# A copernicus/relativity question

• nathan heiney

#### nathan heiney

Can someone explain to me how those who believe relativity also believe that the Earth revovles around the sun. I thought that relativity claims that a moving body considers itself at rest. If so, we on Earth should consider ourselves at rest, such that the sun is revolving around us?

Can someone explain to me how those who believe relativity also believe that the Earth revovles around the sun. I thought that relativity claims that a moving body considers itself at rest. If so, we on Earth should consider ourselves at rest, such that the sun is revolving around us?

So you are saying that someone who is spinning very fast at the edge of a Merry-Go-Round feels the SAME set of forces as someone who is standing still on the ground?

I think you have problems with classical, Newtonian concept of inertial reference frame, even before getting to Relativity.

BTW, we don't "believe" in relativity. Physics concepts do not get accepted simply based on "belief". There are tons of experimental evidence which you seem to have somehow neglected.

Zz.

PF Mentor: I don't get your answer. Are we on the moving Earth considered still, such that the sun revolves around us?

If not, then we are moving, and we should be able to measure the so called ether wind.

PF Mentor: I don't get your answer. Are we on the moving Earth considered still, such that the sun revolves around us?

If not, then we are moving, and we should be able to measure the so called ether wind.

For starters there is no ether, so the ether wind can't be measured.

As far as the solar system is concerned, like any mechanical system, the "natural" center is the center of mass, which is essentially at the center of sun. You could set up the system with the Earth as the center, but the planetary orbits become horrendously complicated.

I agree that according to General Relativity the heliocentric solar system is no more correct than the geocentric one.

PF Mentor: I don't get your answer. Are we on the moving Earth considered still, such that the sun revolves around us?

If not, then we are moving, and we should be able to measure the so called ether wind.

What does having an "ether wind" have anything to do with your misunderstanding of an inertial frame?

You can't distinguish between something moving in an orbit versus something in an inertial frame. Look at your original question. Now push it to the extreme. If the Earth is moving VERY FAST around the sun, do you think you STILL can't tell if it is the Earth moving around the sun or if the sun is moving around the earth? At what point to you try to justify your dizziness?

This has nothing to do with Relativity. It is Newtonian mechanics!

Zz.

Zapper:

I understand what your saying that a revolving thing is different from something moving steadily. My problem, however, is as follows.

The whole relativity thing started because people thought that our movement on Earth should affect the speed of light, but their mesurements said otherwise.

The answer, I thought, was that we don't consider ourselves moving. This is the whole spaceship thing with the beam going straight up and down for the guy on the ship.

But if we revolve around the sun, and this means that we do consider ourselves in movement, then the whole answer to the light problem seems to have disappeared.

I am not saying there is no answer. I would just like for someone to tell me it.

I thought that relativity claims that a moving body considers itself at rest.
No, although the way it is usually presented that is probably an unfortunate impression that many students receive. General Relativity says that you can use any coordinate system you like and describes how to transform between them. Choosing a particular coordinate system is done in order to simplify the equations, but it is possible to use any coordinate system you wish. You can use a coordinate system where the Earth is at rest, but the equations will be messy.

The answer, I thought, was that we don't consider ourselves moving. This is the whole spaceship thing with the beam going straight up and down for the guy on the ship.
Here you're talking about the http://www.aei.mpg.de/einsteinOnline/en/spotlights/equivalence_principle/index.html [Broken], but one thing to note is that in GR the equivalence principle only works in a local sense--it basically says that if a freefalling observer defines a coordinate system which only extends to a very small spacetime region around himself, then the laws of physics in this coordinate system will be arbitrarily close to the laws seen by an inertial observer in SR. For a large-scale coordinate system like one containing both the Earth and Sun, you wouldn't have this sort of equivalence between the measurements of an observer in curved spacetime and those of one in uncurved SR spacetime (although an orbiting observer is in freefall so their local observations in a small region of space and during a small interval of time would be like those of an SR observer).

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Can someone explain to me how those who believe relativity also believe that the Earth revovles around the sun. I thought that relativity claims that a moving body considers itself at rest. If so, we on Earth should consider ourselves at rest, such that the sun is revolving around us?
A body moving in straight line, with constant speed, can consider itself at rest and derive the same laws of physics. The Earth moving around the sun does not move in a straight line nor does it have constant speed.

I still don't get one thing. If we consider ourselves as revolving around the sun, such that we are moving, why will our measurements of light be as if we are not moving.

I still don't get one thing. If we consider ourselves as revolving around the sun, such that we are moving, why will our measurements of light be as if we are not moving.
Which particular measurements are you thinking of ?

nathan heiney said:
I still don't get one thing. If we consider ourselves as revolving around the sun, such that we are moving, why will our measurements of light be as if we are not moving.

Because light is constant, it's speed never changes... If you're measuring light it will always appear as if you are stationary, because there's no change in light's velocity to tell you otherwise...

I still don't get one thing. If we consider ourselves as revolving around the sun, such that we are moving, why will our measurements of light be as if we are not moving.
Perhaps because are measurements are "close enough" to satisfying the definition of locally inertial measurements? (you didn't respond to my post #9, can I take it you already understand that observers in freefall can be considered to be locally inertial in GR?) Of course if we're making measurements on the surface of the Earth we aren't in freefall so it's not a perfect case of the equivalence principle, but the G-forces are pretty small (I imagine an observer accelerating at 1G in flat spacetime would have a hard time noticing any deviations of the path of light beams), and the Earth itself is in freefall around the Sun. Certainly the equivalence principle would say that if someone on board the space station was doing experiments, their observed results should match those predicted by SR, since the space station is in freefall around the Earth.

Because light is constant, it's speed never changes... If you're measuring light it will always appear as if you are stationary, because there's no change in light's velocity to tell you otherwise...
Its speed never changes in inertial frames, but in non-inertial frames the speed can vary...I think nathan's question was about why our results seem to match those of inertial frames if we aren't actually moving at constant speed and direction in flat spacetime.

A body moving in straight line, with constant speed, can consider itself at rest and derive the same laws of physics. The Earth moving around the sun does not move in a straight line nor does it have constant speed.
This is potentially a little confusing, because the Earth moving around the Sun is moving in the closest possible approximation to a straight line in curved spacetime, no? The issue is that in curved spacetime there are no non-local frames that can be considered "inertial" ones, so no matter what possible path you take, if you make non-local measurements using a coordinate system where you're at rest, you'll find that the laws of physics do not match those of an inertial observer in SR.

Its speed never changes in inertial frames, but in non-inertial frames the speed can vary...I think nathan's question was about why our results seem to match those of inertial frames if we aren't actually moving at constant speed and direction in flat spacetime.
Can you briefly define a non inertial frame please?

A non-inertial frame is one in which free particles do not move uniformly.

Can you briefly define a non inertial frame please?
As dx said, one way of defining it would be a coordinate system where objects with no forces acting on them move at constant coordinate velocity. Another way might be to say that if you have an accelerometer moving at constant coordinate velocity, it will not measure any G-forces (in a non-inertial frame this would not be true for accelerometers at arbitrary positions--an object with nonzero proper acceleration as measured by accelerometers can have zero coordinate acceleration in non-inertial frames).

You could say that the Earth is at rest, and according to General Relativity you would be correct. However you need to explain the movements of the rest of the planets as well, too then. This didn't go well for the church...

Also, this relativistic moving is not effected by curves in spacetime in the sense that the Earth was know to revolve around the Sun far before 1904...