Expert: Tom Whiting - 8/31/2008

Question
QUESTION: Hi Tom

My question is about the Gravitational field around the objects. I have two questions,

1. What will happen if the earth stops its rotation about its axis but still continues to orbit around the Sun. Will the population be thrown away in space ?
Since the SpaceTime around the earth is bending around the earth the population will still retain its same position as usual.
May be am wrong, What causes the objects on the Earth stick to it.
To create a artificial gravitation in a space crafts, I have seen in the movies, the space station will look like a donut rotating along its axis, so the centrifugal force acting on the space station generates an artificial gravitational fields, is that what happening to the Earth also. Correct me !

2. What will happen if the Earth quits the orbit around the sun but continues to rotate around the Sun ?
These questions arrived when I was studying General theory of relativity.

When an object (eg, Earth) is pulled out of its orbit and reaches in space where there is no appreciable mass surrounding it, than all the laws of nature will hold good for that object, ie in viewing from the local co-ordinate system, in this case the object (Earth).
So even if the Earth quits its orbit around the Sun and reaches in space similar to above said conditions, than the Earth will still behave normally eliminating the weather conditions and considering the gravitational point of view.
Please correct me if am wrong.
Thanks for your answers in advance !

BR //Mohan

ANSWER: Hi Mohan,
Since you are studying Einstein's General Relativity, why are you
then still using the word 'force' and 'stick' for gravity?
Einstein shows us that gravity is just a curvature of the space-time
continuum, due to the presence of mass.
1a. So if the Earth stopped it's rotation about it's axis, would not
the population be subjected to even a higher 'force'?  Yes, you
would actually weigh more, on a scale...ie, the gravitational field
would appear even stronger, because there would be no centrifugal
force of spinning to partially counter gravity.
[It is true that on a scale, we weigh slightly more at the North Pole, where the rotation rate is zero, compared to a scale on the equator.]
The Gravitational field strengths are only dependent on the mass of the body, and not their velocity or their rotation, or their speed
thru space. And actually, Sir Isaac Newton's concept of gravity
was only dependent on the mass also, not the velocity.
F = G m1 m2/distance squared.  You see no 'v's (velocities) in that equation either.

1b. The extremely small bending of space-time around the Earth is
hardly measureable with very sensitive satellite equipment (spinning
gyroscopes on special satellites) that it's effects on would not even be noticeable on the Earth's surface.
So yes, everyone would maintain their same position.
1c. Yes, we can create artificial centrifugal "force" by spinning a donut type space station, but no, that is not how the Earth does it. The Earth does it the normal way....a presence of mass simply curves the space-time continuum, as does the sun or any other massive body.

2. Nothing....Einstein's general relativity explaining what gravity
is and how it acts, has nothing to do with the rotation or revolution
of the body, so it does not matter what the Earth is doing as far
as it's orbit or rotation.  However, to answer your question, if the
Earth stopped in it's orbit, it would fall directly into the sun's
gravitational well, and fall into, and burn up in the sun. (And I don't think rotation or non-rotation would matter, at that point.)

3. Sorry, I don't quite understand part 3....our weather is highly dependent on the sun's solar energy, and not solar gravity.  Any departure from the sun would drastically affect our weather, not because of gravity, but just because of the loss of heat and infrared energy from the sun. It's the sun's gravitational well
that we orbit, that keeps us from straying too far (or too close) from the sun, and keeps us in a near-circular orbit around it.

Any thoughts about 'eliminating' gravity are in the science fiction
department. Since gravity is simply a curvature of space due to
a presence of a mass, how does one eliminate a curvature of space?
We can simulate gravity by spinning a donut shaped space station, but that's simply substituting centrifugal force...in place of gravity, if we wish to, for human 'convenience'.
Or we can produce a zero gravity condition in Earth orbit by having a spacecraft orbit the Earth fast enough to counteract gravity, by simply 'falling' around the Earth, as the ISS and all satellites
in orbit do...at a velocity of about 5 miles per second (18,000 miles
per hour) around the Earth.  I think that's about our limits as to what we can do.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

FOLLOW UP:
All these questions on gravity and the elimination of same, are all
highly hypothetical and just can't happen. If you are taking a course
in astronomy, tell your instructor that you don't learn much about
astronomy from these way out hypothetical questions, but you want to learn some REAL astronomy...how big is Jupiter and where is it at right now?
(88,000 miles in diameter, and it's in Sagittarius right now).
How far away from the sun? 5th planet, 484 million miles.
What's the life story of a star, and how does a star die?
This is REAL astronomy, not way out hypothetical questions which
are a total waste of time, in my opinion. Sounds like your instructor
is a physics instructor who probably hates real astronomy, so he's
converting an "astronomy" course into a 'math and physics' course,
because he probably knows nothing of REAL astronomy, and could care
less about REAL astronomy, so he gives the students weird hypothetical simulations, and all difficult mathematical problems.
Next time, take Astronomy 101 under the "Earth Science" Department,
and not the Physics Department...then you'll learn some REAL descriptive and observational astronomy! Stuff worth knowing and
studying!
FWIW,
Tom Whiting
Erie, PA


---------- FOLLOW-UP ----------

QUESTION: Hi Tom

Thanks for the answers !
I kinda over confused my 2nd question, here it is,

2. What will happen if the Earth quits the orbit around the sun and travels to a place in Space where there is no appreciable mass around it?
From your answers I understand that the gravitation will still be there around earth.

And about your follow up yesterday,
Currently am not taking any courses on astronomy. I  am a Mechanical Engineer working for Sony Ericsson R&D. To feed my own curiosity am studying Relativity and books of Michio Kaku in my free time. Thats it. And I truely agree the real astronomy as you said. Am planning to buy a telescope too soon. Next time sure I will be one of the guy observing the sky on the other part of the world.
Thanks !!

//Mohan


Answer
Hi Mohan,
Yes, out in deep space, the Earth would still have it's gravity
because it's all based on the mass only, of the object.

Well, that's very commendable of you to study on your own.
Congratulations on a fine subject (mechanical Engr) and working for
a fine company. I also was degreed in engineering (metallurgical) but I elected to go the military pilot route instead of a civilian job.

One thing you might want to consider is that in our fine hobby,
equipment comes last, not first. Knowledge comes first, so hold off
as long as you can from purchasing a telescope. You might want to
read some fine advice to the novice that I posted on our Astronomy club website at
http://www.velocity.net/~bwhiting

And punch on Tom Whiting's Sound Advice for the Novice Observer
for some extra help and advice on this subject.  It's written more
from a North American perspective, but it would apply to any part
of the world generally, except extreme desert regions which never
get very cold.
Clear Skies,
Tom W.
Erie, PA USA