Speed of Gravity

[Lord Antares]

Do u mean that Einstein would say an infinite force tween two infinite plates?? (I reckoned he would say zero force, based on zero bending of space-time).

 

In general relativity, gravity isn't really a force, it's just the bending of space time. So I guess, in a sense, he would say zero force. Someone correct me if I'm wrong.

 

 

Please stop quoting cranks, the speed of gravity is "c".

 

I actually think he's asking a different question.

 

I think he's asking something like this:

If a mass appeared out of nowhere, how long would it take for it to bend all of space?

I would guess instantly because it is mechanical. The same way that moving the end of a 1 light year long stick would be instantenous, instead of taking 1 light year.

 

But that's not really a valid question because, as far as I know, no gravity has ever appeared or disappeared after the big bang, so even that hypothetical scenario is useless as it could never happen.

So all of the gravity there is is already in effect. There can be no ''speed of gravity''.

 

So that's already off-base to mainstream science. I suggest you keep on topic and in line with the plates, as you have been told by a mod.

 

EDIT: Sorry imatfaal, I wrote this before you replied. I basically told him to stay on topic so I won't be veering off mainstream science any further.

Edited February 22, 2017 by Lord Antares

[imatfaal]

!

Moderator Note

 

No worries about the response - I saw that it was immediately cross-posted with my modnote. However, in future, could all members refrain from responding too much to the off topic branches. A report will often lead to a thread split iff done early enough - but after multiple responses we just don't have time/patience to unweave a thread.

 

This thread was created to answer points raised in response to a hijack

 

[zztop]

 

In general relativity, gravity isn't really a force, it's just the bending of space time. So I guess, in a sense, he would say zero force. Someone correct me if I'm wrong.

 

 

 

I actually think he's asking a different question.

 

I think he's asking something like this:

If a mass appeared out of nowhere, how long would it take for it to bend all of space?

I would guess instantly because it is mechanical. The same way that moving the end of a 1 light year long stick would be instantenous, instead of taking 1 light year.

 

Based on his quoting the Van Flandern crank, he's asking exactly what I answered to.

BTW: if the Sun disappeared instantly, the effects on Earth would be felt 8.5 minutes AFTER its "disappearance".

The obverse: if another "Sun" appeared instantly, next to the current Sun, its effects would be felt on Earth....8.5 minutes after its "appearance".

[Lord Antares]

Really? I wasn't aware of that. It seems a bit counterintuitive, but I will suppose you're right.

You do mean sun's gravitational effects, not visual, heating, etc.?

 

Then yes, the speed of gravity is c. But it's not really relevant, because that could never happen. The amount of mass/energy will always stay the same in out universe, to my knowledge. A mass couldn't appear out of nowhere as per his question. But then again, I might have misunderstood his question.

[Strange]

The same way that moving the end of a 1 light year long stick would be instantenous, instead of taking 1 light year.

 

 

It would take more that 1 year. The movement would propagate at the speed of sound in the stick.

Then yes, the speed of gravity is c. But it's not really relevant, because that could never happen. The amount of mass/energy will always stay the same in out universe, to my knowledge. A mass couldn't appear out of nowhere as per his question. But then again, I might have misunderstood his question.

 

But we do see gravity waves (which carry energy away from a system) and these travel at c.

[Sensei]

Let's consider more realistic scenario.

There are lonely stars ejected by f.e. black holes

https://en.wikipedia.org/wiki/Intergalactic_star

 

Such star with significant velocity (perhaps even higher than galaxy escape velocity), is flying through our Solar system.

 

If speed of gravity would be higher than speed of light (or instantaneous),

we would notice arriving of rogue star (as mangling of planets orbits), prior light from it arrived..

Edited February 22, 2017 by Sensei

[Lord Antares]

Really? I don't mean moving the whole way, I just mean by several centimeters.

How is that possible? That would mean that the stick actually gets shorter by several centimeters for some time, just by pushing it.

 

EDIT: Addressed to strange.

Edited February 22, 2017 by Lord Antares

[J.C.MacSwell]

Really? I don't mean moving the whole way, I just mean by several centimeters.

How is that possible? That would mean that the stick actually gets shorter by several centimeters for some time, just by pushing it.

 

EDIT: Addressed to strange.

Nothing has an infinite modulus of elasticity, and in theory nothing can. I would expect it would take very little force to shorten a light year long stick several centimetres, even using the stiffest known materials, though it might take some time.

 

 

If I had to guess, I think we could shorten a 1 inch diameter, light year long stick of any known material by several miles (assuming it was constrained not to buckle), just by lightly pushing with our own strength on each end (assuming we were supported to keep pushing as it moved) before feeling any effects from the other end (many many years later)

 

WRT the speed of gravity, recalling from previous threads on orbits, despite the lag from the distance the gravitational "force" is directed for the most part at the current positions, not the positions the bodies were in the lag time earlier.

Edited February 22, 2017 by J.C.MacSwell

[Lord Antares]

This is actually baffling to me. I don't know why or how it happens.

However, this is getting off topic again (in a topic that was split for being off-topic) so I would implore you not to respond to this anymore. I have questions about this and will probably start a new thread asking them.

 

zztop and Sensei gave good replies which were in line with what was being asked. I will leave it to madmac to address them as they were his questions about gravity.

[Bender]

Just wondering: do we have reasons to assume 'gravitons' are massless? Is there a hypothetical possibility that gravitons have a very small mass and as a result travel slightly slower than c?

[Strange]

Just wondering: do we have reasons to assume 'gravitons' are massless? Is there a hypothetical possibility that gravitons have a very small mass and as a result travel slightly slower than c?

 

 

I think gravitons (if they exist) are assumed to be massless because gravity travels at the speed of light.

 

But nothing is taken fro granted. There are experimental tests of photon mass (these are consistent with it having zero mass). And there will be experimental tests of the speed of gravity (as soon as we are able).

Edited February 22, 2017 by Strange

[swansont]

Just wondering: do we have reasons to assume 'gravitons' are massless? Is there a hypothetical possibility that gravitons have a very small mass and as a result travel slightly slower than c?

 

 

If not massless they would not travel at c and gravity would not have the resulting 1/r^2 behavior.

[madmac]

Sensei, Strange, Lord Antares & co.

It looks like i have been accused of hijacking, but one would think that the speed of gravity was not irrelevant to infinite plates.

However the speed of gravity is one of the most important issues ever (albeit ignored i think), & deserves its own thread.

 

Mainstream says that the velocity of the gravity effects of any changes in mass travel at speed c by virtue of gravitational waves. Likewise quadrupole rotation & spin effects.

And mainstream says that any non-changes in mass act instantaneously (or in effect instantaneously), including for moving mass. Although only locally.

 

Hencely Earth feels the gravity of the Sun at the Sun's true position -- but feels the loss of gravity due to any loss of mass in the Sun at velocity c, ie at the Sun's visible position.

 

Or am i wrong??

 

And i am surprised that it has been said that mass isn't changing. I thought that mass was lost in nuclear explosions (fission & fusion).

And that perhaps mass is created also, i mean real mass (somehow somewhere). Or does mainstream consider that (nowadays) mass in effect only changes form (mass to energy)(& vice versa)??

In all such cases the speed of the change in gravity would be c.

But perhaps the mainstream idea is that a change from one form to another results in no (nett) gravitational wave.

 

Or am i wrong??

Edited February 22, 2017 by madmac

[MigL]

If gravitons had mass, that mass would restrain their 'range'.

Since gravity has an infinite range, its mediating particles must be massless.

And since they are massless, are constrained to move at c .

[swansont]

And i am surprised that it has been said that mass isn't changing.

Who said that, and in which post?

 

I thought that mass was lost in nuclear explosions (fission & fusion).

And that perhaps mass is created also, i mean real mass (somehow somewhere). Or does mainstream consider that (nowadays) mass in effect only changes form (mass to energy)(& vice versa)??

In all such cases the speed of the change in gravity would be c.

But perhaps the mainstream idea is that a change from one form to another results in no (nett) gravitational wave.

 

Or am i wrong??

The mass loss from the sun is in the form of electromagnetic radiation, not gravitational radiation, and is around a part in 10^21 per second, so it's really small.

[madmac]

swansont.

I think my comment re mass isnt changing is referring to Lord Antares post#4 where he says that a mass couldn't appear out of nowhere.

This probably referred to the macro world -- but i think my comment included macro & quantum worlds.

Anyhow the question is whether nowadays there is ever any loss of mass or creation of mass of the kind that would give rise to a gravitational wave (travelling at c).

I guess that in other words this asks whether there is a change in the number of real particles having real mass (or a change in total mass).

And i guess that here radiation doesn't have mass of the type giving rise to GWs.

[swansont]

swansont.

I think my comment re mass isnt changing is referring to Lord Antares post#4 where he says that a mass couldn't appear out of nowhere.

Which is correct. Energy is a conserved quantity. If a mass were to appear, the energy must already exist. The mass would not be appearing out of nowhere, it would be a conversion from some other form of energy.

 

Nobody said that the mass of the sun isn't changing.

 

This probably referred to the macro world -- but i think my comment included macro & quantum worlds.

Anyhow the question is whether nowadays there is ever any loss of mass or creation of mass of the kind that would give rise to a gravitational wave (travelling at c).

I guess that in other words this asks whether there is a change in the number of real particles having real mass (or a change in total mass).

And i guess that here radiation doesn't have mass of the type giving rise to GWs.

How would you get the quadrupole radiation of GWs from mass conversion?

But that's not really a valid question because, as far as I know, no gravity has ever appeared or disappeared after the big bang, so even that hypothetical scenario is useless as it could never happen.

So all of the gravity there is is already in effect. There can be no ''speed of gravity''.

When a mass undergoes acceleration (of certain types), it causes gravity to change. These changes propagate at c.

[Bender]

 

 

If not massless they would not travel at c and gravity would not have the resulting 1/r^2 behavior.

Does it need to be completely massless, or does this only pose a (very tiny) upper limit of the mass it could have? As far as I know, neutrino's also have mass, yet they move at nearly the speed of light (we haven't been able to measure a difference), and have a pretty large range.

In other words: are we able to detect the difference between completely massless and very nearly massless?

[Lord Antares]

Does it need to be completely massless, or does this only pose a (very tiny) upper limit of the mass it could have? As far as I know, neutrino's also have mass, yet they move at nearly the speed of light (we haven't been able to measure a difference), and have a pretty large range.

 

But doesn't this answer itself? Gravitons are thought to be massless and move at c. Neutrinos have a tiny mass, but their velocity is a tiny bit below the speed of light. So everything is in order.

As I understand it, you may be asking ''but is it possible we cannot measure the difference between almost the speed of light and the speed of light?''.

 

If so, two quotes, one from quora, the other from Wikipedia:

 

 

 

First, since the range of the force due to gravity is infinite and it weakens as the distance increases, the graviton must have zero mass. Therefore, like massless photons, gravitons should also travel at the speed of light.

 

 

 

If it exists, the graviton is expected to be massless (because the gravitational force appears to have unlimited range)

 

So, as said above, if it weren't massless, the gravitation formula wouldn't be correct.

Edited February 23, 2017 by Lord Antares

[J.C.MacSwell]

If neutrinos (or gravitons) have a very small mass, and thereby travel "almost" at c, should there not be some pretty slow ones running around these parts, having travelled some distance while the Universe expanded?

[swansont]

Does it need to be completely massless, or does this only pose a (very tiny) upper limit of the mass it could have? As far as I know, neutrino's also have mass, yet they move at nearly the speed of light (we haven't been able to measure a difference), and have a pretty large range.

In other words: are we able to detect the difference between completely massless and very nearly massless?

 

 

You would have to quantify it to get a definitive answer. As MigL points out, having mass limits the range. As a rough guide, you look at the Heisenberg uncertainty relation and know that the lifetime is limited to about hbar/energy (which is the mass energy). If the lifetime is finite, so is the range.

[Bender]

What is the largest distance gravity is observed to operate?

[StringJunky]

What is the largest distance gravity is observed to operate?

About 200 million light years then expansion takes over, so something a bit less than that could be said to be its effective operating distance. This is my own conjecture.

Edited February 23, 2017 by StringJunky

[Strange]

If neutrinos (or gravitons) have a very small mass, and thereby travel "almost" at c, should there not be some pretty slow ones running around these parts, having travelled some distance while the Universe expanded?

 

 

Indeed. And there is presumed to be a neutrino background, equivalent to the cosmic microwave background but earlier. However we can only detect neutrinos with a high energy so, for the time being, that is not testable. (Because they have so little mass, the tiniest amount of kinetic energy means they are moving at near c.)

[madmac]

swansont.

Re How would u get the quadrupole radiation of GWs from mass conversion?.

I suppose that a quadrupole GW is a pulse, lots of single pulses (not really a common wave). Spreading in all directions, but moreso in one direction (lighthouse effect).

And if a bit of mass suddenly appears somewhere, then we have a sudden single pulse (attraction). And if a bit of mass suddenly disappears somewhere then we have a sudden lessening of gravity (a single negative pulse). Spreading equally in all directions.

So its the same thing only different. If indeed there is such a thing as creation & annihilation of mass nowadays.

 

Lord Antares re post#1. U said...

... So all of the gravity there is already in effect. There can be no "speed of gravity".

Yes this is the mainstream theory as i remember it.

This appears ok if the Sun is considered fixed, & the Earth moves through the Sun's space-time field, then the Earth feels the Sun's gravity "instantly" at all times, even though the field itself is limited to speed c.

But then also the Sun must consider itself to be moving through the field of a fixed Earth, to feel Earth's gravity instantly at all times.

And likewise the Sun & every body in the solar system must be considered fixed & yet not fixed for this "instant" theory to work.

Or am i wrong??