How does Gravity travel the speed of light

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Discussion Overview

The discussion revolves around the nature of gravity, specifically how it propagates at the speed of light while objects experience gravitational acceleration at a rate of 9.8 m/s² on Earth. Participants explore the distinction between the speed of gravitational influence and the acceleration due to gravity, raising questions about their relationship and implications.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that the speed of gravity, as per general relativity, is the speed of light (c), while the acceleration due to gravity (9.8 m/s²) is a different concept related to how objects fall towards Earth.
  • Others question why the speed of gravity and the acceleration due to gravity are different, emphasizing that speed and acceleration are fundamentally distinct quantities.
  • A participant illustrates the concept by comparing the propagation of gravitational influence to the way a ship is propelled by photons, suggesting that forces can travel at c while the objects they affect do not move at that speed.
  • Some contributions attempt to clarify the mechanics of gravity, proposing models involving gravitons and the average gravitational pull towards the center of Earth.
  • There are corrections regarding the misunderstanding of gravitational acceleration, with some participants emphasizing that 9.8 m/s² is specific to Earth and varies with location.
  • One participant expresses frustration with another's understanding of the concepts, indicating a lack of clarity in the discussion.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus, as multiple competing views and misunderstandings about the relationship between the speed of gravity and gravitational acceleration remain evident throughout the discussion.

Contextual Notes

Some participants express confusion over the definitions and implications of gravitational concepts, leading to unresolved questions about the nature of gravity and its effects. The discussion includes varying levels of understanding and technical knowledge among participants.

Who May Find This Useful

This discussion may be of interest to those exploring concepts in gravitational physics, particularly students or individuals seeking clarification on the distinctions between speed and acceleration in the context of gravity.

Alpha[X]²
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How does Gravity travel the speed of light, if it accelerates at 9.8 m/s² on Earth? Shouldn't it travel at a constant c?
 
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Don't mix these up:
(1) The speed of gravity--the speed at which a change in gravitational field propagates. That speed, per general relativity, is c.
(2) The acceleration due to gravity of an object near the Earth's surface. That's an acceleration of an object, not the speed of gravity.
 
But, why are they different?
 
Why would you think they would be the same? You do realize that speed and acceleration are two different things--so that right off the bat, equating them makes no sense. (Kind of like aking why is 3 dollars different than 3 feet.) And further: one is the speed of gravity, while the other is the acceleration of an object. What's the connection?
 
Alpha[X]² said:
But, why are they different?

Speed of gravity: Let's say the sun was to disapear, it would take a little over 8 minutes (distance earth-sun sun at velocity C) for the Earth to change gravitational course.
Hence; the "effects of gravity" travel at C.

Acceleration due to gravity: Corresponds to the gravitational force of a mass. On earth: 9.8 m/s²

They are both very different things.
 
A ship with Solar sails is propelled by photons. The force propelling the ship travels out from the Sun to the ship at c, but the ship itself does not. It will be constantly accelerated by the force of the photons, but it will travel much slower than the force that is propelling it.

True for EM force, true also for gravitational acceleration (not saying the two are similar, but the principle of things moving much more slowly than the force that is accelerating them is still relevant).
 
I tend to think of it more like this (ignore unit troubles please): Objects do not fall at c. But gravity get's to the object that fast.
 
Alpha[X]² said:
How does Gravity travel the speed of light, if it accelerates at 9.8 m/s² on Earth? Shouldn't it travel at a constant c?

Gravity permeates space at the speed of light. On earth, objects fall towards its surface at a speed of 9.8 meters per second. The first statement is gravity's own speed while the 9.8 is gravity's affects on other object's speed toward the Earth.
 
I am a newb too but I somewhat get this (I hope)
Here's how I think it works:
Gravity accelerates 9.8m per second. So in 1 sec, it would be 9.8, in 2secs, 19.6m/s(9.8x2) and in 3secs, it would be 29.4(9.8x3). and so on but it would stop until it reaches c.

Am I Right or Wrong?
 
  • #10
Completely and utterly wrong. As the Doc said, 3 dollars and 3 feet are not the same thing, are they?

You do know that g (acceleration due to gravity) is different on every planet and also differs depending on how high above the Earth you are, right? That 9.8 is not a universal constant(g isn't G), it is just about what g happens to be on earth, on average. It isn't even the same everywhere on earth!
 
  • #11
I meant that for Earth only, not universally. Would I be right then?
 
  • #12
I don't mean to be disrespectful to anyone in this thread, but honest to god what kind of physics teachers do you all have?

Raza, no you wouldn't be right. Think before you make any conclusions. I honestly don't know how to reply to make it clear to you (in other ways than have been previously stated).

I'll try this:

Let's say that each particle of Earth emits gravitons which fly outward in all directions. Then, when some object, like yourself, is hit with a graviton, it is pulled towards the origin of the graviton. The average position of the origin of all the gravitons from Earth is the center. So, on average, these gravitons pull you towards the center. They pull with a force proporitonal to your mass. The acceleration you experience is NOT acceleration of the gravitons, it's the acceleration of your physical human body towards the center of the spherical earth. The individual gravitons fly at the speed of light to meet and provide a force onto you.

Would it make sense for gravity itself to begin at rest and accelerate on its way towards an object?
 
Last edited:
  • #13
Raza said:
I meant that for Earth only, not universally. Would I be right then?
You're being rediculous. Please stop.
 
  • #14
ok guys...lets make it clear...raza...i thing u must go and study the chapter onn gravitation...if u did then here how it goes...u stand on the earth...gravitation force...which is defined as (Gm1m2)/r^2 = mg..were m1 is mass of you and m2 is the mass of earth...r is radius of earth..ok?...if u are standing on the earth...the gravitational equation..which is universal must apply to the force that u learn...m1g...hence...in order to find g=(Gm2)/r^2 and this quantity is 9.8ms-1...get it...
 
  • #15
m1g..sorry
 
  • #16
vijay123 said:
ok guys...lets make it clear...raza...i thing u must go and study the chapter onn gravitation...if u did then here how it goes...u stand on the earth...gravitation force...which is defined as (Gm1m2)/r^2 = mg..were m1 is mass of you and m2 is the mass of earth...r is radius of earth..ok?...if u are standing on the earth...the gravitational equation..which is universal must apply to the force that u learn...m1g...hence...in order to find g=(Gm2)/r^2 and this quantity is 9.8ms-1...get it...
Can you stop writing like a 4-year old?
 
  • #17
now suppose...for some reason...the sun was taken out of ur solar system...earth would only get deflect away from the former sun 8mins(approx.) since it takes 8mins for the INFLUENCE OF GRAVITY between the earth- sun system to dispappear...this influence of gravity...i think...which is the gravitational field...dun even think about gravitons right now...
 
  • #18
sorry...i am onli 14 years old and i dun speak english as my first language...sorry for any inconveinece
 
  • #19
arildno...u got a masters in fluid mechanics...omg...can i ask u a question...does bournoullli's equation apply in real life?...is it an approximation of any kind?
 
  • #20
vijay123 said:
..does bournoullli's equation apply in real life?
Yes.
is it an approximation of any kind?
Yes.
 
  • #21
Look at the units. C= m/s; g=m/s^2. As some one said not the same. C is speed; g is acceration (change is speed over time.:smile:
 
  • #22
Can someone explain it me with a graph?
If the accerelation is 5.0m/s2 and you are starting from rest, how would a graph look like?
What velocity would it be in 10 seconds?
 
  • #23
It is a straight line up and to the right. And the velocity after 10 seconds... well, you tell me: v=at.
 

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