Is Time Travel (going backwards) Possible?

[KarinsDad]

However, you can also look at galaxies moving in clusters of galaxies. They also move faster than they "should" based on normal gravity and just normal matter. If you add 5 times as much dark matter, you get a prediction consistent with observation. If you use MOND, you also have to add some kind of invisible, nearly pressureless matter usually identified by MOND enthusiasts as neutrinos (from the Standard Model of particle physics). So MOND doesn't work perfectly on its own.

Thank you for your more detailed and thought provoking discussion. As I mentioned earlier, MOND does have weaknesses when discussing clusters of galaxies.

But, have Dark Matter theories never been adjusted as more data is acquired? The fact that adjustments have to be made to MOND appears to be what good science should do. Adjust theories to match observations.

As for cluster issues, doesn't LCDM some times have problems with clusters? For example, in the Bullet Cluster, shouldn't the relative velocity between the two clusters be faster using Dark Matter? The lensing works, but the velocity doesn't? Isn't the Bullet Cluster a problem for both types of theories?

So, dark matter makes good predictions, MOND, not so much.

Actually from what I've read, MOND makes very good predictions in many areas (and not just galaxy speed), not so good ones in others. To focus on just those where it falters and say that it is probably wrong because of those isn't objective science. No doubt, MOND is minimally incomplete and even possibly incorrect because it cannot explain everything. Dark Matter also has areas where it does not make good predictions (or at least consistent predictions from one galaxy to the next).

There are a lot of recent articles that are starting to support MOND more and more, but some of the more (apparently, who can actually tell) objective sources that I've recently read seem to indicate that it is not winning the fight quite yet. And, this is how you appear to view it.

But, I wouldn't be surprised if the final answer is a combination of a few current theories. The gravitational equations might be wrong and there might be invisible matter out there. There also might be other forces at work beyond just gravity or gravity might not work exactly as thought.

http://blogs.scientificamerican.com...atter-a-glimpse-of-a-deeper-level-of-reality/

But, when someone like Physics Nobel Prize winner Martinus Veltman (who helped architect the standard model of particle physics) states that he doubts that Dark Matter exists at all, other scientists should at least listen:

http://www.scientificamerican.com/article.cfm?id=lindau-dark-energy

He might be wrong. Only time and a lot of hard work will tell.

I also hope you can see that it's not like what KarinsDad has said about Neptune's winds; it's not that we think we know what's going on and make a prediction in the absence of data (though that is supposed to be one part of the scientific method). There really is a lot of data consistent with and predicted by the dark matter hypothesis.

An absence of data in the case of Neptune? Actually, this analysis is a bit flawed.

Scientists did have data. They knew the distance from the Sun to Neptune. They had an idea of its mass and chemical composition. They had information on the amount of solar wind that arrived at Earth and models to predict how much would arrive at the other planets. They did have some information on wind speed on other gas giants in the solar system like Jupiter and Saturn and had even more wind speed data once Voyager 1 and 2 got to Jupiter and Saturn, a decade before Voyager 2 got to Neptune.

To insinuate that scientists had no data and then "Wow, now we have data" is incorrect. They had quite a bit of data there. Their theories and models were just incorrect based on the data that they had at the time. But, there wasn't a total lack of data there.


If one does a matrix diagram of Dark Matter theories and Gravity Modification theories, one finds that both types of theories cover some observations very well and other observations, not so well.

The problem that has been creeping more and more into at least the literature (and opinions like Martinus Veltman's) is a) DM has its flaws just like MOND or other theories, but more importantly b) DM just hasn't been found yet. Period. Not even a hint of it. At least to the lay person, DM sounds like magic. We don't know what it is, we've spent many hundreds of thousands of manhours of some of the brightest people on the planet, and many millions of dollars trying to figure out what it is and/or detect it, but we are teaching our students that it must be true because the equations tell us that it is true.

Well, of course one is going to have thousands of scientists the world over that believe it is true if that is what they were taught in school and there is no alternative theory that explains it all better. Just look at how quickly the students in the video above were ready to defend DM. Why? Because that is what they were taught as true science. They know about alternative theories, but they discard them out of hand without putting any real work into them.

 

[Janx]

Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.

What you just proved is that many people do not read what is actually written, but put their own spin on it.

well duh!

Case in point, is it possible, that you have put your own spin on what you've read?

You don't like Dark Matter.

Right now, Dark Matter is just a placeholder that makes the math work out. In turn, that math turns out to correctly account and predict more things than not.

In another thread, I once challenged Umbran on whether any of this quantum mechanic stuff existed or was applicable to the rest of the world. Because we can't see or prove a quark or other sub-atomic particle actually exists. He pointed out that things like computer processors were designed using the math and concepts from quantum mechanics.

If the ideas were wrong, then the math wouldn't work and the processor wouldn't process.

It's kind of like what we know about basic chemistry and how atoms try to pair up with other atoms to get 8 electrons in their outer orbit (I suppose some chemist will tell me that all changed since high school). While I can't see an atom or its parts, I kinda gotta assume its accurate when the model tells me what bonds to what and I can take those chemicals and produce the effect the model describes.

All without actually seeing the protons, neutrons and electrons to prove they exist.

frankly, I'm not sure why you give a monkey's arse so much about this Dark Matter topic. It is or isn't real. Don't bloody matter. the idea of it will change, as scientists futz with it. I suspect its more of a place-holder concept anyway. By the time scientists actually discover it, it'll get rebranded as [MENTION=177]Umbran[/MENTION]ium or something.

 

[Umbran]

Columbus didn't live thousands of years ago (i.e. a minimum of two thousand years ago), the time frame that I mentioned. So, your example here is a strawman at best.

What you just proved is that many people do not read what is actually written, but put their own spin on it.

No, sir. I was giving you some benefit of the doubt that your "thousands of years ago" was hyperbole.

If it wasn't hyperbole - citation needed. At this point, I don't accept your assertion of what scientists or scholars of any period believe or believed, much less what was believed by those who left no written records.

 

[KarinsDad]

No, sir. I was giving you some benefit of the doubt that your "thousands of years ago" was hyperbole.

If it wasn't hyperbole - citation needed. At this point, I don't accept your assertion of what scientists or scholars of any period believe or believed, much less what was believed by those who left no written records.

Benefit of the doubt??? You change the minimum time frame I was discussing by 1500 years, you then ridicule the notion in the time frame that you chose, and then claim you were giving me the benefit of the doubt?


Here's a citation with references:

http://en.wikipedia.org/wiki/Flat_Earth

A lot of people do not like wikipedia as a strict reference, but if you want to investigate the topic beyond that, please do your own research. I won't be doing it for you. I wrote what I intended and then you ignored what I wrote and changed it to make me sound "less informed" in some way. That's a fallacy in logic. Sorry, it doesn't work that way.

 

[Umbran]

Benefit of the doubt??? You change the minimum time frame I was discussing by 1500 years, you then ridicule the notion in the time frame that you chose, and then claim you were giving me the benefit of the doubt?

My opening note is very simple - I am making zero effort to ridicule anyone or anything. I am, as usual, concerned about scientific accuracy, not in making anyone look foolish. Until you understand that, we aren't going to get very far.

I was giving you the benefit of my doubts, yes.

We were talking about scholars and scientists, and their behavior. I took it to mean that your "prevailing view" in context meant "prevailing among scholars". You brought up Flat Earth as a demonstration of those who *should* have known better sticking to an outmoded theory. Given that there's a known and documented case in American education where folks we actively taught that example, you should by no means be surprised my mind went there.

I, personally, don't fault anyone who doesn't have the necessary tools for not knowing a thing. Development of the math to show the Earth is round coincided with (and is generally required for) the building of those towers and masts you mentioned, tall enough to regularly observe the phenomenon. Which is why Pythagoras and Aristotle worked out the round Earth, as you would say, thousands of years ago.

So, can you demonstrate that those who *should* have known the Earth was round frequently stuck to Flat Earth? If not, then Flat Earth is not really relevant to our discussion. Except...

"Dark matter," is exactly analogous to, "look at masts coming over the horizon, and figure out the world is round." You look at galaxies - you can estimate the amount of mass that's visible. You can also measure their motions. As Freyar noted, the two don't match.

That there's extra mass that you cannot see at intergalactic distances (thus "dark" or "missing") is the absolute simplest explanation for the observed phenomenon. And it is hardly a leap, given that we all live on a chunk of matter that would be "dark" in that sense.

If you figure anyone who can climb a mountain should guess that the world is round, you should also figure that anyone who can measure the motion of galaxies should figure there's mass present that we cannot see.

 

[Janx]

Benefit of the doubt???

ya know dude, you sound like you are much more knowledgeable about physics and more read up on it than I am.

Possibly more than Morrus, who is also pretty well versed in physics stuff. He probably could have watched the tone of his responses a little better.

But reading this thread, you came on a little hot and from an odd angle from the rest of the folks on this thread.

Ask yourself, could you have entered and handled this thread more smoothly?


Ignore whatever beef you had with Umbran and Morrus's responses. Could you yourself have expressed your ideas differently?

I say yeah. Why are we talking about Columbus? Or if the guys behind Dark Matter are wrong? The whole topic is a thought experiment. Ain't none of it actually scientifically provable yet. And some theories are quackier than others. The limit on time frame of the machine's existance just isn't backed up by logic that it MUST be that way.

That's how this whole thing started. Some dude said something matter of factly, that was restrictive and outside of the just as likely solutions some science dudes said. I'm inclined to side with the science dudes when they say a thing is possible, it more likely is.

 

[Janx]

"Dark matter," is exactly analogous to, "look at masts coming over the horizon, and figure out the world is round." You look at galaxies - you can estimate the amount of mass that's visible. You can also measure their motions. As Freyar noted, the two don't match.

That there's extra mass that you cannot see at intergalactic distances (thus "dark" or "missing") is the absolute simplest explanation for the observed phenomenon. And it is hardly a leap, given that we all live on a chunk of matter that would be "dark" in that sense.

That makes sense to me.

Personally, I'd be wondering how a dude could know how much mass is in a galaxy. At best, with a great telescope, you can make out all the stars. Assign an average mass per star type and add it all up.

I'm also guessing that you have math that says if you know how much mass these objects have, you can calculate their orbit, speed, etc. Something that us layman might not realize, as we figure that space stuff is positioned and moving at whatever speed it coagulated at. this would be by nature of the bowling ball on a bed effect of gravity, I assume.

It sounds like that basic math came out a little short, and the preferred reason was that they guessed wrong on the total mass. The difference between the "required" mass to match reality and the calculated mass being the existancce of "dark matter"

It's also possible the formula just doesn't scale to galactic proportions yet. I reckon that's what the MOND theory stuff is about, finding a different formula instead.

Which might really be matter the scientists forgot to count (maybe every star has way more heavy atoms at the center), or the existance of a mystery matter that is invisible, or planets, as we sure haven't found all of them yet. Given that we can't get there to just scoop some up, I reckon it's understandable on why it remains a mystery.

I see no reason to get cranky about Umbran's dark matter thing. I trust that if he finds a better explanation, he'll switch to that instead.

 

[Umbran]

Personally, I'd be wondering how a dude could know how much mass is in a galaxy. At best, with a great telescope, you can make out all the stars. Assign an average mass per star type and add it all up.

They don't go about noting every single star - a typical galaxy can have hundreds of billions of stars, so taking them individually is not tractable. Instead, it is done by measuring overall brightness, and knowing enough about stars to estimate how much mass is required per unit of brightness.

I'm also guessing that you have math that says if you know how much mass these objects have, you can calculate their orbit, speed, etc.

You can pick out individual stars at various points in the target galaxy, and work out their speeds via noting their doppler shifts.

It sounds like that basic math came out a little short, and the preferred reason was that they guessed wrong on the total mass. The difference between the "required" mass to match reality and the calculated mass being the existancce of "dark matter"

The basic math came out more than a little short. It came out a lot short. The mass of glowing, visible matter is much smaller than you'd expect watching how quickly the objects are orbiting. Now, if it were a little off, you'd just say that the estimation due to brightness was a little off. But the difference is so great, most agree there needs to be another explanation.

Moreover, there's an oddity. In a normal case of objects orbiting a central mass (like, say, in our solar system), you expect things near the center to be moving quickly, but objects far out to be moving slowly. But, in most galaxies, you see stuff farther out orbiting just as quickly as those things closer to the center.

You can get a model to act like this, if you adjust it to assume there's a halo of mass distributed differently than the mass you see. Thus, dark matter.

It's also possible the formula just doesn't scale to galactic proportions yet. I reckon that's what the MOND theory stuff is about, finding a different formula instead.

Yes. The idea is that Newtonian gravitation has only been tested when the acceleration due to gravity is relatively large. MOND (MOdified Newtonian Dynamics) says Newton didn't get it quite right. Newton says acceleration is directly proportional to force (F=ma). MOND says that, for small acceleration, it isn't directly proportional to the force.

MOND is not the only modification of Newtonian mechanics to try to explain the effect. It is merely (I think) the first and simplest of them.

 

[KarinsDad]

I am, as usual, concerned about scientific accuracy

Then we are concerned about the same thing.

 

[freyar]

Thank you for your more detailed and thought provoking discussion. As I mentioned earlier, MOND does have weaknesses when discussing clusters of galaxies.

You're welcome. Sorry for the delay on this response, busy day.

But, have Dark Matter theories never been adjusted as more data is acquired? The fact that adjustments have to be made to MOND appears to be what good science should do. Adjust theories to match observations.

As for cluster issues, doesn't LCDM some times have problems with clusters? For example, in the Bullet Cluster, shouldn't the relative velocity between the two clusters be faster using Dark Matter? The lensing works, but the velocity doesn't? Isn't the Bullet Cluster a problem for both types of theories?

The basic framework of DM models, specifically, "there exists a non-luminous, nearly pressureless matter with approximately 5x as much total mass as normal matter," has not been changed, no. The reason is that it hasn't needed to be changed. The error bars have gotten smaller regarding how much DM you need as the data has gotten better, but that's it. It has worked very well. There is a variation in the ratio of normal to dark matter in different galaxies, but you'd expect that the very complicated evolution of structure in the universe would lead to that, and the best computer simulations agree in broad terms. I should say that structure formation isn't completely understood in either DM or MOND theories.

As for the Bullet Cluster, the observations agree very well with what you expect for DM and not what you'd expect for MOND. In particular, you expect the DM to go through, like the galaxies (ie, to miss everything), while the gas piles up in the middle. From what I've read on the MOND description of it, you'd expect extra long-range attraction in MOND, so it's MOND where you'd expect the galaxies to be moving faster than they are.

Actually from what I've read, MOND makes very good predictions in many areas (and not just cluster speed), not so good ones in others. To focus on just those where it falters and say that it is probably wrong because of those isn't objective science. No doubt, MOND is minimally incomplete and even possibly incorrect because it cannot explain everything. Dark Matter also has areas where it does not make good predictions (or at least consistent predictions from one galaxy to the next).

MOND makes very good predictions for galaxy rotation curves but is not so good for other things. I want to stress the measurement of the CMB. The reason that is so important is that the universe was much simpler back when the CMB was formed (looking at the CMB is looking very far back in time). What that means is that, given a theory (either DM or MOND), you can make precise predictions with very few assumptions and compare them to precise experiments, and there aren't many confounding factors. So CMB measurements carry a lot of weight because we know what's going on. There isn't a complicated history to things, no astrophysics (like stars blowing up, etc). And this is where DM works very well and MOND does not. Well, last I saw, there was argument about how to do this calculation even. EDIT: Let me clarify this last statement based on re-reading what I'd remembered, which is worse for MOND than I'd recalled. Even a fairly strong proponent of MOND agreed that the standard theory taken as the relativistic version of MOND can't reproduce the CMB and furthermore that it's hard to imagine a modified theory of gravity that could if you didn't include dark matter. Link

I have to go but want to address the rest of this later.