
#1
Nov2903, 02:50 PM

P: 103

I given to understand that you can't tell the difference between accelerating in space and standing in a gravity well, say, the Earth. Isn't it true than in one setting you are adding kinetic energy and is this the difference between the two? Thanks for patience with ignorant questions.




#2
Nov2903, 03:20 PM

P: 837

The point of the equivalence principle is this: if you're shut up in a completely closed room (such as an elevator), so you can't tell if you're on Earth or in space, is there a physical experiment you can do, entirely within the elevator, that can determine whether you're at rest on a planet or accelerating smoothy in outer space? 



#4
Nov2903, 06:11 PM

P: 103

Equivalency question
If gravity is curved space then how is it a force?
If there were only two objects in space and one moved uniformly towards the other such that its path didn't have to curve at all why would it speed up? All of this reminds me of the way the outside part of something speeds up compared to the inside part of something when rounding a corner; but I don't know why. Many thanks for patience with Cromagnonman. 



#5
Nov2903, 06:31 PM

P: 837





#6
Nov2903, 06:37 PM

P: 103





#7
Nov2903, 06:59 PM

P: 837





#8
Nov2903, 07:32 PM

Emeritus
Sci Advisor
PF Gold
P: 4,005

So, if the elevator walls are not transparent to EM, of any wavelength; block all cosmic rays; are opaque to neutrinos, ... it wouldn't matter what equipment you had. I guess if you put LIGO into your elevator, ...[:)] 



#9
Nov3003, 09:56 AM

P: 103

I should have said, I don't have any physics training. I don't know what you mean by "space" as oppose to "spacetime". 



#10
Nov3003, 11:30 AM

P: 38

Check out this thread... http://www.physicsforums.com/showthr...hlight=gravity It asked basically the same question. 



#11
Nov3003, 01:44 PM

P: 837





#12
Nov3003, 02:49 PM

P: 103

That's sort of what I thought you meant. I imagined that diagram of a cube moving through time (animation I saw on pbs once a long time ago) and tracing lines or curves along the way... Are you saying that my two objects on a straight path for each other actually travel on a curved path as they move through time as well? And I'm not even sure what I just said... Many thanks. 



#13
Nov3003, 03:37 PM

P: 837





#14
Dec103, 01:49 AM

P: 103

I can hear someone asking, "what's the difference between my freely falling bodies heading straight for each other on a straight line (as oppose to one body trying to rush past but getting caught by gravity and curving off its straight course) and one of the bodies being propelled by rocket?" Isn't everything in free fall until it lands, sometimes violently, on something? When I picture a space/time diagram, which looks like a cube moving up through time like an elevator, I see an object moving straight through space from, say, left to right, but tracing a diagonal line in spacetime... To what degree am I making sense? I'm wondering, is the model of the 2d net being dipped by a ball so that when you roll another ball on the net it falls into the pit formed by the first ball  is that just an illustration or did the reasoning for curved spacetime actually devolve from that? I have to go scratch, now. Many thanks! 



#15
Dec103, 09:40 AM

P: 837





#16
Dec103, 11:06 AM

P: 88

Being just as confused as Vosh, and looking at Ambitwistor's analogy, I had this simpler question about a glass elevator.
Relativistically speaking, if you're accelerating in a spaceship, there are all sorts of weird things that are supposed to happen, what with light coming in at an angle, mass increasing and red shift as a result of gravitational fields, and changes in the perceived time frames of distant objects. I know could never explain those sorts of things very well, so I hope I've at least labeled them decently. My question is with regard to the earth, our frame of reference, as a "space ship." Long ago I've seen numbers describing how incredibly fast we move through spacewith respect to the Solar System, Milky way, etc. But I'm not sure I've ever seen any numbers describing our rate of acceleration, if any. With eliptical orbits certainly there must be some acceleration. Is there any way to measure this with respect to our galaxy, or whatever cluster/supercluster we're in, or maybe the universe as a whole? 



#17
Dec103, 12:41 PM

P: 103

So gravity curves my diagonal line meaning that the object moving from left to right will necessarily move faster on the curved part of the diagonal line; right? 



#18
Dec103, 12:44 PM

P: 837




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