Expert: Daniel Mazur - 6/11/2009

Question
Hello Daniel,

I am curious about something so decided to ask you. We know that according to Newton, the planets orbiting the sun do not fall into it because their centrifugal force (due to their motion) keeps them into the orbit, which happens when it cancels the gravity from the sun.

Now, in Einstein’s view, the space time is curved. So gravity is nothing more than the bent mesh of space-time which is what essentially keeps everything in the space going the way it is. In our solar system, the sun puts a curve in space time and the planets are simply rolling over it.

So if their isn’t a gravity, so to speak, the planets are bound by nothing but are simply going around the sun ‘thinking’ all this while that they are moving in a straight line. But again, gravity is not an illusion, it really does exist and we know that from experiments done here on Earth which tell us that everything that has a mass attracts everything else with mass.

1)Then, why do planets not fall into the sun?
      
Also, I understand that space is not a perfect vacuum but its perfect enough so there isn’t anything which is bothering the planets and slowing them down. The only other plausible reason (in my opinion) could be the contribution of dark energy which is known to push everything away from everything else.

2)Then, could dark energy be responsible for the cancellation of ‘gravity’ in the Einstein’s universe, in context of our own solar system?

Please help me understand this problem, its driving me nuts :) Thanks.

Regards,
Siddharth Ahuja

Answer
Dear Siddharth,

When you try to describe gravity, you can EITHER do it using a "straight" Universe and gravitational forces acting between massive objects, OR using a curved Universe, where mass of objects is merely a measure of how much they curve the space-time. It is a misconception to take a curved space-time and keep thinking in terms of forces the same way you do using the first approach.

1) Straight or curved space-time is merely a matter of choice of descriptive tools, it does not change any physics involved. There is no reason for planets to fall into the Sun, thanks to the same centrifugal force - in a curved space it is expressed by different maths than in the classical approach, but it is always there. Had the general relativity been unable to account for centrifugal and other forces, it would never have made it as a viable theory.

To your note about Sun curving our planetary system's space-time I feel compelled to add that the planets, too, are curving it... everything with mass does, because that's the definition of curved space-time.

2) Planets speed up or slow down due to (temporary) proximity of other planets, asteroids and even man-made probes. As I said, gravity does not "cancel" in either description of space-time, so there is no need to look for a culprit. Dark energy can be responsible for many things, but not for something that is not actually happening. When it comes to expansion of Universe, it seems clear that we mustn't treat it as object in the Universe flying apart, but as the space-time "network" (in which objects exist and move) expanding and carrying the objects with it.

If you want to see some mathematical backing of what I wrote, I must suggest that you ask a theoretical astrophysicist. As an experimental physicist specializing in solid material I do not feel qualified to explain these things beyond qualitative level.

All Best,
Daniel