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Comment by close04 | Hacker News Reader

> zero-gravity

There's a common misconception that LEO has zero gravity. You're barely off the ground so to speak and gravity is almost the same as on the surface. The difference is that you're constantly falling so you experience weightlessness.

So you are experiencing zero-g, not 0 gravity. You'd be hard pressed to go anywhere with an absence of gravity acting in that spot.

This being said the question is if the quality increase justifies the price (even for a hypothetical future mass implementation). Plenty of products get incremental upgrades, it doesn't have to be a massive jump. Again, if you can justify the cost.

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> So you are experiencing zero-g, not 0 gravity.

You’re doing it, too. “g” is the acceleration due to gravity. Heck, scientists call drop tower experiments “microgravity” — it’s much less of a mouthful than “really quite a good approximation of free fall”. [0]

And if you believe in general relativity, LEO is a perfectly valid reference frame with no acceleration due to gravity (ignoring drag) but plenty of second order effects thrown in.

[0] I spent a summer at NASA’s Glenn Research Center doing microgravity combustion experiments studying how combustion is affected by the absence of gravity. I say that entirety unapologetically.

I was under the impression that zero-g is the accepted terminology for when you experience free fall (in orbit, in an elevator). Is that wrong?

Regardless of terminology, I tried to highlight the fact that floating in space is not due to the absence of gravity as many people assume. You're experiencing about 90% of the gravity at the planet's surface just that everything else around you is accelerating along with you.

As far as I know, people do call it zero-g. My point is that gravity is always around, so the more interesting thing to talk about is the acceleration due to gravity in one’s reference frame. For example, here on Earth, we usually neglect the Sun’s gravity unless we’re making accurate predictions of tides.

Yup. Creating a pedantic definition of zero-g to mean "no gravitational field" is pretty useless since you'll always be in the presence of a gravitational field no matter where you are in the universe. The accepted definition is just that your g-meter reads zero.

The term microgravity get used because things in orbit due to tidal forces, different orbital planes, air resistance, solar wind, and radiation pressure.

There's a lot of really strong feelings on the subject though. I'd like to think that GR would let people reach some common ground, but not so much.

https://en.wikipedia.org/wiki/Talk:Micro-g_environment

Hey, it's HN, it's the perfect place to clear up misconceptions. It's definitely not pedantry.

I would consider "misconceptions" which are irrelevant to the point being made to be the exact definition of pedantry. It detracts from the point being discussed, often is centered around some or other strict definition of a word, and is all-in-all about as constructive as being a grammar nazi. Yes, there is a gravitational field in LEO, but LEO is so negligibly close to an actual inertial frame of reference that for all intents and purposes we could (and do) call it one. The people and materials in LEO are in fact experiencing "zero gravity".

Calling it zero gravity is only technically wrong in Newtonian physics, but Newtonian physics are more than just technically wrong when it comes to the reality of general relativity.

Weird, your own explanation didn't stop you from discussing something that is not even tangentially relevant to the point.

My comment was meant to help and if it helped one person that's good enough for me. If you think calling me a grammar nazi (against your stated principles, might I add) helped anyone then by all means, trawl the web for opportunities to do it some more.

> You'd be hard pressed to go anywhere with an absence of gravity acting in that spot.

Okay I think I've found somewhere, but I'm not sure. What experiment should I perform to test that gravity is really absent?

You can't. You can only measure acceleration (with an accelerometer). If you get zero acceleration, you're in an inertial frame of reference and for all intents and purposes in zero gravity. Newtonian physics says you're technically wrong, but general relativity says Newton is wrong so you're okay.

Given the size of the universe and the fact that it's not entirely observable makes it... impractical to even consider measuring gravity. Where we can measure it it's done with accelerometers or doppler shift and measuring the orbital perturbation [0]. Objects close to home are studied more intensely.

You can't really conclude there is no gravity acting in some random point across the universe but you can use other methods to determine when it is acting even if you have no chance of putting an instrument there. Like gravitational lensing. [1]

There's definitely not enough space in a comment to discuss general relativity and measuring gravity of an object in free fall. But if you're curious for more and have time to read Wikipedia has one of the most accessible explanations that don't involve a pile of imperfect or misleading analogies. [2]

[0] https://www.youtube.com/watch?v=cMdjAKn_uXw

[1] https://en.wikipedia.org/wiki/Gravitational_lens

[2] https://en.wikipedia.org/wiki/General_relativity

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