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The two postulates and the special theory of relativity
Transcript of The two postulates and the special theory of relativity
special theory of relativity Jane Clements An inertial frame of reference Light speed constancy Galilean relativity To illustrate: Constancy of light speed How do we know it’s constant? Contradiction? There’s no such thing as an objective state of rest
(for example, we’re all on a planet that spins on its
axis and moves around the sun).
An inertial frame of reference is one moving at a constant velocity – in a straight line with no acceleration.
From an inertial reference point all movement outside of that frame of reference is relative to it. The laws of physics Even though there are different frames of reference the laws of physics apply equally in all frames of reference.
Therefore, a person who propels an object in any direction from a vehicle moving through a vacuum at a constant velocity will measure the velocity of the object without reference to the velocity of the vehicle.
A person outside the frame of reference of the
moving vehicle will measure the velocity
of the object also in relation to the
speed of the vehicle. When moving through a vacuum, the speed of
light is measured at a constant speed of
300,000 km per second, regardless of one’s
frame of reference.
In other words, light emitted from a moving
vehicle is measured at the same speed by a
person on that vehicle and by a person
outside it. The speed of the moving vehicle
is neither added nor subtracted. James Clark Maxwell’s equations predicted a constant speed
of light, but because light was known to be a wave and waves require a medium to travel through, it was believed that the equations only applied within a certain reference frame – an electromagnetic ether. As light travels through the vacuum of space it was believed that this ether must be omnipresent in the universe.
Michelson and Morley’s experiment demonstrated there is no ether. (Later, light was shown to be a particle as well as a wave.) So Einstein theorised that Maxwell’s equations (demonstrating the constancy of the speed of light) were true for ALL observers in any frame of reference.
Although the speed of light can be slowed when travelling
through certain mediums (this is because it is first absorbed by
an obstruction and then continues travelling) it has never
been shown to travel faster than 300,000km per second,
regardless of the speed of the object that emits
the light, from any frame of reference. How can both of these postulates be true?
They seem to be contradictory, and certainly most physicists at the turn of the twentieth century thought so.
So which postulate is true? Creative assignment for Understanding Einstein: The Special Theory of Relativity
a Coursera course conducted by Dr Larry Randles Lagerstrom A person on a moving vehicle propels an object at speed in the direction of travel –
The person on the moving vehicle – ONLY measures the speed of the object
A stationary observer measures the speed of the object PLUS the speed of the moving vehicle
A person on a moving vehicle propels an object at speed in the opposite direction of travel –
The person on the moving vehicle – ONLY measures the speed of the object
A stationary observer measures the speed of the object MINUS the speed of the moving vehicle How Galilean relativity measures velocity Proof it works: Consequences of Einstein’s theory: Simultaneity is relative How Einstein discovered the special theory of relativity Other consequences of Einstein’s theory: E=MC2.
Energy and mass are inter-related – in fact they are equivalent. They are also affected by the speed of light. To propel an object close to the speed of light takes an enormous amount of energy and at the same increases the mass of the object which requires more energy to move it. The closer the object gets to the speed of light, the closer its mass reaches to infinity, requiring an almost infinite amount of energy to propel it any faster. Thus it is not possible for any object with mass to move as fast as the speed of light.
Thanks for watching And thanks to Dr Larry Randles Lagerstrom whose patient teaching FINALLY allowed me to reach this basic level of understanding. Intuitively it seems that these two postulates cannot both be correct. Einstein did not believe they were contradictory, however, and thought that both could be true at the same time.
He formulated a theory that outlined the logical consequences if both these postulates were true – and that theory is the special theory of relativity.
It’s ‘special’ because it only applies when we are looking at an inertial frame of reference. His general theory of relativity applies to any frame, and includes the effects of gravity.
The special theory of relativity may seem unintuitive – it certainly has some counter-intuitive logical consequences – but it makes some of our modern technology possible, like our GPS navigation and DVD players.
So, thanks Einstein! Einstein’s special theory of relativity Time slows down (dilates) relative to another frame of reference when a vehicle moves away from it at very fast speed.
Objects become shorter (contract) when measured relative to another frame of reference, when moving away at very fast speed. Other consequences of Einstein’s theory: The special theory of relativity and the laws of physics The special theory of relativity shows that faster-than-light (FTL) travel is not possible. This leaves the laws of physics as we currently understand them intact – effect cannot precede cause. If FTL travel were possible effect could precede cause. In our everyday experience we never experience that phenomenon.
However, the theory does have some counter-intuitive outcomes like time dilation and object length contraction, and these are not phenomena we encounter in our everyday experience either. The laws of physics can therefore be adjusted and understood even though they are outside of our everyday experience.
I therefore cling to the dream that FTL travel could one day be a reality! References Youtube.com – Galilean relativity by Luc Tremblay; Einstein and The Special Theory of Relativity by minutephysics; Simultaneity - Albert Einstein and the Theory of Relativity by MyEarbot; Mythbusters – soccer ball shot from truck by Frank Noschese.
Larry Randles Lagerstrom, Lectures and handouts from Understanding Einstein: The Special Theory of Relativity, a Coursera course.
Wikipedia, Special relativity, en.wikipedia.org/wiki/Special_relativity
University of NSW, Einstein Light www.phys.unsw.edu.au/einsteinlight/