Einstein & Gravity: Explained by HallsofIvy

In summary: And they are the shortest distance between two points in spacetime. This is how Einstein explained gravity - as a curvature of spacetime. In summary, Einstein's theory of relativity explains gravity as the curvature of spacetime, and objects move along geodesics unless acted upon by an external force. This can be difficult to understand because we are used to thinking in straight lines, but precise experiments have shown that the theory of curved geodesics works better than the traditional idea of forces.
  • #1
aychamo
375
0
Hello there!

I asked this question in a different forum, and was directed to ask this here:

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My next question also involves Einstein. It's about gravity not really being a force or whatever. According to the program the Earth revolved about the sun because the sun warps the space time around it. So if I drop my cell phone and it hits the earth, is this because the Earth warps the space time around it? I don't understand this part.

Thank you kindly
AYCHAMO
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I received this answer from HallsofIvy which I really don't understand. May someone break this down for me?

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Geometrically, any surface or "curved" 3 dimensional volume has "geodesics" that act like straight lines: they are the shortest distance between points. Gallilean relativity says things move in a straight line unless acted on by an external force. Einsteinian relativity says things move in a geodesic unless acted on by an external force. Since we are used to "thinking in straight lines", when we see things move on a geodesic that is not a straight line, we interpret it as a force. For normal experience it doesn't matter which "model" you use but very precise experiments indicate that the "curved geodesic" model works better than the "force" model.

You might get better answers by putting this in the "special and general relativity forum".
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Thank you!
 
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  • #2
First think of the surface of the earth. Geometry suggests that "The shortest distance between two points is a straight line". But this isn't true on the surface of the Earth because it is curved. Instead the shortest distance is an arc of a great circle (a circle whose center is at the center of the Earth - I am asuming a perfect sphere here for simplicity). You probably know that airliners follow a great circle course on long distance flights. The great circles are the geodesic on the surface of the earth. The air currents of the world atmosphere also follow great circle paths - or would if it weren't for the coriolis force.

Now imagine another surface, the inside of a whirlpool. Sort of a cone with an exta trumpet shape curvature. If you tried to go from one point to another staying on this surface, you would have to go around the curvature. And you might have to go right aroung the eye of the whirlpool in a circle. This would be another geodesic, the quickest way from one point to another, within this geometry. If you drop a bit of paper into the eddy, is will folow such a curve because a geodesic is the least energy curve.

Now step up in imagination to four dimensional spacetime - one time dimension and the usual three space dimensions. We can't imagine this being curved, we can't visualize it. But we can see the results in the movement of ordinary things. You drop your phone and it follows a bit of a radius toward the center of the earth. If on the other hand you had thrown it as far as you could, it would follow an arc, another path. Leaving out the atmospheric effects, once you drop or throw it, there will be no forces acting on it until it hits the ground. So the set of points the phone passes throug, with the times it passes through them, make a curve in spacetime, and in both cases that curve is a geodesic wending through the geometry resulting from the Earth's gravity.

Some geodesics make orbits - the thrown phone was briefly in orbit. Other geodesics are the ones light moves along. They have a special name and special physics, NULL geodesics. But in spite of the special physics, they are still geodesics.
 
  • #3


Hello there,

I am not an expert in physics, but I will try my best to break down the explanation given by HallsofIvy.

First, let's define some terms. Geodesics are the shortest distance between two points on a curved surface. In this case, the curved surface is the space-time that is warped by massive objects like the sun and the Earth. Gallilean relativity is the concept that objects move in a straight line unless acted upon by an external force. Einsteinian relativity, on the other hand, states that objects move along geodesics unless acted upon by an external force.

Now, let's apply this to the example of dropping a cell phone on Earth. When the cell phone is dropped, it follows a geodesic, which is the shortest distance between the starting point (your hand) and the end point (the ground). However, because the Earth's mass warps the space-time around it, the geodesic that the cell phone follows is not a straight line, but a curved one. This is what we interpret as gravity - the cell phone appears to be pulled towards the Earth, but in reality, it is just following a curved geodesic.

The reason why we are more familiar with the concept of forces is because we are used to thinking in straight lines. But when it comes to very precise experiments, the curved geodesic model of gravity works better than the force model.

I hope this helps to clarify the explanation given by HallsofIvy. If you have any further questions, I would recommend posting in a forum specifically for special and general relativity, as suggested by HallsofIvy. They will be able to provide more in-depth and accurate explanations.
 

1. Who was Albert Einstein?

Albert Einstein was a German-born theoretical physicist who is widely regarded as one of the most influential scientists of the 20th century. He is best known for his theory of relativity and his famous equation E=mc², which describes the relationship between mass and energy.

2. What is Einstein's theory of relativity?

Einstein's theory of relativity is a groundbreaking concept that revolutionized our understanding of space and time. It consists of two main theories: the special theory of relativity and the general theory of relativity. The special theory of relativity explains the relationship between space and time for objects moving at a constant speed, while the general theory of relativity describes the force of gravity as a curvature of space and time caused by massive objects.

3. How did Einstein explain gravity?

Einstein's theory of general relativity explains gravity as a curvature of spacetime caused by the presence of massive objects. According to this theory, objects with mass cause a distortion in the fabric of spacetime, creating what we perceive as the force of gravity. This is in contrast to Isaac Newton's theory of gravity, which described it as a force acting between two objects with mass.

4. What is the significance of Einstein's theory of gravity?

Einstein's theory of gravity has had a profound impact on our understanding of the universe and has been confirmed by numerous experiments and observations. It has also led to advancements in technology, such as the development of GPS systems, which rely on the principles of general relativity to function accurately. Einstein's theory of gravity has also inspired further research and theories in the field of physics.

5. How can Einstein's theory of gravity be applied in everyday life?

Einstein's theory of gravity has practical applications in various fields, including astronomy, space travel, and technology. For example, it is used in the navigation of spacecraft and the prediction of the movements of celestial bodies. It also plays a crucial role in the development of accurate maps and GPS systems. Additionally, the principles of general relativity are used in the design of modern technologies, such as satellite communication and nuclear energy.

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