This is unnecessarily negative. Physics progresses in fits and starts, with plenty of blind alleys and red herrings. Maybe we're in a phase akin to the aftermath of the Michelson–Morley experiment. Or maybe it really is that complicated out there.
I find dark matter seems to fit into the same pattern as epicycles. We can add additional complexity to the theory to make our models better match observational data, and that's useful but also strongly hints that something more basic about the model is fundamentally incorrect. That's depressing.
> Then again, almost no one with a formal education in cosmology or partical physics at the PhD level does.
This is also just wrong. I think we're seeing a number of established voices starting to question the underpinnings of dark matter as a theory.
As for
>Which additional complexity has been added to dark matter?
Let's start with the fucking article we're discussing, where we now need to have "self-interacting" dark matter, as a new spin on the theory to account for the forces needed to explain what we're observing...
The article is also not adding complexity. We didn't get new evidence that required modification of DM. We got a phenomenon which has many potential explanations as the "fucking" article points out, one of which could be a subset of all DM theories. The final parsec problem isn't challenging DM at all, instead one particular flavor of DM could help explain the observation. If anything, it could constrain DM. Why wouldn't you explore this possibility?
We didn't get any new evidence that required modification of epicycles! We got a phenomenon which constrains the epicycle theory by adding more circles! <-- you.
> If anything, it could constrain DM. Why wouldn't you explore this possibility?
You're the only one suggesting that we throw out DM as a model right now... at no point have I even hinted at not exploring it. You are bashing someone who is simply suggesting that this model might not be correct (how fucking dare I...).
But the thing about models... no model is correct, but some models are useful.
Again - I'm not contesting that DM is currently a useful model for explaining some of things we're seeing. I'm suggesting that it might also be worth considering alternatives (how fucking dare I...).
Go back to trying to burn Galileo alive. It's the same energy.
Just today I heard a talk about people looking at stellar motions in ultra faint dwarf galaxies, because standard dark-model theories predict they should have centrally cusped dark-matter density profiles -- and fuzzy dark-matter models predict different profiles, which they could potentially discriminate between.
However, it is very much true that different galaxies have different amounts and distributions of dark matter. There are many broad categories, and certain models about galaxy formation try to predict these, but not all galaxies fit these models. This has two effects: for one, for any galaxy with unexpected dynamics, you can fit a dark matter distribution to explain the dynamics (but! This is not unbounded, it still has to match certain other observations). The other effect is actually in favor of a dark matter approach: it means modifying parameters of our theories in a general way has little chance to reproduce the same properties, as each different galaxy is different.
Loving this comparison, and I hope this is the case.
The problem with cosmology is that it's based almost purely on theory and not data, to the point of speculation.
The period after the apparent failure to the present success has been incredibly fruitful.
When the original aether theory failed, they pointed out that you squish when moving in the aether, and so they detected aether waves rather than aether motion.
With an upgraded version of the same device.
Wilczek for instance points out that modern theories are aether theories.
That sounds like “stuff” is there.
> Stuff/substances is where you can talk about a chunk of it, and say where it is at some later time.
In what sense can you do this with a liquid but not spacetime?
Things that are Lorentz invariant are unlike fluids.
So you’ve failed to show a difference between spacetime and a fluid.
And to address your point directly:
Gravitational waves leave a wake that perturbs the medium.
https://www.sciencealert.com/gravitational-waves-could-be-le...
We can also coherently talk about a chunk moving, eg subluminal warp drives.
https://phys.org/news/2024-05-subluminal-warp.html
So we have two ways “where you can talk about a chunk of it, and say where it is at some later time”.
Fluids aren’t Lorentz invariant.
So I’m not following.
Perhaps you could explain how either of those operates without being able to identify chunks of spacetime as moving.
Then, in flat spacetime, as there is no curvature, then (by the contrapositive) there is no “movement of spacetime”?
But if some stuff is not moving in one inertial frame, then in another inertial frame it is moving.
But that implies that, if spacetime is stuff and such that, if it is moving then there is local curvature, it follows that in a different inertial frame, the spacetime is not flat.
But, whether spacetime is flat doesn’t depend on the choice of an inertial reference frame.
As for how to describe those things without talking about spacetime moving,
How spacetime works has the general coordinate invariance or covariance or whatever.
You have a manifold with a pseudo-Riemannian metric tensor. You can talk about like, geodesics and such.
For a warp bubble (suppose a sub-luminal one) you could talk about a non-accelerating time-like path that remains within the bubble, and talk about how the spacetime interval along it relates to the spacetime interval of some time-like path that starts in the bubble, leaves it, “travels besides it”, and finally re-joins the original path.
(Of course, in some reference frame, the bubble would not move.)
I don’t think it really makes sense to talk about spacetime moving, even in the case of a warp bubble. A position doesn’t have a position, or a time a time, but rather a position is a position, and a time is a time.
You can talk about how, in some particular coordinate system, with some particular foliation into space-like slices, some aspects/pattern of the metric tensor at some position and time coordinate values, correspond to those at a later time coordinate at a different position coordinate value, but this is relative to a choice of coordinate system.
I’m still not following your objection based on relativity:
We have one frame where I’m on Earth; we have another where I’m in a rocket traveling away at a fixed velocity. In one case we see the rocket traveling away; in the other, we see Earth traveling away. But neither one tells us that the spacetime itself is moving — they’re both just local coordinates.
In the case we see the spacetime itself move, eg with a gravitational wave, both the frame from the rocket and the earth agree that the manifold has been changed in a way that left a wake — moving relative to its local, original shape.
As a side note, “inertial reference frames” are a useful fiction: we’re always being accelerated, eg, by the galactic center, fellow galaxies, dark matter filaments, etc. Conclusions based on such a fiction may be practical, but cannot be used to derive conclusions about physical reality — as they don’t exist.
2) Why would something have to be in an inertial reference frame in order for inertial reference frames to “exist”? Doesn’t make sense to me.
In any case, that seems like just an excuse to ignore Lorentz boosts. One should address SR before addressing GR.
3) If spacetime were stuff, then it would be stuff always, not just when there are ripples or whatnot. But, in flat spacetime, you cannot describe it as a stuff in a way that is compatible with Lorentz symmetry, because the velocity of stuff changes under boosts, and so stuff can’t be stationary both when one doesn’t and when one does apply a boost. Therefore, it isn’t stuff.
The solution for both was posited to be a physical medium permeating space that had certain special properties, including very low interactions with other matter (explaining why we didn't directly detect it). Light would only propagate in this medium, and since this medium interacted very little with other matter, it explained why it wouldn't be dragged along by, say, a moving train. The speed of light would be mostly determined by the speed of this universal medium, explaining the constancy of the speed of light.
Current physics has nothing to do with these theories. The medium for light's propagation is the electric field, which is not a material medium at all. And Galilean relativity doesn't agree with Maxwell's equation because it is wrong - we have Einstein's special relativity as the successor explanation, and this doesn't require any kind of medium, and doesn't say anything about space-time waves.
General relativity does see space-time as something that looks more like a medium, except that, like the electric field and unlike aether, it is still not a material medium. And gravity waves have absolutely nothing to do with the aether wind. Gravity waves are specific phenomena triggered by specific events. They have a source and a direction of propagation like any other wave, and differenet gravity waves move in different directions. The aether wind was a fixed universal thing: all of the aether in the entire universe moved in a single direction, and had no source.
If you want to look at something that's actually closer to a modern aether, the quantum models of the void are actually much closer. Per quantum mechanics, all of space-time is permeated by fields and random fluctuations in those fields ensure that there exist virtual particles getting created and destroyed at every point in spacetime. So, in QM, there actually exists a physical medium that does permeate all of spacetime (but it doesn't have any of the other properties of the aether).
Light is an excitation of the EM aether — or if you prefer, “field”. (Which you then explain later, yourself.)
We failed to detect the aether with MM because we’re made of aether-stuff and so we squish in a way that cancels out what they were trying to detect with the original experiment.
> Current physics has nothing to do with these theories.
> [explanation of how fields are aethers]
Wilczek also points out modern theories are aether theories.
LIGO works because it detects waves in the aether — which do not suffer from the same squishing problem as in the original experiment. But that’s why a scaled up version of the original experiment worked: we’re fundamentally detecting the same thing.
Aether theories are more legitimate than dark matter theories: the MM test for the aether failed, but then they reformulated the theory and successfully observed an effect; dark matter has yet to find a working reformulation that is observable.
Also, LIGO is detecting something fundamentally different from what MM was looking for.
MM was trying to prove that light speed measured from the frame of reference of the Earth varies between times when the Earth is moving in the same direction as the Aether vs times the Earth is moving perpendicular to the Aether (since the Earth is orbiting the sun, and the Aether moves in a single direction, at some point in the year a beam of light sent in the same direction must pass between moving along a 0° angle and a 90° angle versus the Aether's direction).
This really has nothing to do with what LIGO observed. LIGO observed waves in space-time. The equivalent waves in the EM field/luminiferous aether are called "light" and we didn't need the MM experiment to discover that they exist.
So again, I agree that we can call fields "aethers", and that the EM field is in some ways similar specifically to the luminiferous aether. But it still doesn't have the property that MM were looking for: the EM field doesn't move globally, even if it fluctuates locally.
Both MM and LIGO were looking for evidence of “the stuff waves move through” — and LIGO found it by changing the experiment, because you can’t measure our own motion because we squish, as we’re also made of aether stuff.
Per wiki:
> The experiment compared the speed of light in perpendicular directions in an attempt to detect the relative motion of matter, including their laboratory, through the luminiferous aether, or "aether wind" as it was sometimes called. The result was negative, in that Michelson and Morley found no significant difference between the speed of light in the direction of movement through the presumed aether, and the speed at right angles.
You also have the MM experiment wrong — they weren’t looking for a pervasive wind, but for signal from our motion through a static aether, by looking for a difference in speed of light aligned with that motion versus perpendicular.
Which is why it’s cancelled by relativity.
The mistake of MM was thinking that the aether only pertained to light and matter was something else that could move through it — but it turns out everything is aether, including us.