PDA

View Full Version : General relativity 101

Cheap Astronomy
2009-Feb-16, 11:04 AM
How's this sound?

An object is moving in 'empty' space at a constant velocity and then comes into the vicinity of a massive object.

To explain the acceleration of an object entering a gravity field I would say this object (which would otherwise have proceeded in a straight line at a constant velocity) suddenly finds itself entering an environment where the clocks run slower and lengths are contracted such that it (from the perspective of an external observer) is suddenly accelerating as a result of the momentum it was initially carrying when it entered this new enivronment (i.e new frame of reference).

I am trying to explain that the result of an object with a fixed momentum, entering a gravity field, is acceleration (at least from the perspective of an external observer hovering above the massive object).

Thanks!

mugaliens
2009-Feb-16, 09:18 PM
To explain the acceleration of an object entering a gravity field...

Well, that's just it - it's always in a gravity field. There's no entry or exit. There's only a change of magnitude and direction based on their relative locations and velocities.

Jeff Root
2009-Feb-17, 12:28 AM
Mugs's objection would seem to apply to the use of the word "suddenly".
I'd drop those suddenlys. :)

I'm not sure about the GR aspects of your question, but an object does
not accelerate "as a result of the momentum it was initially carrying".
The object could as well start accelerating from a standstill, because
as Mugs says, it does not move from a place of no gravity to a place
of gravity. Gravity's reach is infinite.

-- Jeff, in Minneapolis

m74z00219
2009-Feb-17, 05:33 AM
Gravity's reach is infinite.

What exactly is meant by that? The gravity emitted from the matter that became the Earth 13.7 billion years ago should only reach something like a comoving distance of 46 billion light years. Is it said to be infinite because every object in the universe has perturbed an object some 46 billion light-years away from itself? So this "hop-scotching" allows for the concept that gravity's reach is infinite? Or am I off base??

Jeff Root
2009-Feb-17, 08:01 AM
Gravity's reach is infinite.
What exactly is meant by that? The gravity emitted from the matter
that became the Earth 13.7 billion years ago should only reach something
like a comoving distance of 46 billion light years. Is it said to be infinite
because every object in the universe has perturbed an object some
46 billion light-years away from itself? So this "hop-scotching" allows
for the concept that gravity's reach is infinite? Or am I off base??
Infinity is a mathematical abstraction. In both Newton's law of
gravity and general relativity, gravity is described mathematically
in such a way that it always has some effect nomatter how great
the distance. That is all that is meant. Neither Newtonian gravity
nor general relativity take into account the finite age and probably
finite size of the Universe. They don't need to. That isn't their
function. If the Universe were infinitely old and infinitely large, the
Newtonian and general relativistic descriptions of gravity would still
work just fine, and gravity's reach would be infinite in those cases.
The theories cover all the possibilities, even if those possibilities are
not the actual case.

On the other hand, if gravity is quantized, then it probably cannot
have infinite reach. But there is no evidence that it is quantized.

-- Jeff, in Minneapolis

hhEb09'1
2009-Feb-17, 08:16 AM
What exactly is meant by that? The gravity emitted from the matter that became the Earth 13.7 billion years ago should only reach something like a comoving distance of 46 billion light years. When did it first exist? That's an unanswered question. Even converting energy into matter does not change its gravitational effect, right?