If you accept the law of universal gravitation to work then how come doesn't light bend. You can calculate mass by E=MC^2

its all circular reasoning....

Actually it does bend light and this has been measured for our Sun.

During an Eclipse, we can see stars where then light passes close to the Sun, We can measure the angle between that star and another star whose light does not pass close to the sun at that time. Later, we can look again at those pairs of stars and measure the angle between them again.

If the angle has change between the two measurements, then we can say that indeed the light form the star has bent.

We can then compare the amount that the light was bent by to the predictions of various theories (eg: That gravity bent the light, that it was refraction by an "Atmosphere" around the Sun, etc). The theory (or combination of theories) that give the most correct predictions will be the ones that are most likely to be correct.

This measurements have been done and the result matched the prediction by Einstein's Gravity.

Interestingly, even under Newtonian Gravity, it predicts that light should be bent by gravity, but the amount that the light is bent by is different to what is predicted by Einstein's Gravity. As the results of the measurement were different to the prediction by Newton's Gravity and were correctly predicted by Einstein's Gravity, then we can say that Einstein's Gravity is more correct than Newton's Gravity.

Under Einstein's Gravity, light is not "actually" bent or curved. What occurs is that light always travels in a type of Straight line called a Geodesic.

A Geodesic is a straight line in a type of geometry called Non-Euclidean geometry. Euclid was an ancient Greek mathematician that gave us a lot of our maths for geometry. However, the maths that Euclid gave us only apply if the surface we are doing the maths on is flat, like a piece of paper.

Euclid's formulas fail if the surface is curved (like a ball, or even just a hill). Because of this later mathematicians made changes to Euclid's formulas so that they took into account the shape of the surface that you are working on. This new maths is called Non-Euclidean geometry.

Now, because of the differences between Euclidean and Non-Euclidean geometry the equivalent structures in Euclidean geometry were given different names in Non-Euclidean if the formulas that govern them where changed so that they worked in non flat spaces.

One of these was the "Straight Line". In Euclidean geometry a Straight line is defined as: The shortest path between two points. In Non-Euclidean Geometry, the "Geodesic" has the exact same definition (and hence why it is a type of straight line). However, because in Non-Euclidean geometry the surface is not flat, it means that a Geodesic, from certain angles will

*appear* as curved where as from others, it will still appear as straight.

Specifically, the geodesic line will appear as straight when viewed from he line itself. This is actually very important to understanding Einstein's gravity.

Now, light under Einstein's gravity will follow a geodesic. That is it will take the shortest path between two points. Now from the perspective (frame of reference) of the Light, the path it takes is perfectly straight, however, from a perspective (frame of reference) that is not the same as the light (say an Astronomer on Earth), it will

*appear* as if the light curved or bent.

Now the really cool thing about all this is that you can map the geodesics and this will give you the actual shape of the surface even to the point of being able to determine how many dimensions the surface is curved in.

For light around gravitating abject, this shape of the "surface" is a 4 dimensional curve (hence why in relativity they talk about space-time being curved).

Now this also has implications for the difference between a Flat Earth and Round Earth. Non-Euclidean Geometry, if the underlying surface is flat, is identical to Euclidean geometry, so you can trace the Geodesic on a Flat Earth and trace the Geodesics for a Round Earth. You can then check these against actual measurements made on the ground to determine if the Surface of the Earth is flat like on a disk, or curved like on a ball. And you can go out and walk the lines, this avoids any chance that Bendy light can effect the results.

When you do this, the results of these direct geodesic measurement are that the Geodesics that exist for the Surface of the Earth describe a Sphere.

So, in response to the thread, the reason I

*don't* believe in a flat Earth is because Mathematics and Geometry state that the Earth has to be a Sphere (well specifically a spheroid, that is an object that is almost, but not exactly, a perfect sphere).