Let's pretend that you have a basket with 100 apples. You know apples are about 100g each, because you weighed 10 of the them and all of the apples seem about the same size. You know that basket weighs 1000g. You put the whole thing on a scale and find it weighs 500,000g. You know something else is in that basket. You aren't sure what, and frankly it doesn't make sense, but trying different scales and remeasuring more individual apples gives the same result. So you decide that there must be something you can't see but must exist. That's dark matter/energy.
The scale probably just can't measure the apples all together that way. Maybe it's not calibrated to see all the different ways apples can interact. Maybe time to go back to the scale drawing board.
That's that funny thing, they've tried different scales. They've tried radically different ways of measuring it, and always come up with the same discrepancy.
If summing energy works differently on a large scale, why? Since we don't know what we can do is start measuring the difference between observable energy and the "extra" that appears when we add it up. We could call that "unobservable energy" so we can see if there is a pattern, or if it's actually something else. You know "unobservable energy" is a mouthful, why not just call it dark energy?
We don't know what it is. We have tested lots of theories and dark energy doesn't seem to fit any answer, hence the name. I get thinking that it can't be that hard to reconcile and scientists must be missing an obvious conclusion, but it's likely that your theory has already been tested. Maybe you have the solution and can resolve the discrepancy, but right now all data shows that dark energy is a large part of the universe.
I'm not sure what you're getting at. Dark matter has been proven numerous times, is a predictive model, and is the only explanation that has held up to scrutiny and observations. It's very clearly the right explanation and we know how dark matter generally behaves, we just don't know specifically what it is.
See, for example, the behavior of the bullet cluster merger.
Sorry, but...aren't modified gravity theories gaining some more traction recently? Not enough to say that modified gravity is the most likely explanation for observations, but at least enough to avoid saying that dark matter is "clearly the right explanation"?
edit: I've just realised that some people would describe modified gravity as a specific theory to describe the observational effect of dark matter. Is that what you were doing here?
Ok this gave me a laugh.
But to be serious, isn’t the opposite true? Like, my understanding is that string theory is basically dead, and only getting deader. But I thought modified gravity as explanations for the dark matter observations is seeing a bit of a resurgence lately.
my understanding is that string theory is basically dead, and only getting deader.
Huh... where is this impression from? String theory isn't dead, it's just a very narrow field in which most of the participants specialize in a subset of it that's less concerned with completing it as a whole. It's incredibly difficult work, progress is slow, and it's currently too broad to be applicable to reality (which is important for funding). The tests we can think of to validate the correspondence of math to the physical world are... significantly out of reach due to the energy requirements.
But it's still the leading theory of quantum gravity and there's active work in, say, AdS/CFT correspondence - which shows that string theory can line up to reality and be predictive. It's the best idea we have right now, it's satisfyingly elegant, and it's working as a useful tool at the very least.
There are competing alternatives that get their own research, of course. We should persue them all until a clear victor emerges!
But I thought modified gravity as explanations for the dark matter observations is seeing a bit of a resurgence lately.
Modified gravity, so far, is non-predictive and does not account for things like the bullet merger while also accounting for ultra diffuse galaxies and our observations of the CMBR. All proposed modified gravities have failed to pass experimentation compared to general relativity. Modified Newtonian Dynamics (MOND) fails in the face of light and gravity having the same speed. And even if MOND were to be true, it still requires the presence of (albeit possible baryonic) dark matter to be even considered due to existing mass measurements of galaxies.
So, again, dark matter is simply the best model we have.
I probably overstated the case, but I was mainly going on what this video said about string theory. It’s had no reliable test results, no predictive power. It’s a useful mathematical model, but not actually a good theory to explain the real world.
I don’t even know where I got that idea about modified gravity from. I think I vaguely saw a few headlines about it recently but I didn’t even bother to read the articles they came from, and I somehow allowed that to stick in my brain. My bad.
Sorry for not responding earlier, I don't seem to be getting notifications! My other reply further down in the thread hopefully answers all of your (wonderful) questions, though. Have a great day!
Yeah no worries. I’ve had the same problem with notifications sometimes not appearing. No idea why. Thanks for the feedback though!
I was working on a machine last week that's supposed to turn on when it's plugged into a battery. And it did that, but the battery it was plugged into was faulty, so we changed it. And then it wouldn't turn on when plugged in. We tried it with several different batteries, and it would only turn on when plugged into that first one. We couldn't figure it out.
The next day, I came into work and told my coworker that battery is magic. Because it was the only explanation that could accurately predict the results we saw.
Then the battery stopped being magic.
You have a fundamental misunderstanding of physics if you think that analogy is even remotely similar to dark matter.
Let’s say I construct an apple orchard with harvesters that run on batteries. The batteries send power to the machines when they are inserted into them, and the harvesters use the battery power to harvest 100g apples. If I weigh the battery before putting it in a machine, and the battery weighs 1,000g, would that be fucked up, or what?
Look, all I know is that when I bake an apple pie with those apples, my family is happy and no longer hungry, and isn't that what dark matter is all about? The things that matter.
I'm happy to talk about this more, but I'm afraid I don't understand your analogy. I'm sorry! If you'd like to rephrase it, I'll make myself available to respond. 🙂
Why isn't it called "cold matter"?
Don't we just not see it because it's not burning?
Couldn't the unseen mass be clouds, planets and black wholes?
As far as we're aware, dark matter only interacts with the universe gravitationally. It doesn't even interact with itself, which is why we don't see dark planets/stars/galaxies popping into existence. It only follows normal matter around.
As for why it's not called cold, is for two reasons:
If it happened to be clouds of gas and dust that overall had a net gravitational effect on the background galaxies, we'd be able to detect the spectral lines of these clouds. Same for just about all the other objects in that list. In some cases we do detect intergalactic gas clouds. But in places where there's very clearly unaccounted for gravitational lensing, there isn't any sign of this. So far the only things we can match up to the observations is a mathematical model of the stuff.
No, because those would also interact with other things like electromagnetic radiation, light, etc. Dark matter only interacts with normal matter via gravity.
Dark matter is admittedly a bit of a misnomer but that's what everyone's been using for years 🤷🏻♂️
1+1=3
Hypothesize there's an extra 1 out there we don't know about yet
Relentlessly test hypothesis for decades
Every test lines up with hypothesis
Relentlessly move humanity's understanding of physics forward in a search for more information
Get made fun of by dumb fucks who struggle with division
This is the most-tested, best-proven scientific theory humanity has known. For some reason, it completely breaks if you remove these Wingdings characters we added to the code.
You forgot to scale by an arbitrary "heuristic" constant to mask the excess of simplification of the models.
Details in the supplementary information
Although we haven't observed dark matter on a microscopic scale, we have been measuring the effects of dark matter on a macro level for decades.
As far as I understand it, the way stars, planets, gas clouds and galaxies move wouldn't make sense with just the gravity of the visible objects. There needs to be so-far-undetectable (directly, at least) dark matter in the otherwise empty space? I don't really remember if it was understood to be a universally equal distribution, or if it is understood to be concentrated in blobs/clouds/noodles or whatever.
I don't really have a lot at stake riding on it, while it's interesting physics and to an extent I care about it, I don't really care enough to either ridicule, push for solid answers "NOW", hunt down frauds, etc.. Some people really care about that, I don't really take it that seriously. I don't think life should be a rat race where you're either useful and correct weithin a deadline, or dismissed, cut off and dead. The scientific consensus seems to be that there isn't really a good alternative to dark matter. Was it string theory that tried? I might be misremembering, but as far as I've heard, that's basically dismissed and disproven regardless of whether it had anything to do with dark matter.
On a final note, is there any reason to expect that the giant deep Antarctic ice-telescope will be able to observe dark matter? Apparently it's supposed to be able to pick up on wavelenghts that we so far haven't had the chance to observe, iirc by triangulating rare flashes of light from neutrino collisions with particles in the extraordinarily clear ice.
The scientific consensus seems to be that there isn’t really a good alternative to dark matter. Was it string theory that tried?
The alternative to dark matter is modified gravity/modified Newtonian dynamics. Neither of which have held up to scrutiny and have major flaws that would need to be worked out before being a legitimate competitor to dark matter. In every single permutation thought of today, these theories directly conflict with the reality we observe, while dark matter has been in happy agreement with new data.
that’s basically dismissed and disproven regardless of whether it had anything to do with dark matter.
String theory is not disproven and still remains the leading train of thought. It's just a very niche field and progress is hard/underfunded! But so far we've seen things like AdS/CFT correspondence and it's a more "elegant" solution than its competitors.
is there any reason to expect that the giant deep Antarctic ice-telescope will be able to observe dark matter?
Are you talking about the IceCube? If so, no, that's a neutrino telescope. Although, in general, the answer would also be no; dark matter does not interact with itself or with regular mass in any way other than through gravity. It's simply impossible to measure it directly - it must be done by measuring it's gravitational effect on other things.
And because of that very property dark matter's smallest observable structures are galactic in scale, so it's also rather hopeless to try to observe them locally with current technology.
String theory in all it's permutations is still fundamentally untestable and had been since it's inception. I can't speak for academia (though I did watch a YouTube video that claimed to, and it ended up with this same conclusion, fwtw) but in scientific media string theory is more or less absent now. Early 2000s it was everywhere but the standard model kept plugging along without string theory, and now I only see it if I look for it specifically. It's not the leading train of thought and I doubt it ever really was - it was just popular in media. The standard model for quantum stuff and special relativity/general relativity for macro stuff is still the generally accepted view
If your view of string theory is through the lens of media, you aren't going to be up-to-date. String theory was and is the leading theory for quantum gravity, is actively worked on, and has only been supported in recent findings through quantum field theory.
But you're talking about a field with little funding, that requires some of the most brilliant mathematical minds who have specializations, and in which experimentation requires super technology to build particle accelerators the size of the moon. It's not a glamorous field and once the buzz of "theory of everything" wording died off, it was forgotten in media. Just like so many other topics before it.
The standard model for quantum stuff
The standard model doesn't handle quantum gravity, which is kind of important. Nor does it address a slew of other very real phenomenon (dark matter, for instance). It's not a theory of everything, just a good model. It's also something that can be derived from string theory. The two are not competing ideas.
