A week or so ago, I wrote about my skepticism concerning the claim of a “detection” of the photon ring that’s widely expected to lie hidden within the image of a black hole. A nice article in Science News appeared today outlining the current controversy, with some quotes from scientists with differing views (including a dissenter from the EHT itself). It’s a good read: to the point, well-written, and with high standards.
Figure 1: Image of the accretion disk surrounding M87’s black hole [Credit : EHT collaboration]
Background: The image in Figure 1, created by the Event Horizon Telescope (EHT) in 2019, shows the “accretion disk” of material circling and eventually falling into the giant black hole at the center of the galaxy M87. But the image is too blurry to easily reveal the photon ring, whose appearance in a perfect telescope might look like that in Figure 2, which shows a simulation of the region around a black hole.
Unlike the accretion disk, the photon ring would be an effect purely of curved space-time around the black hole, which is expected to lens and focus light from the accretion disk into a narrow circlet. As such, the ring would be a smoking-gun signature of Einstein’s gravity at work. That’s why any claim of detection, using fancy image processing or any other method, merits both attention and critique.
Figure 2: A simulation of what a perfect telescope image might reveal, with a narrow bright ring appearing withing a swirling inhomogeneous disk. [Credit A. Chael]
(where e— stands for “electron” and e+ for “positron”) can be used to verify the electric charges of the quarks of nature. [In this post I’ll usually drop the word “electric” from “electric charge”.] Specifically, the ratio R, at different energies, is both sensitive to and consistent with the Standard Model of particle physics, not only confirming the quarks’ charges but also the fact that they come in three “colors”. (About colors, you can read recent posts here, here and here.)
To keep the previous posts short, I didn’t give evidence that the data agrees only with the Standard Model; I’ll start today by doing that. But I did point out that the data doesn’t quite match the simple prediction. You can see that in the figure below, repeated from last time; it shows the data (black dots) lies close to the predictions (the solid lines) but generally lies a few percent above them. Why is this? The answer: we neglected a small but noticeable effect from the strong nuclear force. Not only does accounting for this effect fix the problem, it allows us to get a rough measure of the strength of the strong nuclear force. From these considerations we can learn several immensely important facts about nature, as we’ll see today and in the next post.
Figure 1: Data (black dots) showing R as a function of the collision energy 2Ee. Horizontal colored lines show the three predictions for R in the regions where the data is simple and 3, 4 or 5 of the quarks are produced. The minor jumpiness in the data is due to measurement imperfections.
A post for general readers who’ve heard of quarks; if you haven’t, you might find this article useful:
Yesterday I showed you that the usual argument that determines the electric charges of the various types of quarks uses circular reasoning and has a big loophole in it. (The up quark, for example, has charge 2/3, but the usual argument would actually allow it to have any charge!) But today I’m going to show you how this loophole can easily be closed — and we’ll need only addition, subtraction and fractions to close it.
Throughout this post I’ll shorten “electric charge” to just “charge”.
A Different Way to Check Quark Charges
Our approach will be to study the process in which an electron and a positron (the electron’s anti-particle) collide, disappear (“annihilate”), and are converted into one or another type of quark and the corresponding anti-quark; see Figure 1. The rate for this process to occur, and the rate of a similar one in which a muon and anti-muon are produced, are all we will need to know.
In an electron-positron collision, many things may happen. Among the possibilities, the electron and positron may be converted into two new particles. The new particles may have much more mass (specifically, rest mass) than the electron and positron do, if the collision is energetic enough. This is why physicists can use collisions of particles with small mass to discover unknown particles with large mass.
Figure 1: (Top) an electron and positron, each carrying energy Ee, collide head-on. (Bottom) from the collision with total energy 2Ee , a quark and anti-quark may emerge, as long as Ee is bigger than the quark’s rest mass M times c2.
In particular, for any quark of mass M, it is possible for an electron-positron collision to produce that quark and a corresponding anti-quark as long as the electron’s energy Ee is greater than the quark’s mass-energy Mc2. As Ee is gradually increased from low values, more and more types of quark/anti-quark pairs can be produced.
This turns out to be a particularly interesting observation in the range where 1 GeV < Ee < 10 GeV, i.e. when the total collision energy (2 Ee) is between 2 and 20 GeV. If Ee is any lower, the effects of the strong nuclear force make the production of quarks extremely complicated (as we’ll see in another post). But when the collision energy is above 2 GeV, things start to settle down, and become both simple and interesting.
A post for general readers who’ve heard of quarks; if you haven’t, try reading here:
The universe has six types of quarks, some of which are found in protons and neutrons, and thus throughout all ordinary material. For no good reasons, we call them up, down, strange, charm, bottom and top. Today and tomorrow I want to show you how we know their electric charges, even though we can’t measure them directly. The only math we’ll need is addition, subtraction, and fractions.
This also intersects with my most recent post in this series on the Standard Model, which explained how we know that each type of quark comes in three “colors”, or versions — each one a type of strong nuclear charge akin to electric charge.
Today we’ll review the usual lore that you can find in any book or on any website, but we’ll see that there’s a big loophole in the lore that we need to close. Tomorrow we’ll use a clever method to close that loophole and verify the lore is really true.
The Lore for Protons and Neutrons
Physicists usually define electric charge so that
the proton has electric charge +1
the electron has charge -1,
the neutron has charge 0 (i.e. electrically neutral, hence its name).
[Throughout the remainder of this post, I’ll abbreviate “electric charge” as simply “charge“.]
As for the six types of quarks, the lore is that their charges are [using notation that “Qu” means “electric charge of the u quark“]:
Back in 2019, the Event Horizon Telescope (EHT) made history as its scientists used it to create an image of a huge black hole — or rather, of the “accretion disk” of material surrounding a black hole — at the center of the galaxy M87. The dark central gap reveals where the disk’s material vanishes from view, as it presumably flows toward and disappears into the black hole.
EHT’s image of the M87 galaxy’s black hole’s accretion disk, created from radio-wave measurements. [How do we know there’s a black hole there? I left an answer in the comments.]
What the image actually shows is a bit complicated, because there is not only “light” (actually, radio waves, an invisible form of light, which is what EHT measures) from the disk that travels directly to us but also (see the Figure below) light that travels around the back of the black hole. That light ends up focused into a sharp ring, an indirect image of the accretion disk. (This is an oversimplication, as there are additional rings, dimmer and close together, from light that goes round the black hole multiple times. But it will be a decade before we can hope to image anything other than the first ring.)
Left: A glowing accretion disk (note it does not touch the black hole). Light from the right side of the disk forms a direct, broad image (orange) heading toward us, and also a focused, narrow, indirect image (green) heading toward us from the left side, having gone round the back of the black hole. (Right) From the entire accretion disk, the direct image forms a broad disk, while the indirect image would be seen, with a perfect telescope, as a narrow circle of bright light: the photon ring. Unfortunately, the EHT blurs this picture to the point that the photon ring and the disk’s direct image cannot be distinguished from one another. [Long and careful explanation given here.]
Regrettably, that striking bright and narrow “photon ring” can’t be seen in the EHT image, because EHT, despite its extraordinary capabilities, doesn’t yet have good enough focus for that purpose. Instead, the narrow ring is completely blurred out, and drowned in the direct image of the light from the wider but overall brighter accretion disk. (I should note that EHT originally seemed to claim the image did show the photon ring, but backed off after a controversy.) All that can be observed in the EHT image at the top of this post is a broad, uneven disk with a hole in it.
However, having looked at the paper, I’m skeptical of this claim, at least so far. Here’s why.
Normally, if you claim to have detected something for the first time, you make it clear to what extent you’ve ruled out the possibility it actually isn’t there… i.e., if there’s only a 0.01% chance that it’s absent, that’s a strong argument that it’s present. I don’t see this level of clarity in the paper.
Almost everyone is pretty darn sure that in reality the photon ring is actually present. That introduces a potential bias when you search for it; at least unconsciously, you’re not weighing the present vs. absent options equally. For this reason, it’s important to demonstrate that you’ve eliminated that bias. I don’t see that the authors have done this.
Simulations of black hole surroundings and theoretical estimates both suggest that the photon ring should have significantly less overall brightness than the broad accretion disk. However, the ring measured in this paper has the majority of the total light (60%). The authors explain this by saying this is typical of their method: it combines some of the disk light near the photon ring (i.e., background) with the actual photon ring (i.e. signal). But normally one doesn’t claim to have detected a signal until one has measured and effectively subtracted the background. Without doing so, how can we be sure that the ring that the authors claim to have measured isn’t entirely background, or estimate how statistically significant is their claim of detection?
I’ve included more details on the following section, but the bottom line is that I’d like a lot more information before I’d believe the photon ring’s really been detected.
Is it possible that the particle physicists hard at work near Geneva, Switzerland, at the laboratory known as CERN that hosts the Large Hadron Collider, have opened a doorway or a tunnel, to, say, another dimension? Could they be accessing a far-off planet orbiting two stars in a distant galaxy populated by Jedi knights? Perhaps they have opened the doors of Europe to a fiery domain full of demons, or worse still, to central Texas in summer?
Mortals and Portals
Well, now. If we’re talking about a kind of tunnel that human beings and the like could move through, then there’s a big obstacle in the way. That obstacle is the rigidity of space itself.
The notion of a “wormhole”, a sort of tunnel in space and time that might allow you to travel from one part of the universe to another without taking the most obvious route to get there, or perhaps to places for which there is no other route at all, isn’t itself entirely crazy. It’s allowed by the math of Einstein’s theory of space and time and gravity. However, the concept comes with immensely daunting conceptual and practical challenges. At the heart of all of them, there’s a basic and fundamental problem: bending and manipulating space isn’t easy.
