# Equivalency question

by Vosh
Tags: equivalency
 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.
P: 837
 Originally posted by Vosh 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?
Well, if you're accelerating in space, then you're not adding kinetic energy relative to someone who is also accelerating in space right next to you. Conversely, from the perspective of someone falling past a person standing on the Earth, the standing person's kinetic energy will be increasing.

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?
 PF Gold P: 3,690 what equipment are you allowed to have in the elevator
 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 Cromagnon-man.
P: 837
 Originally posted by Vosh If gravity is curved space then how is it a force?
In general relativity, gravity isn't 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?
If you're talking about how one body gravitaitonally influences the motion of the other, the important point is that a straight trajectory in spacetime is usually a curved trajectory in space.
P: 103
 Originally posted by Ambitwistor In general relativity, gravity isn't a force. If you're talking about how one body gravitaitonally influences the motion of the other, the important point is that a straight trajectory in spacetime is usually a curved trajectory in space.
Huh?
P: 837
 Originally posted by Vosh Huh?
Huh what?
Emeritus
PF Gold
P: 4,014
 Originally posted by wolram what equipment are you allowed to have in the elevator
Any equipment you like, as long as you can't 'see' outside the elevator!

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, ...
P: 103
 Originally posted by Ambitwistor Huh what?

I should have said, I don't have any physics training. I don't know what you mean by "space" as oppose to "spacetime".
P: 38
 Originally posted by Vosh If gravity is curved space then how is it a force?

It asked basically the same question.
P: 837
 Originally posted by Vosh I don't know what you mean by "space" as oppose to "spacetime".
Space is the collection of all possible locations: it is 3-dimensional. Spacetime is the collection of all possible events (locations and times): it is 4-dimensional. If you pick all the spacetime events that occur at the same time (and there are many ways of doing this, because the concept of simultaneity is relative), then you get a 3-dimensional subspace of spacetime, which is space at a given time. (If you pick a different time, you get a different 3-dimensional surface representing space at the different time, because space can change with time.)
P: 103
 Originally posted by Ambitwistor Space is the collection of all possible locations: it is 3-dimensional. Spacetime is the collection of all possible events (locations and times): it is 4-dimensional. If you pick all the spacetime events that occur at the same time (and there are many ways of doing this, because the concept of simultaneity is relative), then you get a 3-dimensional subspace of spacetime, which is space at a given time. (If you pick a different time, you get a different 3-dimensional surface representing space at the different time, because space can change with time.)

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.
P: 837
 Originally posted by Vosh 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?
Yes, although for a freely falling body, it's the other way around: they travel along straight paths in spacetime, but curved paths in space.
P: 103
 Originally posted by Ambitwistor Yes, although for a freely falling body, it's the other way around: they travel along straight paths in spacetime, but curved paths in space.

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 space-time... 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 space-time actually devolve from that?

I have to go scratch, now. Many thanks!
P: 837
 Originally posted by Vosh 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?"
Freely falling bodies are in free fall: they're weightless.

 Isn't everything in free fall until it lands, sometimes violently, on something?
Everything is in free fall if there are no non-gravitational interactions acting on it.

 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 space-time... To what degree am I making sense?
You've correctly pictured the worldline of an inertial observer in flat spacetime (i.e., in the absence of gravity).

 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 space-time actually devolve from that?
It's just an illustration.
 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 space--with 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?
P: 103
 Originally posted by Ambitwistor You've correctly pictured the worldline of an inertial observer in flat spacetime (i.e., in the absence of gravity).

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?
P: 837
 Originally posted by Vosh 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?
Well... the diagonal line is the object's path through spacetime (worldline). In the presence of gravity, the path is still a "straight line", it's just that the space itself is curved... (Take the surface of the Earth: the equator is a "straight line" on that surface, but the surface itself is curved.)

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