Shouldn't gravity travel at light speed immediately?

Question

If gravity travels at $c$ (light speed), why aren't objects pulled to Earth at that speed?

Since the velocity of gravity is 9.8 meters per second squared, will it eventually accelerate until it maxes out at $c$ then hold constant?

And if that is the case, then why doesn't the gravitational pull between objects and earth immediately travel at $c$ like photons?

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So the acceleration of gravity is 9.8 meters per second squared only on earth.

The gravitational pull is contingent on the body off mass and the distance between the masses.

Gravity waves travel at the speed of light.

So if a gravity wave extending from one primary object of greater mass to another object of lesser mass was to move a lights speed it wouldn't affect the speed or acceleration of the secondary object. the secondary object would just react to the primary object at the speed of light, but the reaction it self is dependent upon the size and distance between them?

Answer 1

I think there's a combination of terminology and information misunderstanding going on here, so let me try and explain this at an appropriate level. First, the phrase "travel at light speed immediately" doesn't make much sense. In physics, there's not really any such thing as "immediately." "Immediately" is synonymous with "instantaneously," and there's nothing we've ever measured that we can call instantaneous communication. But, regardless of a lack of instantaneous anything, phrased this way, I hope it's clear that saying something moves instantaneously at a finite speed is nonsensical from a conceptual point of view. It's like saying something is moving at 5 m/s and 20 m/s at the same time; 5 just doesn't equal 20, no matter how you slice it.

When you talk about the velocity of gravity, you're talking about the speed at which the force carrier of gravity (or the spacetime disturbance) propagates outward. That value is c, as far as we can tell. When you start talking about 9.8 meters per second squared, you're talking about the acceleration due to gravity, which is not the same thing. How hard you push and something and how fast it moves are related, but they're not the same, right? It's the difference between velocity and acceleration.

Now, if something provides a continuous acceleration, the object that is accelerating will keep going faster and faster, approaching a velocity of c. It doesn't matter what provides the acceleration; could be gravity, could be a rocket booster with infinite fuel. The point is, the speed that gravity propagates has nothing to do with how hard it pulls on objects. Those are completely different properties that are unrelated.

Finally, the gravitational pull between all objects does respond at speed c. This includes the earth and moon. But just because gravity "gets from" the earth to the moon at a speed c, that doesn't mean it causes the moon to move toward the earth with velocity c. I hope that clears up some of the confusion.

Answer 2

9.8 m/sec/sec is not the speed of gravity, it is the acceleration due to gravity at the surface of the earth. At the surface of the moon it is a good deal less. At the surface of the sun it is a lot more.

It is true that if you could fall in a straight line, gaining 9.8 m/sec every second, after about a year you would approach the speed of light, but you would never surpass it. It would be hard to find such a building to jump out of. You could do it in space if you had a good enough rocket motor and enough fuel.

Think about sound in air. The speed of sound is about 340 meters/second, but that does not mean if the wind blows something around, it blows it at that speed. What it means is if someone claps their hands 340 meters away, you hear it one second later.

Gravity is like that. If a big piece of matter, like a planet, suddenly moves into position 30 000 kilometers away, its gravity is felt by you 1/10 second later. But that only means you feel the force at that time, not that you are traveling at that speed.

Answer 3

How Gravity Works?

Why the things do not reach speed of light due to gravity? Because the gravity's effect goes sideways. Its because gravity do not work by just pulling. This is because both bodies pull. The pulls are of equal force. There is therefore no net pull. The bodies instead start moving around a point. This is how orbits are made.

Orbits, Orbits, Everywhere

There is no movement in straight line. There always are curves.

The curves are always some kind of orbits. No matter a thing is falling, in a stable orbit, is spiraling inwards, or is breaking away, its in an orbit.

Falling Is Orbiting

A fall is a failed orbit. The object was just trying to move in an orbit, the planet got in the way. The object hit it and we say the object has fallen.

If the object didn't hit and just continued moving we say the object is in an orbit.

A Thought Experiment

Suppose you are in an aircraft and the aircraft is applying thrusters to always hover over a spot on earth. Now you drop a ball, would it fall to the spot on ground? No. As soon as the ball is out of the aircraft it no longer have the push of the thrusters, so earth moves under it.

Would it fall to the portion of earth that should be under it because of rotation of earth, when it reach the ground? No. There is a curve in play here.

The Curve Is Independent Of Spin Of The Planet

Where Do The Force And The Energy Comes From For The Sideway Motion / Orbiting?

The force that start the revolutions would just appear. Forces do not follow law of conservation. Any force that is needed just appear and disappear when not needed.

Energies follow law of conservation. Revolutions do not consume energy.

The Center Point Of The Orbit

Law of gravity is such that when a body attracts another body then it got attracted back by the other body with an equal force. The pull is of same intensity from both sides so there is no net pull. There would be a sideway motion though. Both bodies will start moving around a point that lies on the line that is between the centers of the bodies.

Acceleration Is A Vector Therefore Straight. Orbital Motion Is Curved. Both Are Result Of Same Gravity. How Do That Play Out?

Velocity is a vector. Vectors are like arrows, always straight. There are no curve vectors.

The acceleration due to gravity, is a vector. All accelerations are vectors. Its because all velocities are vectors and accelerations are built on velocities.

So, you have a curve motion, motion in an orbit, due to gravity, which is acceleration and therefore a vector. How do the two act together when they are opposite of each other? They act together by attempting to cancel each other. Sometimes one wins sometimes the other. No orbits are stable because the two are never equal.

How To Have A (Relatively) Very Stable Orbit

If a heavenly body is a perfect sphere and the object revolving around it has no mass but still somehow experience gravity then the curve of the orbit would exactly be such that the change in direction would be exactly enough to compensate for acceleration.

Why Earth Not Fall To The Sun

The point around which earth revolves is inside the sun. Its not at the center of sun ofcourse but near enough for the orbit to be stable for billions of years.

Its All About Distance

Consider moon coming very near to earth. Say to 40,000 km. Its currently 400,000 kms away. Would it be able to continue in its orbit? No. It will fall to earth. Now consider moon going to 1,000,000 km away from earth. Would it continue its orbit? No. It will break away from the orbit and go in some other orbit around some other body.

Stability Of An Orbit

Stability of orbit depends on where its center is. In an unstable orbit an object either fall to what it revolves around or break away.

Closer the center of orbit is to the center of body, stabler the orbit is.

Its because closer the center of orbit is to the center of (one of the) bodies, the more massive it is compared to the other. Therefore it gravity dominates.

Stability Of Orbit Of A Small Object VS Orbit Of A Large Object

Small objects fall because the body gets in the way of their orbit. If they start the fall from a bit higher they would move around in a (relatively) stable orbit. Its because being small compared to the body the center of orbit would be very close to the center of body. The gravity of the body therefore dominate.

For a larger object the center of orbit would be more away from the center of body. The more away it is the less stable the orbit is. Two stars of almost equal mass revolving around at almost the center of the line connecting their centers are in a considerably unstable orbit.

What Could Have Fallen Has Already Fallen In Our Timescale Among What We Can See

When we see heavenly bodies we see old things relative to our live spans and we see big things like planets and stars. We see old things because it takes a very long time for a planets or a star to be formed. We see only planets and stars because the small things, like house sized or car sized are too far away to be seen.

The old things, the planets and stars work at very slow time scale. Their orbits even if unstable are unstable on scale of atleast hundreds of thousands of years. As those that were less unstable have already fallen hundreds of thousands of years ago atleast.

Source

https://spaceplace.nasa.gov/barycenter/en/