Is Time Travel (going backwards) Possible?

Umbran · Nov 8, 2012

KarinsDad said:
There should be a way with our current technology to exactly measure the speed and shape of these ellipses as planets go around the sun and determine how much they are distorted by the gravity of the galactic core since there is considerably more gravity in that direction than in the opposite direction.

The term "considerably more gravity" does not have a well-defined technical meaning.

There's a whole lot more mass out there, yes. But it is very, very far away. And the force exerted by that mass drops off with the square of the distance. So, the acceleration due to gravity from the galaxy is actually small.

Now, what you're actually suggesting we look for is the difference in forces on the planet - when it is close to the galactic core in its orbit, and when it is farther away. Compared to the distance to the galactic core, the distance across the orbit is very, very small. The planetary orbit is really just a dot by comparison.

Hm. Let me see...

It is about 27,000 light years to the galactic core. Meanwhile, the Earth's orbit is about 8 light minutes in radius. That's about 1.5x10^-5 light years. So, the differences are going to be very small indeed.

I think you overestimate our technical abilities, if you think we should, with current tech, be able to measure such differences.

freyar · Nov 8, 2012

KarinsDad said:
Well, if this is true, then there would be an experiment that could (more or less) prove it once and for all.

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.

<KarinsDad's planet experiment>

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.

Nagol said:
I saw a report of a similar experiment using stellar motions of nearby stars. http://www.eso.org/public/news/eso1217/

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.

Umbran · Nov 8, 2012

freyar said:
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.

That makes sense. Especially if it is interacting through the Weak nuclear force, the composition of the targets will matter a great deal - and the differences in observed interactions between various detectors should tell you something about the stuff, even.

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.

This one seems a bit more arbitrary. Generally, you expect collisions with things from off-planet to be pretty high energy. How often would we expect the relative velocities to be so low that there's not enough for a bit of excitation. Not impossible, but odd.

KarinsDad · Nov 8, 2012

Umbran said:
I think you overestimate our technical abilities, if you think we should, with current tech, be able to measure such differences.

No?

I was thinking of actually measuring the motion of Neptune against the background of distant galaxies. The Hubble telescope or alternatively, the James Webb or Large Synoptic Survey telescopes (once working) could take photographs (wavelength dependent on telescope) of it at regular intervals over 6 or 8 months of each year. The timing of the photographs would need to be precise, but they could be done with an atomic clock or alternatively, many photographs could be taken over an extended time, but an atomic clock might need to be used to determine exactly when they were each taken. Since the data might need to be collected over many decades, it might take a long time to get a precise answer. But, even early observations would give us a good idea, and it should be technically feasible. There would be problems with the brightness of Neptune versus the brightness of background galaxies, retrograde motion, wobble due to Triton, calculating how much DM affects the Earth itself, etc., but nothing that is technically or mathematically insurmountable.

As for your calculation of Earth, the average diameter of Neptune's orbit around the Sun is ~9 billion km. It is nearly a circular orbit as well (hence, the acceleration / deceleration due to the sun's gravity is very tiny). The amount of dark matter in a spherical halo within the galactic radius when it is closest to the galactic core versus when it is away from the galactic core would be ~99.999990 percent (assuming an equal distribution, a more densely packed distribution would result in a higher percentage). The distance to the galactic core from closest to furthest would be ~99.9999963 percent. But by collecting a lot of data over large time scales, the precision of the measurements increases (by comparing data collected a long time ago with recent data).

Now, this assumes that the sun doesn't significantly change its distance from the galactic core over these time frames. Considering that it takes the sun 225+ million years to orbit the galaxy, even if it has an elliptical orbit, the change in distance would be extremely small in a time frame of decades. But, it should also be measured as best as possible and taken into account for the calculations.

This also wouldn't prevent us from measuring Mars (or other planets) instead, or in addition. The amount of time observing for Mars would be a lot less (3 to 4 years, maybe double that to get more data), but the precision would need to be a lot greater (probably at least two orders of magnitude). Course, since Mars is closer, precision might be greater because of it.

If we can measure that supernovas are accelerating away from our galaxy based on extremely precise temporary brightness from them and hence, calculate that 72% of the universe is Dark Energy, we can measure this. It's a lot closer, easier to see, we can view at optimal times, etc.

Note: I did not state that such an experiment would not be expensive or time consuming, I merely stated that it should be technically feasible. There are also faster and more precise, but even more expensive but technically feasible ways, to measure this (e.g. multiple telescopes in orbit on various sides of the Earth viewing the planet with different angles to acquire a more precise change in background, distance from Earth/sun, etc.). But, it should still be doable with technology already built or in the works today.

Nagol · Nov 8, 2012

Neptune, at 30 AU, is still insignificantly far away compared to galactic core.

When Nepture is closest to the core it is 99.9999997% (26999.999984 / 27000.000016) of its maximum distance. Or it has a variance of 0.00000012% (1.2x10**-9).

So at its closest, Neptune experiences 100.0000002% of the force felt at its furthest.

Call the mass of the Milky Way 10**12 solar masses.

That means the difference in force is about the equivalent of 2400 solar masses across 27,000 LY... or half a solar mass at 400 LY Or about 1 tonne about 100 km over the planet.

KarinsDad · Nov 9, 2012

Nagol said:
Neptune, at 30 AU, is still insignificantly far away compared to galactic core.

Ok, I ran the numbers (using Newton's gravity equation) and it worked out that the difference between DM and no DM over 40 years of observing Neptune heading towards the galactic core as it swung around the Sun (1/4th of an orbit) was just under a kilometer. So, I give on this one.

Umbran · Nov 9, 2012

KarinsDad said:
Ok, I ran the numbers (using Newton's gravity equation) and it worked out that the difference between DM and no DM over 40 years of observing Neptune heading towards the galactic core as it swung around the Sun (1/4th of an orbit) was just under a kilometer. So, I give on this one.

Yeah.

And, here's another point - assume you can take the measurement. It tells you how much total mass is in a shell 27,000 ly radius, and the thickness of the diameter of Neptune's orbit.* Note that is total mass, not dark-matter-mass. You're measuring with gravity, so you can't tell the difference.

Do you happen to otherwise know how much normal matter also resides in that shell? If not, you can't speak to how much of the effect is dark matter.

*For those of you who like perspective games - assume that the sphere is an apple. The shell is about 2 nanometers thick. Not the thickness of the skin of the apple. Not the thickness of the outer layer of cells. The thickness of one DNA molecule.

Nellisir · Nov 9, 2012

Umbran said:
*For those of you who like perspective games - assume that the sphere is an apple. The shell is about 2 nanometers thick. Not the thickness of the skin of the apple. Not the thickness of the outer layer of cells. The thickness of one DNA molecule.

I don't. My brain hurts. Please stop.

freyar · Nov 9, 2012

Umbran said:
That makes sense. Especially if it is interacting through the Weak nuclear force, the composition of the targets will matter a great deal - and the differences in observed interactions between various detectors should tell you something about the stuff, even.

Actually, the weak nuclear force interacts (roughly) equally with protons and neutrons, so that won't work. DM and normal matter must interact through two different forces. I'll give you a simplified model that illustrates the idea but wouldn't actually work (I think). Imagine that DM interacts with both protons and neutrons equally via the weak force but that DM also carries an electric dipole moment (still electrically neutral, so ok). The dipole interacts with protons (charged) but not neutrons. If the sign of the two interactions is different, then the scattering can depend a lot on the target nucleus. I wouldn't say that working models are more complex but do require a bit more background to explain. These ideas were pretty popular a year or so ago but have dropped off a bit recently partly because none of the experiments are coming out with new data right now --- it's kind of a wait-and-see phase.

This one seems a bit more arbitrary. Generally, you expect collisions with things from off-planet to be pretty high energy. How often would we expect the relative velocities to be so low that there's not enough for a bit of excitation. Not impossible, but odd.

The collisions are shockingly low energy, actually. The earth's orbital speed is only about 1/1000 the speed of light, which is similar to what we believe is a typical DM speed in the galaxy. But that means the kinetic energy available to excite the DM in a collision with something on earth is only about (1/2) mv^2, or about 1 part in a million compared to the mass of the DM particle (energy mc^2). So only a small relative gap between DM states is enough to prevent the scattering. (For typical WIMP masses of around 100 GeV, you're only talking about a 100 keV splitting, which is a fraction of an electron mass).

Since we've been talking dark matter so much (in a time travel thread, oddly enough), I figure you all might be interested in the latest news. The big news is that, about 6 months ago, analysis of data from the Fermi gamma ray telescope discovered a signal of high energy gamma rays with a set energy coming from the galactic center. People have for years thought this would be a "smoking gun" for dark matter annihilating very rarely into photons just because there aren't astrophysical mechanisms that can do this easily. So there's quite a lot of work going on right now to (1) figure out if the signal is real (not a problem with the instrument or how the data is analyzed), (2) see if it has other characteristics consistent with what we'd expect from dark matter, and (3) what models of dark matter can create this signal. Very exciting times right now in the field; if this holds up, it will mark the first known non-gravitational detection of dark matter and effectively the discovery of a particle outside the Standard Model. There are of course the usual caveats that this is being scrutinized very carefully as indicated above and that there's also a lot of thought going into whether some kind of standard astrophysics can produce this. And everyone agrees that more data is needed. So we'll see how this plays out over the next year or two.