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quiet · 05-17-2003, 10:25 PM

Quote:

Originally posted by KRL

So it would seem.

Alleged Gravitational Attraction
People who are not familiar with Einstein's discoveries about gravity tend to think that the reason for gravitational phenomena has been the universal gravitational attraction. It has been thought to be one of the four fundamental forces of nature. Indeed it looks like the objects in the universe attract each other. Yet they look like that only if we don't look too closely.
It is like with all magic tricks. If we observe everything that makes the trick it turns out that really it is not what it looks like it is. It turns out that it is really something else.

The same thing happens with the alleged gravitational attraction. If we measure everything with high accuracy it turns out that there is no way of fitting the gravitational attraction into the picture. The gravitational attraction is a fake explanation for what's going on. The objects in the universe just happen to move in a fashion that simulates attraction but there is no force that forces them to do that. It is just their natural free movement. The explanation why this free movement is so strange is the subject of Einstein's theory of gravity.

In times of Newton such free movement in absence of forces was thought to be possible only with constant velocity (meaning with constant speed along a straight line). It has been thought that any other movement requires a force to make it different than movement with constant velocity. Einstein proposed, and precise measurements confirmed what he proposed, that all objects when left alone move not along straight lines in space but along straight lines in certain more general "super space" that is composed of space and time together and called spacetime for short. It is like we call a man and a woman who live together a couple rather than "a man and a woman living together" because "couple" tells us more about their relation. Many of their actions depend on each other (e.g. they don't have any more that much of a freedom of choice as to whom they marry; they may be even already married to each other). It is similar with the spacetime in which behavior of space depends on behavior of time and vice versa. As if space had been married to time. Something that was not known in times of Newton when it was thought that time and space don't form a couple in which behavior of one is related to behavior of other in any way. In Newtonian theory the time and the space are totally independent from each other. At the times of Newton's it was not known that the time at some circumstances may run slower and in other faster. Einstein knew this already (especially after discovering his earlier theory called special relativity) and applied it to gravity. It turned out that in places where the time runs faster (more time) the space shrinks a little bit (less space) and v.v. As if the nature tried to keep the same the total amount of both all the time.

Gravitational phenomena
To understand how gravitational phenomena arise without gravitational attraction let's imagine that we are inside of a big container with a slippery floor, carried by a truck and the truck is just negotiating a tight turn on the road.
If we are someplace in the middle of the container we start sliding toward the container's wall. We might think that the wall attracts us. Similarly as when we fall toward the earth we tend to think that the earth attracts us.

Physicists, who analyze such things more carefully, thought that two different forces act in those two cases: inertial force in the former and gravitational in the latter.

The modern physics recognizes only one force in both cases, an inertial force called also a pseudo force. The name comes from the fact that e.g. in the case of the turning truck we are really not pushed against the wall with any force. It is the wall that is moving toward us when the truck is turning while we are still moving in a straight line as the truck had been moving before getting to the curve. From the point of view of the wall that does not know about its acceleration it looks as some force pushes us against it.

When, in the process of the truck making a turn, we finally collide with the wall, it starts pushing us with certain force depending on how tight turn the truck makes and with what speed. Therefore depending on how fast the wall accelerates towards us, now together with us pressed against it. If there were an open door in that wall we might never collide with the wall, just flew out through the door and never feel any force despite that from the wall's point of view there might have been a force that has thrown us out of the container.

So pseudo forces are forces that seem to exist from the point of view of one system and may not exist at all from the point of view of another system. In this case the one system would be container's wall that is seeing us moving faster and faster toward her (presumably pushed by some force), and the other system is us who just keeps moving along a straight line as we did before the tuck got to the curve.

In the case of falling on the earth the situation may seem not identical but it is surely a similar one. Like in the case with a moving container we don't feel any force while we are falling toward the earth (we are weightless then). When we finally collide with the earth, presumably survive the collision, and then lie pressed against it we are feeling a force pushing us up. If there were a hole in the earth leading to the other side of the earth so that we wouldn't collide with the earth, we would never feel any force as in the case of flying out of the container.

The difference between the two cases is that we think that the earth's surface does not accelerate towards us when we are lying on it, as the container's wall has been. Einstein has proposed that it actually does accelerate towards us all the time and we accelerate with it when we stay put on it. And that's why we feel the force that we used to call gravitational attraction that still is a gravitational force but not attraction anymore, just plain inertial force caused by acceleration.

We don't feel any force when we are falling on the earth, ignoring negligible gravitational effects generated by our own body called tidal effects. Those effects arise from the change in the time rate generated by the earth mass around itself that is still called gravitational field but does not mean field of force any more. Neither we feel any force (except small tidal forces about which later) when we happen to be on an orbit around the earth. We feel a force that looks like attraction only after we are pressed against an object whose surface then accelerates together with us, creating an inertial force that presses us to that object. The nature of that strange acceleration seemingly without any movement is the main point of Einstein's discovery.

To understand Einstein's discovery and why the gravitational force is the same inertial force we feel at other occasions e.g. when we are tossed around in a moving vehicle, we should first understand what the acceleration really is as a physical phenomenon and how it is generated.

i was keeping it simple by stating classic/newtonian theory. cut and paste is really impressive.