A Couple of Questions About Gravity

[FluffyUnbound]

I have been having some trouble thinking gravity through and am looking for some insight on it.

1. Does gravity act instantly regardless of distance?

In other words, does the sun's gravitational attraction of the Earth act instantly across space, or does it act at the information limit of the speed of light?

2. Is it theoretically possible for a sense organ to exist that "sees" gravity?

Our eyes evolved to provide us information about our surroundings by detecting the impact of light in the visible spectrum on objects.

Is there any reason that it would be impossible for evolution to produce a species with a sense organ that somehow detects the gravitational effect one object has on another, "seeing" gravity the same way we "see" light and "feel" heat?

3. If #1 is true and #2 is possible, would a species in possession of such a sense organ live in a non-relativistic universe, since they would be able to get information about the universe instantly, without being bound by the limit of the speed of light? When that species "looked" up into the sky, would it "see" the whole universe as it actually is in real time, rather than seeing the light-ghosts of the past as we do?

 

[iguana_tonante]

Quick answers:

1. Gravitational interactions are bound by the speed of light. Gravity is not instantaneous. (In fact, in a relativistic universe, the very concept of "instantaneous" is meaningless.)

2. We have an organ that sense gravity, the vestibular system of the inner ear. Together with your visual system and your proprioception system, it gives the sense of balance, or equilibrioception.

 

[JarodRussell]

1. Gravitational interactions are bound by the speed of light. Gravity is not instantaneous. (In fact, in a relativistic universe, the very concept of "instantaneous" is meaningless.)

Has that ever been tested, and how? If the Sun suddenly vanished, would it take 8 minutes before Earth's orbit is affected? That's kinda weird.

 

[YellowSubmarine]

1. Gravitational interactions are bound by the speed of light. Gravity is not instantaneous. (In fact, in a relativistic universe, the very concept of "instantaneous" is meaningless.)

Has that ever been tested, and how? If the Sun suddenly vanished, would it take 8 minutes before Earth's orbit is affected? That's kinda weird.

Yes. It would take 8 minutes before the Earth's orbit is affected. But that's not what's weird, what's weird is that the Earth is always falling towards the actual position of the Sun, not towards the observed position of the Sun. If it were the latter, it would eventually fall out of the solar system.


http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html
https://secure.wikimedia.org/wikipedia/en/wiki/Speed_of_gravity

 

[Wanderlust]

Now that is weird.

 

[KJbushway]

Yeah. I missed the chance to comment but the seond post about the ears and balance is what I was going to say.

 

[YellowSubmarine]

2. Is it theoretically possible for a sense organ to exist that "sees" gravity?

Actually, I don't think that's feasible. The vestibular system is not what you're asking for.

If you're in a free fall, you can't feel gravity at all. You can measure the acceleration caused by gravity if it's different in different parts of your body (it would be more correct to say that you can measure difference in acceleration). For example, if you're firmly on the ground, your inner ear can sense the resulting vector of all gravitational sources pulling you down because the ground is pushing you up. But that would be exactly the same as a platform that's accelerating upwards with 1g with no gravity at all. Conversely, astronauts on the space station aren't feeling anything with their vestibular system, even though the gravity of the Earth is almost the same.

Since gravity is a very weak force, small gravitation sources don't have almost any effect, and another problem is that gravity doesn't change very much from point to point.

To see gravity you need to create a big enough grid of interconnected balls and measure the forces that the balls exert on each other. That way you can tell the differences in the forces that the difference balls feel, and therefore the difference in gravity. You can detect how the gravitation changes and maybe even isolate different gravitational sources. But gravity is way too weak for anything like that to be feasible.

We already have one such grid: The Earth. It always “sees” the position of the Moon, and the way to have the reading is to measure the tides. That's possible because 1) the moon is close to us, so the force of gravity changes “significantly” as it crosses the Earth, 2) the Earth is enormous so that the same effect can be measurable and 3) the Moon's massive enough to be measurable.

My expectation is that a human-constructed gravity sensor of a similar design could only work on merging black holes from a close range.

Now, if you could “focus” gravity somehow to concentrate and intensify it before it reaches your grid, you might be able to do something, but to be honest, I have no idea what this would even mean, let alone how possible is it to do it.

Now, to not sound too pessimistic, Neptune has been found due to its gravity being “seen” in the effect on the orbit of Uranus. You could position multiple small objects around the solar system and measure speed differences between them and use that to find bigger gravitational sources. Now, I don't see how that would be that useful, but it's certainly possible. We could use something similar to construct our first early warning systems for rogue black holes approaching the solar system.

 

[JarodRussell]

But that's not what's weird, what's weird is that the Earth is always falling towards the actual position of the Sun, not towards the observed position of the Sun. If it were the latter, it would eventually fall out of the solar system.

That makes no sense to me. If gravity works at speed of light, then it should be the observed position.

 

[FluffyUnbound]

But that's not what's weird, what's weird is that the Earth is always falling towards the actual position of the Sun, not towards the observed position of the Sun. If it were the latter, it would eventually fall out of the solar system.

That makes no sense to me. If gravity works at speed of light, then it should be the observed position.

Yeah, that seems weird to me too.

If it's falling towards the actual position and not the observed position, then a piece of information [the actual position] has arrived at a speed faster than light.

 

[FluffyUnbound]

Quick answers:

1. Gravitational interactions are bound by the speed of light. Gravity is not instantaneous. (In fact, in a relativistic universe, the very concept of "instantaneous" is meaningless.)

2. We have an organ that sense gravity, the vestibular system of the inner ear. Together with your visual system and your proprioception system, it gives the sense of balance, or equilibrioception.

Thanks!

#2 is less important based on your answer to #1. I was looking for a way around the light speed limit on information.

 

[YellowSubmarine]

That makes no sense to me. If gravity works at speed of light, then it should be the observed position.

What I said was slightly misleading and possibly slightly wrong.

The idea is that the motion of the observer doesn't change the direction of the field. The sun might be orbiting around the centre of the galaxy, but since the acceleration is minimal you can say that it's moving with almost constant speed. This means that there is an inertial frame of reference in which the sun is almost stationary. If the sun is stationary, all planets would fall towards it, not to some other random point. If you switch to another observer, the planets would still continue to fall in this same direction even if you don't “see” the sun there because of observer delay effects.

My claim that we are pulled towards the “actual” position of the sun might be wrong or at least misleading, I can't quite understand how the picture changes if you're looking from the frame of reference of the Earth. I don't understand the equations, and I can't seem to find a way to figure that out without them.

(I made some sketches here, and so far I still think that what I said is correct, but I can't verify if I'm using the right assumptions for the drawings, so don't assume is correct without a word from someone knowing relativity. )

 

[Wanderlust]

I asked this question to a friend who is knowledgeable in science and this is his answer.

In the Einsteinian understanding of gravity, gravity is an emergent property of the curvature of space-time in the presence of mass. It's range is infinite, but it's strength drops off as the inverse cube of the distance. Gravity doesn't "propagate" (although there are such things as gravitation waves, which are variations in the curvature of spacetime that propagate as waves, emitted by very massive co-orbiting objects [like two orbiting black holes]), so it doesn't have a "speed". Technically, you are always in a gravity field, because it's range is infinite. If the sun were to suddenly disappear, I'm not sure how long it would take for space to "flatten out", though. That's an interesting question. I would assume it would flatten out almost instantly, if the sun were to simply disappear.

 

[John O.]

I think it's been incorrectly insinuated that there is a definite answer to this question, the question of the finiteness or non-finiteness of the "speed" of the propagation of the effects of gravity.

It's important to remember that gravity is exactly the crux of the problem in unifying quantum mechanics and general relativity. Even though GR says that massed particles cannot be accelerated beyond the speed of light, there are proposed loopholes ABOUND on this question. It assumes too much to describe, with certainty, our actual known universe as a "relativistic" universe, since that's just saying "our universe as governed universally and absolutely by the laws of general relativity", which, I need not remind you, fall apart at the Planck scale. A brief list of these potential loopholes:

1) Force mediating particles of negative mass
2) Higher dimensionality
3) Existence of chiral symmetry in the S(0) boson group
4) There are arguments for why causality may not be broken by information transit>c


I've seen the studies done that show the speed of gravitational propagation to be .8-1.2c and the handwavey claim that "statistically that supports the argument that it's actually exactly c". The reality is that the methodology is still controversial and we will not know anything for certain until a gravitational force mediator is identified and understood.

On the applied question of 'sensory organs' - the short answer is 'no' IMO, the long answer is if that even if there is for certain a force mediating particle like the graviton, and unless something truly bizarre turns out to result in a wildly off-base value for its rest mass (i.e., not the predicted values), then it's hard to imagine biological tissue could detect it. The theory is that gravitons are either massless or of extremely low mass. If they are finite positive mass, then obviously the answer to the first question is that they are of finite velocity, but the answer to the second question is - it's difficult to imagine a cellular mechanism by which this kind of particle influence could be felt. We only 'see' light because it's of sufficient energy to excite electrons in our optic nerve that turn into neuronal impulses, and electrons are "big shots" on the atomic landscape compared to particles even remotely near the size scale we're talking about being likely for gravitons (like on the neutrino scale or smaller). We can barely detect solar neutrinos by forcing them to pass through a 20 million gallons of water, that's how little they interact with the atoms they're moving through. The nature of the graviton is so crucial to this question that much of this is speculative, but if it's on the mass range of a neutrino, the idea that it could somehow trigger a cellular mechanism (like a neuronal impulse) is difficult to imagine.

 

[Bill Morris]

Yes, the effect of gravity is limited to the speed of light.

With regard to General Relativity, let me first quote Albert Einstein himself:

There could be no fairer destiny for any physical theory than that it should point the way to a more comprehensive theory in which it lives on as a limiting case.

I wonder about the mechanism of gravity in terms of zero-point energy, which is more than just a quirk of quantum mechanics. Inability to freeze helium is attributed to zero-point energy, and the Casimir effect is thought to be a result of zero-point energy, where such energy in the longer wavelengths is prevented from counteracting that in the opposite direction, suggesting also that gravity and perhaps even the strong force are pushing rather than pulling actions of packets of energy nearly infinite in quantity but individually too weak to detect with current technology. (Another theory about that Casimir effect has to do with "virtual particles.")

Anyway, this is sort of 19th-century ether physics, perhaps relying on the assumption that even the best of the far-superior modern versions of the Michelson-Morely experiment are still not sensitive enough to even indirectly detect zero-point energy. But there are definitely problems with current theory, and it may take a long time to sort all this out.​

 

[YellowSubmarine]

That makes no sense to me. If gravity works at speed of light, then it should be the observed position.

(I made some sketches here, and so far I still think that what I said is correct, but I can't verify if I'm using the right assumptions for the drawings, so don't assume is correct without a word from someone knowing relativity. )

To make a simplified version of my sketch:

Observed from the Earth, the Sun doesn't move much. If you wait 8 minutes it will still occupy almost the same position in the sky. This means that the observed position and the actual position (seen 8 minutes later) are nearly the same, so you can safely say that we are accelerated towards either one.

Imagine an object moving damn fast on the tangent to the orbit of the Earth. From its perspective, the Sun moves, and there is a 8 minute delay effect, so it will observe the sun in a different position than we do. Now the big assumption that I'm making. Both Earth and this hypothetical object will observe the same gravitation field and will be pulled in the same direction. As a result, the object will fall towards Sun's actual position, even though it “sees” it somewhere else.

Someone with understanding of relativity to tell if this is correct.

Now, if the Sun had vanished less than 8 minutes ago, the effect would presumably travel with the speed of light, and both the earth and the unknown object would fall in that same direction even though there's no Sun there any more.

Of course, nobody has measured the speed of gravity reliably so far, and I'd guess that this effect contributes to how hard it is to measure it. If gravity acts instantly, then yes, there would be means for communicating faster than light with current technology, but they would be incredibly expensive and possibly impossible with our limited resources.

 

[JarodRussell]

I don't get it, I think.

 

[John O.]

Yes, the effect of gravity is limited to the speed of light.

That's an opinion, not a fact. It shouldn't be represented as such. The fact is that this question has not be satisfactorily answered.

Ask Ronald Held if you don't believe me.

 

[Bill Morris]

Yes, the effect of gravity is limited to the speed of light.

That's an opinion, not a fact. It shouldn't be represented as such. The fact is that this question has not be satisfactorily answered.

Ask Ronald Held if you don't believe me.

Agreed. I should have checked the lastest on that.

 

[YellowSubmarine]

Yes, the effect of gravity is limited to the speed of light.

That's an opinion, not a fact. It shouldn't be represented as such. The fact is that this question has not be satisfactorily answered.

We certainly don't know for sure what the speed of gravity propagation is. It's true that there is a lack of experimental evidence that could directly or indirectly confirm certain limit for its speed. However, I think we can assume certain things about those results.

Instantaneous propagation of the effect of gravity would almost certainly imply faster than light communication. Faster than light communication would imply communication back in time. The implications of not having the effect limited by the speed of light would be extreme and would make the universe much much more complicated. And that's when you know it's unlikely.

We wouldn't know for sure before we test it, that's why we have to test it, but until we know, I'd say that the assumption that it's limited by the speed of light is the correct and safer one. Assume that the unlikely option is not true, and if it turns out to be true – well, the best thing that can happen in science is when the results contradict the expectation! There's nothing like a good surprise.

Come on! “Don't worry, we won't die in a car crash today!” “Hey, that's an opinion, not a fact!” At what sigma confidence level does an opinion become a fact exactly?

 

[Bill Morris]

Okay, well, I thought I had lost this battle about speed of gravity several years ago with that observation involving Jupiter and some distant celestial object, but that has not been accepted as reliable.

But if the zero-point energy or virtual particles or so-called gravitons or whatever responsible for gravity are not of wavelike nature and not limited to the speed of light, that doesn't mean going back in time. Time dilation of a massive object traveling at 90% lightspeed or so doesn't really necessitate the colorful SR description that time itself has slowed down, at least from a mechanistic point of view, but just that its physical processes, like beta decay or even how fast an electron orbits, are slowed.

And exceeding lightspeed doesn't mean going back in time. You can see back in time just by looking at the stars, since their light took years to reach you, but to go back in time is sci-fi.

And those mysterious tiny packets of energy just might be responsible for determining the speed at which photons travel, as expected in wave mechanics, since they could well be the medium and perhaps not be subject to wave mechanics in their own movements.