Relativity

[Pogue]

There was something I was wondering about because I’ve been thinking about thr implications of Einstein’s theory of relativity(which I haven’t studied much).Relativity says that if you are on a train and someone else is on the ground,then relative to the person on the train, the person on the ground is moving the other way. So, could you say that relative to the earth, the sun is revolving around it as opposed to the earth moving around the sun which is relative to the sun. Maybe one of those ancient philosophers(aristotle) were correct and the earth is at the centre of the universe if you use a certain frame of reference.

[Brian Peacock]

You could say that if you set the Earth as an unmoving fixed point by which to reference all other stellar bodies. You'd be silly to do so, not least you'd have a tough time accounting for Sun's orbit round all the other planets and the other planets' orbits around the Earth, but if you reduced the system to only two bodies, the Sun and the Earth, then you could do it, on paper as it were.

[Pogue]

So there really isn’t any center of the universe if you don’t use a fixed point frame of reference?

[laklak]

Yes! The center of the Universe is Our Lord and Savior, Jesus Christ! All things are possible in Him! Praise His Holy Name!

[Scot Dutchy]

Another one? Get out the tambourines.

[Rum]

The sun and the earth (as well as the other planets) revolve around there common centre of gravity, which varies slightly all the time depending on their relative positions. Luckily is is very close to the centre of the sun most of the time. Of course.

[Brian Peacock]

And of course, the solar system orbits the super-massive black hole at the centre of our galaxy at a speed of c. 828,000 kmh and which, given our distance from the centre of the Milky Way, makes one 'Sol year' about 230 million Earth years.

[JimC]

The blink of a cosmic eye, really...

[mistermack]

There was something I was wondering about because I’ve been thinking about thr implications of Einstein’s theory of relativity(which I haven’t studied much).Relativity says that if you are on a train and someone else is on the ground,then relative to the person on the train, the person on the ground is moving the other way. So, could you say that relative to the earth, the sun is revolving around it as opposed to the earth moving around the sun which is relative to the sun. Maybe one of those ancient philosophers(aristotle) were correct and the earth is at the centre of the universe if you use a certain frame of reference.

While straight line inertial motion is relative, rotation is absolute.

So you can't say the the Earth is stationary, and the Universe rotates around it under relativity.

Acceleration is also absolute. If you have two moving bodies, moving parallel to each other at the same constant speed, it's valid to consider them both as stationary, in one frame of reference.

But the same two, if they are both accelerating at the same rate, might look as if they are stationary relative to each other. But they are actually BOTH accelerating. And the acceleration is absolute, not relative.

Objects like the Earth, orbiting the Sun under gravity, are constantly accelerating towards the Sun. So the orbiting is also absolute, not relative.

[pErvinalia]

There was something I was wondering about because I’ve been thinking about thr implications of Einstein’s theory of relativity(which I haven’t studied much).Relativity says that if you are on a train and someone else is on the ground,then relative to the person on the train, the person on the ground is moving the other way. So, could you say that relative to the earth, the sun is revolving around it as opposed to the earth moving around the sun which is relative to the sun. Maybe one of those ancient philosophers(aristotle) were correct and the earth is at the centre of the universe if you use a certain frame of reference.

In terms of Special Relativity, no. That's because special relativity deals with "inertial frames", which are frames of reference that aren't accelerating. But both the sun and the earth are accelerating (both of them will be orbiting (i.e. accelerating) around their shared centre of gravity - which I assume would be very close to the sun, probably within the body of the sun). General relativity deals with space-time in gravity fields (or something like that). But the real point is that both the Earth and Sun revolve around each other (at their shared centre of gravity).

[mistermack]

What makes the idea in the OP obviously wrong is the forces involved in rotation.

If you sit on a playground roundabout and spin, you will feel a force pulling you outwards.
You have to hold on and pull towards the centre to stay on.

It's those real forces that tell you that it's you that's rotating, and not the universe around you.
So it's not a case of inertial relativity, where either case is equivalent and true.

[pErvinalia]

What I'd like to know is how acceleration affects special relativity. An example: We all probably know the train analogy regarding simultaneity. The order of events as observed from two inertial frames (i.e. non-accelerating) depends on which frame you are observing from. So event A might precede event B as observed by a person on a train going a set velocity past a station where a person there observes event B preceding event A. That's all standard Special Relativity. But don't forget that both the train and the platform are accelerating around the centre of the galaxy at some stupendous rate. But because they are both accelerating at the same rate, can we disregard the acceleration and apply the equations of Special Relativity as if no acceleration was occurring?

[mistermack]

I think the acceleration towards the centre of the galaxy is actually incredibly tiny.
If you consider that we only rotate around 360 degrees, and it takes 250 million years, compare that to the child on the roundabout, who does the same rotation in a few seconds.
So for practical purposes, I would say that it can be safely ignored.

[pErvinalia]

Acceleration is m/s/s, not degrees/s/s.

[laklak]

Relativity differs from location to location. Your sister, for example, isn't a relative in Arkansas but is in New York.