Science a GoGo's Home Page Forums General Discussion Physics Forum Gravity

If I hold a stone weighing (for example) 1kg. in the air, then release it, gravity causes it to fall to the centre of the Earth. Gravity is a 2-way thing, so does the Earth move, albeit imperceptibly, towards the stone?

Absolutely. The Earth moves toward the stone as well. But don't forget...the Earth also moved away from the stone a little bit when you first lifted it.

I don't have a big problem with the idea of the Earth moving towards (or away from) the stone. What does give me pause for thought is the insistence that gravity is not a force. If gravity is a distortion of spacetime, I find it difficult to visualise the tiny distortion round a stone, or a grain of sand, being able to influence the movement of the Earth.

Another thought that comes to mind is that if space, at the quantum level, is grainy, and if the Planck length defines the size of the grains, would we have a situation, similar to the photoelectric effect, in which the Earth would move only if the influence of the smaller object was sufficient to move it a distance equal to, or greater than, the Planck length?

Bill S.

I think your confusion about gravity being about a distortion of
spacetime rather than a force is rather natural. The thing
is that our perception of the distortion is that it appears
to be a force. Of course quantum theory considers it to be
a force, because that is one of the differences between
GR and quantum theory. Hopefully when a full theory of
quantum gravity is developed it will clarify the matter.

In the mean time, since quantum theory has been extremely well
developed for small things then you are quite correct that
the Earth has to move in quantum jumps. The jumps are so
small that they are undetectable to our measurements, but
that is they way it works. Just one more of those weird
things we understand about quantum theory. It has been pointed
out that if you think you understand quantum theory you
should sit down and relax, the doctor will be able to
give you some medication that will help. You probably
won't be admitted to the psychiatric unit.

Bill Gill

I had thought of this possible limit immediately, even while I was making the declaration. I have an engineering background and I have learned to look for possible interferences as soon as possible.

I have never looked over Planck’s work in an attempt to size up all the assumptions and final accuracy. I have no doubt that the basic idea is correct. Indeed, the universe has an underlying discrete nature to it. I know that personally, I would not be enthusiastic if I was charged with the job of attempting to derive Planck’s Limits. In my opinion; I think that it is most likely that the final published resulting values are in error. I’m sure that there is probably an erroneous assumption somewhere but, I still stand by the concept and I still believe that there are a lot of truths about it.

One thing that you can immediately notice about Planck’s Length is how much smaller it is than the width of a neutrino (10 -35 vs. 10 -24). Obviously, the premise for Planck’s Length is not an over-simplification. In effect, Planck is compensating for situations like this problem as well as all other contingencies.

I think of the concept of PL as --- The smallest possible happening in the shortest amount of time. I take it on faith that in “Planck’s world”, the stone is looking more like a heavenly body and that all actions at this scale easily fit within the limits.

I’m glad that you mentioned it.

I think that if we were to somehow come across a situation where PL is violated that; it just means Planck made a “mistake” and came up with an erroneous value…but he still wouldn’t be “wrong”. I think that the idea (intention) of it is rock solid.

“Another thought that comes to mind is that if space, at the quantum level, is grainy, and if the Planck length defines the size of the grains…”

I wanted to comment on this separately.

Quantum foam. If the structure of the foam is inline or equal to Planck Length…isn’t this just a sly way of “placing a grid on the board”?

I had another thought.

As I had stated before, I think that the stone and the Earth interact gravitationally and that PL hasn’t been reached. Heck, I think that even a pebble-sized moonlet is causing Saturn to wobble. Without attempting calculations, I really doubt that Saturn is reacting in any meaningful way to any distant star’s gravitational field. PL has been reached and regardless of my ability to calculate a substantial value, gravity has been “cutoff”. In effect; Saturn and the star don’t “see” each other.

This caused me to wonder if a Black Hole at the center of a galaxy is “aware” of any of the stars that are orbiting it. In other words, does the BH wobble in response to anything that stars are doing? Has PL been reached in someway? There’s no doubt that the stars see the BH. Does the BH see the stars? And if not, could this unusual gravitational arrangement result in an unusual planetary pattern?

This has to be a case of "inter-visibility", doesn't it? If A can see B, B must be able to see A.

Could be; but it would have to be a grid that influenced possible activity on the board, rather than just being a measuring aid.

"This has to be a case of "inter-visibility", doesn't it? If A can see B, B must be able to see A."

When I said “see” I meant gravitationally but it’s still interesting that you put it that way.

Visually, the phenomenon would be opposite. From the star, I can’t see the BH. From inside the BH, I have no problem seeing the light from the stars. No inter-visibility…Perhaps there’s no appreciable two-way inter-gravitation. For instance: the center of gravity between the star and the BH is located <.5 PL from the star's center. In effect all gravitational influence from the star has been negated…ineffectual…non-existent.

Just a thought…certainly not a conviction.

I think you have lost me there.

I made an important correction on my previous post.

Sorry, Kirby, I couldn't find the correction. I'm loosin' it!

In my previous post, I corrected the location of the center of gravity from BH to star...(big difference).

That's OK. this isn't a subject that I wanted to get side tracked onto anyway.

I'll elaborate if you wish.

Basically I was saying that when the center of gravity between BH and star gets so close to the star that it is virtually at the star's center then the star has no gravitional influence on the BH. I don't know if this holds true in reality or practice...just a thought.

I thought that it was an interesting, extreme condition of the rock/earth relationship.

Presumably this supports the idea that unless sufficient energy is involved, the Earth would not move in response to gravitational "attraction" by a smaller body.

The big question seems to be, does the scientific community regard gravity as a force, or a distortion; or is it a matter of choice, depending on the point being made at the time?

Gravity is a force mediated by the graviton, if you are looking
at it as a quantum interaction. This mainly applies at very
small scales. Gravity is the result of the warping of
space time, if you are looking at it from the GR point of view.
This of course applies at large scales. Both points of view
work extremely well at their appropriate scales. That having
been said, for the scales used by most quantum physicists
gravity can be ignored. The force of gravity is many orders
of magnitude smaller than the forces that operate in the atom.
Where the problem comes in is that when you get to much smaller
scales, such as in association with black holes the two views
are incompatible. That is why string theory, and other theories
attempting to combine the two views are being so heavily
investigated. Of course I'm not sure that string theory and
the others are really theories. So far they haven't been
able to come up with any way to test them.

Bill Gill

Thanks Bill. So far, so good, that's more or less as I see it. Unfortunately, many P S authors seem to blur the boundaries when it suites them.

There's a thought about orbits going round in my head. (No comments about puns, please).

Let's take the Earth/moon as our example.
Because the moon is constantly changing direction, it is accelerating.
Gravity (on this scale) is a distortion of spacetime.
The moon is not being attracted to the Earth, it is following a geodesic, which is defined as the shortest route through spacetime: i.e. a straight line through curved spacetime.
If the moon is following a straight line, at a constant speed, in what sense is it accelerating?

Bill S.

Ok now you are getting into kind of a philosophical realm.
As I see it, and I might be wrong, acceleration is a product
of our classical view of the universe. Newton codified
this view in his theory of gravitation and the laws of
motion. I'm not quite sure how Einstein handled it,
GR is quite a bit more complicated than Newtons laws.
But the distortion in space that results in what we view
as gravity is the result of mass (or the energy equivalent).
This produces gradients in spacetime, and everything follows
those gradients. So in a sense there is no acceleration,
but the effect is almost the same as if there was. GR
makes slightly different predictions as to the motion of
things. Normally this is not a problem for us, although
it does make enough difference that the GPS does have to
take GR into account.

Now, does that confuse you enough? I know I am getting
myself confused.

Bill Gill