A Variety of Notes

Relativity and the Absolute

• Electricity and magnetism were once thought to be distinct but are now known to be part of the same field, the electromagnetic, the two affect each other
• This field travels in waves at the speed of light which means light is an electromagnetic wave
• Luminiferous aether was proposed as a substance through which light travels so that the speed of light could be relative to something (that something being the aether)
• But experiments show that the speed of light is the same whether it is traveling towards or away
• Einstein declared there is no aether and “that the speed of light is 670 million miles per hour relative to anything and everything”
• This can only be explained by saying that observers moving relative to each other will not have the same observations for distance and time
• No matter how fast one moves, light will always appear to travel 670 million mph
• Stationary objects travel through time but when they begin to move in space some of that movement in time is diverted to movement in space which means time slows down for that object (my though: wow!)
• The combined speed of an object moving through time and space is equal to the speed of light
• This means time stops at the speed of light!
• Einstein and other claimed you would still feel spinning in an empty universe, even in relativity some things are absolute
• A region of spacetime is all things that occur in a region of space over a period of time
• Relativity of simultaneity – an observer moving relative to another will say that events are not simultaneous although the second observer declares they are
• Depending on the motion and point-of-view things appear to happen at different times and both orders are correct
• The observer in relative motion views events as if on an angled page which angle can be no more than 45 degrees because of the limit set by the speed of light
• If Newton had been correct about absolute space observers would always agree about when an event occurs
• Not everything in relativity is relative, absolute spacetime replaces Newton’s absolute space
• Objects traveling in a straight line through spacetime will be seen as such by all observers though they may disagree on how much time passes or distance is covered in each segment of spacetime
• In special relativity nothing travels faster than light, but gravity appears to be instantaneous so Einstein had to reconcile the two which he did with “general relativity”
• At a constant velocity you would feel no motion; in accelerated motion you feel pressure, as if you are being forced into something
• The force felt from gravity and acceleration are the same, this is the “principle of equivalence”
• Therefor, if you feel gravity’s influence you are accelerating; those in free fall are not accelerating, they don’t feel gravity
• Matter in space (planets, stars) warps gravity which changes the path of objects moving through space like a marble on a warped floor
• Einstein derived that gravity travels at exactly the speed of light, not instantaneously
• Gravity felt by us comes from all matter in the universe no matter how distant (unless it is so distant that the light of it hasn’t yet reached us)
• According to general relativity, even in an empty universe the water in the bucket (ch. 2) spins and moves outward so Mach was incorrect

(I apologize if some of these notes don’t appear to make sense, some are dependent on having read the book and being able to look at Brian Greene’s diagrams)