[nilkn]

"Einstein cast gravity not as a force but as the geometric bending of space and time. In a popular analogy, the fabric of space-time is like the flat expanse of a mattress, and a massive object like a star is like a bowling ball sitting on top. The weight of the bowling ball compresses the mattress, forming a dimple — matter tells space-time how to curve.

In this analogy, a planet is like a smaller ball. If it rolls close enough to the bowling ball, its path will be altered by the dimple in the mattress — space-time tells matter how to move."

This analogy is wrong in a way that even people who've studied physics often don't realize.

On an everyday scale like the Earth orbiting the Sun, almost none of that gravitational interaction is from the bending of space. Far beyond 99% (actually, about 99.999999%) of it is from the bending of time.

[AnimalMuppet]

Could you be more specific? How does bending time cause the Earth to orbit the Sun?

[lubujackson]

Imagine you are driving in a car coming up parallel to the sun on your left. Time moves a bit faster for you on the left side than the right side. This slight speedup makes your left side traverse space faster than the right side, which causes a slight drift to the left (and also makes you spin).

Now just add massive scale and distances.

[bobbylarrybobby]

How does this cause a point particle to accelerate towards the sun? Must be something about the gradient, but how does the gradient of time cause you to curve towards the sun?

[whatshisface]

That's a great question. The answer is, the stuff you are reading in this thread is not right (you figured it out). The real version of the story is, there is this thing called the "Christoffel symbol," which tells you where, at every point in space, you would end up if you went in a certain direction, including which way you would be facing if you went that way. It relates three vectors: your direction of motion, the direction you are currently facing, and the delta to your direction of facing that would result from taking that direction of motion.

If you let your current momentum be your direction of facing, and let the same momentum also specify your direction of motion, the Christoffel symbol tells you what your momentum vector would be after an infinitesimal amount of motion. This can be integrated to find the version of a straight line appropriate for a curved surface (imagine an ant walking straight forwards on the surface of a cone or something), a geodesic. A changing momentum is like a force is acting, so that's gravity.

There is more to learn than that, of course. Many many many books have been written about general relativity and you can read them.

[mjcohen]

For a nice introduction to relativity, look at The Einstein Theory of Relativity: A Trip to the Fourth Dimension by Lillian R. Lieber. $15 on Amazon. Written in 1945 and still quite good.

[Retric]

With QM there’s no pure point particles.

[throwaway27448]

No, but you can talk about changes in perturbations of fields over time in QFT (which has its own representational issues). A particle is a useful metaphor.

[nilkn]

Their point in this case is that a wavefunction is spread out over space, which would cause it to be subject to a local clock gradient in curved spacetime. If you wanted to use particles, you'd need to use a Feynman-style "integrate over all possibilities" approach, which would again be subject to a clock rate gradient over space.

The mathematics of this is a bit too complex to reproduce in a comment here, but in, say, the Earth's gravitational field, taking this effect into account (approximating GR as a field of locally varying clocks, then allowing, e.g., an electron's wavefunction to evolve on that spacetime) would reproduce gravitational acceleration / free fall towards the Earth.

Said differently: this is precisely the kind of nuanced scenario where getting sloppy with metaphors gets you into trouble very quickly. Quantum mechanics in curved spacetime is not to be dabbled with lightly.

[Retric]

> A particle is a useful metaphor

I disagree, an actual point particle with a mass should have an event horizon. Using terms without baggage helps avoid such misleading assumptions.

[russdill]

A point particle? You mean that useful mathematical approximation for excitations in a field?

[antonvs]

No, the actual point particle described by Bohmian mechanics.

[russdill]

I don't believe that theory incorporates gravity.

[alfiedotwtf]

I’m going to ask the obvious next question… so if the sun and me in the car are next to each other but stationary, where is the attraction coming from now? As in, time may make the closer side slower, because we’re stationary, there’s no drift etc

[kridsdale1]

You always have to define stationary when it comes to relativity.

There is no way to have a “zero speed orbit”. You’d be on a trajectory straight in to the middle of the sun or away from it (under your own power). The only way to stop is to push away with equal constant acceleration (which looks like “force”). This is what rockets do.

[AnimalMuppet]

One nit: Time moves a bit slower on the sun's side.

Other than that, thank you for a very clear explanation.

[throwaway894345]

If a particle was dropped into the sun’s gravity (not with “horizontal” motion that might cause it to orbit), is it time dilation that causes it to accelerate toward the sun somehow?

[djsamseng]

My guess: In the reference frame where the particle is not moving, the sun would be either a) moving (with a perpendicular component) and be ever so slightly moving toward the particle or b) not moving but a third body would be, moving both the sun and the particle at different “strengths” (different mass and distance, different time dilation) thus the particle and the sun would appear to move closer to each other. That means in either case (sun in the middle or particle in the middle) the third body moving closer must make it look like the particle and sun are gravitationally pulling each other. If we then shift reference frame back to the third body being stationary and the particle and sun moving, we should see that. It would be really cool if we could simulate this to test it but I believe that would require solving the 3 body problem.

[TACIXAT]

That's a very cool analogy but I might not be understanding something here. Why then do objects that have no light have gravity? If 99% comes from time dilation, why am I stuck to the earth rather than drifting toward light sources?

[simiones]

Light has nothing to do with this.

The point is that mass bends space-time. The amount of bending is dependent on the size of the mass and on the distance from the mass. Even though the Sun is incomparably heavier than the Earth, it is also MUCH farther away from you. So, space-time around the Earth is curved much more towards the center of the Earth than it is towards the center of the Sun. In the mattress analogy, consider a large mattress, with a bowling ball and a car sitting on it. The car will obviously bend the mattress much more, but if you're close to the bowling ball, you'll still fall towards the bowling ball first before both you and it fall towards the car.

So, say you're in an airplane moving directly forward, with the Sun just overhead (and the Earth obviously just below you). The Earth curves spacetime towards it a lot in this area, while the Sun curves it towards itself just a little bit. The overall curvature is such that time still moves more for the bottom of the plane (closer to the Earth) than the top of the plane (closer to the Sun). So, the bottom side moves a little slower than the top side, but the structural integrity of the plane pulls the top side towards the bottom, causing a slight motion towards the Earth - gravity [note that the GP's explanation got the signs a little wrong - time flows slower, not faster, closer to a big mass]. Conversely, if the Earth disappears from the picture and only the Sun remains, now the top part of the plane will move slightly slower, pulling the bottom part towards it, and thus towards the Sun.

[djsamseng]

If the sun is on my left, doesn’t that mean time moves a bit slower on my left and the slowdown on the left means I’ve traveled less on my left side? Thus I turn left toward the sun.

[__turbobrew__]

I believe that is correct.

[wizzwizz4]

All objects move through spacetime at the speed of light, but a stationary object is moving in the time direction. (And the time dimension has opposite sign to spatial dimensions, so (Lorentzian) rotation's effect on length works opposite to what you'd expect from Euclidean rotations: https://commons.wikimedia.org/wiki/File:Spacetime_diagram_of....) Suppose we drop a test mass from the top of the leaning tower of Pisa. The "forwards through time" direction takes the object deeper into the local gravity well: as far as the test mass is concerned, it's just moving forwards through time according to Newton's First Law, and everything else is accelerating towards it for no apparent reason.

It may help your intuition to consider the extreme case of a black hole. The event horizon is where time is so warped that no possible future trajectories lead outside of the black hole, and you need a magical time machine to escape. (Of course, the best way to gain intuition is to work through the mathematics, either symbolically or with diagrams, rather than reading English-language descriptions.)

There is a sense in which an orbit is a straight line. Obviously, an orbit is not a straight line through space (unless you count the perfect and unobtainable orbit of a beam of light around a black hole, some distance from the event horizon), but we often think of them as spirals through spacetime: there's an argument that really we should think of them as straight lines through spacetime, much like how a great circle is a straight line along the earth's surface.

[ck2]

it's the speed of causality (limit of information transfer)

https://www.youtube.com/watch?v=8yhk1EZq9tY

fortunately that video is more gentle but the math in that youtube channel absolutely melts my brain some days, I can keep up for the first minute but then all bets are off as he dives in and I realize there are some insanely brilliant people out there

[bavell]

This was a helpful visualization: https://youtu.be/U_sI9agWmEw?si=MItDfnTx1-oT_qX_

[canjobear]

Also very well explained by PBS Spacetime here https://youtu.be/UKxQTvqcpSg

[jfyi]

I skimmed the video, but it's definitely the same example that finally made this make sense to me.

I always hated the ball and sheet example simply because it was describing gravity with gravity. It felt fundamentally wrong.

[meindnoch]

It changes the geodesic that Earth follows, from a straight line (in 3D space) to a curved one (an ellipse).

[stuff4ben]

And also, how does one bend time?

[nilkn]

It means clocks tick at different rates depending on where they are.

Imagine spacetime as a field of local clocks. Far from the Sun, clocks tick faster. Near the Sun, clocks tick slower. A freely moving object tries to follow the straightest possible path through spacetime. But because the “time axis” changes from place to place, what counts as “straight ahead into the future” tilts slightly inward near the Sun. So the Earth’s path through spacetime curves toward the Sun.

Earth’s spatial speed around the Sun is about 30 km/s. But through spacetime, its “timeward” motion is basically c, 300,000 km/s. So even a tiny tilt in the time direction creates a significant spatial acceleration. That is why the time-warping term dominates for slow massive bodies.

[bgilroy26]

Does the atmosphere stay attached to the earth due to the bending of time?

[nilkn]

That's right -- the atmosphere stays attached to the Earth mostly thanks to gravity, and the Earth's gravity in GR is almost entirely the gradient in clock rate near the Earth.

Near Earth’s surface, clocks lower down tick very slightly slower than clocks higher up. The change in tick rate is on the order of 10^(-16) per meter. While extremely small, that's enough to generate the familiar 9.8 m/s^2 spatial acceleration we experience. Such a small gradient in clock rates generates macrosopically noticeable spatial accelerations because the "translation" factor is c^2, a tremendously large number.

Now, if I wanted to cover all my bases here, I'd need to point out that gravity does also bend space -- that is just not a relevant factor for "ordinary" gravity acting on relatively slow moving matter (like the Earth itself, or the Earth's atmosphere). For instance, for light itself, spatial bending is just as important (in fact, the gravitational deflection of light by a weak static gravitational field is controlled by a near 50/50 split between spatial and temporal effects). Near a massive black hole, it's not that simple and can't meaningfully be understood in terms of "time" and "space" effects being independently separated.

[mixmastamyk]

I’ve seen this described before in terms of how GPS is able to do what it does, so not surprising. But still haven’t explained why time would so dominate the space dimension.

Edit: The response below is dead for some reason, please vouch.

[cellular]

It's much simpler than anyone thought:

https://youtu.be/A-2XQQDD6QQ

[mr_mitm]

Same way you bend space. In GR, time is just another dimension of a slightly different flavor.

[ikekkdcjkfke]

Whats the actual mechanism though?

[mr_mitm]

You contribute to the stress-energy tensor by having mass or energy. That directly influences spacetime curvature. The Einstein field equations tell you how precisely.

If that answer doesn't satisfy you, I'm afraid that's all we can say. It reminds me of Feynman's answer on how magnets work: https://www.youtube.com/watch?v=MO0r930Sn_8

[throwaway27448]

I'm having a bit of an issue teasing apart space and time when it comes to reasoning what bending time even means without space, let alone comparing the magnitude of the two. They're inextricably linked—as I understand it, both space and time can be seen as a magnitude of causality.

[snowwrestler]

That’s very interesting, but the analogy is not wrong, because the mattress is analogous to space-time, not just space. Essentially it is a 2D analogy for a 3+1D reality.

[nilkn]

The beauty of a useful analogy is it allows you to make correct inferences, even without a full understanding.

If you view gravity as a mattress, you're stuck. There's nothing to do with it that you couldn't already do, because it's fundamentally wrong. Another way to say this is that it's actually an analogy for Newtonian gravity, not for GR, despite apparently including something curved.

If you view gravity as a field of local clocks that tick at different rates, you can make many correct predictions:

- Clocks at different heights will tick at different rates.

- GPS needs gravity corrections.

- Light climbing out of a gravitational field is redshifted.

- Radar signals passing near the Sun should be delayed (the Shapiro time delay).

- You can have no gravitational pull but still have time dilation (inside a perfectly spherical shell, Newtonian gravity seems to cancel out).

- From the outside, it appears to take an infinite amount of time for something to fall into a black hole's horizon.

- Aging can be path-dependent.

- Gravity affects every physical process: chemical reactions, radioactive decays, biological aging, atomic transitions, molecular vibrations, computer processors, pendulums, pulsars.

- A sufficiently precise clock can measure height.

- Objects in eccentric orbits should have periodic clock-rate changes.

- Quantum matter waves should accumulate gravitational phase shifts.

- Spectral lines from compact stars should reveal compactness.

- Thermal equilibrium in gravity should involve a temperature gradient.

[camel_gopher]

Aka the Einstein tensor

[colordrops]

I never liked the mattress analogy, because if gravity is the bending of the mattress, what is pulling the ball down the slope?

[zehaeva]

It's just an analogy, you're not supposed to think too deeply about them.

The main take away for a lay person is that _like_ the mattress space is being deformed. That's where the analogy stops. Taking it further, like with all analogies, breaks the analogy.

If the analogy was a perfect one, then it would just be the reason and not an analogy.

My main gripe is how hard for most people it is to extrapolate that deformed mattress into a 3d space.

[colordrops]

I get that, I'm just wondering if there's a more effective analogy. Maybe there isn't.

[twentyfiveoh1]

I am not an expert at all but I believe the rough idea is that in General Relativity, everything follows a geodesic. The straightest possible path through curved spacetime.

They use that to explain why you don't feel acceleration when you jump off a building or out of a plane.

They say you DO feel it when you are standing, because the earth is impeding you and pushing you away from the geodesic you naturally want to follow.

So it is counterintuitive. the standing still person is being accelerated/pushed. The freefall person is free.

John wheeler said "Spacetime tells matter how to move; matter tells spacetime how to curve". I think it explains the circular nature.

Mass curves spacetime, that changes how mass moves , mass moving to a new position changes the curvature, that changes how the mass will move next. The dimple in the mattress staying with the mass.

[hollerith]

You've got it right. Our normal environment is an accelerating reference frame. The resting frame (i.e., free fall) is easier to analyze using physics, but is unfamiliar (indeed tend to feel alarming) to us.

[defaultcompany]

I agree. The analogy is using our intuition of how objects move because of gravity to explain gravity. It has always seemed circular to me. I bet there is a better analogy.

[twoodfin]

What’s an example where it would be accurate to say 99%+ of the interaction is from the bending of space?

[nilkn]

That is a harder question than it sounds. The answer might actually be "there are no such examples", but I'm not confident enough to jump to that with any certainty by any means. Near and inside black holes, there can certainly be significant warping of space, but it's unlikely to be near exclusive warping of space.

The only plausible example I can think of that isn't purely theoretical / speculative would be gravitational waves.

[vardump]

Does this mean universe can only support certain amount of energy (or motion) in a certain volume?

[UncleSlacky]

Well, interesting things are expected to happen above the Schwinger limit:

https://en.wikipedia.org/wiki/Schwinger_limit

[canjobear]

Close. One approach is to conjecture that the universe only supports a limited about of information per unit surface area.

https://en.wikipedia.org/wiki/Entropic_gravity

[goldylochness]

i agree with you but i'd go even further

time and gravity are the same thing, the history of understanding physics is basically of the same nature, understanding that two things are actually one thing, which is more like philosophy but with physical confirmation