Physics question of the day - what speed is gravity?

Juddog

Diamond Member
Dec 11, 2006
7,851
6
81
Einstein thought that gravity would be limited by the speed of light, since information is limited to the speed of light in special relativity.

Yet the refutation posted here states
"The fact that the Earth is not accelerating toward the visible location of the Sun, but rather 20 arc seconds in front of the visible Sun (where the Sun will visibly be 8.3 minutes in the future) is very strong evidence against gravity propagating at the speed of light. "
http://www.policymic.com/articles/1...y-why-einstein-was-wrong-and-newton-was-right

So was Einstein wrong about the speed of gravity?

There are other experiments which seemingly verify that gravity does indeed occur at light speed:
http://www.spacedaily.com/reports/Chinese_scientists_find_evidence_for_speed_of_gravity_999.html
 

z1ggy

Lifer
May 17, 2008
10,004
63
91
Can you or somebody else explain how gravity has "speed"? Gravity is measured in m/s^2 and a speed (velocity) is measured in m/s. So I don't quite get how gravity would have speed per say.
 

lozina

Lifer
Sep 10, 2001
11,711
8
81
gravity is a force not an object accelerating through a space.

its like asking what is the speed of a magnetic field
 

z1ggy

Lifer
May 17, 2008
10,004
63
91
gravity is a force not an object accelerating through a space.

its like asking what is the speed of a magnetic field

If gravity was a "force" then F=Ma wouldn't be the proper equation. To measure the force you exert while on earth, you use gravity as "a" and your mass (in kg) for "m".

Magnetic fields travel at the speed of light, just like any other electromagnetic field (radio waves, etc).
 

Gibsons

Lifer
Aug 14, 2001
12,530
35
91
Can you or somebody else explain how gravity has "speed"? Gravity is measured in m/s^2 and a speed (velocity) is measured in m/s. So I don't quite get how gravity would have speed per say.

What it's asking is how fast does the force of gravity propagate through space. Is the earth attracted to the suns current exact position or where it was when gravity waves (or gravitons or whatever) were emitted?
 

lozina

Lifer
Sep 10, 2001
11,711
8
81
If gravity was a "force" then F=Ma wouldn't be the proper equation. To measure the force you exert while on earth, you use gravity as "a" and your mass (in kg) for "m".

Magnetic fields travel at the speed of light, just like any other electromagnetic field (radio waves, etc).

Humankind is lucky that I am not a scientist then.
 
Feb 25, 2011
16,907
1,552
126
Can you or somebody else explain how gravity has "speed"? Gravity is measured in m/s^2 and a speed (velocity) is measured in m/s. So I don't quite get how gravity would have speed per say.

Any two objects will exert a gravitational force on each other. (Depending on distance and mass.)

So let's say you have an object in space, hooked up to all manner of sensors.

Now you place another object into space X distance away.

How long until the sensors attached to object A notice the effect of the gravity from object B?

That's the "speed" of gravity.

In the Newtonian model, it's supposedly instant. (If you've read any of the Honor Harrington novels by David Weber, his in-universe physics follow this model to do FTL Comms.)

But we've come a long way since the 18th century, and have actually measured the speed of gravity propagation as being "meh, around the speed of light."
 

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
49,601
166
111
www.slatebrookfarm.com
It propagates at the speed of light. Many of Van Flandern's beliefs that are in disagreement with mainstream science have long since been discredited as being in error. He seems to have a history of believing the wrong thing - believed in a steady state universe, rather than Big Bang. While there's no evidence that the Big Bang is wrong, the steady state hypothesis has long since been thrown out. He also had some sort other belief about gravity in that it caused planets to explode and that, in fact, Mars was a moon to the planet between Mars and Jupiter which exploded (the asteroid belt) - and that mankind had originated on Mars. He was convinced the features on Mars were man-made in origin. Van Flandern also accused mainstream science of never reexamining new evidence once a theory was accepted as true. I really don't know a lot about the guy, but he sounds like one of those conspiracy theorists (though a smart one) from a couple decades ago.

Also, I hate to discredit sources, but really - an article from a site whose current top three stories are austerity ahead, looser concealed carry laws, and pole dancing?
 
Last edited:

z1ggy

Lifer
May 17, 2008
10,004
63
91
What it's asking is how fast does the force of gravity propagate through space. Is the earth attracted to the suns current exact position or where it was when gravity waves (or gravitons or whatever) were emitted?

That's what I thought, but I wasn't certain.

I just want to know how gravity is related to electromagnetism D: Get to work scientists
 

Juddog

Diamond Member
Dec 11, 2006
7,851
6
81
It propagates at the speed of light. Many of Van Flandern's beliefs that are in disagreement with mainstream science have long since been discredited as being in error. He seems to have a history of believing the wrong thing - believed in a steady state universe, rather than Big Bang. While there's no evidence that the Big Bang is wrong, the steady state hypothesis has long since been thrown out. He also had some sort other belief about gravity in that it caused planets to explode and that, in fact, Mars was a moon to the planet between Mars and Jupiter which exploded (the asteroid belt) - and that mankind had originated on Mars. He was convinced the features on Mars were man-made in origin. Van Flandern also accused mainstream science of never reexamining new evidence once a theory was accepted as true. I really don't know a lot about the guy, but he sounds like one of those conspiracy theorists (though a smart one) from a couple decades ago.

Also, I hate to discredit sources, but really - an article from a site whose current top three stories are austerity ahead, looser concealed carry laws, and pole dancing?

My apologies about the site in question - I thought it summed up the statement nicely and so it was used solely for that purpose. I didn't realize that Van Flandern's ideas had been debunked.
 

Ventanni

Golden Member
Jul 25, 2011
1,432
142
106
That's what I thought, but I wasn't certain.

I just want to know how gravity is related to electromagnetism D: Get to work scientists

Have you been watching YouTube videos about gravitomagnetics? Like gravitomagnetic propulsion? =P
 
Feb 25, 2011
16,907
1,552
126
What is the color of 440 Hz?

It's not visible at all. It'd be a VLF radio wave.

Visible light is a narrow frequency range around five hundred trillion hertz. You can't hear it.

Or are you trying to insinuate that the propagation speed of gravity is somehow nonsensical in the vein of "what color is this sound?" Because somebody with synesthesia would probably consider it a perfectly cromulent question.
 

exdeath

Lifer
Jan 29, 2004
13,679
10
81
Max speed of gravity = speed of gauge bosons meditating gravitational fields = c
 

Jeff7

Lifer
Jan 4, 2001
41,596
19
81
Can you or somebody else explain how gravity has "speed"? Gravity is measured in m/s^2 and a speed (velocity) is measured in m/s. So I don't quite get how gravity would have speed per say.
Imagine if an object abruptly popped into existence.

How long would it take for its gravitational effects to be felt a distance away?

That's what this is talking about.


Also, m/s² is a measure of the acceleration experienced by an object within a gravitational field.

Even that is a measure of the rate at which a velocity is doing something.
Meters per second, per second.
 

Juddog

Diamond Member
Dec 11, 2006
7,851
6
81
Imagine if an object abruptly popped into existence.

How long would it take for its gravitational effects to be felt a distance away?

That's what this is talking about.


Also, m/s² is a measure of the acceleration experienced by an object within a gravitational field.

Even that is a measure of the rate at which a velocity is doing something.
Meters per second, per second.

^^ This - since we can't pop objects into and out of existence, there must be other ways that we can come up with to measure the effect of gravity and whether or not it equals the speed of light.
 

Mark R

Diamond Member
Oct 9, 1999
8,513
16
81
^^ This - since we can't pop objects into and out of existence, there must be other ways that we can come up with to measure the effect of gravity and whether or not it equals the speed of light.

There is. For example, in the solar system, it is commonly said that the moon orbits the earth. This is not actually correct, as it is an over simplification.

What actually happens is that the moon and the earth both orbit their combined center of mass.

However, as both the earth and the moon orbit, so moves their center of mass. In other words, as the moon orbits, so it drags the center of its orbit with it. The question then is, given that the center or the orbit has moved, how long does it take for the moon to feel the pull from the *new* position.

Newton did a lot of work with his equations of gravity, and worked out that if there was any delay at all, between when the center of the orbit moved, and when the orbiting object "felt" the movement, that the orbit would rapidly become unstable, and that a solar system would rapidly fall apart.

Newton never worked out how this "instantaneous action at a distance" could work. In fact, for many years, he fretted that his theory of gravity was somehow wrong, as he simply could not believe this instantaneousness of gravity. He never found a solution to this problem.

The problem was eventually solved by Einstein in his theory of gravity, general relativity. In Einstein's model, there is indeed a delay between when the center of an orbit moves, and when the pull from the new position is felt. The delay is the time it would take light to travel the distance. In Einstein's model, it does appear that this does allow stable orbits. Or perhaps, as this article suggests, maybe it doesn't.
 
Last edited:

DownTheSky

Senior member
Apr 7, 2013
785
154
106
Bleh. We measured some stuff we have no idea how it works or why it exists. Actually we don't know shit about gravity. All we know is how fast it causes mass to accelerate.

If gravity has a measurable speed it's greater than the speed of light.
 

z1ggy

Lifer
May 17, 2008
10,004
63
91
Have you been watching YouTube videos about gravitomagnetics? Like gravitomagnetic propulsion? =P

No but I read the wiki page on the former. To me it seems obvious that Newtonian physics and electromagnetism would resemble one another mathematically...But what's obviously puzzling is how gravity acts on the sub atom scale, yadda yadda yadda.

Read some other interesting things today about how a scientist is trying to test to see if space is "digital" at the Planck level. Forgot to copy the link but results could be interesting.
 

Blitzvogel

Platinum Member
Oct 17, 2010
2,012
23
81
I always assumed that since gravity is a force and a "field" in a sense. Once a source of gravity is turned on or appears, it's influence is instant, no time needed for it to spread.
 

Lithium381

Lifer
May 12, 2001
12,452
2
0
I thought the whole thing about black holes is that light cannot escape.. therefore gravity is at least (c + 1)...?
 

Gibsons

Lifer
Aug 14, 2001
12,530
35
91
There is. For example, in the solar system, it is commonly said that the moon orbits the earth. This is not actually correct, as it is an over simplification.

What actually happens is that the moon and the earth both orbit their combined center of mass.

However, as both the earth and the moon orbit, so moves their center of mass. In other words, as the moon orbits, so it drags the center of its orbit with it. The question then is, given that the center or the orbit has moved, how long does it take for the moon to feel the pull from the *new* position.

Newton did a lot of work with his equations of gravity, and worked out that if there was any delay at all, between when the center of the orbit moved, and when the orbiting object "felt" the movement, that the orbit would rapidly become unstable, and that a solar system would rapidly fall apart.

Newton never worked out how this "instantaneous action at a distance" could work. In fact, for many years, he fretted that his theory of gravity was somehow wrong, as he simply could not believe this instantaneousness of gravity. He never found a solution to this problem.

The problem was eventually solved by Einstein in his theory of gravity, general relativity. In Einstein's model, there is indeed a delay between when the center of an orbit moves, and when the pull from the new position is felt. The delay is the time it would take light to travel the distance. In Einstein's model, it does appear that this does allow stable orbits. Or perhaps, as this article suggests, maybe it doesn't.

I played golf with a guy once. Smart guy, apparently wrote software for flight simulators before he retired (for General Dynamics, not microsoft).

Anyway, his hobby was setting up planetary orbit simulations and letting them go for thousands/millions of orbits. He told me that if he did the Einstein simulation, the orbits went kablooey very quickly. With Newtonian mechanics they were stable. Maybe he was doing something wrong I don't know.
 

Mark R

Diamond Member
Oct 9, 1999
8,513
16
81
I played golf with a guy once. Smart guy, apparently wrote software for flight simulators before he retired (for General Dynamics, not microsoft).

Anyway, his hobby was setting up planetary orbit simulations and letting them go for thousands/millions of orbits. He told me that if he did the Einstein simulation, the orbits went kablooey very quickly. With Newtonian mechanics they were stable. Maybe he was doing something wrong I don't know.

I don't know. I've never tried to simulate orbits.

My understanding was that Newtonian gravity was rapidly proven not to work as a model of the solar system; even while Newton was alive, astronomers were starting to be suspicious that the orbit of mercury didn't quite fit. Shortly after Newton's death, telescope technology had progressed to the point where the measurements were clear; Mercury's orbit was clearly inconsistent with Newton's gravity.

A series of additional anomalies inconsistent with Newtonian mechanics were discovered by astronomers over the next couple of centuries.

I'm led to believe (but I'm not an expert in this) that Einstein's GR was able to provide an exact match to the observations without any anomalies.