Why is #1 an issue? You’re assuming physics at a subatomic level works the same as that at a macroscopic level, but they don’t. Things don’t have well defined boundaries.
I don’t think it’s likely that there is a minimum volume, at least not a discrete quantized one. It would have to be a [regular honeycomb tessellation](en.wikipedia.org/wiki/Honeycomb_(geometry)) that shows no bias towards any particular direction (i.e. no corners). There are no shapes that fulfill both of those conditions in 3D space.
You might be interested in Tomasello’s “The Evolution of Agency” where he kind of addresses this very question. It really depends on how you define “making decisions” and “purely chemical reactions” doesn’t it - all life is chemical reactions, including when we make decisions, and it’s easy for us to apply decision-making language even to systems that are simple enough that we can see them as “purely chemical reactions”.
Tomasello defines the notion of “agents” as “feedback-control systems” that he distinguishes from pure stimulus-response systems. In his examples a nematode for example is “stimulus-response”; its behavior is very directly related to its immediate environment. If it runs into food it eats, otherwise it doesn’t, and there isn’t really a notion of it seeking out food when it’s hungry and not when it’s not. In contrast and “agent” is a feedback-control system with goals, a perceptual system that checks whether the goal is accomplished at any given time and a behavioral repertoire aimed at accomplishing the goal. In our lineage he sets the appearance of this agency around the evolution of vertebrates, and uses lizards as an example of the most basic level. (he doesn’t address other lineages other than to say that various levels of agency clearly evolved convergently a few times; so octopuses and social insects for example would also have these systems). So where a nematode has feeding behavior that’s triggered by running into food and other behaviors when food isn’t present, a lizard’s behavior depends not only on the immediate stimulus but on more abstract goals - in a given environment it might be currently hungry and looking for food, or sated and looking for shade or sun to rest or hide or thermoregulate, or looking to reproduce, etc, and its behavior will depend on and be directed towards accomplishing that goal.
It’s interesting that you say “thinks through and makes decisions” as if they’re on the same level but the book actually claims that human agency is actually the result of the evolution of several successive layers of feedback-control mechanisms that each allow more flexibility and responsiveness - so for example lizards have a feedback systems that adjusts behavior to achieve goals, and mammals have that and also a higher-level feedback system above that to adjust the goal-seeking behavior itself, mentally “playing out” different ways of accomplishing the goal in order to pick the best one. He describes four such levels for humans and it suggests a variety of ways we could define “think through and make decisions”, with different species qualifying or not depending on which we choose.
This is what I thought - I just wanted to make sure I hadn’t failed to consider something obvious. Am meeting up with some old friends who are science geeks next month and wanted to throw out the line “for all we know, the center of the galaxy exploded 25,999.9 years ago and we could all die tomorrow” and I didn’t want anyone coming back with “well actually…we would have detected that by now thanks to technology xyz that was in ivented in 20XX”.
I totally misread your post as you were meeting up with some old friends who are science geckos and I wanted the story behind all of that, but then I read it again and was disappointed in the lack of geckos.
If the black hole specifically disappeared, it would have no effect on us. The solar system would not even be launched on a 100 million year trajectory out of the galaxy, as galactic rotation is dependent on the masses of stellar and interstellar matter in the disk and dark matter in the halo. The supermassive galactic black holes, despite being supermassive, still only make up a tiny percentage of total galactic mass.
If you want to wow your friends, tell them about false vacuum decay. We could have bubbles of true vacuum expanding out in space from multiple directions towards us at lightspeed, and no way of knowing about them, stopping them, or outrunning them. Any point in space could nucleate a new true vacuum bubble at any time, just like a given uranium atom could decay now or in 5 billion years or never. Even spookier, by principle of quantum immortality, the Earth could have been engulfed by vacuum bubbles many times before, and we are just the one tiny sliver of probability space where by luck alone we survived long enough to talk about it here and now.
Thankfully false vacuum is just an idea and there is currently no evidence that it is real.
Many worlds is a fun idea, too. But also being regarded as not real for a while now. The cat in a poison box living or dying doesn’t mean it lives and dies.
I never heard about the false vacuum before, that’d be some good sci-fi
Nothing will happen to the black hole, except for its continued growth. At least not anything on time scales that are meaningful for humanity. We’ll be long gone before any observable changes happens to any black holes.
If you had a planet that was hollow in the center*, the entire hollow region would have zero gravity. You could have a thin-skinned planet with the entire interior an empty weightless void. I doubt any planets like this actually exist.
Assuming radial symmetry. If you can represent the planet as concentric spherical shells then you’re good.
You’re right but that was not the point. The comment just explained that at any point inside a hollow sphere gravity forces cancel out so that effectively there is no gravity.
If you had a planet that was hollow in the center*, the entire hollow region would have zero gravity. You could have a thin-skinned planet with the entire interior an empty weightless void. I doubt any planets like this actually exist.
Assuming radial symmetry. If you can represent the planet as concentric spherical shells then you’re good.
Yeah it’s a pretty counter intuitive result. I’d expect a greater pull of gravity towards the nearer side, but it turns out to be exactly cancelled out by the greater mass on the further side.
E: oops, looking at your edited comment, I should stress this is only for hollow bodies. Your comment pre-edit was correct for non-hollow bodies. If you’re part way to the middle of a planet, you can think of the planet as two sections, a small sphere for the part that’s below you, and a larger hollow shell for the rest. You experience no gravity from the outer shell, so only feel gravity of the smaller mass below. 10m from the earth’s center, you feel equivalent gravity to if you were on a 10m radius iron sphere.
“If the body is a spherically symmetric shell (i.e., a hollow ball), no net gravitational force is exerted by the shell on any object inside, regardless of the object’s location within the shell.”
Inside a sphere of constant density, gravity is linearly related to distance from the center.
So for example the Earth has a radius of ~4000 miles. Assuming it has constant density, a 200 pound man would be weightless at its center, weigh 0.2 pounds at 4 miles from the center, weigh 2 pounds at 40 miles from the center, weigh 100 pounds at halfway to the surface, and so on.
So the larger the star, given that most (or all) aren’t uniform, there will come a gradient of gravity at its center that one can’t even call it low gravity - it’s heavy material is simply churning too much for their to be a stable center of gravity?
I think the best way to visualize it is that when you are inside a star, you are effectively “standing” on a smaller star. Everything behind you can theoretically be ignored. When you are very close to the center, you are standing on a very tiny star.
Imagine you are standing on the surface of Earth, and you weighed 200 pounds.
Now imagine Earth were magically transported to the center of the sun, completely replacing an equal volume of solar core. Inside the very middle of the sun, standing on planet earth, you would still weigh 200 pounds. The gravity of all the solar mass surrounding the Earth would cancel out.
If you traveled upwards, to the surface of the sun, your weight would increase. At the sun’s surface, you would weigh 5400 pounds.
It’s basic math. You can do the gravitational calculations yourself. Basically any sphere of uniform density is going to exert gravity uniformly. So if you’re in the center the pull from the mass on any direction will be counteracted by the pull in the opposite direction. It’s one of the basic introduction to physics calculus examples.
So to your question about what the zone of negligible gravity would be, you can define negligible gravity, and then figure out how large that zone would be based on the material on the outside of the shell.
Basically the further you get from the exact center of the sphere, you’re going to have more gravity from the closer edge pulling you, and less gravity from the further edge offsetting that. So there’ll be a gradient of increasing gravity as you get further away from the center
This feels very close to answering the question in a way my brain can interpret it. So, going outward makes complete sense to me but the area at the center, the way I under your answer is, yes, the area or zone will increase proportional to its mass?
This may be asking too much, but, have any idea the size of that low gravity zone of earth bs our Sun?
I can’t answer that question for you. Because you’re using a relative term. Only the exact center will have no gravity. Anything outside the exact center will have some gravity. So you have to define what negligible it means.
So once you define low gravity. You can do the math to figure out the size of that zone of low gravity.
I highly recommend doing the math anyway. Follow along with a YouTube example or a written example on gravitational attraction of a sphere. It’s really good calculus. Then you can you know put it into octave and get the exact answer for yourself. Just plugging in numbers for the relative density of the Earth and the mass and the sizes. These will be approximate of course. Because nothing is perfectly uniformly dense so it’s just a rule of thumb anyway
Depends what you’re trying to do. If you want to balance something so it never moves you can only use the exact center. If you want something to stay relatively in the center for a period of hours then you’re going to have a much larger area. If you’re okay with minutes it’s going to be much much larger area. If it needs to be stable for years in the area is smaller. Gravity is going to apply a force of acceleration and on an object, and if there’s nothing resisting that acceleration things will just fall off the center. You know imagine trying to balance something on top of a cone.
My original question stemmed from thinking about the possible different area sizes of low gravity within different size stars - and if that area was gradient.
Not quite. If by “edge” you mean the surface of the earth, then the force of gravity from the closer edge will always exactly offset the gravity from the farther edge. So if the earth were hollow, then you would experience zero gravity at any point in the hollow portion.
Of course, the earth is not hollow. And any mass under you (i.e. closer to the center than you are) will not be offset, and all of it will pull you towards the center. As you move further away from the center, more of the earth’s mass will be closer to the center than you and therefore the force of gravity will increase.
So if the original poster wanted to have a 10km sphere in the center of the earth of zero gravity (earth gravity at least), then all they have to do is hollow out 10km and they are good to go?
Yes you can, as long as you are inside a perfectly spherical shell.
The net gravitational force on a point mass inside a spherical shell of mass is identically zero! Physically, this is a very important result because any spherically symmetric mass distribution outside the position of the test mass m can be build up as a series of such shells. This proves that the force from any spherically symmetric mass distribution on a mass inside its radius is zero.
Possible, yes. Not likely though with most things if you have them regularly - your body kind of recognises it as something that isn’t a threat.
I’m of the opinion that more often than not allergies are your body reacting to the wrong thing. Take seafood allergies for example. The first time your face swells up and gets itchy after eating seafood isn’t the time you had bad seafood - that happened the time before. When you ate the bad seafood, your body reacted and got over it with minor fuss. However, it then tags seafood as the cause of what happened, and the next time you eat seafood you have an allergic reaction.
It is also sometimes possible to overcome allergies, although this is incredibly difficult.
You should feel bad for giving advice that can kill. You should not feel bad for correcting that bad advice and potentially saving lives.
Trust me. It’s my day job to make decisions where the wrong one can result in deaths. If you don’t feel bad when you inevitably make such a mistake, you are a sociopath and need to be told as much.
Besides the only reason I brought up my opinion of how he should feel is he tried to say I was attacking him, which at no point have I done.
What are you talking about? I do not mean “you” specifically. I certainly only said he was wrong exactly once, and I clearly referred to op in third person.
Where did I give any advice? Where have you offered any other information instead? All you’ve done is jump off the handle to say that I’m wrong and criticise me personally.
Yeah, that’s fair. I don’t think any lines have been crossed (yet) but the tone in the thread is certainly veering close to it. It’s also possible I missed a comment. Please don’t hesitate to report any uncivil or otherwise rule breaking comments.
Say a ship leaves Earth traveling half the speed of light, but it carries a communication device that can communicate with Earth “instantaneously” (i.e., faster than light). From Earth’s frame of reference, time on board the ship is slowed down by a factor of 0.866, while from the ship’s frame of reference, time on Earth is slowed down by the same factor. (This isn’t some trick of perception—the geometry of spacetime distorts in such a way as to make both these observations true simultaneously.)
Now suppose a year has passed on Earth, and we use the device to communicate with the ship ”instantaneously”. From Earth’s frame of reference, the ship has currently experienced only 316 days of elapsed time, so that’s when they receive the signal according to their clock. But from their frame of reference, Earth at that point has only experienced 274 days of elapsed time—so when they send their “instantaneous” reply, it arrives on Earth three months before the original signal was sent.
From Earth’s frame of reference, time on board the ship is slowed down by a factor of 0.866, while from the ship’s frame of reference, time on Earth is slowed down by the same factor
Why is time on earth slowed down from the ship’s perspective? Shouldn’t it be faster? Like if earth perceives that the time on the ship is passing slower shouldn’t the people on the ship perceive the time on earth as passing faster to compensate?
Also, I have quite a hard time understanding how time exactly slows down. Is it sort of as though we adjusted the time step duration (tickrate, more precisely) of a physics simulation in an area (making everything happen slower/faster there in relation with the rest, where the original timestep is kept)? (Without losing precision and all those problems that occur in a simulation normally) Or is this analogy flawed and that is why I’m not getting it?
Why is time on earth slowed down from the ship’s perspective? Shouldn’t it be faster?
According to special relativity, all non-accelerating frames of reference are equally valid, so the observations are symmetric: both Earth and the ship see the other moving away at 0.5c, so they both see the other slow down.
Now it’s true that if the ship turned around and returned to earth at 0.5c, it would be the ship’s clock that was behind earth’s, and not the other way around—but that’s because, when the ship turns around, it accelerates, and while it does so the whole non-accelerating frame of reference thing goes out the window. After it finishes turning around, the point in earth’s timeline the ship judges to be simultaneous with its own will have jumped into the future—so that even though it observes earth-time moving slower than its own during both the outbound and return trips, the time jump as it turns around will more than compensate.
Okay. Thank you! This explanation made it click for me (now I think I get the original example too). Here the real cause of the violation is the instant communication, isn’t it? If the communication was done via radiowaves (which as far as I know also travel at the speed of light) it would not be violated, because of the time it takes for the information to arrive from the Earth to the spaceship and back, is that correct? Is this why (as I have read/heard on several occasions) the upper bound for the speed of information is also the speed of light?
Has anyone actually proven no violation of causality? Wikipedia seemed to suggest that it’s not physically impossible to have a wormhole, take one of the ends on a round trip so that it doesn’t age as much, and you’d be left with a situation where you can go in one end and come out in the past.
No—“no violation of causality” isn’t a physical law that can be formulated, much less proven. It’s just our intuitive feeling that anything physically possible should also be comprehensible.
iirc the only method of faster than light travel that doesn’t violate laws of physics involves warping spacetime. We can now detect ripples in spacetime, and scientists postulate that in the far future it might be possible to manipulate spacetime by warping it with technology not currently in existence.
Of course you could argue this method isn’t really faster than light travel, since you’re actually bending the distance between you and the destination.
I’ve wondered why no one seems to be seriously putting effort into a genealogical ship. I’d be okay with being the first generation; I can’t possibly be the only one.
I guess there’s a place to be conserned that eventually a society might emerge within the genealogical ship that might cause them to loose their allegiance to the rest of humanity and go their seperate way. We saw this when european colonists came to the new world, they didn’t stay loyal to their home governements because of the difficulty to communicate across the ocean and the difficulty the home government would experience projecting their authority. Communication would be just as difficult with a genealogical ship and they might leave us forever, like we’ll never see any benefits from the genealogical ship.
And when you think about it that would make the most sense, because even when the final decendants of the genealogical ship find a new home world they’ll never come back to earth, their will be no travel. That world would become a different world for people who might not even consider themselves as human.
Conclusion: there’s no way to space travel unless a person can travel between worlds and still have enough of their lifespans ahead of themselves to do stuff to contribute to the wider galactic human soceity. Unless you want to live in the cowboy bebop world where the government is too weak to do anything so they have to hire bounty hunters to suppress criminal organizations competing governments, and you don’t know who has your better intrest and who’s going to protect you from who, be my guest, fracture human soceity before we’re truly ready to go out into space. It sure worked out well 100 years ago.
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