# Gravity is Not Intuitive

I’m going to display some specific ignorance I have about physics, but I have a question for @physicists.

Why is it that gravity interacts with matter in such different ways at different scales? Is it just because matter acts differently at different scales? Or because the time scale of evolution is different? A couple (am I sure illusory) paradoxes:

1. Gravity is totally neglect-able at molecular scales.
2. At human scale it draws things together.
3. Up at solar system scales, it makes planets round, and enables them form from gas (something we would never see at a human scale).
4. At a cosmic scale, it makes the universe like foam, with galaxies in dense zones.

I’m not even touching on dark matter, dark energy, or singularities. My best explanation is that matter works differently at different scales, so even though it is the same formula for gravity, we see different dynamics at different scales. Is that correct? What ever the case, it does seem that gravity does some fairly non-intuitive things. It is the same fundamental law, but doesn’t look the same when it operates on planets as when it operates on us.

Help me make sense of this?

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As far as I am aware, gravity acts the same at all scales, except perhaps at the quantum level:

The same formula is used for the attraction between the Earth and Moon and for the attraction between two molecules separated by 100 angstroms.

The differences you experience probably have more to do with the relative strengths of the electromagnetic and gravitational forces. Electromagnetism would dominate when there is low mass which is why you can use a magnet to counteract Earth’s entire gravitational pull on a paperclip. Electromagnetic forces also decreases quickly with distance whereas gravity doesn’t decrease quickly which is why you need the magnet close to the paperclip in order to counteract gravity. The same applies to two molecules in solution.

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I was simply going to answer with “The Gravitational Constant is so tiny” but I like T_aquaticus’ answer much better.

Newton was a sharp guy.

(I recently tried to explain to a Flat-Earther how gravity would affect us if we lived on a flat-disk. I tried to use the aforementioned equation to explain how we would experience gravity at different places on the earth’s surface if we weren’t living on an spheroid----but it was like talking to a brick wall.)

Actually, both electromagnetism and gravity follow the same inverse square law.

The reason why the effects of electromagnetism are less obvious at large scales is because like charges repel but unlike charges attract, so overall, positive and negative charges cancel each other out. (Magnetic fields from free electrons in ionised gases, plasmas, the Earth’s core etc notwithstanding.) On the other hand, with gravity, everything attracts, period, so you don’t see things cancelling out in the same way.

The reason why everything seems weird is because you’re talking about scales that are outside our everyday experience as humans. The classic essay “On Being The Right Size” by J B S Haldane might put things into perspective a bit better: https://irl.cs.ucla.edu/papers/right-size.html

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All of the “paradoxes” you mentioned can be explained by the fact that the 4 fundamental forces simply have different relative strengths. At different distances, different forces are dominant. So you are roughly right in saying that “matter works differently at different scales.”

As @jammycakes pointed out, @T_aquaticus is wrong in saying that gravity decreases more quickly than the EM force. They are both inverse squared laws. But he is right in pointing out that gravity is much weaker than the EM force, if we look at the everyday objects around us and calculate how much gravity vs. EM force they would result in. James is also correct in pointing out that the EM force is very easy to cancel, as there are positive and negative charges all around us (in fact, in every atom).

In contrast, as far as we know there is no such thing as “anti-gravity” to cancel normal gravity. (Interestingly, some physicists at CERN are testing that right now by investigating if antimatter drops the opposite way from gravity.) Gravity is a very simple force: it exists between any two massive bodies. Einstein showed that you can understand it as the existence of mass distorting the fabric of spacetime. I’m not an expert on GR, but for me, the universality of gravity to me is what makes such a theory intuitively plausible. (It’s hard to imagine explaining the EM force as coming from distortions of spacetime.)

At large scales, gravity becomes the dominant force: besides the weakness of the other forces, there are large masses of matter in space whose gravitational attraction cannot be canceled out, such that the small value of the gravitational constant is made up for. If you pack enough matter in a small enough volume, the force will be enough to make the mass collapse on itself and form a black hole. However, it’s hard to imagine this happening with EM force, as positive charges attract negative ones, so you can never create a black hole by amassing tons of positive charges or negative charges.

(One can imagine, though, a universe where the gravitational constant is much, much larger, such that even small masses of things will collapse into a black hole. That would make planet formation impossible. So perhaps this is an example of fine-tuning (?))

I found this article by Matt Strassler to be very comprehensive and well-written about exactly this issue:

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@dga471, this is a very well written answer! Let me add two small corrections that perhaps you already know, but might be interesting for non-physicists in this thread:

In GR, gravity exists even for bodies of zero mass, as in the case of photons.

The EM force can be seen as coming from a distortion in spacetime if one allows for a 5th dimension (which could be compactified) - this is the impetus behind string theory.

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So is gravity really a force? GR seems to be a relationship between matter and the curvature of spacetime. Matter shapes spacetime, and spacetime says how matter moves. Zero mass photons moves in curved spacetime near massive galaxies. Is the photon experiencing a force or a curved spacetime?

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No. But even GR people still often say “gravitational force” out of habit and sloppiness.

To be precise, the Einstein Field Equation is a relationship between energy+momentum (note that mass is a form of energy) with the curvature of spacetime.

It’s the latter. Photons only want to move as straight as possible, as fast as possible.

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This most makes the most sense to me, because it explains this:

Which makes me happy I didn’t say something really stupid. I still find this very non-intuitive and surprising from a naive point of view. For example, how they heck does rarified gas collapse into a planet or a star? All the gas I deal with in real life is in no danger of suddenly collapsing into a solid. I know the answer, but we are kidding ourselves if we don’t catch how crazy it sounds.

Appreciating the face-value absurdity of all this should give us some empathy for YECs that come here with incredulous questions. Science brings us to deeply non-intuitive understanding of the world. Then, it ironically reshapes our intuition to understand things as obviously true.

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Well think of the earth’s atmosphere. The oxygen molecules just don’t float into space they are held in the sky by gravity. Now what if the mass of the earth was missing? A tiny section will have move mass than another section. Given a lot of time and this section grows into in a Jupiter, a star or a black hole.

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I do understand how it works on reflection, of course. The issue is that on face-value it is not-intuitive. You have to work your way to the right intuition.

Of note, subtle disagreements and mistakes were made in trying to respond to my original question. That is an indicator of some subtle complexity. It is a good rationale for empathy.

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Given a lot of time, the impossibility of a homogeneous and isotropic distribution over time, some regions
have to get clumpy and get bigger and denser due to just gravity.

At just 380,000 years after the Big Bang, the universe is nearly perfectly homogeneous and isotropic (the difference from blue and red is in microKelvins) the red turn into massive filaments of the galaxies and the blue turns into massive voids.

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DOH!! Thank you all for the correction.

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I agree. The idea when I jump off a chair I am moving across curved space-time is not intuitive at all. Newton’s description was much more intuitive yet did not account for planetary orbits.

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You don’t have to jump off a chair to move across curved space-time. Just sitting there you are moving in the “time” direction at the speed of light. Indeed, you are always moving in spacetime, and it is impossible for you to stop!

Everything moves at the speed of light in spacetime. Light just uses as much of this velocity as possible to move in space, while typically objects use most of this speed to move in the time direction instead.

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Does that have to do with why time slows down as you approach the speed of light, traveling within space?

Yup, it absolutely has something to do with time dilation!

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Isn’t this question (about whether gravity is really a force or not) something of an interpretation of the data over and above what the experimental evidence, and even what the mathematics, says? Sure, the orthodox interpretation of GR is that gravity is nothing more than spacetime curvature. But GR hasn’t been reconciled with quantum theory, and it may well turn out that the correct theory of quantum gravity makes gravity out to be a force (in the same way as electromagnetism and the weak and strong forces).

There’s even a reformulation of GR (or at least, GR of a restricted class of manifolds) by Julian Barbour that breaks spacetime back down to space (as a 3-dimensional entity) evolving over time, so that “spacetime” would just be a mathematical description, not corresponding to what actually exists. (Barbour actually thinks this formulation is “timeless”, but the same mathematical structure can be easily used by someone who is a philosophical “A-theorist” about time.)

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Of course! I am answering from the point of view of currently known physics. Bear in mind that it might also be possible that everything is spacetime curvature and nothing is “force” in the traditional sense. Indeed this avenue seems to be promising from the point of view of string theory.

I am aware of Barbour’s formalism, but have never actually read any of its mathematical details, so I can’t say much about it.

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By the way, I know time from the physics perspective but don’t really follow the philosophical literature. Therefore, I never grasp the issue of the A/B-theory in the philosophy of time, or their philosophical context and the debates between their proponents. Perhaps this is something that @structureoftruth can clarify/discuss in a different post. I am sure there are others in this forum that would find this interesting.

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