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Is Time Travel (going backwards) Possible?
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<blockquote data-quote="freyar" data-source="post: 6044392" data-attributes="member: 40227"><p>The results of the CMB measurements are true and publicly available (even the raw data is for many of the independent experiments which have done this measurement). Furthermore, it's a fairly easy calculation to show that normal matter by itself could not produce those measurements. Trying to revise gravity to make normal matter make those patterns would require something even more bizarre than MOND (see the link I provided in post 170). The point is, to physicists trained in this subject, the CMB measurements are far cleaner and more definitive than the experiment you propose.</p><p></p><p></p><p></p><p>This is actually a nice idea but with a few problems. One, as Umbran points out, I don't think we have the technical capacity to make those measurements. Two, we know from stellar rotation curves in other galaxies that something affects orbits, be it dark matter or something like MOND. Well, presumably MOND would affect planetary orbits in some way similar (if not identical) to the way dark matter would.</p><p></p><p>You might also ask about the gravity from dark matter that is in our solar system. But there's just not enough of it to make a difference. The way dark matter ends up being so much more than stars, etc, is that it fills the vast empty regions between stars.</p><p></p><p></p><p></p><p></p><p>Yes, looking at the motions of stars is how we measure how much DM we think is near the sun, which is of course important for experiments on the earth that hope to see DM particles hitting normal stuff. The paper you mention there was a big deal earlier this year. It uses a new method to look at stellar motions (which is good and interesting) but claimed that there isn't really any dark matter! That's why it got a lot of press. However, a reanalysis of their study was done by other authors within a month and found a very serious mistake in how the data was interpreted. A correct interpretation actually finds agreement with previous studies of DM near the sun. And the authors of the original paper agreed. So this is a good example of self-correction in science and how scientists can be open-minded that they were in fact wrong about something.</p><p></p><p>......</p><p>A while back I mentioned that there are models of dark matter that could interact reasonably strongly with normal matter (well, strongly enough that you'd naively expect to have "caught" some DM in an experiment by now) but would not actually turn up in experiments looking for DM on earth. Umbran said he was interested in hearing about some. Here are a couple:</p><p></p><p>1) Assume that some of the experiments which are claiming possible DM detections are right. Then why don't the other experiments detect DM? Well, the experiments use different types of atoms --- particularly the nuclei --- to look for DM. What if DM interacts differently with protons and neutrons? Then it's possible that interactions with the protons and neutrons in particular nuclei could nearly cancel out.</p><p></p><p>2) Maybe dark matter has excited states. Then it's entirely possible that DM interacting with normal matter can't stay in the same state but must always jump between the ground state and excited state. If the excited state is only slightly more energetic than the dark matter itself, then there is simply not enough kinetic energy for DM hitting a nucleus to jump that excitation gap. So no scattering is possible.</p></blockquote><p></p>
[QUOTE="freyar, post: 6044392, member: 40227"] The results of the CMB measurements are true and publicly available (even the raw data is for many of the independent experiments which have done this measurement). Furthermore, it's a fairly easy calculation to show that normal matter by itself could not produce those measurements. Trying to revise gravity to make normal matter make those patterns would require something even more bizarre than MOND (see the link I provided in post 170). The point is, to physicists trained in this subject, the CMB measurements are far cleaner and more definitive than the experiment you propose. This is actually a nice idea but with a few problems. One, as Umbran points out, I don't think we have the technical capacity to make those measurements. Two, we know from stellar rotation curves in other galaxies that something affects orbits, be it dark matter or something like MOND. Well, presumably MOND would affect planetary orbits in some way similar (if not identical) to the way dark matter would. You might also ask about the gravity from dark matter that is in our solar system. But there's just not enough of it to make a difference. The way dark matter ends up being so much more than stars, etc, is that it fills the vast empty regions between stars. Yes, looking at the motions of stars is how we measure how much DM we think is near the sun, which is of course important for experiments on the earth that hope to see DM particles hitting normal stuff. The paper you mention there was a big deal earlier this year. It uses a new method to look at stellar motions (which is good and interesting) but claimed that there isn't really any dark matter! That's why it got a lot of press. However, a reanalysis of their study was done by other authors within a month and found a very serious mistake in how the data was interpreted. A correct interpretation actually finds agreement with previous studies of DM near the sun. And the authors of the original paper agreed. So this is a good example of self-correction in science and how scientists can be open-minded that they were in fact wrong about something. ...... A while back I mentioned that there are models of dark matter that could interact reasonably strongly with normal matter (well, strongly enough that you'd naively expect to have "caught" some DM in an experiment by now) but would not actually turn up in experiments looking for DM on earth. Umbran said he was interested in hearing about some. Here are a couple: 1) Assume that some of the experiments which are claiming possible DM detections are right. Then why don't the other experiments detect DM? Well, the experiments use different types of atoms --- particularly the nuclei --- to look for DM. What if DM interacts differently with protons and neutrons? Then it's possible that interactions with the protons and neutrons in particular nuclei could nearly cancel out. 2) Maybe dark matter has excited states. Then it's entirely possible that DM interacting with normal matter can't stay in the same state but must always jump between the ground state and excited state. If the excited state is only slightly more energetic than the dark matter itself, then there is simply not enough kinetic energy for DM hitting a nucleus to jump that excitation gap. So no scattering is possible. [/QUOTE]
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