Starts With A Bang

Comments of the Week #120: from the Great Attractor to the highest-energy particles

The various galaxies of the Virgo Supercluster, grouped and clustered together. Each individual group/cluster is unbound from all the others. Image credit: Andrew Z. Colvin, via Wikimedia Commons.

“Even though the future seems far away, it is actually beginning right now.” -Mattie Stepanek

It’s been a fantastic week here at Starts With A Bang, where we’ve covered even more ground than normal! First off, for those of you not following me on SoundCloud, we’ve got a new science podcast out, on the last star in the Universe.

Have a listen and enjoy; that’s all possible thanks to the generous donations of our Patreon supporters, as are the re-runs of each article, ad-free, on a 7-day delay over on Medium. Here were the new pieces of this past week:

And now lets take a look at your inquiries, ideas and more on this edition of our comments of the week!

Image credit: ESO/L. Calçada, of the illustration of the dark matter halo surrounding the luminous disk of our galaxy.

From Sinisa Lazarek on the MACHO content of our galaxy: “Maybe I’m nitpicking, but IMO “ten times more” is not “slightly more”. This of course doesn’t influence the DM halo AROUND the galaxy (where most of DM is). But I expected that 10x more machos would influence the mass content of our galaxy more than slightly.”

It’s not nitpicking; you’re asking a questions about magnitudes. If you thought something was 2% and it turns out to be 20%, that’s suddenly very significant. But if you thought something was 0.001% and it turns out to be 0.01%, then it doesn’t matter. When we talk about the mass content of our galaxy, we are still much less than 1% when talking about MACHOs over the mass range we’re discussing. So no, ten times more MACHOs still only influences the mass content of our galaxy slightly.

A modern high field clinical MRI scanner. MRI machines are the largest medical or scientific use of helium today. Image credit: Wikimedia Commons user KasugaHuang, under a c.c.a.-s.a.-3.0 license.

From eric on the consciousness problem: “Every night, the pattern of electrical activity that is the conscious “me” disappears. It doesn’t go underground somewhere in my brain or go into some standby/dormant mode, this pattern ceases to exist. Not there. Gone. Noneexistent. New patterns, associated with sleep and dreaming, take its place. Then when I wake up, my brain recreates the pattern that is “me” from stored information…probably not exactly the same as it was before, I just don’t notice the differences.”

Now, this is a hard problem and I don’t have an answer. But I would submit that MRI results would show that there is some brain (electrical) activity that is identifiably you that is still present while you are asleep. I would also submit that it is that electrical pattern that occurs in your brain that is identifiable as you. Here are some unanswered questions:

Tom and Will Riker, together on the Enterprise after the former’s rescue. Image credit: Memory Alpha Wiki, by user ThomasHL, from the TNG episode Second Chances.

In other words, I don’t know that, for example, Will and Tom Riker aren’t both exactly 100% the same Will Riker that was born in Alaska? And yet, perhaps neither one actually is; perhaps the original “Will Riker” died the first time his body was deconstructed and reconstructed, with that line-of-consciousness coming to a total end. And perhaps all the memories and continuity experienced by the copy doesn’t mean that it isn’t murder every time one goes through the transporter. Is this different or the same than going to sleep and waking up?

I don’t even know how to test this with a working transporter. Ideas?

The LUX underground detector, installed and in the tank. Image credit: C.H. Faham and the LUX collaboration.

From Alan L. on dark matter and LUX: “So have DM particles yet managed to achieve a level of such extreme puniness that it would make DM, as envisaged post LUX, an extremely unlikely candidate to be one capable of pushing around Andromeda sized galaxies so to ensure a uniform rotational speeds across their width, as if DM particles in galactic haloes formed gangs of some kind of super powerful schoolyard bullies?”

You must understand that it is not the size, magnitude or puniness of dark matter that makes it viable or not as capable of accounting for the gravitational effect of the Universe. It is its density and its clustering properties, the latter of which are determined by its kinetic energy as a function of its mass. A class of dark matter that doesn’t interact with normal matter or itself at all, that has only gravitational interactions with anything in this Universe, would be the ultimate nightmare scenario for experimental physicists, and yet it is a very real possibility for what the nature of dark matter could be.

It is up to us to push those limits in all mass ranges as far as we can go. We are constantly re-evaluating what the science tells us with a view to the full suite of evidence available, and as a result DM was replaced by CDM was replaced by Lambda-CDM, and now we are starting to find that the density profiles do not match simulations quite as well as we had hoped, which is leading to modified models of CDM as part of the Lambda-CDM model. Just because you may not like the path that science is being led doesn’t mean that scientists aren’t doing exactly the job that the data tells them to do.

Composite image of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). Image credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS

From Jerry on dark matter collisions: “Since two of the key pieces of evidence for dark matter is that galaxies rotate to fast to hold together and that dark matter can be mapped separating from galaxy clusters in collisions, what would happen to a galaxy that became completely separated from its dark matter halo after hitting an especially dense area of intergalactic matter?”

Intergalactic matter would only be able to stop the other intergalactic matter in a galaxy: things like plasma, dust, and neutral gas. You want to stop a star? You need something as dense and massive as a star. You want to stop a galaxy? You need to something that’s going to stop not an “averaged galaxy,” but each of the 100 billion+ stars in it. That’s why, in the Bullet Group, above, the luminous stars move unimpeded through the group, while the gas (in pink) separates. If you truly wanted to allow the dark matter to continue moving while making your imaginary super-star-stopper stop the galaxy, you would forever alter the stars by nature of stopping the baryonic matter in the galaxies.

In other words, the answer isn’t a universal physics one, but rather is dependent on how you do this thing that requires a severe, non-natural intervention.

Closeup of a large region of the Andromeda galaxy’s disk, containing hundreds of open star clusters (identifiable as bright blue sparkles). Image credit: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.

From PJ on the galaxies behind Andromeda: “Interesting to note the visibility of other galaxies through Andromeda in the closeup photo, lower right of photo, reddish appearance.”

This is the less common type of galactic reddening we see: not due to redshift, but rather due to dust in a foreground galaxy! In fact, you will notice what appears to be a large population of red stars in this galaxy as well, and that’s because the “dust” tends to exist in a thin plane in the galaxy’s center. The stuff in front of the dust isn’t reddened, but everything behind it — including stars and galaxies — experiences this extinction effect.

A crude model of a spiral galaxy, with equal numbers of stars throughout the entire volume of it. Image credit: E. Siegel.

Dust grains are of a size where smaller wavelengths are blocked much more easily than longer ones, and so the more dust we pass through, the redder things appear. (Even though there’s less red light, there’s a higher percentage of red light as compared to everything else!) If you were to look at a region that wasn’t dusty in Andromeda, the galaxies behind it would only appear red as a function of their redshift.

Isotropic random walk on the euclidean lattice Z^3. This picture shows three different walks after 10 000 unit steps, all three starting from the origin. Image credit: Zweistein, under c.c.a.-s.a.-3.0.

From Denier on the concept of dimensional reduction: “Would this mean it is possible to collapse 4 dimensions down to 2, or that the 4 dimensions that we perceive all around us are in fact 2 dimensions when viewed on the QM scale?”

It actually doesn’t mean either of those. The former is definitely not what’s being said, so drop that from your mind. The latter, though, is kind of close. Imagine you go to take a step in our three dimensional world. What direction will you go in? Realistically, you’ll likely go some distance in the x direction, some in the y and some in the z direction. The odds that you’ll come back to within a certain distance of your starting point on the second step are fairly low; you’d need to simultaneously get the exact opposite of each of those three directions that you took in your first step. If you were only two dimensional, you’d have better odds and in one dimension, even better odds.

What dimensional reduction says is that the “quantum mechanical fuzziness” of reality means that if you were to take the odds of returning to your starting point in four quantum dimensions, it’s the same as in two classical dimensions, meaning that quantum mechanics increases significantly your odds of a random return. That’s the big finding.

Image credit: DC Comics.

From See Noevo on superhero physics: “Do you have any articles in the works on Superman, or on each of the Marvel superheroes?”

I sure do. Go read it; you may enjoy it!

Saitama having just punched through the meteor, stopping its momentum and breaking it apart. From the Anime, “One Punch Man.” Image credit: from

From Denier on the physics of One Punch Man: ““No sir” came the quick reply. “The damage is from the force of One Punch Man’s foot pushing against the Earth with enough force to instantly propel him to 99.99999997% the speed of light”.”

The whole comment is accurate, and pretty spot on. As James Kakalios often says, you need to set out what the laws of physics are and how they are different or violated/not violated at the outset, and then you can construct comic book realities in a consistent fashion. One Punch Man’s leap from the Moon back to Earth is more destructive to the Moon than his meteor-stopping punch is to Earth, even though the latter requires more energy. There must be something in that fist of his…

An illustration of the Casimir effect, and how the forces on the outside of the plates are different from the forces on the inside. Image credit: Wikimedia commons user Emok, under a c.c.a.-by-s.a.-3.0 license.

From Wow on a plausible dark energy explanation idea: “Imagine the [Casimir] plates half a universe apart. The energy density is lower inside, right? And as the plates get closer, the energy inside gets lower.
Now imagine that these plates are “unit metric” in the multidimensional universe of string theory.
Where all dimensions have the same metric, the energy is equally distributed in all dimensions. As the three dimensions expand, the higher dimensions “roll up”, and the “size” of the universal dimension shrinks and excludes more and more wavelengths, reducing the energy in those smaller dimensions.
However the energy goes SOMEWHERE, energy isn’t destroyed or created, it’s a constant total.
That energy goes into the remaining three dimensions.”

As far as we can tell, the vacuum expectation value (we can call that energy) in the space inside the plates is different (lower) from the energy outside of the plates. As the plates get closer, more EM modes are forbidden, and hence the energy gets lower still. Your analogy is saying, rather than close down one dimension, forbidding modes (as in the space between the plates, you can still move arbitrarily in the other two), close down all the “extra” dimensions of string theory, thereby increasing the vacuum energy of our space.

All we need is a full theory of string theory where we can calculate the string vacuum from first principles, and the size of the dimensions that are compactified, and we can test this theory. Unfortunately, “string vacua” are undetermined from first principles, and this is one of the biggest frustrations of the whole string theory enterprise. It’s a plausible idea, but not one that’s in currently calculable territory today.

The production of a cosmic ray shower, produced by an incredibly energetic particle from far outside our Solar System. Image credit: Pierre Auger Observatory, via

From eric on cosmic rays: “I find cloud chambers fascinating to watch. Any time I see one in a museum or science exhibit, I usually end up standing there much longer than planned. Evidently they’re relatively easy to build (there are loads of DIY videos and guides on the web), but I haven’t yet taken my minor obsession to that stage. “

And for that, here is a video of a cloud chamber with cosmic rays flying through it (timelapse):

plus one of a cloud chamber with a radioactive (uranium) source inside:


The spectrum of cosmic rays. Image credit: Hillas 2006, preprint arXiv:astro-ph/0607109 v2, via University of Hamburg.

And finally, from Elle H.C. on cosmic rays and the LHC: “Objection: “The Large Hadron Collider (LHC) will collide in 2015 protons at √s ≃ 14 TeV. This impressive energy is still about a factor of 50 smaller than the centre-of-mass energy of the highest energy cosmic ray so far observed, assuming primary protons.”
While for the LHC the collision rate is even higher then in nature. It’s like saying on elephant is more intense than all the +1 billion chinese people in the world.”

This is an invalid objection based on a misunderstanding of the different between energy, collision energy and center-of-mass energy. Let’s explain. A particle has a certain amount of kinetic energy relative to our reference frame: the energy of its motion. The highest energy we’ve ever created for a single particle (e.g., a proton, not counting particles made up of multiple protons) on Earth is ~6.5 TeV, which is an LHC proton. If you collide this proton with a fixed target, which is to say a proton at rest, you “only” get √(2mE) worth of energy for new particle creation, where m is the mass of the proton and E is the kinetic energy of the LHC proton. This is pretty lame for the LHC; we’d only get 114 GeV of energy, maximum, per collision for new particle creation. If you like, you can replace the LHC’s energy with an ultra-high-energy-cosmic-ray’s energy: 10^11 GeV, and find it reaches approximately 500 TeV of energy available for new creation. This is the center-of-mass energy referred to.

Image credit: CERN, via

The way the LHC reaches 13 TeV for particle creation is by colliding 6.5 TeV protons with other 6.5 TeV protons moving with the opposite momentum. Assuming there are multiple UHECR sources in the Universe (there are), and that they shoot UHECRs at one another, it stands to reason that there are plenty of ultra-high-energy collisions, where “m” in that equation can be replaced by the energy of the other particle with an approximately equal-and-opposite energy. The Universe has had collisions that are ~10^7 times as powerful as what we make at the LHC. I’m not sure what your point is with the elephant analogy, but that’s what the physics says and means. The energy of these cosmic rays is real and unique, and it’s only that the center-of-mass collisions are both high energy and incredibly precisely localized that make the LHC interesting at all.

Thanks for a great week, and can’t wait for another fantastic one starting tomorrow!