BB84
@BB84@mander.xyz
- Comment on I guess we are fucked now 3 days ago:
As stupid as that sounds, you are not totally wrong.
@don@lemm.ee and @kopasz7@sh.itjust.works you are misunderstanding what “observable universe” means. The observable universe is defined by the particle horizon, but the universe that can affect us in the future is defined by the event horizon. en.m.wikipedia.org/wiki/Cosmological_horizon says
The particle horizon differs from the cosmic event horizon, in that the particle horizon represents the largest comoving distance from which light could have reached the observer by a specific time, while the cosmic event horizon is the largest comoving distance from which light emitted now can ever reach the observer in the future.
But even the cosmological event horizon distance is dependent on our model of the universe’s expansion, which in turn depends on the content of the universe. An event such as a vacuum collapse will drastically alter the content and the expansion rate, rendering our calculation of the event horizon invalid. So “snap changes…” may in fact be the case.
- Comment on vibes-based astrophysics 3 weeks ago:
I think we just differ on the terminology of invention versus observation. What draws the line between a well-supported theory and an observation in the end comes down to how tangible you think the data is.
- Comment on vibes-based astrophysics 3 weeks ago:
The concept needs to be able to predict and explain new observations, or else it has no utility and is still essentially just a placeholder. They first came up with it to explain galactic rotation curves. After that, many new observations came in and the model successfully explained them. To name a few: bullet cluster dynamics, gravitational lensing around galaxies, baryon acoustic oscillation.
Like, relativity, you have to accept and account for or GPS wouldn’t work nearly as accurately as they do. It is neat that general relativity is used in GNSS, but I’d bet that GNSS could still be invented even if we don’t know general relativity. Engineers would probably have came up with a scheme to empirically calibrate the time dilation effect. It would be harder, but compared to the complexity of GNSS as a whole not that much harder.
There’s no real value in having an explanation (other than personal satisfaction, i.e. vibes) for something unless that explanation helps you to make predictions or manipulate objective reality in some way. Are you saying there is no value in like >95% of math, astronomy, and probably many other fields of research? That’s a harsh take, but perhaps you’re right.
At least let me say this: finding explanations to satisfy personal curiosity (doing it for vibes, as you put it) is different from projecting personal feelings onto objective understanding of reality (the vibes-based astrophysics I was referring to in the meme).
- Comment on vibes-based astrophysics 3 weeks ago:
I must admit I don’t know that much about MOND being tested. But yeah, from a Lambda CDM point of view it is unsurprising that MOND would not work well for every galaxy.
- Comment on vibes-based astrophysics 3 weeks ago:
Yeah it’s not settled by any means. Far from it.
But the hypothesis that it exists and is some kind of matter is pretty well supported through observing gravitational effects.
- Comment on vibes-based astrophysics 3 weeks ago:
It’s a classic MEMRI TV meme. What MEMRI TV is would require a … “nuanced” explanation that I don’t want to get into here. Look it up on Reddit or start a thread on !nostupidquestions@lemmy.ml
- Comment on vibes-based astrophysics 3 weeks ago:
WIMP is only one model of dark matter. A favorite of particle physicists. But from a purely astrophysics point of view there is little reason to believe dark matter to have any interaction beyond gravity.
- Comment on vibes-based astrophysics 3 weeks ago:
But it is a model we invented no? To explain the astrophysical and cosmological observations.
Among all those observations, a commonality is that it looks like there is something that behaves like matter (as opposed to vacuum or radiation) and interact mostly via gravity (as opposed to electromagnetically, etc.). That’s why we invented dark matter.
The “it is unsuited” opinion in this meme is to poke at internet commentators who say that there must be an alternate explanation that does not involve new matter, because according to them all things must reflect light otherwise it would feel off.
Once you believe dark matter exists, you still need to come up with an explanation of what that matter actually is. That’s a separate question.
(I’m not trying to make fun of people who study MOND or the like of that. just the people who non-constructively deny dark matter based on vibes.)
- Comment on vibes-based astrophysics 3 weeks ago:
Particle physicists love the Weakly-Interacting Massive Particle dark matter model. But from a purely astrophysics point of view there is little reason to believe dark matter to have any interaction beyond gravity.
- Comment on vibes-based astrophysics 3 weeks ago:
I’m still far from convinced about MOND. But I guess now I’m less confident in lambda CDM too -_-
I’m inclined to believe it’s one or many of the potential explanations in your second link. But even then, those explanations are mostly postdictions so they hold less weight.
- Comment on vibes-based astrophysics 3 weeks ago:
MOND is a wonderful way to explain rotation curves but since then with new observations (bullet cluster, gravitational lensing, …) MOND doesn’t really hold up.
- Comment on vibes-based astrophysics 3 weeks ago:
I’ve heard of something similar that is able to predict an effect of dark matter (the rotation curves), but AFAIK it couldn’t match other observations (bullet clusters, etc.) correctly.
Do you have a link for the model you’re talking about. I’m curious.
- Comment on vibes-based astrophysics 3 weeks ago:
This is a very fair take, but I’d say dark matter is harder to falsify, but not totally unfalsifiable.
You can’t see it, true. But what makes sight so special? We can’t smell stars either. You just need to sense dark matter in some other way. Namely gravity! We have seen the way visible matter orbit, and that points to dark matter. We have seen gravitational lensing due to dark matter. Hopefully soon we’ll observe gravitational waves well enough to sense dark matter around the regions the waves are being emitted from.
Most individual dark matter models are falsifiable (and many have already been falsified) through non-gravitational means too. People have been building all sorts of detectors. The problem with this is that detectors are expensive and there are always more models beyond any detector’s reaches.
- Submitted 3 weeks ago to science_memes@mander.xyz | 37 comments
- Comment on Call me SKEPT1KAL 1 month ago:
Unfortunately cosmic string and string theory are completely different things.
en.m.wikipedia.org/wiki/Cosmic_string says
Not to be confused with String, the subject of String Theory.
- Comment on Can't install app because it isn't "certified" by the government 2 months ago:
Since when do they have those rules? A year ago I unlocked my Xiaomi phone. Outside China. Did not have a Chinese phone number. It took less than an hour.
- Comment on your mom falls significantly faster than g 2 months ago:
even light can stop following null geodesics because the curvature can be too big compared to the wavelength
Very interesting! How do you study something like this? Is it classical E&M in a curved space time, or do you need to do QED in curved space time?
Also, are there phenomena where this effect is significant? I’m assuming something like lensing is already captured very well by treating light as point particles?
- Comment on your mom falls significantly faster than g 2 months ago:
So if I have a spherically symmetric object in GR I can write the Schwarzschild metric that does not depend on the radial mass distribution. But once I add a second spherically symmetric object, the metric now depends on the mass distribution of both objects?
Your point about linearity is that if GR was linear, I could’ve instead add two Schwarzschild metrics together to get a new metric that depends only on each object’s total mass?
But even in a situation with one source, does the shell theorem work in GR? Say I put a infinitely light spherical shell around a black hole. Would it follow the same geodesic as a point particle?
- Comment on your mom falls significantly faster than g 2 months ago:
For the bowling ball, Newton’s shell theorem applies, right?
- Comment on your mom falls significantly faster than g 2 months ago:
Earth is in this case not an inertial reference frame. If you want to apply Newton’s second law you must go to an inertial reference frame. The 9.81m/s/s is relative to that frame, not to earth.
- Comment on your mom falls significantly faster than g 2 months ago:
That is one very impressive feather.
Restricting ourselves to feathers made by non-human animals
🤔🤔🤔
- Comment on your mom falls significantly faster than g 2 months ago:
the original title was “your mom false significantly faster than g”
- Comment on your mom falls significantly faster than g 2 months ago:
Re your first point: I was imagining doing the two experiments separately. But even if you do them at the same time, as long as you don’t put the two objects right on top of each other, the earth’s acceleration would still be slanted toward the ball, making the ball hit the ground very very slightly sooner.
Re your second point: The object would be accelerating in the direction of earth. The 9.81m/s/s is with respect to an reference frame (say the center of mass frame). The earth is also accelerating in the direction of the object at some acceleration with respect to the inertial reference frame.
- Comment on your mom falls significantly faster than g 2 months ago:
Nope. The argument only works if you conjured the bowling ball and feather out of
thin airvacuum. lemmy.world/comment/13237315 discusses what happens when the objects were lifted off earth. - Comment on your mom falls significantly faster than g 2 months ago:
I didn’t think about that! If the object was taken from earth then indeed the total acceleration between it and earth would be G M_total / r^2, regardless of the mass of the object.
- Comment on your mom falls significantly faster than g 2 months ago:
Okay how about now
- Comment on your mom falls significantly faster than g 2 months ago:
@WolfLink@sh.itjust.works and @theturtlemoves@hexbear.net are correct
- Comment on your mom falls significantly faster than g 2 months ago:
fixed it sorry
- Comment on your mom falls significantly faster than g 2 months ago:
I meant cross-section area, not surface area. Sorry. Edited my comment above.
- Comment on your mom falls significantly faster than g 2 months ago:
If your bowling ball is twice as massive, the force between it and earth will be twice as strong. But the ball’s mass will also be twice as large, so the ball’s acceleration will remain the same. This is why g=9.81m/s^2 for every object.
But the earth’s acceleration would not remain the same. The force doubles, but the mass of earth remains constant, so the acceleration of earth doubles.