tag:blogger.com,1999:blog-37936507.post2891987746580545696..comments2023-08-08T10:25:47.529+01:00Comments on McCabism: Can anything move faster than the speed of light?Gordon McCabehttp://www.blogger.com/profile/09151162643523937086noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-37936507.post-29352058483704140422007-03-27T16:55:00.000+01:002007-03-27T16:55:00.000+01:00You're on the right path, Duck, (if you'll excuse ...You're on the right path, Duck, (if you'll excuse the pun). A geodesic is a curve of zero acceleration. Acceleratory motion means non-geodesical motion. The astronaut jetting back and forth to Alpha Centauri has to engage in heavy acceleration and deceleration at both ends of his journey, and it is this which causes the discrepancy in personal time, relative to the other astronaut, who is, say, in a free fall orbit around the Earth. It's not speed which makes the difference, but acceleration.Gordon McCabehttps://www.blogger.com/profile/09151162643523937086noreply@blogger.comtag:blogger.com,1999:blog-37936507.post-23138885218989430092007-03-27T15:36:00.000+01:002007-03-27T15:36:00.000+01:00So time dilation has nothing to do with speed, but...So time dilation has nothing to do with speed, but with motion relative to a geodesic? That's not how it is popularly described.<BR/><BR/>Please indulge my ignorance a little longer, because I still don't grasp it yet. I was under the understanding that free-fall was the equivalent of zero gravity, or zero acceleration. So a person standing on the Earth's surface, since he is experiencing an acceleration of 1g, and therefore is not in free fall. However a person in space in orbit around Earth, or in an unpowered trajectory toward some destination, no matter how fast he is going relative to the Earth, is in a zero-g environment, and therefore in free fall. Is this not correct?<BR/><BR/>And how does one measure one's movement relative to a geodesic of space-time, if not by acceleration?Duckhttps://www.blogger.com/profile/08852569465893563139noreply@blogger.comtag:blogger.com,1999:blog-37936507.post-82055140424995096472007-03-26T20:35:00.000+01:002007-03-26T20:35:00.000+01:00Zero-mass objects follow curves in space-time call...Zero-mass objects follow curves in space-time called geodesics, and a massive object changes the geometry around it, including the geodesics. Geodesics are the generalisations to curved geometries of straight lines in Euclidean geometry.<BR/><BR/>In the case of the so-called 'twin paradox', the situation isn't symmetrical. One astronaut (twin) is in free-fall, following a geodesic of space-time, whilst the other one is jetting back and forth, following non-geodesic curves, and expending less personal time.Gordon McCabehttps://www.blogger.com/profile/09151162643523937086noreply@blogger.comtag:blogger.com,1999:blog-37936507.post-78699247335381717972007-03-26T17:46:00.000+01:002007-03-26T17:46:00.000+01:00OK, this is where I get confused. If photons have...OK, this is where I get confused. If photons have zero mass, why is their trajectory curved around massive objects? There should be no gravitational attraction at all.<BR/><BR/>Here's another question I've had about relativity for a long time. It has to do with the time dilation effect for objects moving very fast. An astronaut in a spaceship going some fraction of the speed of light will age more slowly than an astronaut on Earth. But their motions are relative to each other, not absolute, correct? The astronaut on Earth is travelling just as fast relative to the astronaut in the spaceship as that astronaut is traveling relative to the one on Earth. So why is the astronaut on the spaceship aging slower?Duckhttps://www.blogger.com/profile/08852569465893563139noreply@blogger.com