Expert: Tom Whiting - 4/1/2009

Question
QUESTION: Hi
Can you give a detailed explanation about what is gravity in astronomy and why does it exists
i know that the gravity of the earth is the force that the earth pulls the masses towards it center but why does it exist.also let say gravity of the sun it's a force right? so it should have an acceleration where does it come from?i thought before gravity was an easy subject but it seems other wise. i mean why do bodies attract each other and why don't we have gravity?

ANSWER: Hi Hamad,
Anything that has mass has gravity.  So we, as people do have
gravity, but since our mass is small, it's barely measureable.

Gravity is the same in astronomy as it is in the....real world,
there is no difference.  Gravity is gravity.

Why it exists is a philosophical question...why are we here?
Why does Saturn exist?  Why is there gravity?  The best non-scientific answer I can come up with.... comes from a cartoon I once saw where an Angel was heading back down, from heaven, to the Earth under construction at the beginning, and he said to a passing, returning Angel, "Hey, Hold up on the Velcro!  HE's come up with a new and better idea called 'gravity'."

Where does it come from?  We don't know, but what we do know is its
strength is directly related to the surrounding mass. At the quantum level, the particle is called a 'graviton'...as yet to be identified.

So gravity is one of the 4 primary forces in nature (even though
gravity itself is not a force....it only acts like a force...see
### below).  The 4 fundamental forces are:

1. Electromagnetic force - chemical reactions.... electron transfer.
2. Strong Nuclear force  - over very short distances, binds the positively charged protons in the nuclei of atoms (we use it in fusion and fission type bombs).
3. Weak nuclear force - responsible for spontaneous radioactive decay.
4. Gravity...very very weak, but its effects are felt over a very large distance.

### Ok, as we had covered before, gravity is not really a force
as Einstein has shown with his General Relativity, but simply a curvature of space... or more accurately the space-time continuum, due to the presence of a mass.  The more the mass, the tighter the
curvature, all the way to a black hole where spacetime is completely
enclosing the contained mass.  {Some people call this the warped
space-time continuum instead of using the word curvature.}
Where the space curves completely around the black hole, is called
the "event horizon" which once passed, one can never escape because
the escape velocity is greater than light speed (which is impossible
for mass to attain).  Space travelers should note that this "event
horizon" could be invisible, so be careful not to stray into one.
At other event horizons, ripped up material from stars that ventured
too close to the black hole and got ripped apart from tidal forces,
the material is spiraling around the event horizon and heated to
millions of degrees, called an accretion disk. Don't go near those
either because they emit UV, hard X-rays, and gamma radiation due to the very high temperature of the material in the accretion disk
spiraling around the event horizon.
These are how we can study black holes....while the black hole and
event horizon are not visible, the hot accretion disk on the outside
of the event horizon, IS visible.

But going back to gravity simply curving the immediate space surrounding a massive object, that's why gravity "acts" like a force
because matter travels in that curvature.  In effect, matter tells
space how to curve, and space tells matter how to move.

Picture the sun like a iron cannonball sitting on a rubber trampoline...it makes a big dip in the trampoline rubber, steep walls in close to the cannonball with shallow walls further out.
This simulates the sun's real situation of...gravity and the sun's gravitational influence.

The inner planets are circling the cannonball, on the rubber, in
close on the steeper part of the rubber wall, so they have to travel fast to maintain their orbits around the cannonball.
(They don't dare slow down, otherwise they'd spiral down into the
sun).
Mercury orbits once in only 88 days for one orbit, Venus in 243 days.
The Earth further out, takes one year to make one
orbit around the cannonball, while the outer planets, located on
a much shallower part of the rubber trampoline, travel very slowly
around the cannonball because they are farther out from the central
mass, the cannonball. Neptune takes about 165 years for one orbit.
This is how it works.
Of course the sun is traveling thru space around the Galaxy, so the entire trampoline is moving about 200 miles per second around the Galaxy, so the planets are actually on a spiral course around the sun, and not elliptical orbits. They are only pure elliptical orbits
if you pretend the sun's real motion is frozen in time.  But in
reality, we are always breaking 'new  ground' as we travel into
new space that the sun, earth, solar system, has never been before...
Because the Milky Way Galaxy is moving relative to the Local Group,
and the Local Group is moving relative to the Virgo Supercluster,
of which we all are members of.
So there is no such thing as absolute rest...everything moves
relative to everything else. And we're always seeing...new space in
which we've never been in before because we're constantly moving.
Hope this helps,
Clear Skies,
Tom Whiting


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

QUESTION: "Of course the sun is traveling thru space around the Galaxy, so the entire trampoline is moving about 200 miles per second around the Galaxy, so the planets are actually on a spiral course around the sun, and not elliptical orbits. They are only pure elliptical orbits
if you pretend the sun's real motion is frozen in time.  But in
reality, we are always breaking 'new  ground' as we travel into
new space that the sun, earth, solar system, has never been before...
Because the Milky Way Galaxy is moving relative to the Local Group,
and the Local Group is moving relative to the Virgo Supercluster,
of which we all are members of.
So there is no such thing as absolute rest...everything moves
relative to everything else. And we're always seeing...new space in
which we've never been in before because we're constantly moving"
so wont we get out of space?? or in like billiob years out of the universe?

ANSWER: Hi Hamad,
No, no one in our Universe ever 'runs out of space' because the Universe (the space-time part) is expanding....and it's accelerating it's expansion as time goes by. In fact, we don't even know the
size of the space component. With hyper-inflationary expansion occuring immediately after (within a microsecond) the Big Bang, it's estimated that our Universe, the space part, could be super-enormous compared to that portion that contains the matter and energy (all the galaxies and galaxy clusters)....upwards of 10 to the 80th power, lightyears in size.
(Far greater than the known visible Universe of matter and energy).

How can that be?  Because space and spacial expansion is not limited to lightspeed like matter and energy is.  So the initial expansion rate of the spacetime continuum, a micro-second after the Big Bang, was enormous compared to the near-lightspeed energy expansion.
To check it, just google inflationary expansion of Universe.
Clear Skies,
Tom

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

QUESTION: since gravity acts like a force it should be subjected to newton's second law ? f=ma and in case of the earth w=mg
g=9.8
i want to understand what does the acceleration of the earth means i mean acceleration is the change of speed per sec (v2-v1)/s how does that applies to the earth? which speed are we talking about the speed the earth rotates around it self or the sun or what????

Answer
Hi Hamad,
Einstein's correct view of gravity replaced Newton's view of
gravity.  So technically, all of Newton's Laws and formulae are not correct.
Technically, to be super-accurate, they have to be modified for the relavistic effects.
But the beautiful thing is, the relavistic effects are so miniscule
at velocities at less than 10% of lightspeed, and very small
masses like the Earth and Sun (compared to Black holes, neutron stars, etc)....that for all practical purposes, we can still successfully and accurately use Newton's laws and equations for most all student work and normal forces and velocities.

{Because you remember in the Lorentz transformation equations,
the denominator is square root of 1 - (v squared over c squared)
Which provides a very minimum change when velocity is only a small
small fraction of light speed, and masses are well below those
of black holes.}

So we can still 'pretend' that gravity is a 'force' and treat it
as such.....and still use Newton's formula's and laws successfully, until either the velocity gets up to a substantial fraction of light speed, OR you are dealing with masses which get up close to black holes, neutron stars, etc.

No, little g of 9.8 has nothing to do with the rotation or revolution
velocity of the Earth. (Those are tiny tiny nothings, compared to lightspeed).  Little g of 9.8 is the force (pull of gravity) of gravity while 'pretending' that gravity is still a Newtonian Force.
And it will provide an accurate answer to 4 or 5 places after the
decimal point, maybe even further out....but not out 10 or 12 places past the decimal point as you will need to apply relavistic correction effects out that far.  Because Einstein and his General
Relativity was right, and Newton was actually wrong about gravity.

But since we only see major relavistic corrections out on the extreme
end, we (and you) can still use the Newton's formulae without
fear of being incorrect.  Little g still represents the "pull" (or
acceleration of) of 'gravity' acting as a "force", even though it's true nature is a curvature of space, and not a force....because it "acts like" a force.  So you can still use F = ma and all the
others....as long as you aren't computing gravity of a black hole, or something over say, 30% lightspeed. Then, relavistic corrections
need to be applied, to be accurate.
Hope this helps,
Clear SKies,
Tom Whiting