9 ms·

There's inconsistencies, and then there's real inconsistencies. Recall some predictions that were later confirmed: the top quark (18 years later) or the Higgs

There's inconsistencies, and then there's real inconsistencies. Recall some predictions that were later confirmed: the top quark (18 years later) or the Higgs boson (48 years!). These theories were a jigsaw puzzle with a single piece-shaped hole. You could call "inconsistency!" and break apart the whole puzzle, or look behind the couch for the missing piece. (The piece was found!) We also have real inconsistencies, such as between QM and GR. Not a missing piece but a yawning gap: no single observation can fill in that hole. Dark matter is a "missing piece" problem. The dark matter hypothesis fills in the cosmological missing mass problem, galactic rotation curves, Bullet-cluster-style gravitational observations, the expectation of non-EM interacting particles, and others. The gap between prediction and observation is the size and shape of a single jigsaw piece; let's just find it behind the couch. Dark energy is a "real inconsistency" problem. Predictions and observations are orders of magnitude apart. Well hell.

raverbashing 7y ago
This is a good explanation, but I'm not so sure the missing piece is so clear in the case of dark matter (and the blog post explained this nicely). Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results. Maybe gravitational attraction is not r^2 at some non-obvious scale that we failed to measure (meters? mm? Casimir effect on cosmic dust? Solar system sized gravitational anomalies?) Why relativity makes Mercury's orbit precess is very not obvious (I mean, the two-body problem is already hard without relativity). The bending of light is a more obvious effect for example.
- Tuna-Fish 7y ago
  > Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results. How do you propose for a modified equation to explain the bullet cluster? That is, we have multiple good observations that show situations where the distribution of mass differs substantially from the distribution of visible matter, and good theoretical explanations why this should happen in the case of dark matter. This is not adequately explained by modified gravitation or other such effects.
  - raverbashing 7y ago
    Disclaimer: I do think that the Bullet Cluster is good evidence for dark matter, but we can speculate a bit One is that there's also a possibility that dark matter exists but it's only part of the story. For the Bullet Cluster I wonder how reliable the estimation of gravitational mass is for cluster of objects, as I understand, it is the images of the galaxies behind it that are lensed by mass in front of it https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:MACS_J0025.4-1222.jpg https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:... For one heavy object in front of another, it's easy to see how lensing works and estimate masses from there. But with clusters and "fuzzier" mass distributions I'm not so sure.
- wiml 7y ago
  Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle. Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.
braindead_in 7y ago
Is there a well defined boundary between QM and GR?
- fsh 7y ago
  The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.
  - baq 7y ago
    and yet the universe is full of these objects - black holes, neutron stars, quasars, etc.
    - krastanov 7y ago
      No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.
      - fsh 7y ago
        Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.
        deleted 7y ago
        [deleted]
  - enkid 7y ago
    I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.
marvin 7y ago
Here's an image explaining the "bullet-cluster" observation. It's a compelling piece of evidence. Two galaxy clusters have collided. The hypothesis is that normal matter will be slowed down by the collision, while dark matter, not interacting (as strongly?) with itself or normal matter, passes through in the collision. We can determine the locations of the gas by X-ray telescope, and the supposed location of the dark matter (assuming it exists) with gravitational lensing. https://ircimg.net/slide_33.jpg https://ircimg.net/slide_33.jpg (I am not a physicist, just had this explained by a physicist friend of mine).
- sandgiant 7y ago
  Indeed, this is the observation most usually cited in support of the matter / particle interpretation of dark matter. It's very non trivial to come up with a decent theoretical extension to GR that explains it. Which is why, I think, that a lot of people feel more confident proposing a new dark matter particle, which does in fact explain it quite nicely. Perhaps we're wrong in assuming that all of these observations on very different scales (galaxy rotation curves over galaxy cluster collisions to large scale structure) are tied to the same unknown (dark matter), but if you want to explain them all in a simple way, a particle still seems like the way to go. Remember that elementary particles have been predicted and found many times over the past years, whereas modified theories of gravity haven't seen the same kind of success. Of course this is only anecdotal, and may not mean anything, but I think it's part of the reason for most people's preference for a particle explanation.
- _carl_jung 7y ago
  The final bullet point in that image contains the answer to the question of dark matter.
- sarbaz 7y ago
  The counterargument given by one of the proponents of MOND (a non-dark-matter counterargument) is that the bullet cluster can be explained by a combination of slight modification of GR, and additionally by the presence of a significant (but not crazy) amount of cold regular matter. Cold regular matter can't be easily detected because it isn't emitting any light, but it is just regular matter. I'm not actually familiar with the specifics so I don't know whether this is actually a good explanation, but it's at least worth discussing.
sorenn111 7y ago
Thank you for posting this. I often read commenters who dismiss dark matter the way dark energy is dismissed without people addressing the bullet cluster style observations which in my layman view is the most compelling piece of evidence for dark matter

raverbashing 7y ago

This is a good explanation, but I'm not so sure the missing piece is so clear in the case of dark matter (and the blog post explained this nicely). Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results. Maybe gravitational attraction is not r^2 at some non-obvious scale that we failed to measure (meters? mm? Casimir effect on cosmic dust? Solar system sized gravitational anomalies?) Why relativity makes Mercury's orbit precess is very not obvious (I mean, the two-body problem is already hard without relativity). The bending of light is a more obvious effect for example.

Tuna-Fish 7y ago
> Sure, dark matter can explain those, but what if there's some not yet imagined change to the equations that gives the same results. How do you propose for a modified equation to explain the bullet cluster? That is, we have multiple good observations that show situations where the distribution of mass differs substantially from the distribution of visible matter, and good theoretical explanations why this should happen in the case of dark matter. This is not adequately explained by modified gravitation or other such effects.
- raverbashing 7y ago
  Disclaimer: I do think that the Bullet Cluster is good evidence for dark matter, but we can speculate a bit One is that there's also a possibility that dark matter exists but it's only part of the story. For the Bullet Cluster I wonder how reliable the estimation of gravitational mass is for cluster of objects, as I understand, it is the images of the galaxies behind it that are lensed by mass in front of it https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:MACS_J0025.4-1222.jpg https://en.wikipedia.org/wiki/MACS_J0025.4-1222#/media/File:... For one heavy object in front of another, it's easy to see how lensing works and estimate masses from there. But with clusters and "fuzzier" mass distributions I'm not so sure.
wiml 7y ago
Right. A lot of "modified gravity" theories have been proposed in recent decades, investigated, and compared to observations; with work they can be made to match up. Dark-matter theories on the other hand do a somewhat better job of matching observations, which is why they have become the best-liked explanation for the galactic rotation curve puzzle. Scientists didn't just jump to dark matter at the start, and no cosmologist will tell you that dark matter is definitely the explanation — it's simply the explanation that currently seems most likely.

braindead_in 7y ago

Is there a well defined boundary between QM and GR?

fsh 7y ago
The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.
- baq 7y ago
  and yet the universe is full of these objects - black holes, neutron stars, quasars, etc.
  - krastanov 7y ago
    No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.
    - fsh 7y ago
      Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.
      - deleted 7y ago
        [deleted]
- enkid 7y ago
  I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.

marvin 7y ago

Here's an image explaining the "bullet-cluster" observation. It's a compelling piece of evidence. Two galaxy clusters have collided. The hypothesis is that normal matter will be slowed down by the collision, while dark matter, not interacting (as strongly?) with itself or normal matter, passes through in the collision. We can determine the locations of the gas by X-ray telescope, and the supposed location of the dark matter (assuming it exists) with gravitational lensing. https://ircimg.net/slide_33.jpg https://ircimg.net/slide_33.jpg (I am not a physicist, just had this explained by a physicist friend of mine).

sandgiant 7y ago
Indeed, this is the observation most usually cited in support of the matter / particle interpretation of dark matter. It's very non trivial to come up with a decent theoretical extension to GR that explains it. Which is why, I think, that a lot of people feel more confident proposing a new dark matter particle, which does in fact explain it quite nicely. Perhaps we're wrong in assuming that all of these observations on very different scales (galaxy rotation curves over galaxy cluster collisions to large scale structure) are tied to the same unknown (dark matter), but if you want to explain them all in a simple way, a particle still seems like the way to go. Remember that elementary particles have been predicted and found many times over the past years, whereas modified theories of gravity haven't seen the same kind of success. Of course this is only anecdotal, and may not mean anything, but I think it's part of the reason for most people's preference for a particle explanation.
_carl_jung 7y ago
The final bullet point in that image contains the answer to the question of dark matter.
sarbaz 7y ago
The counterargument given by one of the proponents of MOND (a non-dark-matter counterargument) is that the bullet cluster can be explained by a combination of slight modification of GR, and additionally by the presence of a significant (but not crazy) amount of cold regular matter. Cold regular matter can't be easily detected because it isn't emitting any light, but it is just regular matter. I'm not actually familiar with the specifics so I don't know whether this is actually a good explanation, but it's at least worth discussing.

sorenn111 7y ago

Thank you for posting this. I often read commenters who dismiss dark matter the way dark energy is dismissed without people addressing the bullet cluster style observations which in my layman view is the most compelling piece of evidence for dark matter