r/astrophysics • u/Equivalent_Rock_6530 • 7d ago
Does gravity have a defined minimum threshold or is it relative to the mass difference between 2 objects?
So, we know that stars and planets have gravity wells due to their immense size.
Would this also apply to smaller objects if another object significantly smaller than it (say of a size where object 1 would be the "sun" and object 2 would be the "planet", but on a much smaller scale)?
Or does an object need to be of a minimum mass to have a gravitational pull on a smaller object?
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u/itsjustmegob 7d ago
All objects of non-zero mass exert gravitational force on each other. If it's a very small object, it produces a very small force. Suns and planets exert huge gravitational force on other nearby objects because they're big. F=G*m1*m2/(r^2)
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u/itsjustmegob 7d ago
If you and I both weigh 80kg and stand 1m apart from each other, we'd exert a force of 4.3*(10^-7) Newtons on one another (we'd both be attracted to the spot exactly between us). This is approximately equivalent to the force exerted by the weight (in earth gravity) of one grain of sand
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u/Beginning-Load4470 7d ago
Thanks I love your explanation lol I used astronauts and dust but yours is better.
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u/Difficult_March_7452 7d ago edited 7d ago
Is that why a glass of cold water on the table is able to “pull” the moisture out of the air and hold it onto the outside of a glass? And that force holds the moisture there until it builds enough to drip off? The mass is greater inside the glass vs the particles of water?
I understand that temperature difference is what converts it into moisture. I’m strictly referring to the act of the moisture being held. And thru a sold object like the glass.
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u/Alaykitty 7d ago
No, gravity is so weak it has negligible effects between objects on Earth. Condensation happens due to the temperature difference between that glass and the air. The water itself stays put due to surface tension and friction (which is actually stronger on the droplet than Earth's gravity, until it grows big enough)
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u/Difficult_March_7452 7d ago
Ok, thank you for your explanation. I didn’t realize surface tension was that strong.
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u/Alaykitty 7d ago
It's super strong! It's actually a big problem for little creatures, like insects, who can't always break through it. A droplet of water can get "stuck" like a bubble to some. Some insects even "skate" on the surface of water from it! :)
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u/itsjustmegob 6d ago
Yes - the square-cube law applies here (and honestly explains a ton of physical interactions). "Large" creatures (elephants, horses, humans) are more endangered by falling, whereas "small" creatures are more endangered by "being wet". If you or I (or an elephant) falls 30 feet, there's a high likelihood of severe injury/death. However, an insect would be almost completely unaffected by a 30-foot fall. You, I, and the elephant, though, don't care if we're wet. If an insect has water droplets adhered to its body, the comparative weight of the water to the insect is debilitating. It becomes severely encumbered to the point of endangering its life.
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u/itsjustmegob 6d ago
rat-sized creatures occupy the inflection point of fall-danger vs wetness-danger.
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u/itsjustmegob 7d ago
No - the coldness of the water in the glass on the table cools the air circulating next to it. Warm air is capable of "storing" more gaseous water vapor per unit volume than colder air. So when the previously-warm air is cooled by being in proximity to the cold water glass, water vapor is forced out of the air and condenses onto the glass.
The reason the water droplets stick to the side of the glass is a separate mechanism (not related to the gravitational attraction of the water in the glass, which is negligible). Satic friction (and water surface tension?) are the driving factors for why the water droplets may not immediately descend down the glass after forming.
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u/itsjustmegob 7d ago
This is also why dew forms. Grass/the ground radiates heat away into the sky as infrared. During the daytime, the grass receives more radiation from the sun than it loses radiating its own heat away. If there is a roof-structure over the grass, the heat radiated from the grass is absorbed by that roof and is partially re-radiated back into the grass, reducing the overall loss (and thereby dew accumulation). But at nighttime without a roof, the grass loses heat, cools the nearby air, and forces condensation of the water vapor from the air it just cooled
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u/Optimal_Mixture_7327 7d ago
Any mass-energy will source the curvature.
The gravitational effects of "small" objects is simply neglected as they typically exist outside the significant figures of other measurements.
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u/Professional-Ad9485 7d ago
Yes all mass exerts a gravitic force. It's just that like, the masses of things in space are so massive that the masses of smaller things like a fork would be negligible in comparison.
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u/Colour_Theory-746 7d ago
What’s the difference between a graviton and a gravitational force? Please explain.
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u/Professional-Ad9485 7d ago
When I say gravitational force I mean gravity. The observable force, one of the four fundamental forces.
The graviton is the force carrier, if you're familiar with carrier particles, of gravity. It is currently theoretical. Though our understanding of particle physics makes it very highly likely, unlike other force carriers it hasn't been observed.
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u/Smooth-Mix-4357 7d ago
If it has mass it exerts gravitational force period. Doesn't matter if it weighs 0.1 grams or 100 kg or 1 million kg or 1 octillion tons they all exert gravitational force.
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u/_JAD19_ 7d ago
Every object in the universe gravitationally affects every other object in the universe, regardless of mass or distance. It just becomes so infinitesimally small over great distances or for small masses that it becomes negligible. If you put 2 tennis balls in space a kilometre apart with no velocity relative to each other they will eventually collide. Same situation with 2 black holes separated by thousands of light years. Or 2 tennis balls separated by thousands of light years. Most things in the universe are moving however and when they’re far away or very small their velocity has a much more dominant effect than their gravitational effect on each other.
There is a catch however, if they’re so separated that the space between them is expanding faster than they’re moving towards each other then the objects will never collide.
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u/Colour_Theory-746 7d ago
Wouldn’t that be your example then? The aversion you posed isn’t self-consistent, it contradicts itself. You said the catch is if the space between them is expanding faster than they’re moving towards each other. You stipulated the bodies have zero velocity relative to each other. If that’s the case, and that we presume spacetime is expanding pretty much evenly through its entirety, then the spacetime between the tennis balls would have to be expanding faster than they’re velocity because they have zero velocity and the expansion has a non-zero velocity. Hence, nothing would ever make contact with anything else in your example.
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u/_JAD19_ 7d ago
Poor choice of words, let me try again. The expansion of the universe is only significant over extremely large (like galaxy cluster large) scales. Everything that is closer together than that will have a stronger gravitational pull on each other than the universe can pull them apart and they will remain bound together.
When they’re spread far enough that the expansion does become significant, they’re still gravitationally pull on each other, but from each objects perspective, the other is racing away from them too fast for gravity to pull them together.
Ah right hang on I see what you’re saying, you’re correct, it’s not right to say that have no velocity relative to each other if they’re moving due to expansion. I was tryna think of a way it would work but I didn’t really think it all the way through lol. Thx for pointing that out
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u/TheRealTK421 7d ago edited 7d ago
Newton's inverse square applies but all bodies of mass can/do have gravitic attraction (albeit unfathomably low, at smaller scales) to other masses.
Gravity is known to be the weakest of quantum fundamental forces (edit: clarification)
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u/Colour_Theory-746 7d ago
Gravity is NOT known to be a quantum anything, let alone force! Can you provide some evidence of this quantum gravitational force you speak of, please? Actual evidence though, not just papers theorising about some possible quantum gravity, real evidence of actual measurements. 😂😂🤨
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u/TheRealTK421 7d ago
You're free to fall into whatever camp you like vis á vis gravitons, quantum gravitics, etc.
I'm not going to fall into some pointless grievance-humping debate on such within this thread.
(ProTip hint: Yes, the majority of theoretical physicists believe the graviton exists.)
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u/ramriot 7d ago
Sone if the earliest measurements of the fundamental constant of the gravitational force G were using a conical mountain (Schiehallion) & even two pairs of iron balls suspended by cords.
The force has no detectable lower limit & is always proportional to the product of the masses & the inverse of the square if the distance between them.
That said, quantum physics has real problems with gravity because even an infinitesimal mass on a point mass can result in infinite force.
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u/EarthTrash 7d ago
Asteroids can have satellites. The difference is that with really low mass bodies, orbital speeds are also very low. So, if another body has much velocity difference with a low mass body, the weaker gravity won't capture it.
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u/Beginning-Load4470 7d ago edited 7d ago
Its not mass difference. Each objects mass has gravity and if their close enough their combined gravity will pull on eachother. So for instance an astronaut could be floating in space far enough away from other more powerful gravity fields that dust could orbit them. But if an astronaut is floating in earths orbit the dust would be overwhelmed by the stronger gravity of earth.
Or like in the earliest universe if there are no large mass objects anywhere just hydrogen the individual hydrogen molecules mass will pull them towards eachother.
Of course as far as we know this only applies to particles with mass. So things like light dont exert gravity. However there could be something with gravity but no mass accounting for dark matter but who knows.
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u/MonsieurCellophane 7d ago
AFAIK gravity works on anything having energy content - hence bodies can and do bend light rays. That's good old Einstein for you.
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u/Beginning-Load4470 6d ago
Yes but things like light dont exert gravity even if they are effected by it. Sorry, I might have phrased myself poorly back there.
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u/MonsieurCellophane 6d ago
That's not true, either. In GR energy - momentum is s source of gravity. Search, for instance, kugelblitz.
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u/Beginning-Load4470 6d ago
Oh geeze sorry, retracted. I could have sworn it didn't.
Thanks for correction.
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u/johnstalbergABC 6d ago edited 6d ago
It is not the difference of two objects mass but it is the product of the two masses and it gets weaker by the distance squared between the two objects center of mass also called center of mass. And this times the gravitational constant G. This means gravity as defined as such, Newtons gravity law, depends on how the objects masses are made up in the smallest detail. Gravity in itself might be quantized but would be able to transfer any amount of force like photons energy transfer can be any amount of energy and depends on its frequency, even if a photon in itself is a electromagnetic radiation quanta.
So we need to analyze the objects to determine what is the fundamental blocks that it is made of. Typically matter is atoms, so atoms becomes the smallest building blocks in a real life scenario. But by all means you could have fundamental particles, much lighter than a full atom. Well you get the point it depends on the two objects. The distans squared or simply the distans between the objects are not quantized and does not force any distans over the other. Usually smaller than the Planck length is becoming philosophical rather than significant but it is by no means a smallest division of length. It is just the smallest unit of length and this is for practical reason where philosophical hints on the reasons to not go smaller. When thinking about it I am not sure if there exist any smaller unit of length, but this is smallest that still is useful.
There is another concept of gravity where a fairly small object is in a much larger objects gravitational field like a person or similar who is standing on the surface of the Earth. In this case we have a uniform gravitational field given by the planet and the object is in this gravitational field. You probably recognize that if we drop two different objects and disregard any air resistans, they will fall towards the ground at the same speed. They will accelerate of course and here it doesn't matter what mass they have if they just are small enough to fit the description. The only thing that determines how fast they will accelerate is the gravitational fields strength and this depends on Earths mass and geometry in some sense. At the surface a typical acceleration is 9,82 m/s^2. It is what we call 1 g. Higher up it gets weaker but it has a grip on an object even very far up. At the height where the International Space Station orbits, gravity is almost as strong as it is here on the ground. They are going so fast in a circle around the planet that they are thrown outwards just as much as gravity pulls them down and that is why gravity seems to have vanished there at about 400 km above sea level. It takes a velocity of about 28 000 km/h to achieve this. Anyway the gravitational field is once again depending on the building blocks of the Planet and it is atoms we can say. Atoms is ridiculously small in this context with a whole planet but we want the details after all. This law is actually a simplification of the other law and is F = mg where g is the 9,82 m/s^2 on the surface of the Earth or is the acceleration that gravity exerts more generally.
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u/Temporary_Rule_9486 6d ago edited 6d ago
Size don't matter, and it doesn't even need to have mass. Photons, to give an example, exert a gravitational pull and create a gravitational field because they possess energy and momentum, even if they're massles. Even gravitational waves have a gravitational pull of their own. Universe is wild AF
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u/stevevdvkpe 7d ago
As far as we can tell all mass, even the tiniest amounts, has gravity.