Acceleration is change in speed, so it is, by its very nature, relative just like speed is.
If I fall, I might accelerate at G meters per second, relative to the earth, but I don't absolutely accelerate. If the earth decelerates at the same time, I'm now both accelerating an decelerating. It's relative.
It’s absolute in the sense that you can determine your acceleration without any external reference. You feel a certain force (like what you feel in an elevator). That’s your acceleration. You don’t accelerate relative to Earth, or relative to anything else. You accelerate relative to when you wouldn’t be accelerating (your inertial rest frame, a state of free fall).
If you are in space accelerating and the Earth would decelerate (which is just an acceleration in the other direction), you would still feel exactly the same force (minus Earth’s gravity, to the small extent you’d still feel it), and people on the Earth would feel the Earth’s acceleration. (For them it would feel like “down” isn’t perpendicular to the Earth’s surface anymore, or as if the Earth’s surface was tilted.)
When you sit on a chair on Earth, the pressure you feel on your butt is your acceleration upwards. If there was no chair and no ground (and no air), so that you’d be in free fall, that’s when you’d have zero acceleration. Your inertial rest frame is the trajectory you’d take in free fall. When you’re sitting on a chair, or lying in bed, or standing on the ground, you’re accelerating upwards relative to that rest frame, and that’s the pressure you feel on your butt, or on your body, or under your feet.
I am in a gravitational field. I have no idea what my acceleration is, I just know that I feel 1G (I could be falling in a stronger gravity and only feel 1G, or I could be climbing in a weaker gravity and feel 1G). The only way of determining it is to see if I'm moving relative to the stuff around me. Even then, that's not definite - I could be in an elevator and everything around me is also accelerating.
I'm not disagreeing with you, I'm just pointing out that there are circumstances where "you can determine your acceleration without any external reference" isn't true. You might even say that this is relative to your circumstances ;)
According to general relativity, you (and the ground) are accelerating at 1g, and feel weight because your inertia resists that acceleration. If you jump off a cliff, you'll stop accelerating for a bit, until the ground hits you.
Edit to reply:
> I am standing on the ground. I feel 1G acceleration. My speed is not changing. How much am I accelerating?
You are accelerating at 1g through curved spacetime. Newtonian "speed" behaves strangely in curved spacetime.
> According to general relativity, you (and the ground) are accelerating at 1g
I don't believe this is correct. If I lock two rockets in opposition to each other, they aren't accelerating. They're pushing at each other. And their propellant is accelarating away. But their displacement and orientation are unchanging, which means their velocity is zero which means acceleration isn't happening.
Similarly, the normal force resists your gravitational force to produce zero net acceleration. (An object at rest in a gravity well is its own local frame.)
> If you jump off a cliff, you'll stop accelerating for a bit, until the ground hits you
I don't believe this is correct. In GR, free fall is still inertial motion. You're just free of fictitious forces and thus following the curvature of spacetime.
As I understand it, in GR acceleration is indistinguishable* from gravity, so while you're on the ground feeling 1 gee, you're being accelerated up at 1 gee, and so is the ground.
When you're in free-fall, that's when you're in a non-accelerating frame, even though a non-relativistic description** would say that you are, in fact, accelerating.
Caveat: I only do physics as a hobby, neither academically nor professionally, so take with appropriate degree of doubt.
That is incorrect. Acceleration generates a force that is indistinguishable from gravity (and vice versa) but that does not mean they are the same thing.
It is correct, and you're also right that two rockets tethered to each other would not feel acceleration. The acceleration we feel in Earth's gravitational field is affecting our speed, though - it's slowing down the speed at which we move towards the future.
You say later that you think gravity and acceleration look the same but cannot be the same , which is funny since that’s exactly what relativity says: if two things are indistinguishable from each other even in principle, then they must be the same. Which is what led Einstein to realize that gravity really is just a curvature in space time. Hard to wrap your head around that! But if you study relativity, you eventually understand what being relative actually means.
Your speed relative to what? There is no absolute speed. Relative to an inertial rest frame, you're accelerating upwards at 1G, which is what you are feeling and what an accelerometer is measuring. Of course, relative to the non-inertial reference frame of the ground, your speed doesn't change.
You need to take into account your entire 4-vector for speed. You don't just have a speed in the 3 spatial coordinates, you're also moving thorough the "time" coordinate, and that is happening at a slower pace near a large mass like the Earth than it would of you were far away from here.
Not really, no. The ground isn't moving. I'm not moving. I get that if the ground wasn't there, I would be moving, but that's not the same thing, I think?
Like I said in another response, I have always been told that acceleration is change in velocity over time. If my velocity is not changing, I don't understand how I'm accelerating?
I do understand that gravity exerts a force that is indistinguishable from acceleration, which was my original point. But that doesn't mean it is acceleration.
You can always hold an accelerometer in your hand. If you did so now, assuming you're on Earth's surface, it'd register approximately 9.8m/s/s pointing in the upward direction.
You could also perform one of many historical experiments, such as dropping an object from an elevated height with careful timing, or rolling a round ball down a gently sloped track, and so on.
I don't think I understand the difference. I have always been told that acceleration is change in velocity over time. Is that wrong? Are there other types of acceleration?
"Similarly, standing on a non-rotating planet (and on earth for practical purposes) observers experience an upward proper-acceleration due to the normal force exerted by the earth on the bottom of their shoes."
Hi Nick, first of all, very cool of you to respond here instead of letting us all sit in the dark. I think that's what makes HN special.
That said, is it not a little bit weird that you want to protect yourself from scraping and bots, when your entire company, product, revenue, and your employment, depends on the fact that OpenAI can bot and scrape literally every part of the internet? So your moat is non-hydrated react code in the frontend?
While I would generally sympathize on that front, it doesn't really apply here.
None of the management-level desiderata he appealed to require that the user experience be broken this bad. There is very little bot deterrence from prevention of typing at that stage, while it heavily impacts user experience, especially on mobile.
Make a friend. Keep talking to that friend. Put reminders, say good morning, or write it. Call them, ask how it's going.
Send them a 5 minute audio about something interesting in your day. Schedule a meet up, like you did as kids. "Lets hang out after ~~school~~ work today".
It's not as easy to make friends when you don't meet new people all the time, and it's hard to keep them when you're not forced to see them every day (because of school or something).
Make a friend or maybe two, and do this. It takes effort, but you'll be more fulfilled.
It's okay to just meet up, hang out, watch some YouTube together, play a game, go on a walk, or do legal drugs (coffee, beer, whatever)
Some definitely are, but I think you're right to keep an eye out. I don't think that the thing open source needs is more foundations with compensated presidents and community managers and fundraising departments
Much of the article is about getting people to pay for services around open source, specifically package registries. Big users paying to use a package registry hardly sounds unreasonable.
its not actually clear what the article is about, and it has the usual journalistic conflation of concepts (market cap is not the same thing as income!).
I don't expect to get paid for the open source work I do in my free time. But I would also really like it if I could work on open source software full time (or for the software I work on at my day job to be open source), but to do so I would need a source of income from somewhere.
If I fall, I might accelerate at G meters per second, relative to the earth, but I don't absolutely accelerate. If the earth decelerates at the same time, I'm now both accelerating an decelerating. It's relative.
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