A gravity question

[jajrussel]

If light has no mass, why is it affected by gravity?

[ajb]

Gravity should be thought of as the local curvature of space-time, that is the space-time around a massive body is not flat. As the space-time does not look like a flat sheet of paper or your desk top it is not really surprising that light will not follow a straight line path as defined on a flat space-time.

 

For example, the shortest path between two point on a sphere is part of what is known as a great circle. These are the nearest thing to a straight line on a sphere. Something similar happens on space-time and light follows an analogue of these great circles.

[swansont]

Alternate answer: Newtonian gravity doesn't give the whole picture, so the question turns out to be based on a false premise.

[jajrussel]

So, are you both saying that gravity is not a force; but a direction? Or maybe you are saying that gravity is a force that determines direction? But in either case there is no pull?

 

I didn't think this through. Any change of direction has to be the effect of a force. but there is still the question of pull? If it is just a matter of curved space what happens to that space when I use enough energy to leave? Am I somehow changing the curve to the point that I simply flow away or am I forcing my way through the curve?

Edited March 29, 2011 by jajrussel

[ajb]

So, are you both saying that gravity is not a force; but a direction? Or maybe you are saying that gravity is a force that determines direction? But in either case there is no pull?

 

I myself do think of gravity as a force, it can be seen as an acceleration. However, when people say that it is not a force I understand what they are saying. If you look at the action describing a test particle in free-fall there is no "force term".

 

I didn't think this through. Any change of direction has to be the effect of a force. but there is still the question of pull?

 

In some respect under free-fall there is no change of direction as it is the underlying space that is changing.

 

If it is just a matter of curved space what happens to that space when I use enough energy to leave?

 

I don't understand.

 

Am I somehow changing the curve to the point that I simply flow away or am I forcing my way through the curve?

 

 

It is the energy-momentum of a body that acts as the source of gravity, if that helps.

[jajrussel]

The moderators may decide to move this thread to speculations because my original question seems to have led to a speculative thought and questions.

 

{The greater the intensity of a gravitational field the greater the curvature of space-time. (This statement seems to agree with what ajb said?)}

 

Gravity should be thought of as the local curvature of space-time, that is the space-time around a massive body is not flat. As the space-time does not look like a flat sheet of paper or your desk top it is not really surprising that light will not follow a straight line path as defined on a flat space-time.

 

For example, the shortest path between two point on a sphere is part of what is known as a great circle. These are the nearest thing to a straight line on a sphere. Something similar happens on space-time and light follows an analogue of these great circles.

 

The speed of light in a vacuum is said to be constant, but what if vacuum is an assumption?

 

If vacuum were an assumption couldn’t we say that what we normally call a constant (speed) is actually an average velocity in an assumed vacuum? Then say that the greater the change in a gravitational field the greater the change in velocity?

 

The implication being that an assumed vacuum is always occupied by a gravitational field and is not actually a vacuum.

 

I am assuming that we could then say a photon has mass, but not necessarily at rest.

Then think of gravity as an attractive force between two masses and explain the curved path of light as it passes an intense gravitational field this way?

 

Is this thinking too far outside the box?

Is there something I am not considering here?

 

After thinking about it I decided to change a word. I used the term (constant velocity) when perhaps I should have used the term constant speed.

Edited April 11, 2011 by jajrussel

[ajb]

The speed of light in a vacuum is said to be constant, but what if vacuum is an assumption?

 

In general relativity the vacuum is space-time with out any non-gravitational fields. We can still have a non-trivial local geometry.

 

The speed of light is dependant on the coordinates employed. Now, general relativity on small enough scales looks indistinguishable from special relativity. That is we can always find suitable coordinates such that in small enough regions the space-time is flat. In these coordinates the speed of light is the same as in special relativity.

[jajrussel]

I am trying to organize my thoughts on what you have written and to include what others have written and sometimes it is difficult, because I am not always exactly sure of the meanings of what I have read.

It seems to me that if something is following a curved path then there is a constant change of direction. Free fall would have to exist in its purist sense before the path would be straight. The concept of curved space surrounding two body masses does not seem to allow a straight path to initially exist, but in its purist sense a straight path would be created between two body masses as the opposing curves cancelled each other out.

In a single body of mass space seems to curve inward toward the center of mass. Without mass, space would then in the purist sense be flat, so any resulting curvature of space is the effect of present mass.

Some seem to want to associate gravity to the curve, the greater the curve the greater the accelerated-momentum. Yet, remove the mass and there is no curve, so it seems to me that by attributing accelerated-momentum to curved space they are in a sense saying that space prefers to be flat, and that accelerated-momentum is the result of space trying to be flat.

It also seems to me that the final result of an evaporating black hole would be flat space. So, maybe they are right.

 

Only one single piece of matter regardless it size would result in curved space. Gravity would exist. The curved space is the sign of its existence.

I had thought that two pieces of matter would be needed for gravity to exist. The two pieces of matter thought doesn’t seem to make much sense anymore, but at the time I wasn’t looking at curved space as proof that gravity existed, I was looking at F_g.

 

So is gravity, curved space trying to be flat?

 

I probably shouldn’t add this final thought, since the question already seems a bit foolish; but I can’t seem to stop thinking.

It would seem to me that if gravity were space trying to be flat; then space would expand at a rate equal to its effort.

Edited April 18, 2011 by jajrussel

[jajrussel]

I remember reading one time, and I can not cite it, because I can not remember where or when I read it, but the line of reasoning was that quantum physics was a continuum of classical physics. The reasoning being that some things were difficult to accept in the classical view therefore a quantum view was necessary, but the end reasoning was that quantum physics would fall apart if classical physics were not maintained as the foundation.

I remember thinking that writer was likely a purist grudgingly admitting as humbly as possible the need for fuzzy thinking.

I tend to see the uncertainty principle as a mechanical problem. You can’t see something in its natural state if every time you shine a light on it you disturb its natural state. Assumption becomes necessary. To be used only when needed. If F_g works, or there is a mechanical reason why it doesn’t work you stick to classical thought. Quantum physics should only explain how the fuzzy part fits into the classical part, and not attempt to explain the whole.

[jajrussel]

My thinking tends to get very fuzzy very fast. Some of it has to do with a lack of understanding; well most of it has to do with a lack of understanding. Really, I am trying to work on that. The concept of a photon having zero rest mass is difficult to understand. What is happening when it goes from rest to c? Does it have mass at c? If it has mass at c can I assume that between zero and c it also has mass; and if it does how can it be said that anything that has mass can not be accelerated to c? Do they mean that at its starting point its rest mass has to be zero and that at any point beyond its starting point mass is acceptable and said mass can be accelerated to c because its rest mass was zero?

[Mark R]

If an object is to free fall, where or what direction will it fall?

In the direction the universe is expanding?

[ajb]

If an object is to free fall, where or what direction will it fall?

 

Massive test particles will follow timelike geodesics, exactly what this is will depend on the space-time and the initial conditions of the test particle. Massless particles, eg. light follows null geodesics.

 

You should think of geodesics as the analogue of a straight line for a curved space.

 

In the direction the universe is expanding?

 

Well, you can examine the geodesic equation for the FRW metric. Based on your questions so far I am not sure how much I can explain easily. This is most conveniently described in hyperspherical coordinates; see here. I will have to think about how best to explain this, but for now I will say that in these nice coordinate system you can get a closed form for the geodesic equations. I am not sure about what nice solutions these have. I would have to do some more reading, or calculating.