Why does gravity get weaker with distance?

[Rdivine]

So we know that gravity is a wave. Light, and the electromagnetic spectrum, is a wave. However, light does not seem to get weaker with distance.

Brightness does get weaker with distance as the gap between photons increases. However, since gravity has no particles, why does it get weaker the further you get away from it?

If anyone has any scientific background and realises im misintepreting something, please correct me!

[Steel]

So we know that gravity is a wave

First of all, do we know gravity is a wave? There's excellent evidence to support gravitational waves (disturbances in spacetime), but none as far as I know supporting that gravity is a wave (unfortunately, despite those two sounding the same, they are in fact two very different things)

Secondly, there is a theoretical, but currently unobserved, particle called the graviton that could be responsible for mediating gravitational interactions

Thirdly, while its true photons do not get weaker over distance, the intensity of the electomagnetic field due to the photons (brightness as you called it) does, and its this field that we observe and effects things. In the same way, a graviton would not weaken as it travelled but the intensity of the gravitational field (or if the like the 'brightness' of the gravity of an object) will weaken

I hope this helps

Edited July 27, 2015 by Steel

[magnemoe]

So we know that gravity is a wave. Light, and the electromagnetic spectrum, is a wave. However, light does not seem to get weaker with distance.

Brightness does get weaker with distance as the gap between photons increases. However, since gravity has no particles, why does it get weaker the further you get away from it?

If anyone has any scientific background and realises im misintepreting something, please correct me!

Light get weaker at distance, look at the sun, wait until night and look at an random star, notice the difference in light from it.

Its the inverse square law who works for all point sources who radiates in all directions. Gravity works the same way.

[Ralathon]

So we know that gravity is a wave. Light, and the electromagnetic spectrum, is a wave. However, light does not seem to get weaker with distance.

Brightness does get weaker with distance as the gap between photons increases. However, since gravity has no particles, why does it get weaker the further you get away from it?

If anyone has any scientific background and realises im misintepreting something, please correct me!

Light also gets weaker following the same inverse square law. This is why spaceprobes past Jupiter can't use solar panels for energy production. Or why a small light bulb isn't enough to light up your entire house.

The reason this happens is a simple consequence of living in a 3 dimensional universe (with time as a 4th, but lets not get into GR here). Say you have a bubble with 100 small dots on its surface. The bubble has a surface area of 1m^2, so the density of the dots is 100 dots per square meter. Now say we inflate the bubble so its twice as big. The bubble now has 4m^2 of surface area, but the number of dots stays the same. So the number of dots per square meter is now 25. Twice the radius means the dot density becomes 4 times as low.

Same thing happens in the universe. Gravity (or light) is equivalent to the dot density. The number of photons/gravitons is the same but they're smeared out over a larger area. And because that area increases by the square of the distance you end up with the familiar inverse square laws.

[Darnok]

Light get weaker at distance, look at the sun, wait until night and look at an random star, notice the difference in light from it.

Its the inverse square law who works for all point sources who radiates in all directions. Gravity works the same way.

So past Jupiter (like @Ralathon mentioned) each photon is carrying less energy? That would means for me it is getting weaker.

[andrewas]

No. Well, slightly, since photons lose energy as they climb out of the gravity well, but its negligible. But the important thing for solar panels and illumination is how much energy is delivered to a given area. Double your distance from the star and you get 1 quarter of the energy density.

[shynung]

So past Jupiter (like @Ralathon mentioned) each photon is carrying less energy? That would means for me it is getting weaker.

No, but there are less photons hitting a particular area. Cumulative energy by area becomes smaller, but not energy of the individual photons.

Edit: Ninja'd.

[SuperFastJellyfish]

I'm a fan of visualizations. This might help some people to understand what has been said so far in this thread:

[peadar1987]

Each photon has the same amount of energy (more or less), but the amount of photons hitting a square metre is smaller, as they have spread out.

Barring weird cases like the photoelectric effect, the energy you receive is the energy of one individual photon multiplied by the number of photons that hit you, so both the energy of a photon and the "density" of the photon field affect energy.

[Darnok]

Each photon has the same amount of energy (more or less), but the amount of photons hitting a square metre is smaller, as they have spread out.

Barring weird cases like the photoelectric effect, the energy you receive is the energy of one individual photon multiplied by the number of photons that hit you, so both the energy of a photon and the "density" of the photon field affect energy.

I know, but now IF gravity works same way just in opposite direction that would means every particle on Earth can be pulled by "gravity wave" send by planet discovered last week and being 1400ly away. Maybe that is source of "randomness" in quantum physics?

[peadar1987]

I know, but now IF gravity works same way just in opposite direction that would means every particle on Earth can be pulled by "gravity wave" send by planet discovered last week and being 1400ly away. Maybe that is source of "randomness" in quantum physics?

Yep, earth is affected by the gravity of every object in the observable universe. However, most of them are too far away to have any measureable effect.

Subatomic particles are dominated by forces many orders of magnitude stronger than gravity, acting over far smaller distances, so I think it's fairly safe to say that the gravity of distant objects has a negligibly small effect over, say, the motion of electrons.

[Hcube]

I know, but now IF gravity works same way just in opposite direction that would means every particle on Earth can be pulled by "gravity wave" send by planet discovered last week and being 1400ly away. Maybe that is source of "randomness" in quantum physics?

Whaaaaat ? no, i don't think that you understand what gravity waves are. Just think of the regular gravittional force here. Yes, every single thing in the universe that has a mass is attracted by said planet. But in the case of earth, it is so far away that this force is next to nothing, (it can be calculated easily btw and would be incredibly close to 0) so the earth isn't going to be cruising towards Kepler-452B

[YNM]

One thing to clear off : We're going to talk gravitational wave, which is generated mostly by close orbiting massive objects, and not gravity wave, which are waves that is normalized (assisted ?) by gravity (gravitational acceleration), like undulating clouds and ocean waves.

Gravitational wave are generated by the "lag" in spacetime curvature whenever a huge object creates that curvature, and is propagating at speed of light (guess this is more of causal matter than anything, or metric matter). When you have two massive objects (means making a huge "dent") going fast (most likely around each other), that means the curvature felt by an observer is going to alternate much like ocean waves. If you want to know why does gravitational effects goes down by (approx.) ratio of the square of distance, my thinking is that they're a consequence of volume and surface area. Not sure how to explain them in GR, I barely understand tensors but those are some of my bet, based on stress-energy tensor and the metric, which implies less energy density for larger space (and so less curvature).

[saquito]

That it could be actually true. According to one of the most famous theories which are trying to "tune up" the Newton-Gauss-Birkhoff theorems for explaining things like the rotation curves of the galaxies removing the dark matter from the equations, gravity force never weakens to zero. This theory is called MOND and it replaces the inverse-square law with a little term of acceleration. This theory says that every point in a hollow mass sphere distribution (the universe, in this case), is subject to an small force towards the center of mass. This way, any point in this universe would affect, still relative to distance, with at least the force this potential gravitational acceleration provides to it. Check this out for more info: http://www.scielo.br/pdf/bjp/v39n3/15.pdf

Randomness in quantum physics is indeed a problem which probably relies in the imposibility for us to observe anything below the Planck distance. It seems there is a lot of things happening there, a huge sea of events with tides, waves and posibly catastrophic events. The random behaviour we see is the foam of this sub-Planck events. You probably heard about the quantum foam: the closest you try to look (the narrowest you try to make your observation), the most frantic the universe gets (and still, this tiny events can lead to macroscopic effects, like the spontaneous generation of particle-antiparticle pairs causing the Hawking's Radiation at the event horizon of a black hole).

I know, but now IF gravity works same way just in opposite direction that would means every particle on Earth can be pulled by "gravity wave" send by planet discovered last week and being 1400ly away. Maybe that is source of "randomness" in quantum physics?

[Hcube]

One thing to clear off : We're going to talk gravitational wave, which is generated mostly by close orbiting massive objects, and not gravity wave, which are waves that is normalized (assisted ?) by gravity (gravitational acceleration), like undulating clouds and ocean waves.

Gravitational wave are generated by the "lag" in spacetime curvature whenever a huge object creates that curvature, and is propagating at speed of light (guess this is more of causal matter than anything, or metric matter). When you have two massive objects (means making a huge "dent") going fast (most likely around each other), that means the curvature felt by an observer is going to alternate much like ocean waves. If you want to know why does gravitational effects goes down by (approx.) ratio of the square of distance, my thinking is that they're a consequence of volume and surface area. Not sure how to explain them in GR, I barely understand tensors but those are some of my bet, based on stress-energy tensor and the metric, which implies less energy density for larger space (and so less curvature).

yup, and none of those gravitational waves has been detected yet... there is this instrument in italy made of 3km-long "arms" that form a cross, and should get longer (of the order of the nm, very small elungation !)when a gravitational wave passes by... I can't remember the name of that instrument though... can anyone refresh my mind ?

[saquito]

yup, and none of those gravitational waves has been detected yet... there is this instrument in italy made of 3km-long "arms" that form a cross, and should get longer (of the order of the nm, very small elungation !)when a gravitational wave passes by... I can't remember the name of that instrument though... can anyone refresh my mind ?

I understand that you are talking about the Lisa Pathfinder mission, which we have scheduled to launch in 2015. It implements an interferometer capable of nanometer resolution, aimed to detect low frequency (1-30MHz) gravitational waves. It will check the variations of a reference mass with this instrument in the order of nanometer, but measuring the relative motion of both the spacecraft with the instrument and the reference mass, it is capable of measuring with a picometer resolution, which it is suspected it is an acceptable resolution to observe this gravtiational waves.

By the way, you also have the LIGO (Laser Interferometer Gravitational-Wave Observatory) comprised of two separated laboratories located in Lousiana and Washington states, both working in paralel for remove false positives (seismic events, instrument failures...)

Edited July 27, 2015 by saquito

[Hcube]

I understand that you are talking about the Lisa Pathfinder mission, which we have scheduled to launch in 2015. It implements an interferometer capable of nanometer resolution, aimed to detect low frequency (1-30MHz) gravitational waves. It will check the variations of a reference mass with this instrument in the order of nanometer, but measuring the relative motion of both the spacecraft with the instrument and the reference mass, it is capable of measuring with a picometer resolution, which it is suspected it is an acceptable resolution to observe this gravtiational waves.

By the way, you also have the LIGO (Laser Interferometer Gravitational-Wave Observatory) comprised of two separated laboratories located in Lousiana and Washington states, both working in paralel for remove false positives (seismic events, instrument failures...)

Haha, no i wasn't talking about the LISA pathfinder mission ! I was talking about something like the LIGO : as you said, there are two dispositives like it in the USA, but there is a third one very similar to those located in Italy (that's the one i know) I'ts a huge 3km-wide X with lasers continuously measuring the lenght of the "arms". I guess we are talking about the same thing

[saquito]

Haha, no i wasn't talking about the LISA pathfinder mission ! I was talking about something like the LIGO : as you said, there are two dispositives like it in the USA, but there is a third one very similar to those located in Italy (that's the one i know) I'ts a huge 3km-wide X with lasers continuously measuring the lenght of the "arms". I guess we are talking about the same thing

I just googled it. Is the VIRGO interferometer you are talking about, right? So cool what I just read about: https://en.wikipedia.org/wiki/Virgo_interferometer

[Hcube]

I just googled it. Is the VIRGO interferometer you are talking about, right? So cool what I just read about: https://en.wikipedia.org/wiki/Virgo_interferometer

Yes that's it ! it's a really interesting instrument. There was recently a TV program about it on Arte with an interview of an italian astrophysician that works there, i loved it !

EDIT: What would the celerity of such a wave be ?

Edited July 27, 2015 by Hcube

[saquito]

Yes that's it ! it's a really interesting instrument. There was recently a TV program about it on Arte with an interview of an italian astrophysician that works there, i loved it !

EDIT: What would the celerity of such a wave be ?

According with general relativity they would have the speed of light in vacuum. As you know, everyhing in the universe is traveling at the speed of light, just balancing the speed in four dimensions, three spatial and time, moving along a Minkowski space. Since the graviton is stated would have mass zero, all its energy would be cinetic, as the photon. The difference with the photon is the nature of the force (the nature of the field) they transmit: electromagnetic for photons, gravity for gravitons. However, there are theories saying that a gravition with a mass slightly above zero would be an excelent explanation of Einstein´s cosmological constant with no need of the dark energy. However these theories are highly speculative, more if considering an increasing number of evidences of its existence through dramatical events like supernovae explosions or Doppler effects. But, who knows?

And then, of course, you have string theory and quantum gravity. Too much for me

[Jim DiGriz]

yup, and none of those gravitational waves has been detected yet... there is this instrument in italy made of 3km-long "arms" that form a cross, and should get longer (of the order of the nm, very small elungation !)when a gravitational wave passes by... I can't remember the name of that instrument though... can anyone refresh my mind ?

The orbital decay of double neutron star binary systems has been observed and is evidence of gravitational waves. The weak-field linearized Einstein field equations also describe gravitational waves and are sufficient to explain the observed orbital decay. A quantum theory of such a linear gravity would describe a spin-2 massless graviton which would be the quantum particle mediator of gravity, which has not been observed at all.

A weak-field linear theory of gravity is going to look very similar to E+M only that it will be a tensor (spin-2) force mediated by a graviton with only "positive" charge (mass). It will still obey the inverse square law and the force will drop off with distance. Individual gravitons would be similar to individual photons and would not lose force over time, but the graviton flux per unit area as you back away from an object would drop off giving rise to the inverse square law.

Of course real gravity is non-linear and that is where quantizing it explodes in messes of infinities.

I also suppose it is possible that gravity is not mediated by a particle at all, but that waves on spacetime are somehow fundamental (dodging the question for now about space-time itself being quantized, but lets say its not) in which case you'd still get an inverse square law as the strength of the wave dissipated -- just like how ripples from tossing a rock into a pond dissipate as the energy-per-unit-area of the wave decreases as the size of the ripple increases -- or how earthquake energy dissipates with distance from the epicenter, etc, etc, etc.

There's really no mystery here and as you back off of a point source of energy the area increases so the energy per unit area decreases. Affects E+M, affects earthquakes, affects ripples on a pond...

[Woksaus]

Haha, no i wasn't talking about the LISA pathfinder mission ! I was talking about something like the LIGO : as you said, there are two dispositives like it in the USA, but there is a third one very similar to those located in Italy (that's the one i know) I'ts a huge 3km-wide X with lasers continuously measuring the lenght of the "arms". I guess we are talking about the same thing

I know there are (or have been) plans to build one of these detectors in space. The laser would be at the Earth's L3 Lagrange point, while the two mirrors would be at L4 and L5. I cannot begin to imagine the challenges in building this, let alone in keeping them perfectly aligned and in place...

[PB666]

No. Well, slightly, since photons lose energy as they climb out of the gravity well, but its negligible. But the important thing for solar panels and illumination is how much energy is delivered to a given area. Double your distance from the star and you get 1 quarter of the energy density.

Photons red-shift slightly coming out of a mild gravity well. Coming out of a black hole the bend backwards in the direction from which they propagated. I suppose those that are pointed strait out end up red shifting into the long wave-length radio waves. Solar panels are dependent on wavelength, since this is essentially Einstein's photo electric effect. Therefore if the wavelength falls below the wavelength the photoelectric effect could disappear entirely. Although the sun is not a big emitter of X-rays it does produces alot of short-wave length UV and therefore red-shifting would only result in UV light becoming visible and the visible spectrum becoming infrared. Note that the yellow green part of the Sun spectrum is the most intense, a larger more massive star would emit more blue light, none the less massive red-shifting would cause a dimming of intensity at the visible spectrum for a variety of reasons (less EM emitted at short wavelengths, gravity bends light)

[Aethon]

Sorry. I didn't read the whole thing. In response to the op. The four (three sir..) forces each have a range over which they can have an effect. Gravity, as far as we can tell, acts out to infinity.

Every particle is attracted to every other particle in the universe. ( Except most of the particles that make up a surprising lot of girls ).

[PB666]

The orbital decay of double neutron star binary systems has been observed and is evidence of gravitational waves. The weak-field linearized Einstein field equations also describe gravitational waves and are sufficient to explain the observed orbital decay. A quantum theory of such a linear gravity would describe a spin-2 massless graviton which would be the quantum particle mediator of gravity, which has not been observed at all.

A weak-field linear theory of gravity is going to look very similar to E+M only that it will be a tensor (spin-2) force mediated by a graviton with only "positive" charge (mass). It will still obey the inverse square law and the force will drop off with distance. Individual gravitons would be similar to individual photons and would not lose force over time, but the graviton flux per unit area as you back away from an object would drop off giving rise to the inverse square law.

Of course real gravity is non-linear and that is where quantizing it explodes in messes of infinities.

I also suppose it is possible that gravity is not mediated by a particle at all, but that waves on spacetime are somehow fundamental (dodging the question for now about space-time itself being quantized, but lets say its not) in which case you'd still get an inverse square law as the strength of the wave dissipated -- just like how ripples from tossing a rock into a pond dissipate as the energy-per-unit-area of the wave decreases as the size of the ripple increases -- or how earthquake energy dissipates with distance from the epicenter, etc, etc, etc.

There's really no mystery here and as you back off of a point source of energy the area increases so the energy per unit area decreases. Affects E+M, affects earthquakes, affects ripples on a pond...

My guess is that it behaves like a wave when traveling and like a particle on its arrival. Gravity is a measure of warping of space time due to energy density. Since many energies smaller than that of an electron or proton have such energy the particle would have to be much smaller than these and it is far far weaker than any force, thus the particle would have to be very very small. IF you take fluid particles and make smaller and smaller spheres of them keeping the total mass the same, eventually you reach a state where the collection of particles behaves just like a wave, Gravity may be one of these, it may be so small that the only way we can detect it is in mass, where the collections of particles behave like a wave.

Experiment (thought). Create a new empty universe. Place Hydrogen molecule (Proper) in the universe, then add another hydrogen molecule to that universe with energy relative to the first of say 0.0001 Kelvin. Track the motion of the first and second without observers paradox (Heisenberg uncertainty). Without EM we cannot see any acceleration or deceleration, but if we add EM we add energy, if mass can bend light, then light can move mass. The experiment is essentially impossible because the premises of the experiment violate empirical and theoretical physics. However the experiment must be reasonable even if we cannot conduct, a force particle must be sent from one part of space to the next.

How fast does gravity act, over planks time? And how big are for hydrogen atoms in mass, multiplied together the particle would be very small. How could you detect something that produces so little force.

Thus when we look at superstellar events and we see the wave, we are actually seeing particles, but the particles are so numerous, from all realistic points of view they look like a wave, because we do not have a perspective that can resolve the particles. I don't believe you can create a circumstance in the universe where you can see the particle (given that it is massless, traveling faster toward it will not make it bigger).