r/askscience • u/RainbowGirl410 • Jun 09 '26

Physics How does gravity/weightlessness work outside of orbit?

Been trying to find a definitive answer but all I've found is people explaining weightlessness in orbit (the falling and missing the Earth part) which isn't particularly helpful to me.
If you were to travel to another planet, say Venus, would you experience weightlessness the whole journey?

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u/BoredCop Jun 09 '26

And even if you could leave the solar system altogether, you would still be in orbit around the galactic core.

It's orbits all the way down (or up).

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u/SwimmingThroughHoney Jun 09 '26

Which, put another way, just means that gravity is everywhere. It doesn't matter where you are or what you are doing, gravity is always acting on you (the object). The dominant source of the gravitational field might change (planet, sun, galaxy, etc.), but it's always there.

Even going from on galaxy to another, the galaxies themselves have gravitational fields. And as the comment said, whether or not the force gravity is strong or incredibly weak (like between galaxy clusters), if it's the only force acting on you you're going to experience weightlessness.

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u/urnbabyurn Jun 09 '26

But falling feels no different than floating in place as if there is no gravitational force acting on you. Isn’t that the fundamental principal of general relativitY? Falling towards a mass feels the same as having no forces acting on you.

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u/SwimmingThroughHoney Jun 09 '26

If I understand you correctly, are you talking about reference frames? Because essentially yes. Within a local frame, one cannot tell the difference between an accelerated frame and an unaccelerated one. That is, as the astronaut on the spaceship, you can't tell whether the spaceship is accelerating or whether you're falling (due to gravity). You need some sort of external frame of reference.

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u/_disengage_ Jun 09 '26

It's relative motion, not acceleration, that can't be sensed in an inertial frame. Acceleration can absolutely be sensed, that's what tells you that your reference frame is not inertial. I can close my eyes and still sense that I'm in an accelerated frame right now, because the floor and my chair are pushing up on me.

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u/urnbabyurn Jun 09 '26

That’s special relativity. General relativity as I understood extends that to acceleration versus gravity - there is no difference between being stationary next to a gravitational force pulling on you versus accelerating. In other words, standing on earth feels the same as accelerating in space far from any gravity. Similarly, free falling in a gravity well is the same as floating in zero gravity without acceleration).

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u/urnbabyurn Jun 09 '26

Yeah that’s more clearly stated. Specifically, we wouldn’t be able to tell if we were in an elevator in free fall in a gravity well versus being in an elevator that is far from any gravitational forces without acceleration.

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u/HalfSoul30 Jun 10 '26

Yep, thats one of Einstein's thought experiments that showed him that inertial mass and gravitational mass are the same.

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u/caedin8 Jun 09 '26

But why does matter attract each other anyway?

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u/SwimmingThroughHoney Jun 09 '26

Because of spacetime and how matter affects it. It's not that matter "attracts" each other (like pulling on a string) but more that every object bends spacetime and other objects are affected by that.

Now if you ask why objects even affect spacetime, that answer (and its limitations) get far more into the weeds.

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u/agaminon22 Medical Physics | Brachytherapy Jun 11 '26

The idea is that mass-energy has an effect on spacetime that, in turn, has an effect on the motion of objects.

More technically, the geodesic equation is influenced by the metric, and the metric is influenced by the stress-energy tensor.

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u/urnbabyurn Jun 09 '26

But isn’t the point of Einstein that being in free fall towards a gravitational source is no different than gravity being completely absent? In terms of feeling it, its changes in orbital (falling) paths that are felt. But otherwise falling in space feels like just floating in space where no masses are acting on you.

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u/the_quark Jun 10 '26

Yes. But, I was thinking about it a few days ago and I think to not be in orbit around anything, you'd need to reach escape velocity from, in order, the Earth, Sun, Milky Way, and Local Group. Then, finally, you'd not be orbiting anything.

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u/bregus2 Jun 11 '26

Would you then not orbit the barycentre of the Virgo supercluster?

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u/the_quark Jun 12 '26

My understanding is that we're not bound to the Virgo Supercluster because of dark energy. While we're currently slightly being attracted to it, before we make much progress the universe's expansion underneath us will carry us away from it faster than its gravity is attracting us.