For years I have been very fascinated by the "mystery" aspect of gravity. Functionally, we understand it for our applications, but in my (limited to my 3 quarters of undergrad general physics) understanding, we don't know why it exists or how the "pull" works.

To provide an example of what I mean: there has to be some "measurable material" of gravity between, say, the earth and the moon -- some kind of 'graviton' (not that it needs to be a particle or anything, I have zero claims as to the nature of how gravity does what it does). The [butchered] saying of "pluck a flower and move the furthest moon" originates because gravity in theory, has an infinite range (right?) and moving a flower on earth could maybe move an atom on Jupiter ever so slightly.

Anyway, similar to how we had "very good" hypotheses about the existence of Higgs boson that the LHC 99%+ confirmed, do we have any "very good" hypotheses in regards to the fundamental way how gravity does what we have long-known it to do?

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    $\begingroup$ General relativity pretty much covers the how and why of gravity... Gravity is a manifestation of the curvature of spacetime. It is inherent to it. This makes gravity fundamentally different from, say, electromagnetism (EM). EM is described by external fields existing in and moving through spacetime. Gravity follows from the structure of spacetime itself. In this sense, gravity is not even a force. Of course the philosophical question why gravity works this way is not answered by GR, because it cannot be answered by science. $\endgroup$
    – Wouter
    Oct 18, 2013 at 18:34
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    $\begingroup$ @HC_ By definition, science answers how. We leave why to philosophers and priests. $\endgroup$ Oct 18, 2013 at 19:31
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    $\begingroup$ "Why", is something that goes on in our mind, and has no answer; so philosophers can talk about it ad infinitum, and not be called idiots. "What", is something that humans can "detect", (aka "observe") with our "senses" , so we can describe it to others. Allegedly, Isaac Newton saw an apple "fall", to the ground from a tree ("what"), so he asked "why" (big mistake), and he made up the idea of gravity, to answer the silly question. Einstein said that maybe, the ground flew up, along with the tree, and landed on the apple. We've all been talking about that ever since. $\endgroup$
    – user26165
    Oct 19, 2013 at 1:24
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    $\begingroup$ "Why" is a word that, notwithstanding the skeptics here, does have a tendency to grow into "whats" and "hows" and ultimately even to testable "whats" and "hows" if people get together and think about it hard enough. Once it does this, new "whys" will sprout in its place, leaving the false impression that nothing has been learnt. GR is an excellent example. Gravity to Newton was a what and "why" it is there would have seemed inpenetrable, even meaningless. He couldn't have foreseen that someone one hundred years after his death would contemplate "what if" we chop off .... $\endgroup$ Oct 20, 2013 at 6:34
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    $\begingroup$ What and why questions within a formal theory, i.e. it has postulates and mathematical formulae that can describe and predict phenomena, can be answered up to the point of hitting on the postulates. Then the answer becomes "because we assume the postulates are correct". So all why questions will end up hitting the postulates, only how can be answered within the theory. If a theory is falsified then the postulates will have to change and a new chain of mathematical derivation will have to describe the old data and explain the falsifying datum. $\endgroup$
    – anna v
    Oct 20, 2013 at 11:43

2 Answers 2


Gravity may be fundamentally different from electromagnetism, as Wouter says, but it seems to me that, as far as your question is concerned, gravity is not fundamentally different from electromagnetism: there is gravitational field (http://en.wikipedia.org/wiki/Gravitational_field#General_relativity ), which is indeed "SOME "measurable material" of gravity between say the earth and the moon". AFAIK, you are right, and "gravity in theory, has an infinite range and moving a flower on earth could move an atom on jupiter ever so slightly." (I removed your "maybe" and question marks). However, in general, an atom on Jupiter does not "feel" a movement of a flower on Earth instantaneously - gravity is widely believed to have a finite propagation speed (http://en.wikipedia.org/wiki/Speed_of_gravity ) equal to the speed of light.

So again, it seems that, as far as your question is concerned, the situation with gravity is not fundamentally different from that with electromagnetic field: the Coulomb field has an infinite range, and there is "measurable material" - electromagnetic field (in the form of electrostatic field) between two charges, but, if one charge moves, the other charge does not "feel" that movement instantaneously. Thus, both gravitational and electrostatic forces are mediated by fields. One could say that gravitational field is just space curvature, but I don't feel that would change much. As for gravitons... Again, AFAIK, while it is not clear yet how gravitational field should be quantized (http://en.wikipedia.org/wiki/Quantum_gravity ), that does not seem to matter much as far as your question is concerned.

  • $\begingroup$ This doesn't seem to answer the "how" of why gravity exists. It seems to only say, "it's like electromagnetism" to me, which isn't very helpful $\endgroup$ May 24, 2016 at 1:01
  • $\begingroup$ @ZachSaucier: the question was not "how gravity exists", it was "how gravity does..." and the OP suggested that "there has to be SOME "measurable material" of gravity between say the earth and the moon -- some kind of 'graviton' (not that it needs to be a particle or anything, I have zero claims as to the nature of how gravity does what it does)". I just confirmed that gravity probably acts locally (and the recent LIGO result seems to confirm that). It may well be that this "how" is not enough for you, but I am afraid I cannot offer more. $\endgroup$
    – akhmeteli
    May 24, 2016 at 1:12
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    $\begingroup$ @ZachSaucier "Why gravity exists?", I think is not even completely on-topic here at physics stack exchange. Though, what u say is also correct. He doesn't answer the question because the question in some sense is unanswerable $\endgroup$ Jun 11, 2017 at 12:13

Here is a theory about why gravity exists:

In a cosmological model that has a constant rate of expansion $\frac{\dot a}{a} = H$ and the expansion happens to all length scales, including the observer as in this link Cosmology - an expansion of all length scales

enter image description here

Then gravity is explained as follows:

In the expansion of the type above each physical quantity $Q$ varies with time as $Q=Q_0 e^{nHt}$ where $n$ is the number of length dimensions.

Gravity is produced in nature to allow a scale-symmetric expansion of the universe without violating conservation of energy.

For an isolated mass its energy $mc^2$ becomes $(mc^2)e^{2Ht}$ and without gravity conservation of energy is violated.

However the total energy due to the mass in the universe of mass $M$ and radius $R$ is $$\left(mc^2 - \frac{GMm}{R}\right)e^{2Ht} \tag 3$$

Energy can be conserved if

$$\left(mc^2 - \frac{GMm}{R}\right)e^{2Ht} = 0 \tag 4$$

and the strength of gravity is

$$ G=\frac{Rc^2}{M} \tag 5 $$

Small numerical constants omitted for simplicity.

The interpretation of this is that gravity is caused to allow the universe to have scaling symmetry. The 'expansion' does not slow down or speed up as the universe expands. Energy is conserved during the 'expansion' due to the gain of internal energy of masses being of balanced by the gain of negative gravitational potential energy.

The flatness problem is naturally explained by (5).

Since no expansion is measurable in such a universe we have a stable, apparently static universe, always at critical density.

So this scaling symmetry is the reason behind the gravitational force and the reason that it's proportional to mass.

  • $\begingroup$ So this is a personal theory of yours? $\endgroup$
    – D. Halsey
    Sep 8, 2021 at 14:59
  • $\begingroup$ @D. Halsey Yes, I like it as it's simple and seems to explain why the matter density is measured, from supernovae and other methods at between 0.25 and 0.33, as described in the link near the top of the answer. $\endgroup$ Sep 8, 2021 at 15:17

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