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Comment by lubujackson | Hacker News Reader

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.

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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?

With QM there’s no pure point particles.

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.

> 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.

No metaphor is perfect.

Many are terrible.

I think it takes a very narrow understanding of physics discourse to call the particle a terrible metaphor. What's next, entirely discarding the teaching of newtonian physics in high school?

“Newton mechanics” has the benefit of a disclaimer suggesting that it is not a representation of reality.

“Particle” isn’t great but it beats “point particle”

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.

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

No, the actual point particle described by Bohmian mechanics.

I don't believe that theory incorporates gravity.

Neither does quantum field theory.

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.

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.

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

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.

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

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

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?

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.

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?

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.

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.

I believe that is correct.

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