3:54 PM (All times Pacific): Welcome to the Live Blog of Erik Verlinde’s talk! For those of you curious about why emergent gravity is such a radical idea, it presents the possibility of explaining the most mysterious force, gravity, in terms of something that is simple and well-defined: the entropy of tiny black holes. If our Universe is truly made of something more fundamental than the particles of the Standard Model, this may be the key breakthrough that leads us to the next frontier!

3:58 PM: It may also be completely wrong. In fact a lot of people, especially dark matter proponents (like, but not including, me), are very skeptical of Verlinde’s idea, even going to the point to advocate that Perimeter shouldn’t be giving his ideas a platform. I strongly disagree with that. A new idea that’s full of possibilities, even if it turns out to be wrong and not descriptive of our physical Universe, can be a fascinating line of inquiry to investigate. New mathematics, new innovations, and new perspectives can often lead to a new way of viewing the Universe, even if the way we’re initially investigating is fruitless. You never know what you’re going to find unless you look. It may be a purely blind alley we’re going down, but we won’t know unless we tread down that road!

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:00 PM: And here we go! Welcome from Greg Dick, the director of outreach at Perimeter, with an hour lecture (by Verlinde) and then a ~15 minute Q&A. Let’s see what we’re in for; if you want to ask a question, use the hashtag #piLIVE on Twitter, which I’ll be following, too.

4:03 PM: If you’re wondering, it’s pronounced “ver-LIND,” not “ver-LIN-de.” I didn’t know that until just now.

Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. Image credit: X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A. Mahdavi et al. (top left); X-ray: NASA/CXC/UCDavis/W.Dawson et al.; Optical: NASA/ STScI/UCDavis/ W.Dawson et al. (top right); ESA/XMM-Newton/F. Gastaldello (INAF/ IASF, Milano, Italy)/CFHTLS (bottom left); X-ray: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University) (bottom right).

4:05 PM: One of the most controversial aspects of Verlinde’s theory is that it attempts to explain away dark matter and dark energy, which appear to be absolutely necessary from a variety of astrophysics observations. Will Verlinde’s theory eventually provide an explanation for these observations? We can hope, but I hope he’s honest that his theory is not quite there yet.

A diagram of a cathode ray tube. Image credit: Theresa Knott / Wikimedia Commons.

4:07 PM: Verlinde talks about particle accelerators as being key to 20th century physics, noting that the television is even a particle accelerator. This was true for the old-style Cathode-Ray-Tube televisions, which accelerated electrons onto a screen, causing the emission of photons that create a picture. But we now use LED-style technology; we have moved beyond CRTs and particle accelerators. It’s possible that we’ll need to go to quantum bits (qbits) of information, beyond individual particles as we understand them, to understand gravity.

Screenshot from the October 4th talk at Perimeter Institute, of Verlinde’s speculative view of the Universe. Image credit: Perimeter Institute.

4:10 PM: Could replacing particles or other fundamental points with a new point-of-view, could we get new answers? This is dangerous ground here, because it straddles the border of what’s actually physics (i.e., rooted in reality), and pure speculation, where there are no observed phenomena or experiments you can perform to tell them apart. Curious to see where this leads.

4:13 PM: It’s true that macroscopic quantities, like temperature and pressure, are made up of a more fundamental set of quantities: the individual particles that make up, for example, a gas. As they move, interact, and collide, they create these macroscopic properties. Entropy, for instance, is one such derivable quantity from the microscopic properties of a system. But is it possible that what we conceive of as fundamental, like particles, gravity, or even space and time, is actually made of something else? Even entropy, at a fundamental level? Be open-minded, but also be skeptical.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:15 PM: At least we’re starting off, from Verlinde, with a solid definition of entropy: the total number of bits of information in a system. This is, in fact, going to need to be promoted to qbits (quantum bits) to explain our quantum Universe, but that’s okay.

It takes a speed of 7.9 km/s to achieve “C” (stable orbit), while it takes a speed of 11.2 km/s for “E” to escape Earth’s gravity. Image credit: Brian Brondel under a c.c.a.-s.a.-3.0 license.

4:18 PM: He wants you to think about gravity, in Newtonian terms, that gravity, going around the Earth, and “falling” are actually all the same thing. This is where you get a Newtonian force law from, and this was one of the great theoretical advancements in human history. For over 200 years, more than twice as long as we had General Relativity, Newtonian gravity was the unquestioned champion of the Universe. Don’t underestimate it’s impact and importance!

The fabric of the Universe, spacetime, is a tricky concept to understand. But, thanks to Einstein’s general relativity, we’re up to the challenge. Image credit: Pixabay user JohnsonMartin.

4:20 PM: Verlinde is now outlining the difference between “dark matter” (i.e., a new, unseen source of mass) and “modified gravity” (i.e., a new theory of gravity). Einstein replaced Newton with spacetime curvature determining the motion of matter and energy, with matter and energy determining in turn the spacetime curvature. The analogy is clear: Verlinde wants to modify how we think about gravity rather than add dark matter. This is a tall order; there are many, many things that work out very well if you add dark matter and fail if you don’t.

The extended rotation curve of M33, the Triangulum galaxy. With the addition of star-formation feedback into simulations, as well as accounting for the ratio of normal to dark matter, could dark matter finally account for these observed rotation curves? Image credit: Wikimedia Commons user Stefania.deluca.

4:24 PM: What fails in modified gravity?

Large-scale structure formation,

the fluctuations in the cosmic microwave background,

the motions of individual galaxies within galaxy clusters,

the amount and shape of gravitational lensing,

the gravitational effects of merging galaxy clusters,

the Sachs-Wolfe and Integrated Sachs-Wolfe effect,

and the varied ratio of dark matter to normal matter (as inferred from motions of individual stars) on galaxies of different scales/sizes.

It’s a tall order for a modified theory of gravity to explain their bread-and-butter, galactic rotation, and simultaneously hit all of these.

Screenshot from Twitter user New Leibniz: https://twitter.com/NewLeibniz/status/915714279726297089 Image credit: Twitter Screenshot.

4:26 PM: There’s a really good question on Twitter that Perimeter isn’t answering: would the discovery of dark matter invalidate Verlinde’s theory (or any modified gravity theory). It would render it totally unnecessary, to be honest. We have lots of evidence for dark matter, and the only place where modified gravity is superior is on individual galactic scales.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:29 PM: Ladies and gentlemen, we have SILLY PUTTY! Verlinde is talking about elasticity, which is a derived (emergent) property of the interactions of all the particles. Verlinde’s contention is that just as the silly putty flows, perhaps gravity, or dark matter, is a derived and emergent property of a more fundamental, underlying theory. Yes, Einstein is right, but perhaps something else is more right, which is to say, fundamental and from which Einstein’s equations can be derived.

4:31 PM: Whoa, buddy! Don’t be unfair to dark matter! Just because we haven’t built an experiment that’s successfully directly detected it in the lab is not evidence that dark matter doesn’t exist! It means that the detectors we’ve been able to build and the scales we’ve been able to probe, mostly looking for WIMP-type dark matter, haven’t been successful. But absence of evidence is not evidence of absence. To argue otherwise is pure silliness.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:33 PM: And yes, gravitational lensing shows the need for dark matter. It’s simply a question of mass, and lensing (for both strong and weak lensing) is a strong indirect verification of dark matter. Again, no modified gravity theory, including Verlinde’s, can explain all of these large-scale observations.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:35 PM: Did you catch this beautiful graphic? Yes, this is a fabulous illustration of the Universe as seen from our perspective, in terms of lookback time. Early galaxies, the dark ages, and the CMB are all visible. Neat!

Constraints on dark energy from three independent sources: supernovae, the CMB and BAO. Note that even without supernovae, we’d need dark energy. Image credit: Supernova Cosmology Project, Amanullah, et al., Ap.J. (2010).

4:37 PM: But again, to look at “95% of the energy in the Universe is something that we don’t know what it is,” in other words, dark matter and dark energy, does not mean that there’s anything wrong with our picture of the Universe. We have people who don’t accept relativity; don’t accept quantum mechanics; don’t accept field theories or the existence of black holes, etc. Why should your intuition, or what makes sense to your puny mind in the context of your limited experience, be a mirror of reality? That is quite a bit of arrogance that we need to guard ourselves against, not embrace to convince ourselves that our intuition is right.

4:42 PM: On a side note, this video of falling into a black hole is really cool. Andrew Hamilton at Colorado is the mastermind behind this, and the simulation is available on Vimeo to view any time. I’ve embedded it above; enjoy and watch it as many times as you like!

Hawking radiation is what inevitably results from the predictions of quantum physics in the curved spacetime surrounding a black hole’s event horizon. This diagram shows that it’s the energy from outside the event horizon that creates the radiation, meaning that the black hole must lose mass to compensate. Image credit: E. Siegel.

4:45 PM: The Bekenstein-Hawking radiation is brought up, in an almost correct fashion. He’s talking about it in terms of quantum mechanics, which is too naive. It’s not “particle-antiparticle pairs” where one of them comes out; it’s mostly photons, and blackbody radiation, that comes out of it, not particles or antiparticles. It requires quantum field theory to describe, and even the improved illustration I made above doesn’t fully encapsulate it. But what is correct is that the area of a black hole (related to its mass) is proportional to its entropy.

4:47 PM: Verlinde says it’s the first crack in Einstein’s theory, but I don’t think that’s fair. Entropy is something that can be derived on top of the background of General Relativity, but it doesn’t imply any problems with Einstein’s theory. In fact, from what Verlinde is arguing, I’d say he thinks dark matter is the first crack in the theory. On that account, I disagree, but at least I see where he’s coming from.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:49 PM: If you replace bits with qbits in a computer, you don’t just have 0s and 1s, you have superpositions, and therefore everything in between. In fact, you can entangle many qbits with one another, which is why quantum computers offer a much more powerful possibility to conventional, “classical” computers.

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:50 PM: Just FYI, this was the picture I was curious about at the beginning of the talk. It seems to insinuate that entangled quantum particles, or quantum information, is what gives rise to dark matter. The amount of entanglement has increased over time, as the Universe has expanded. So what does this mean for the dark matter density (or how it appears to manifest) over time? I don’t have an answer, but I know Verlinde’s theory has it change… which I’m unhappy about. But at least now we’re finally getting the connection between entanglement and dark matter.

4:53 PM: Verlinde asks the big question: does quantum entanglement/emergent gravity explain the deviations from Newton’s laws in galactic rotation? It’s the easiest problem for modified gravity, but the answer so far is, “only at one instant in time.” It doesn’t work the same if we look to greater distances, and greater lookback times.

4:55 PM: What is the entropy of the Universe? From photons, Verlinde says it’s about 10⁹⁰ ‘bits’. From the size of the cosmological horizon, he says it should be 10¹²⁰ bits. (He’s leaving a factor of k out of there, I think.) What’s interesting is that we can actually calculate this, and black holes are the dominant factor. In fact, I’ve calculated it myself, here; go check it out!

Screenshot from the October 4th talk at Perimeter Institute. Image credit: Perimeter Institute.

4:59 PM: This is Verlinde’s big “conclusion”? That entropy, entanglement, the Hubble constant, etc., are all related and explain the galactic rotation problem? This is… very, very suspect. Particularly because the Hubble “constant” is a constant in space, but changes over time. So how does galactic rotation evolve with time/redshift? No answer from Verlinde, because the other constants in his equations do notchange.

5:01 PM: I have to say I’m disappointed; there was really very little quantitative information presented. There’s nothing he presented that you can use, say, a fact to challenge his idea. And that’s tremendously disappointing. That’s the definition of “kid gloves” to me. Someone challenge this, please!

My Tweet (@startswithabang) on Twitter. Image credit: Screenshot from Twitter.

5:02 PM: Alright, I asked it. Let’s see if we get an answer!

5:05 PM: I have to say, this is the first time I’m lessexcited about an idea after seeing a talk than I was before seeing the talk. The ideas Verlinde presented in his paper were shown as being under development, as being aware of the problems with them, as being finely-tuned to match the parameters that describe our Universe. This talk was light on the details, and heavy on the advertising. Too bad.

5:06 PM: Hey! Here’s an admission that if they do discover “too much” dark matter, his ideas are wrong. And everything else is his intuition.

5:09 PM: Too bad; no addressing of the quantitative aspects of his idea. It was interesting to listen and I’m glad I’ve heard it, but I’m more convinced that this is a cherry-picked solution he claims to have come up with, and that the details, particularly as the Universe evolves, won’t turn out to be consistent. I’m also convinced that the physicist who shows this won’t be Erik Verlinde.

In any case, I hope you enjoyed the talk, and thanks for tuning in to the live blog!