Thursday, August 30, 2018

Modifying gravity is called "cheating"

Gizmodo reports:
A fight over the very nature of the universe has turned ugly on social media and in the popular science press, complete with accusations of “cheating” and ad hominem attacks on Twitter. Most of the universe is hiding, and some scientists disagree over where it has gone.

It’s quite literally a story as old as time. Wherever you look in the cosmos, things don’t seem to add up. Our human observations of the universe’s structure—as far back as we can observe—suggest that there’s around five times more mass than we see in the galaxies, stars, dust, planets, brown dwarfs, and black holes that telescopes have observed directly. We call this mystery mass, or the mystery as a whole, “dark matter.”

Several thousand physicists researching these dark matter-related mysteries will tell you that dark matter is a particle, the way that electrons and protons are particles, that only appears to interact with other known particles via the gravitational pull of its mass. But there are a few dozen physicists who instead think that a set of ideas called “modified gravity” might one day explain these mysteries. Modified gravity would do away with the need for dark matter via a tweak to the laws of gravity. ...

Then, in June, the most sensitive dark matter particle-hunting experiment, called XENON, announced it had once again failed to find a dark matter particle. A story titled “Is Dark Matter Real?” followed in the August issue of Scientific American, ...

“It’s only if you ignore all of modern cosmology that the modified gravity alternative looks viable. Selectively ignoring the robust evidence that contradicts you may win you a debate in the eyes of the general public. But in the scientific realm, the evidence has already decided the matter, and 5/6ths of it is dark.”
In other words, it is a cheat to tweak the laws of gravity to accommodate the slow galaxy rotation, but not cheat to hypothesize a new particle.

Hoping for a dark matter particle was one of the main reasons for believing in SUSY, as SUSY requires about 100 new particles. Maybe the lightest one is the dark matter particle.

Another little controversy is whether the evidence for dark matter already contradicts the Standard Model. Not necessarily. Wilczek pushes axions as an explanation that I think is consistent with the SM.

Also, the SM only tries to explain strong, weak, and electromagnetic interactions. Dark matter could be some substance that does not interact with those forces, and thus could exist independently from the SM.
In Gizmodo’s conversations with 13 physicists studying dark matter, a pretty clear picture emerged: Dark matter as an undiscovered population of particles that influence the universe through gravity is the prevailing paradigm for a reason, and will continue as such until a theory comes along with the same predictive power for the universe’s grandest features.
It is odd to call the substance a particle. We only call electrons particles because of how they interact with light, but dark matter does not interact with light.
“Everywhere the dark matter theories make predictions, they get the right answers,” Scott Dodelson, a Carnegie Mellon physics professor, told Gizmodo. But he offered a caveat: “They can’t make predictions as well on small scales,” such as the scales of galaxies.
I am surprised that anyone would brag about a theory that only works on scales much larger than galaxies.


  1. When you can't actually find over 90 percent of something in order to balance your books, and have to make up a fudge factor composed of imaginary substances you can't even detect which happens to be over ten times larger than your observable mass, your theory is just wrong.

    "There's nothing wrong with me (or my math), it must be the universe..." is not science. It's called denial. It ain't in Egypt.

  2. Perhaps I'm looking at this all wrong.

    Maybe what is needed is just a new marketing scheme. Instead of 'Dark Matter' or 'Dark Energy', instead try 'Unobtanium', 'Mithril', 'or 'Impossibillium', or perhaps the somewhat more direct 'Fullofshitanium' with a dash of 'Grande-Waste-a-Moola'. If you pronounce it with a dry British accent it almost sounds plausible.

  3. Does anybody really care anymore? None of these things lead to scientific progress in any practical sense. No government funding should be allowed. It's really boring stuff.