## Send Your Dog Through a Wormhole?

A wormhole! What an amazing concept — a secret tunnel that connects two different regions of space! Could real ones exist? Could we — or our dogs — travel through them, and visit other galaxies billions of light years away, and come back to tell everyone all about it?

I bring up dogs because of a comment, quoted in the Guardian and elsewhere, by my friend and colleague, experimentalist Maria Spiropulu. Spiropulu is a senior author on the wormhole-related paper that has gotten so much attention in the past week, and she was explaining what it was all about.

• “People come to me and they ask me, ‘Can you put your dog in the wormhole?’ So, no,” Spiropulu told reporters during a video briefing. “… That’s a huge leap.”

For this, I can’t resist teasing Spiropulu a little. She’s done many years of important work at the Large Hadron Collider and previously at the Tevatron, before taking on quantum computing and the simulation of wormholes. But, oh my! The idea that this kind of research could ever lead to a wormhole that a dog could traverse… that’s more than a huge leap of imagination. It’s a huge leap straight out of reality!

## What’s the problem?

Decades ago there was a famous comedian by the name of Henny Youngman. He told the following joke — which, being no comedian myself, I will paraphrase.

• I know a guy who wanted to set a mousetrap but had no cheese in his fridge. So he cut a picture of a piece of cheese from a magazine, and used that instead. Just before bed, he heard the trap snap shut, so he went to look. In the trap was a picture of a mouse.

Well, with that in mind, consider this:

• Imaginary cheese can’t catch a real mouse, and an imaginary wormhole can’t transport a real dog!

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## How Do You Make a Baby Cartoon Wormhole In a Lab?

This post is a continuation of the previous one, which you should read first…

Now, what exactly are these wormholes that certain physicists claim to be trying to make or, at least, simulate? In this post I’ll explain what the scientists did to bring the problem within reach of our still-crude quantum computers. [I am indebted to Juan Maldacena, Daniel Jafferis and Brian Swingle for conversations that improved my understanding.]

An important point from last post: a field theory with quarks and gluons, such as we find in the real world or such as we might find in all sorts of imaginary worlds, is related by the Maldacena conjecture to strings (including quantum gravity) moving around in more dimensions than the three we’re used to. One of these dimensions, the “radial dimension”, is particularly important. As in the previous post, it will play a central role here.

## Einstein-Rosen Bridge (ER) vs. Einstein-Podolsky-Rosen Entanglement (EPR)

It’s too bad that Einstein didn’t live long enough to learn that two of his famous but apparently unrelated papers actually describe the same thing, at least in the context of Maldacena’s conjecture. As Maldacena and Lenny Susskind explored in this paper, the Maldacena conjecture suggests that ER is the same as EPR, at least in some situations.

We begin with two identical black holes in the context of a string theory on the same curved space that appears in the Maldacena conjecture. These two black holes can be joined at the hip — well, at the horizon, really — in such a way as to form a bridge. It is not really a bridge in spacetime in the way you might imagine a wormhole to be, in the sense that you can’t cross the bridge; even if you move at the speed of light, the bridge will collapse before you get to the other side. Such is the simplest Einstein-Rosen bridge — a non-traversable wormhole.

What, according to the Maldacena conjecture, is this bridge from the point of view of an equivalent field theory setting? The answer is almost fixed by the symmetries of the problem. Take two identical field theories that would each, separately, be identical to one of the two black holes in the corresponding string theory. These two theories do not affect each other in any way; their particles move around in separate universes, never interacting. Despite this, we can link them together, forming a metaphorical bridge, in the most quantum sense you can imagine — we entangle them as much as we can. What does this mean?

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## Black Holes, Mercury, and Einstein: The Role of Dimensional Analysis

In last week’s posts we looked at basic astronomy and Einstein’s famous E=mc2 through the lens of the secret weapon of theoretical physicists, “dimensional analysis”, which imposes a simple consistency check on any known or proposed physics equation.  For instance, E=mc2 (with E being some kind of energy, m some kind of mass, and c the cosmic speed limit [also the speed of light]) passes this consistency condition.

But what about E=mc or E=mc4 or E=m2c3 ? These equations are obviously impossible! Energy has dimensions of mass * length2 / time2. If an equation sets energy equal to something, that something has to have the same dimensions as energy. That rules out m2c3, which has dimensions of mass2 * length3 / time3. In fact it rules out anything other than E = # mc2 (where # represents an ordinary number, which is not necessarily 1). All other relations fail to be consistent.

That’s why physicists were thinking about equations like E = # mc2 even before Einstein was born.

The same kind of reasoning can teach us (as it did Einstein) about his theory of gravity, “general relativity”, and one of its children, black holes.  But again, Einstein’s era wasn’t first to ask the question.   It goes back to the late 18th century. And why not? It’s just a matter of dimensional analysis.

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## In Our Galaxy’s Center, a Tiny Monster

It’s far from a perfect image. [Note added: if you need an introduction to what images like this actually represent (they aren’t photographs of black holes, which are, after all, black…), start with this.] It’s blurred out in space by imperfections in the telescopic array that is the “Event Horizon Telescope” (EHT) and by dust … Read more

## Black Hole Announcement Expected Thursday

In 2019, the first image of the surroundings of a black hole was produced, to great fanfare, by the astronomers at the Event Horizon Telescope (EHT). The black hole in question was the enormous one at the center of the galaxy M87.

At the time, there was also hope that the EHT would produce an image of the region around the black hole at the center of our own galaxy, the Milky Way. That black hole is thousands of times smaller, but also thousands of times closer, than the one in M87, and so appears about the same size on the sky (just as the Moon and Sun appear the same size, despite the Sun being much further away.)

However, the measurements of the Milky Way’s black hole proved somewhat more challenging, precisely because it is smaller. EHT takes about a day to gather the information needed for an image. M87’s black hole is so large that it takes days and weeks for it to change substantially — even light takes many days to cross from one side of the accretion disk to the other — so EHT’s image is like a short-exposure photo and the image of M87 is relatively clear. But the Milky Way’s galaxy’s black hole can change on the times scale of minutes and hours, so EHT is making a long-exposure image, somewhat like taking a 1-second exposure of a tree on a windy day. Things get blurred out, and it can be difficult to determine the true shape of what was captured in the image.

Apparently, the EHT scientists have now met these challenges, at least in part. We will learn new things about our own galaxy’s black hole on Thursday morning; links to the press conferences are here.

In preparation for Thursday, you might find my non-expert’s guide to a black hole “silhouette” useful. This was written just before the 2019 announcement, when we didn’t yet know what EHT’s first image would show. The title is a double-entendre, because I myself wasn’t entirely expert yet when I wrote it. The vast majority of it, however, is correct, so I still recommend it if you want to be prepared for Thursday’s presentation.

The only thing that’s not correct in the guide (and the offending sections are clearly marked as such) are the statements about the “photon ring”. It took me until my third follow-up post, two months later, to get it straight; that post is accurate, but it is long and very detailed. Most readers probably won’t want to go into that much detail, so what I’ll do here is summarize the correct parts of what I wrote in the weeks following the announcement, repeating a few of the figures that I made at the time, and then tell you about a couple of new things that have been learned since then about M87’s black hole. Hopefully you’ll find this both interesting on its own and useful for Thursday.

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## A Ring of Controversy Around a Black Hole Photo

[Note Added: Thanks to some great comments I’ve received, I’m continuing to add clarifying remarks to this post.  You’ll find them in green.]

It’s been a couple of months since the photo’ (a false-color image created to show the intensity of radio waves, not visible light) of the black hole at the center of the galaxy M87, taken by the Event Horizon Telescope (EHT) collaboration, was made public. Before it was shown, I wrote an introductory post explaining what the ‘photo’ is and isn’t. There I cautioned readers that I thought it might be difficult to interpret the image, and controversies about it might erupt.

So far, the claim that the image shows the vicinity of M87’s black hole (which I’ll call M87bh’ for short) has not been challenged, and I’m not expecting it to be. But what and where exactly is the material that is emitting the radio waves and thus creating the glow in the image? And what exactly determines the size of the dark region at the center of the image? These have been problematic issues from the beginning, but discussion is starting to heat up. And it’s important: it has implications for the measurement of the black hole’s mass (which EHT claims is that of 6.5 billion Suns, with an uncertainty of about 15%), and for any attempt to estimate its rotation rate.

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## The Black Hole Photo’: Seeing More Clearly

THIS POST CONTAINS ERRORS CONCERNING THE EXISTENCE AND VISIBILITY OF THE SO-CALLED PHOTON-SPHERE AND SHADOW; THESE ERRORS WERE COMMON TO ESSENTIALLY ALL REPORTING ON THE BLACK HOLE ‘PHOTO’.  IT HAS BEEN SUPERSEDED BY THIS POST, WHICH CORRECTS THESE ERRORS AND EXPLAINS THE SITUATION. Ok, after yesterday’s post, in which I told you what I still … Read more

## The Black Hole Photo’: What Are We Looking At?

The short answer: I’m really not sure yet.  [This post is now largely superseded by the next one, in which some of the questions raised below have now been answered.]  EVEN THAT POST WAS WRONG ABOUT THE PHOTON-SPHERE AND SHADOW.  SEE THIS POST FROM JUNE 2019 FOR SOME ESSENTIAL CORRECTIONS THAT WERE LEFT OUT OF ALL REPORTING ON THIS SUBJECT.

Neither are some of my colleagues who know more about the black hole geometry than I do. And at this point we still haven’t figured out what the Event Horizon Telescope experts do and don’t know about this question… or whether they agree amongst themselves.

[Note added: last week, a number of people pointed me to a very nice video by Veritasium illustrating some of the features of black holes, accretion disks and the warping of their appearance by the gravity of the black hole.  However, Veritasium’s video illustrates a non-rotating black hole with a thin accretion disk that is edge-on from our perspective; and this is definitely NOT what we are seeing!]

As I emphasized in my pre-photo blog post (in which I described carefully what we were likely to be shown, and the subtleties involved), this is not a simple photograph of what’s actually there.’ We all agree that what we’re looking at is light from some glowing material around the solar-system-sized black hole at the heart of the galaxy M87.  But that light has been wildly bent on its path toward Earth, and so — just like a room seen through an old, warped window, and a dirty one at that — it’s not simple to interpret what we’re actually seeing. Where, exactly, is the material in truth’, such that its light appears where it does in the image? Interpretation of the image is potentially ambiguous, and certainly not obvious.

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