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The Reason We Haven’t Directly Detected Dark Matter

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Re: The Reason We Haven’t Directly Detected Dark Matter

#31

Post by acqq »

analog31 wrote: post #19
beefield wrote: post #16

Earlier quoted context omitted.

> Also, dark matter doesn't interact much with regular matter, which makes the search even harder. This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass? Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average densi…

As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.

"Think of the amount of mass required to generate a gravitational pressure needed to overcome the electromagnetic binding force between molecules inside the mass--the equilibrium occurs, basically, when an object in space becomes spherical. This happens at about 10^20 - 10^21kg. Divided by the mass of a proton implies you need about 10^47 atoms to generate the amount of gravitational pressure to break the electromagnetic strength between atoms." (1)

Richard Feynman would tell you that this immense difference is "what's keeping you from not falling through the floor down to the Earth center." Or why the apple hanging on the "few atoms" of its stalk is not falling from the tree, when the summary gravitation of all atoms of the whole Earth are pulling it down.

Or, again to compare such big numbers, there are "only" 10^86 atoms in the whole Universe observable to us! (2)

1) https://www.physicsforums.com/threads/how-are-the-gravitatio...

2) https://www.universetoday.com/36302/atoms-in-the-universe/

Re: The Reason We Haven’t Directly Detected Dark Matter

#32

Post by wongarsu »

skdjii wrote: post #21

Another reason may be that dark matter doesn't exist. If the universe consists of 85% of the stuff it would seem that it should be easy to find. Occams razor should apply as usual.

We have countless observations that tell us that a lot of places in the universe have a lot more mass than we can see. The two most obvious solutions are that dark matter exists, or that we are wrong about how gravity works. So far we have failed to come up with a satisfying alternative description of gravity and it's not looking good for ever finding one.

So if you really want to apply Occam's razor on this, the existence of matter that only interacts with gravity and nothing else is a much simpler explanation than some elusive alternative theory of gravity.

Re: The Reason We Haven’t Directly Detected Dark Matter

#33

Post by sonnyblarney »

gbrown wrote: post #17
drewg123 wrote: post #12

Earlier quoted context omitted.

We know how stars work, and we know how gravity works...These two numbers don’t match, and they don’t match spectacularly. There had to be something more than just stars responsible for the vast majority of mass in the Universe. I think the problem here is that maybe there is something else we don't understand. We also did not understand the propagation of light in a vacuum in the 19th century, and invented a hidden…

Perhaps we could call that thing: "dark matter" for now. In seriousness, we can observe the effect, and even map the distribution of the stuff.

"In seriousness, we can observe the effect, and even map the distribution of the stuff."

No, we can measure the dissonance between our best theories and reality. And make a conjecture about 'stuff'.

Re: The Reason We Haven’t Directly Detected Dark Matter

#34

Post by acqq »

beefield wrote: post #27
analog31 wrote: post #19

Earlier quoted context omitted.

As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.

But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth…

Fascinatingly, it is known that the interaction of the dark matter with the Earth is so weak that there would be no "clumping" of it around the Earth at all! No "clumping" even around e.g. Sun can be observed. The "hanging around" is on the level of the whole galaxies, and sometimes the dark matter even remains outside of the whole galaxies, being too slow to follow their gravitational interaction(!) That's the famous example of the "bullet cluster":

https://www.forbes.com/sites/startswithabang/2017/11/09/the-...

That's how weakly the dark matter interacts with anything else. And it obviously doesn't even interact strong enough to "fall" to the center of the galaxy. Otherwise it would be there, but it remains to the outside of even where the "normal" matter is (mostly the stars and black holes, the "supermassive black hole" in the center is only at most 1e-5 of the estimated total mass of our Galaxy).

Re: The Reason We Haven’t Directly Detected Dark Matter

#35

Post by siweiz »

beefield wrote: post #27
analog31 wrote: post #19

Earlier quoted context omitted.

As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.

But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth…

They are not distributed evenly throughout space. So if you measure more gravity in a region of space that you cannot see (because dark matter doesn’t interact with em), is it dark matter, or an error in your theory of gravity?

Re: The Reason We Haven’t Directly Detected Dark Matter

#36

Post by improbable22 »

drewg123 wrote: post #25
gbrown wrote: post #17

Earlier quoted context omitted.

Perhaps we could call that thing: "dark matter" for now. In seriousness, we can observe the effect, and even map the distribution of the stuff.

That assumes that it is "matter". Maybe there is something we just fundamentally don't understand about gravity.

This is another possibility, and would be exciting.

But it's very very hard to come up with plausible models for such modified gravity. There are lots of people trying, and their ideas tend to break all sorts of other things we know. For example (IIRC) lots of candidates gave a slightly different speed for gravitational waves, and were ruled out by recent detections where we also saw X-rays from the same event.

Re: The Reason We Haven’t Directly Detected Dark Matter

#37

Post by roenxi »

antidesitter wrote: post #30
skdjii wrote: post #21

Another reason may be that dark matter doesn't exist. If the universe consists of 85% of the stuff it would seem that it should be easy to find. Occams razor should apply as usual.

> If the universe consists of 85% of the stuff it would seem that it should be easy to find. How did you infer this? We are bathed in gravitational waves but they weren’t “easy” to find. > Occams razor should apply as usual. So what would be your razored explanation for the gravitational anomalies that point to the presence of matter we cannot see?

Our understanding of gravity might be completely wrong. If 85% of the gravity is unexplainable that seems like a pretty safe bet. Maybe it just behaves very differently at galactic scale.

Much like the aether, it seemed most likely that light traveled through a medium until it turned out that it probably doesn't.

Re: The Reason We Haven’t Directly Detected Dark Matter

#38

Post by lottin »

So, basically physicists have deduced two facts, and these facts don't agree with one another. From this they conclude that there must be something else in the universe, something unseen and undetectable, that they call dark matter. But what about the other possibility, which seems far more reasonable to me, that either one of the facts is wrong?

Re: The Reason We Haven’t Directly Detected Dark Matter

#40

Post by InclinedPlane »

std_throwawayay wrote: post #39

If we can't detect it or disprove it, it doesn't even belong to science. It belongs to the same realm as witches and demons - until some smart physicist devises a way to actually detect it.

We can disprove it, any one of the many, many lines of evidence that have been collected so far could have disproved the WIMP/CDM theory of dark matter. They haven't. Instead they've eliminated all of the other alternatives. That's science.
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