clock as it I would think the clock had stopped.
© M. Bass
Simple and Beautiful
It is worth noting that Einstein’s concept of a simple and beautiful set of laws to govern the universe might pass for certitude.
It actually leads to a 20th century of change and loss of causality. Einstein for contributing to both relativity and quantum mechanics was both loved and reviled, as much for disturbing people’s peace of mind as for his brilliant contributions. © M. Bass
General Relativity
After 1905 he spent time in Prague and then back to Switzerland. Then, Max Planck recruited him.
Einstein in 1914 moved to Berlin to the prestigious position of Professor of Theoretical Physics at the University of Berlin.
In 1916, at the height of World War I he published his theory of General Relativity – relativity when in accelerating reference frames.
Gravity would bend light!!
In 1919, Sir Arthur Eddington measured the gravitational bending of light exactly as Einstein had predicted it.
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Albert and Elsa
In Berlin he re-connected with Elsa.
She was the kind of supportive and stable wifely woman he needed.
He dictated to Mileva what she had to do to be with him (for example, not to speak to him without being spoken to first).
Finally, a divorce was arranged when he promised Mileva his Nobel Prize money (which he had not yet won).
She returned to and lived in Bern with their children.
© M. Bass
Gravity bends light
Distant star appears at some location in space.
sun Moon eclipses the sun Eddington Earth Distant star appears shifted in location due to bending of light by the sun’s gravity.
Eddington – wow!! Albert was right.
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Black holes
If gravity can bend light then a very large gravitational field can bend light so much that it can not escape – this is a black hole.
We have found black holes including a multi million solar mass black hole at the center of our galaxy Masses or light that cross the boundary get sucked in. There is no escape!!
Masses that pass by have their trajectories altered.
Light that passes close by just gets bent.
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Bending leads to lensing and other funny stuff
Multiple images Einstein rings Measures of distance This really happens!!
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Expanding universes
Our universe is not static – it is expanding and has been since it started about 13.5 billion years ago.
At the time of creation gravity and the other forces were unified into one force.
Almost immediately gravity separated from the others and today there seems to be only 4 fundamental forces in the universe The strong nuclear force The weak nuclear force The electromagnetic force Gravity © M. Bass
Gravity
All but gravity have been quantized so that they explain with incredible accuracy the observed universe.
Gravity is eloquently explained by Einstein’s General Theory of Relativity and defies quantization.
We just don’t know what happens inside a black hole where the force of gravity crushes matter into a singularity of infinite density.
But that was the universe at t = 0.
Everything, all the mass-energy of the universe was confined to a tiny volume.
It was very dense and very hot © M. Bass
So what happened
There must be another force,
the force of the big bang, that could overcome gravity and create a universe that could eventually contain critters that could wonder how it all got started.
Sometimes called DARK ENERGY it may be why the universal expansion seems to be accelerating.
Einstein started the process but it is far from over and new observations such as an acceleration of the expansion must be taken into account. This is the excitement of science and the demand for the scientist to continue to pursue the truth.
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Beyond science
Einstein during his years in Prague became a pacifist.
This was a problem for him while in wartime Germany.
After WW I his fame led him to other interests in particular, to be a voice for other Jews who were being discriminated against and persecuted in Germany.
1932 he moved to the United States.
Institute for Advanced Studies at Princeton He defected from pacifism in the face of Nazi criminality and, in 1940
signed the letter to Roosevelt that led to the Manhattan Project and the nuclear weapons program
.
Interestingly enough, he never took part in the project.
In 1948 he was offered the Presidency of the new state of Israel but had the good sense to turn it down.
Died in 1955 without ever achieving a unified field theory since he was unwilling to apply quantum thinking to gravity.
To date no one else has come even close.
© M. Bass