# Would planets fall out of orbit when gravity has a speed?

• B

## Main Question or Discussion Point

As I understand it, Newton considered the influence of gravity between two objects to be instantaneous.

Now it would appear that gravity has a speed limit. If the earth is influenced by the sun from where it was 8 minutes ago, how does the earth keep up with a traveling sun?

Does relativity and frames of reference play a mathematical role in this? What about simple vector analysis?

Related Astronomy and Astrophysics News on Phys.org
fresh_42
Mentor
As I understand it, Newton considered the influence of gravity between two objects to be instantaneous.

Now it would appear that gravity has a speed limit. If the earth is influenced by the sun from where it was 8 minutes ago, how does the earth keep up with a traveling sun?

Does relativity and frames of reference play a mathematical role in this? What about simple vector analysis?

Gravity has a speed. It is the speed of light, which is fast, but nevertheless a certain finite number. The planets are free falling in space. They basically all fall into the sun, but they are so fast, that they constantly miss it. And this makes the orbits. Not only of planets, but of our satellites as well. It's a bit like jumping over a puddle. If you're fast enough you won't fall into it.

Staff Emeritus
2019 Award
Gravity has a speed, and planets do not fall out of orbit, so the answer to your question is "no:".

Gravity has a speed, and planets do not fall out of orbit, so the answer to your question is "no:".
Very clever. However the correct answer could be that gravity acts instantaneously. Or, maybe you are pulling my leg?

What instruments measure the speed of gravity? Do such instruments have algorithms that are based on finding such speed to be that of light? If so, I would call that unethical science.

To return your answer in kind: the earth exists so that proves there is a god that travels at the speed of light. Have a go at disproving that theory.

Last edited by a moderator:
Gravity has a speed. It is the speed of light, which is fast, but nevertheless a certain finite number. The planets are free falling in space. They basically all fall into the sun, but they are so fast, that they constantly miss it. And this makes the orbits. Not only of planets, but of our satellites as well. It's a bit like jumping over a puddle. If you're fast enough you won't fall into it.
So I assume you are using the extraterrestrial fourth dimension, and those pictures of wells around matter. Okay, but isn’t that complete conjecture?

In Einstein’s colleagues derivation of the field equations, their first step was to claim that gravity existed without mass. That is, they arbitrarily set mass to zero. Is there proof that there is a field that could exist in a universe without mass?

Most people think that gravity and mass are related, but not Einstein’s group. Reality is not a set of equations on a blackboard that we can just step into. There is a big difference between a description of a thing and the thing itself. Ever try to step into a map of Argentina so you could be in that reality.

I am just looking for strong experimental data on gravity’s speed. We trust in God, all others must produce data.

Thank you for your answer. I am familiar with the setting you propose. What you left out was that the sun is moving. By the time Pluto gets its gravitational signal, it would seem like the sun is long gone. How is this explained. Simple vector analysis seems to disprove that gravity has a finite speed. But, what do I know. I am a simple biophysicist.

Cheers

fresh_42
Mentor
PeterDonis
Mentor
2019 Award
Steve Carlip published a good paper on how relativity deals with gravity having a finite speed; it should be required reading for anyone investigating this topic:

https://arxiv.org/abs/gr-qc/9909087

PeterDonis
Mentor
2019 Award
In Einstein’s colleagues derivation of the field equations, their first step was to claim that gravity existed without mass. That is, they arbitrarily set mass to zero.
I have no idea where you are getting this from. This and your posts in other threads indicate that you know a lot less than you think you do about the subjects you are posting from. You really need to build a better understanding.