On Sunday, February 17, 2019 at 2:26:12 PM UTC-7, John Clark wrote: > > On Sat, Feb 16, 2019 at 7:07 PM <[email protected] <javascript:>> wrote: > > *1) Using the EP and the example of an accelerating elevator, it follows >> that light takes a curved path in space (not space-time). * >> > > No, it's spacetime. If a photon of light has moved from one side of an > elevator to the other then it has moved in BOTH space and time because, > although it's the fastest thing there is, light does not move at infinite > speed. Light, just like everything else, always needs time to move through > space. You can't change your position in space without changing your > position in time. >
*Sure, but why does this obvious fact force us to merge space and time in one concept, aka a manifold? Also, why is it that Newton's law of gravity is not Lorentz invariant, yet it seems to work in all inertial frames? TIA, AG * > > > > Wasn't this known by virtue of Newtonian gravity? >> > > That depends on if light had mass or not; if it didn't, and there was no > experimental evidence to indicate that it does, then Newton would say light > wouldn't curve at all near the sun, if light does have a rest mass but was > just too small to be detected then Newton would say light would curve but > only half as much as Einstein said it would. But to Einstein it doesn't > make any difference if it has a rest mass or not light must must curve in a > gravitational field. So no curvature or slight curvature of light by the > sun would be consistent with Newton but only large curvature was consistent > with Einstein. And large curvature was exactly what was found in the > eclipse of 1918. So Einstein won and Newton lost. > > > 2) Assuming a geodesic is the shortest distance between two *spatial* >> points on a curved surface, does it follow from the EP that free falling >> bodies move on geodesics, and if if so how? >> > > Yes Einstein says everything is always following a geodesic path through > spacetime unless it is acted on by a force, and to Einstein gravity is not > considered a force. > *So how does GR explain motion? That is, how does curvature of space-time result in motion? AG* > So if you jumped out the window you'd follow a geodesic path through > spacetime but just standing on the floor you are not because the floor is > exerting a upward force on your feet. If spacetime were flat that force > would let you to float off the ground but at the surface of the Earth > Spacetime is curved so you can't, and we call that spacetime curvature > "gravity". > > It takes light 1/19,184,132 of a second to move 60 feet 6 inches from > pitcher's mound to home plate on a baseball field, Earth's gravity is 32 > feet per second per second so in that time something near the Earth surface > would drop by D=1/2 *AT^2 =16*(1/19,184,132)^2= 2.7*10^-15 inches, that is > the amount the spacetime curvature a baseball field deviates from perfect > flatness, it's about the same amount of curvature as a sphere with a > radius of one light year would have. That's pretty flat but if it were > absolutely flat baseball would be a VERY different game. > *What would baseball look like without that tiny curvature? AG * > > *3) Concerning the above questions, how does "space-time" enter the >> picture since it seems the questions can be asked without referencing >> space-time. * >> > > > As Einstein's teacher Hermann Minkowski said about his former student's > theory: > > "*Henceforth space by itself, and time by itself, are doomed to fade away > into mere shadows, and only a kind of union of the two will preserve an > independent reality*." > > John K Clark > > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
