On 2/2/2014 12:44 PM, [email protected] wrote:
On Sunday, February 2, 2014 3:45:24 PM UTC, jessem wrote:
On Sun, Feb 2, 2014 at 7:13 AM, <[email protected] <javascript:>> wrote:
Jesse - if the assumption is a fundamental geometry akin to the surface
of a
world, and if the speed of light is constant, then you could draw dots
around
that world for exact intervals of the speed of light, in which case the
light
arrives at each point from each other point at exactly the same
moment....isn't
that saying edgar's p-time?
I don't understand what you mean by "dots around the world for exact
intervals of
the speed of light"--in terms of a standard spacetime diagram from SR, what
would
the dots be? Different events along the path of a single light ray, or
events along
the paths of multiple light rays radiating from some other event, or
something else?
It is true that the spacetime "distance" between any two points along a
light-like
path is zero, if that's what you mean, but I don't see the connection
between this
observation and the idea that the light arrives "at exactly the same
moment", I'm
not sure how you're defining that phrase. "Same moment" normally suggests a
judgment
about simultaneity, not a judgment about the proper time along a particular
path
between events. Also, note that by means of a zig-zag lightlike path (like
that of a
light ray bouncing repeatedly between mirrors), you can connect *any* two
points in
spacetime that are within one another's light cones by a path of zero proper
time--for example, there is a path of zero proper time between the
assassination of
Julius Caesar and me sitting here typing this. So if you were to define events
"at
exactly the same moment" in terms of the existence of a path of zero proper
time
connecting the events, you'd have to say that all events throughout history
occurred
"at exactly the same moment" which is pretty clearly not how it works with
Edgar's
p-time.
Jesse
Hi Jess/Brent - thanks for getting back,
Perhaps I should backtrack to my first reaction reading your post. First off, I
definitely am not at your level on relativity so get ready for one big pile of steaming
...misconception. I'll do the right thing, and keep it short.
I thought that one of the big themes from the principle of equivalence and relativity
via frames, was that there wasn't a complete resolution to the absolute,
non-relativistic conception of what the whole universe is like., That you can't
necessarily talk about a landscape in absolute terms at all.
I read liz's thread on block time (very helpful thanks Liz) and will be rephrasing this
issue over there at some point too.
It's funny because this came up for me first because I speculated with some guys (and
dolls) a lot nearer your level than mine - maybe 3/4 years back - that spacetime had a
definite geometry. They came back very firm it did not. That even between the Earth and
the Sun you couldn't look at it that way. I must say I couldn't accept what they were
saying and said so, because for me, the geometry would be very clear that there was this
huge gravity well oneside, and this relativily tiny one the other (earth).
Dunno what was meant by "geometry". Einstein's equations relate the metric of spacetime
to the location of stress energy. That seems plenty definite to me. Of course in
application it's just a model and one neglects various effects thought to be small, e.g.
gravity waves coming in from far away.
But they maintained it wasn't legitimate to think that way and then when I wouldn't buy
but didn't have the expertise to make a case from relativity knowledge, got the usual
dressing down about intuition and how the world isn't intuitive and all the rest.
So where do we actually stand? Was their point legitimate but subtly different to yours
- and it's a case of I don't have the knowledge to tell them apart?
Or has this been a kind of furious debate within the field, or was a furious debate? Is
the matter now resolved for blocktime, or is it still controversial, as say, MWI is in
the quantum field?
Could we start there? I'll obviously understand if your response is along the lines,
you've been able to deduce my level from my words, and explaining this isn't going to
work in a post, and I need to bugger off and read a text book. Of course. However...I
would settle for a roughie, and the text book ain't likely to go down at the moment.
First read Lewis Carroll Epstein's nice little book "Relativity Visualized" which has
essentially no equations and yet manages to be pretty rigorously correct.
Brent
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