First stars spell trouble for dark matter

The HERA telescope array
in South Africa [img src].

In the beginning, reheating created a hot plasma of elementary particles. The plasma expanded, cooled, and emitted the cosmic background radiation. Then gravity made the plasma clump, and darkness was upon the face of the deep, whatever that means.

Cosmologists call it the “dark ages,” the period in the early universe where matter is already too cool to emit radiation, but not yet clumpy enough to ignite nuclear fusion. At this time the universe was filled almost exclusively with rather dilute hydrogen gas. It’s not until a billion years after the Big Bang that the first stars light up, an epoch poetically called “cosmic dawn.”

We cannot directly measure light emitted from those first stars, but we can indirectly infer the stars’ presence by studying the cosmic microwave background. That’s because the early stars emit UV radiation which couples to the hydrogen gas, and for some while this coupling enables the gas to absorb light of a specific wavelength – at about 21cm. This leaves a mark in the cosmic microwave background.

The wavelengths of light stretch with the expansion of the universe, so what was 21cm back then is now deep in the radio regime. That makes it difficult to find cosmological signals because other sources – both on earth and in our galaxy – can contaminate the data.

In February, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) announced they had measured the absorption that stems from the first stars. They found it at the expected wavelength – a few meters – but stronger than the predictions said it should be.

Astrophysicists can make predictions for this absorption by using the concordance model for cosmology. This model has 8 free parameters – one of which is the amount of dark matter –  and the physics of the first stars follows from this straight-forwardly. Besides the cosmological dynamics, it’s only well-known thermodynamics and atomic physics. Compared to the large variety of today’s stars, the first stars were fairly simple. Or at least that’s what astrophysicists thought so far.

It took me a while to get around and read the EDGES paper, and I’ve since tried to find out what, if anything, astrophysicists think about the mismatch with the predictions. The answer is: not much. Most of them think it’ll go away. Maybe a measurement error. I have even been told to not pay attention to the EDGES result because the paper has not been cited all that often. Seriously.

Well, as you can tell, I looked at it anyway. I’m not an astrophysicist and I can’t judge the experimental design of the EDGES collaboration. I can only say that I don’t see obvious flaws with their data analysis. The paper seems fine to me.

Besides the possibility of a measurement error, the theoretical explanations for the signal have so far focused on what type of dark matter could possibly make it work, as the commonly considered ones don’t do the trick.

To explain the EDGES result, dark matter would need a stronger interaction with normal matter than presently assumed. If so, that could shift the temperature between gas and radiation and give rise to more absorption. However, a more strongly interacting type of dark matter is difficult to make compatible with other observations that rule out such interactions.

Stacy McGaugh – the modified gravity dude – had the brilliant idea to see what the absorption signal from the first stars would look like if there was just no dark matter. Turns out this would fit remarkably well with the EDGES data. I say “remarkably well” because the parameters that enter his calculation are known from other measurements already, so no freedom to adjust them.

Fig 1 from McGaugh, PRL 121 (2018) 081305.

The reason why the absorption is stronger without dark matter isn’t hard to understand. The more matter there is in the universe, the faster the expansion decelerates. This means without dark matter, the period in which the gas can interact with the radiation is longer, allowing more absorption.

Now, I have recently developed a soft spot for modified gravity, but I am not terribly convinced of Stacy’s argument. It’s one thing to say that galaxies probe different physics than cosmology and thus a new type of force may kick in on galactic scales. It’s another thing to just throw out dark matter from the concordance model because that screws up the whole fit from which the other parameters stem to begin with. You have to self-consistently extract the whole set of parameters from the data – you need a different model entirely.

Indeed, to recover the benefits of dark matter, Stacy employs rather heavy neutrinos. The masses are on the upper end of what is still compatible with constraints. (That’s not counting the cosmological constraints, which are tighter, because these constraints assume the concordance model, and hence don’t apply for modified gravity.) The neutrinos don’t make a difference for the EDGES signal. Still, the dark-matter-less model does not account for the third acoustic peak of the cosmic microwave background. So you have to choose, get the EDGES absorption right or get the third acoustic peak right. Frankly I’d rather have both.

As we speak, other experimental groups are trying to reproduce the EDGES result. One of them is the Hydrogen Epoch of Reionization Array (HERA) in South Africa that is supposed to be completed next year, another one the Square Kilometer Array (SKA) in Australia, planned to be in full operation by 2024. If they confirm the EDGES result, this may be decisive to distinguish dark matter from modified gravity.

Dear Dr B: What do you actually live from?

Some weeks ago a friend emailed me to say he was shocked – shocked! – to hear I had lost my job. This sudden unemployment was news to me, but not as big a surprise as you may think. I was indeed unemployed for two months last year, not because I said rude things about other people’s theories, but simply because someone forgot to renew my contract. Or maybe I forgot to ask that it be renewed. Or both.

In any case, this happened a few times before, and while my younger self wouldn’t normally let such a brilliant opportunity for outrage go to waste, I now like to pretend that I am old and wise and breathe out bullshit.

After some breathing, I learned that this time my sudden unemployment originated not in a forgotten signature, but on Wikipedia. I missed the ensuing kerfuffle about my occupation, but later someone sent me a glorious photoshopped screenshot (see above) which shows me with a painted-on mustache and informs us that Sabine Hossenfelder is known for “a horrible blog on which she makes fun of other people’s theories.”

The truly horrible thing about this blog, however, is that I’m not making fun. String theorists are happily studying universes that don’t exist, particle physicists are busy inventing particles that no one ever measures, and theorists mass-produce “solutions” to the black hole information loss problem that no one will ever be able to test. All these people get paid well for their remarkable contributions to human knowledge. If that makes you laugh, it’s the absurdity of the situation, not my blog, that’s funny.

Be that as it may, I have given a lot of interviews in the past months and noticed people are somewhat confused about what I actually work on. I didn’t write about my current research in my book because inevitably the physicists I criticize would have complained I wrote the book merely to advertise my own work. So now they just complain that I wrote the book, period. Or they complain I’m a horrible person. Which is probably correct because, you see, all that bullshit I’ve been breathing out now sticks to them.

Horrible person that I am, I don’t even work in the foundations of physics any more. I now work on quantum simulations or, more specifically, on using weakly coupled condensed-matter-systems to obtain information about a different, strongly coupled condensed matter system.

The relation between the two systems stems from a combination of analogue gravity with the gauge-gravity duality. The neat thing about this is that – in contrast to either the gauge-gravity duality or analogue gravity alone – we are dealing with two systems that can (at least in principle) be prepared in the laboratory. It’s about the real world!

This opens the possibility to experimentally test the validity of the gauge-gravity duality, or its applicability to certain systems, respectively. Current experiments (like Jeff Steinhauer’s) aren’t precise enough to actually do this, but the technology in this area is rapidly improving, so I’m hopeful that maybe in a decade or so it’ll be doable.

If that was too much terminology, I’m developing new methods to describe how large numbers of atoms interact at very low temperature.

Today, Tobias Zingg and I have a new paper on the arXiv that sums up our recent results. And that’s what I’ll be working on until my contract runs out for real, in November next year. And then what? I don’t know, but stay tuned and we’ll find out.

Yes, women in science still have a disadvantage.

Women today still face obstacles men don’t encounter and often don’t notice. I see this every day at my front door, in physics, where women are still underrepresented. Among the sciences, it’s physics where the gender-balance is most skewed.

While women are catching up on PhDs, with the ratio now at roughly 20% (US data), women are more likely to leave higher education for good. Among faculty, the percentage of women is down to about 10%. The better the job, the more likely it’s occupied by a man.

Source: APS.

The reasons for this “leaky pipeline” are manifold and no one presently knows for sure which factors are most relevant. Therefore, the question what, if anything, to do about it is hotly debated.

Source: AIP statistics.

On one end of the spectrum are those who think that the current gender-balance correctly reflects qualification and nothing needs to be done. People in this camp explain differences between genders by women’s lack of performance and not discrimination. On the other end are those who think the world will only be a good place if half of physicists are women. In this camp, any under-performance by women is caused by discrimination.

I think both of these extreme positions are unreasonable.

The current gender balance almost certainly does not correctly reflect qualification because a) women have to push back harder on gender stereotypes and b) are more likely to have difficulties combining academia with family life.

Consider this: Last time I got my hair cut, I was informed that women can’t do physics because they can’t think logically. This insight was delivered to me matter-of-factly by a middle-aged, female hair-dresser after I answered her question what I’ve been up to lately. (Wrote a book – About what? – Physics).

I didn’t pursue the matter because I don’t like to argue with people who use sharp instruments near my eyes. But I was glad I didn’t have my daughters with me. They’re still at an age where by default they believe what adults say.

Gender stereotypes like this are everywhere and they almost certainly influence career choices. We all want to belong, and if women feel that science is for men, they’re less likely to pursue this avenue. They lose motivation more easily. They call it quits sooner. This means we are missing out on qualified women and instead fill up the pool with lesser qualified men.

Men tend to underestimate how pervasive these stereotypes are, and how tiresome it is to always be the weird one. It wears you down; doubly and triply so for women who are also members of minorities.

Example: I am constantly accused of being rude, aggressive, and snarky, and have been advised multiple times (by men) to not express myself with certainty. Because that’s offensive, you see. If men behave this way, they’re brilliant geniuses, and one cannot blame them for what is merely an expression of their enthusiasm. I’m left to constantly apologize for being who I am. Or figure this: Last time I won an award, the speaker who was supposed to summarize my achievements felt the need to point out that I do great work despite being short.

Yes, I have a lot of anecdotes, and they can be summarized as a pronounced lack of respect. I’m as much a professional physicist as the men around me, yet I’m not treated the same way. I’m LOOK-A-WOMAN!

However, I try not to draw conclusions from my own experiences because the very fact that I’m still working in physics is evidence I’m not suffering all that greatly. I’d go so far to say most of my colleagues are nice guys, and even the assholes don’t normally mean to be assholes. But I hear what my female colleagues have to say, and most of them are really frustrated about the nonsense they have to deal with. And, yeah, I too have been mistaken for the secretary.

The other major disadvantage that women face is that they are hit harder by the family-unfriendliness of academia. Turn it how you wish, women are still responsible for procreation, and female fertility rapidly declines past the age of 40.

Unfortunately, the years between 35 and 40 are also critical to establishing yourself as researcher. Most physicists presently don’t secure permanent positions until their early 40s. Up to then, job-hopping and frequent international moves are the norm. Taking time off to raise kids is difficult, and the inevitable decrease in academic output and flexibility is a competitive disadvantage. A priori this disadvantage exists both for men and women, but women are on the average the younger part of the couple, and, needless to say, pregnancy and nursing is an extra burden.

Arguments that women just perform worse than men are fundamentally flawed because they’re based on shaky interpretation of data that don’t quantify what they’re supposed to show. Data say, for example, that women in science publish less and their papers are cited less often (references here). Does this mean women are less capable of doing science? No, it means that they publish less and their papers are cited less often. To put it differently, it means that women’s papers are less popular with their – predominantly male – colleagues. How about not judging women by how much their work appeals to men?

The recent case of Alessandro Strumia is an example for such shaky interpretations of data. There’s no simple way to measure whether women are having a harder time with their research. There could be all sorts of reasons, from the lack of role models to difficulties getting funding to being more frequently asked to sit in committees because, well, we need at least one woman, you see? None of these difficulties will reflect in publication records in any obvious way.

Besides, there’s no agreed-upon measure for academic success. Indeed, it is well known that rewarding a high number of publications and citations creates perverse incentives, and therefore many scientists now compete to excel on meaningless performance indicators. So why are we even talking about this?

This is not to say that the differences between publications of men and women are not interesting or should not be studied. Just that one shouldn’t jump to conclusions from them.

But I am also not an advocate of a gender-balance of one-to-one. Biological factors, such as muscle strength, arguably play a role for some professions. I find it highly questionable that a profession like physics, which mostly deals with abstract ideas, is much influenced by genetic factors. But it’s a controversial subject and, as they say, more research is needed. Regardless of whether the reason is nature or nurture, however, women demonstrate preferences different from men, and it doesn’t make sense to push them into disciplines that they might not currently feel well in.

For this reason I cannot support policies that aim to increase the ratio of women based on the premise that 50:50 is the “correct” target. It’s not only that this risks we’ll hire women who are less motivated or qualified than some men who don’t get jobs, it also pisses off the men who feel like they are now the ones at a disadvantage. And this creates yet another bias, namely the belief that women now have an easier time rather than merely having less of a disadvantage. Again the recent Strumia-case is a good example.

Having said this, some people tell me it’s justified to risk hiring lesser qualified women, at least temporarily, to restore what they consider fairness in the long run. But at this point we are down to a value-decisions. What is more important to you: That science works most efficiently, or that women catch up with men as quickly as possible? I think this is the key question which no one wants to discuss.

Be that as it may, the easiest way to disqualify yourself from any discussion on the matter is to simply disregard the existing hurdles that women face. The problems are real, and they’re far from vanishing.

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If you want to make a difference, check out the Practical Guide to Improving Gender Equality in Research Organisations by Science Europe, and raise awareness for the therein proposed changes among your colleagues.

Gender-bias in Academia: The Case Strumia

Dr Alessandro Strumia is a physicist working at CERN, where he is Principal Investigator of an ERC Advanced research grant on the topic “Understanding the mass scales in nature.” Ie, he spends your tax money on the kind of research that I criticize in my book. He also recently published an analysis of publication and citation-rates in his field. At a workshop on gender diversity at CERN last weekend, he used this analysis to argue that women are underrepresented in physics because they are underperforming.

I did not attend the workshop and have not seen a recording of the talk, but I have seen the slides (a PDF version of which is here). The slides contain statements that are both inaccurate and exceedingly unprofessional.

For example, he begins his talk by stating that “smarter people are less affected by implicit bias,” but this is wrong. Studies have shown repeatedly that intelligence does not protect from thinking biases. Yes, intelligence is useful to overcome certain types of biases (mostly those that can be exposed with mathematical reasoning), but only once people are aware they are biased to begin with.

Strumia’s mistaken belief that intelligent people are less affected by cognitive biases does not remotely surprise me. I have encountered this very same attitude (“We are too smart to be biased!”) among almost all high-energy theorists and phenomenologists I have spoken with about the issue. That in itself is a bias, known as the “bias blind spot.”

But that Strumia is ill-informed about the very topic he speaks about at a scientific workshop is not the biggest problem with his presentation. Far worse is that he names and attacks two women, apparently because he is annoyed he did not get a job that he was shortlisted for. Nonsense like this just does not belong in a research presentation.

After complaints ballooned on social media on Monday, CERN pulled the slides from the net quickly and has since suspended Strumia. What will happen to his ERC grant is unclear. A large number of members of the particle physics community have meanwhile signed a statement declaring that they distance themselves from the content of Strumia’s talk.

Now, as you know, I have also recently taken up bibliometric analysis. And I admit I found some of the data Strumia showed interesting. We did, in our paper, also look at gender differences, but not for citation counts. We looked at an entirely different quantity, that of research broadness, and for this we did not find any gender differences.

The gender difference that Alessandro Strumia and his co-author Ricardo Torre find is huge. It’s a more than 100% difference in the total number of citations that researchers accumulate throughout their career.

I don’t think that the number of citations is a good measure for scientific performance, but if the difference between the genders was so large, it might mean that women and men chose their research projects in distinctly different ways. That would be interesting. I thus decided to look into this for a bit.

The key figure that Strumia presents on his slides is the total number of citations that researchers accumulate since the publication of their first paper:

Figure from slide 16 of Alessandro Strumia’s talk.

That the horizontal axis is labeled “scientific age” is unfortunate because this term has been coined to emphasize that the scientific age might differ from the chronological time passed since PhD or first paper. If a researcher takes a career-break, for example because of health reasons or for parental leave, their scientific age goes on hold. However, there is presently no standardized way to determine the scientific age, and in any case, you couldn’t do it from publication data alone, you’d also need biographic information.

Since women are more likely to take leave for child-raising, their citations should on the average increase somewhat slower, simply because they have more breaks in which they don’t publish. However, it seems unlikely that this would make such a huge difference. So, while the label on the axis is inaccurate, I don’t think it’s all that relevant.

When I saw this graph, however, another worry came to my mind immediately. When we did our previous analysis, we found that the vast majority of people who use the arXiv publish only one or two papers and are never heard of again. This is in agreement with the well-known fact that the majority of physicists drop out of academic careers.

I am not sure why this surprised me when it showed up in the data. Maybe because, if you work in the field, the drop-outs are pretty much invisible. They leave and you forget about them. But they are there, in the stats, big and fat.

Now, the total number of citations for such drop-outs will accumulate very slowly because they don’t publish new papers any more. And we know that women are more likely to drop out – that’s the “leaky pipeline” and reason why I find myself increasingly often, if not the only woman in the room, then at least the oldest woman in the room. And I’m only 42.

If you leave the drop-outs in the citation analysis, the leaky pipe will pull down the average of female authors more than of male authors.

I hence asked one of our PhD students, Tobias Mistele, to plot the same quantity as Strumia did for our data sample, but to only keep authors who have more than 5 papers in total, and who have published a paper in the last 3 years. This is sloppy way to shrink down the pool to “active researchers only.” It’s maybe not the most sophisticated way to do it, but it should give us an idea how large the contribution from the drop-outs is.

If we plot the number of citations for active researchers only, we see no noticeable difference between men and women:

arXiv data, active researchers only

When normalized to the number of authors per paper (as Strumia did), there is also no noticeable difference between men and women.

I must add a warning here. We do not use the same data set as Strumia and Torre. They use data from inspire, we use data from the arXiv. This means our data set does not reach back in time as far, and it includes disciplines besides high energy physics. So the absolute numbers are not directly comparable.

Another caveat I must add is that we are using a different method to identify male and female authors. We use the author-id algorithm that is explained in our earlier paper, and then try to match first names with a database for common anglo-saxon first names. Naturally, this means that the authors who remain in our sample are most likely to be of Western origin. By this method we assign a gender to 19% of authors. This is in contrast to Strumia and Torre who use a more elaborate gender-id procedure that allows them to match 60%. The remaining authors in our sample break down to 70,295 male und 53,165 female researchers. After applying the above mentioned cuts, we are left with 12,654 male and 8,177 female. That’s not a huge number, but decent.

Let me also mention that probably a similar effect is behind another finding in Strumia’s talk. He points out that women, on the average, are hired into faculty positions earlier. A paper that appeared on the arXiv yesterday argued that this is not a signal that women have an unfair advantage, but simply a consequence of women leaving at a higher rate. If they aren’t hired early, they’ll not be hired at all, which means the average age of hiring is smaller.

Finally, to state the obvious, this is a blogpost, not a paper. The above is a quick and dirty way to check whether removing dropouts significantly affects the large difference between men and women, and the answer seems to be yes. However, we will have to do a more careful analysis to arrive at definite conclusions. I haven’t checked my biases.

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I want to thank Tobias Mistele for doing the graphs so quickly and Alessandro Strumia and Ricardo Torre for helpful communication.

You say theoretical physicists are doing their job all wrong. Don’t you doubt yourself?

This is me with John Horgan, yesterday.
This photo is only here so
the share widgets work properly.

One of the most frequent critical remarks I have gotten on my book is that I seem confident. I was supposed, it seems, to begin each paragraph with “I’m sorry, but.”

But I am not sorry. I mean what I say. Yes, in the foundations of physics we are financing some 15,000 or so theorists who keep producing useless scientific articles because they believe the laws of nature must be beautiful. That’s exactly what I am saying.

Let us leave aside for a moment that you have to skip half the book to not notice I question myself on every other page. Heck, if you ask me to sign the book, I’m afraid I’ll misspell my own name. I’m a walking-talking bag of self-doubt. Indeed that was the reason I ended up writing this book.

See, I don’t understand what’s going on with this community. Everyone knows there’s no reason that a scientific explanation must appeal to the human sense of beauty. Right? Doesn’t everyone know this? Science is about explaining observations, regardless of whether we like these explanations.

But if it’s clear that putting forward new hypotheses just because they are beautiful doesn’t mean they’re likely to be right, then why do theorists in these fields focus so much on beauty? Worse, why do they continue to focus on the same type of beauty, even though that method has demonstrably not worked for 40 years?

At first I considered there might be a mathematical basis to their arguments which I was missing. That there is a solid reason why a theory must be natural, or that the fundamental forces must be unified, or that the mathematics of a theory must be “fruitful” and “have deep connections” and be “rigid” – to quote some expressions people in the foundations of physics commonly use. But there is no mathematical basis. Arguments from beauty are additional assumptions, and they are unnecessary to make a theory work.

Indeed, some philosophers have suggested I speak of “metaphysical assumptions” rather than “aesthetic arguments”, but I think the latter captures the historical origin better. These arguments trace back to tales about God’s beautiful creations. Also, if I’d call it metaphysics no one would know what I am talking about.

I then considered that using criteria from beauty is justified because it has historically been successful. This would leave open the question why that would be so – I cannot think of a reason such a connection should exist. But in any case, history speaks against it. Relying on beauty has sometimes worked, and sometimes not. It’s just that many theoretical physicists prefer to recall only the cases where arguments from beauty did work. And in hindsight they then reason that the wrong ideas were not all that beautiful. Needless to say, that’s not a good way to evaluate evidence.

Finally, the use of criteria from beauty in the foundations of physics is, as a matter of fact, not working. Beautiful theories have been ruled out in the hundreds, theories about unified forces and new particles and additional symmetries and other universes. All these theories were wrong, wrong, wrong. Relying on beauty is clearly not a successful strategy.

So I have historical evidence, math, and data. In my book I lay out these points and tell the reader what conclusion I have drawn: Beauty is not a good guide to theory-development.

I then explain that this widespread use of scientifically questionable but productive methodology is symptomatic to the current organization of academic research, and a problem that’s not confined to physics.

Now, look, just because I cannot find a reason that beautiful theories are more promising than ugly ones doesn’t mean that relying on beauty cannot work. It may work, if we get lucky. Neither, for that matter, do I think that if we find a new law of nature it must be ugly. Chances are we will come to find a successful new idea beautiful simply because it works. But our sense of beauty changes and adapts, and therefore I do not think that using criteria of beauty from the past is a promising route to future progress.

Needless to say, making a case against a community of some thousands of the biggest brains on the planet has not been conducive to my self-confidence. But I have tried to find a scientific reason for the methods which my colleagues use in theory-development and could not. I wrote the book because I think it’s my responsibility as scientist to say clearly that I have come to the conclusion what goes on the foundations of physics is a waste of money, and that the public is being misinformed about the promise of this work.

I do not think that this will change the mind of people in the field. They have nothing to worry about because the way that academia is currently organized there is safety in numbers.

So, yes, I doubt myself. But I have written a whole book in which I explain why I have arrived at my conclusion. Rather than asking me, you should ask the people who work in these fields what makes them so certain that beautiful ideas are promising descriptions of nature.

Das hässliche Universum [book & travel update]

[See below for travel update in English]

Ab heute ist die Deutsche Übersetzung von „Lost in Math“ in Handel erhältlich unter dem Titel „Das hässliche Universum: Warum unsere Suche nach Schönheit die Physik in die Sackgasse führt.“ Wegen Kommunikationsproblemen mit dem Verlag habe ich die Deutsche Übersetzung nicht im voraus gesehen; tatsächlich habe ich das Buch selbst erst am Freitag erhalten. Ich hab’s bisher auch nicht gelesen. Lasst mich doch bitte wissen, was drin steht.

Ich werde auch in den nächsten Monaten noch Vorträge zum Thema „Mist in der Physik“ geben, sowohl in Deutsch als auch in Englisch. In der ersten Oktoberwoche bin ich in New Jersey (3. Oktober) und in Richmond, Kentucky (4. Oktober). In der zweiten Oktoberwoche bin ich auf der Buchmesse. Am 7. November gebe ich einen Vortrag am Planetarium „Am Insulaner“ in Berlin (und zwar nicht über das Buch sondern über Dunkle Materie). Am 8. November rede ich in der Urania, dann wieder über mein Buch. Am 29. November bin ich an der Chapman University, Los Angeles, und am 10. Dezember in Kaiserslautern. 

Ausser der Deutschen Übersetzung wird es ausserdem Übersetzungen geben in Chinesisch, Japanisch, Spanisch, Französisch, Russisch, Koreanisch, Italienisch und Rumänisch.

On October 3rd I’m New Jersey at the Stevens Institute for Technology. I can’t recall sending either title or abstract, but evidently I’m speaking about “How Physics Went Wrong.” On October 4th I’m in Richmond, Kentucky, for a lecture and book signing.

The week after this I’m in Frankfurt on the International Book Fair. On November 7th I’m speaking at the Berlin observatory “Am Insulaner” about dark matter (not about the book!) and on November 8th I’m at the Urania in Berlin, back to speaking about the book. On November 29th I’m at Chapman University LA, on December 10th in Kaiserslautern, Germany.

Besides German, the book will also be translated to Chinese, Japanese, Spanish, Italian, French, Russian, Korean, and Romanian. The English audiobook is supposed to appear in December. The British, you guessed it, still haven’t bought the rights.

For updates, please follow me on twitter or facebook.

Hawking temperature of black holes measured in fluid analogue

Fluid art by Vera de Gernier.

Stephen Hawking sadly passed away earlier this year, but his scientific legacy is well alive. The black hole information loss problem in particular still keeps physicists up at night. A new experiment might bring us a step closer to solving it.

Hawking notably was first to derive that black holes are not entirely black, but must emit what is now called “Hawking radiation”. The temperature of this radiation is inversely proportional to the mass of the black hole, a relation that has not been experimentally confirmed, so far.

Since the known black holes out there in the universe are very massive, their temperature is too small to be measurable. For this reason, physicists have begun to test Hawking’s predictions by simulating black holes in the laboratory using superfluids, that are fluids at a few degrees above absolute zero which have almost no viscosity. If a superfluid has regions where it flows faster than the speed of sound in the fluid, then sound waves cannot escape the fast-flowing part of the fluid. This is similar to how light cannot escape from a black hole.

The resemblance between the two cases more than just a verbal analogy, as was shown first by Bill Unruh in the 1980s: The mathematics of the two situations is identical. Therefore, physicists should be able to use the superfluid to measure the properties of the radiation predicted by Hawking because his calculation applies for these fluids too.

Checking Hawking’s predictions is what Jeff Steinhauer and his group at Technion in Israel are doing. They use a cloud of about 8000 Rubidium atoms at a temperature so low that the atoms form a Bose-Einstein Condensate and become superfluid. They then use lasers to confine the cloud and to change the number density in some part of it. Changing the number density will also change the speed of sound, and hence create a “sonic horizon”.

Number density (top) and velocity (bottom) of the
superfluid. The drop in the middle simulates the sonic horizon.
Figure 2 from arXiv:1809.00913

Using this method, Steinhauer’s group already showed some years ago that, yes, the fluid black hole emits radiation and this radiation is entangled across the horizon, as Hawking predicted. They measured this by recording density fluctuations in the cloud and then demonstrated that these fluctuations on opposite sides of the horizon are correlated.

Three weeks ago, Steinhauer’s group reported results from a new experiment in which they have now measured the temperature of the fluid black hole:

Observation of thermal Hawking radiation at the Hawking temperature in an analogue black hole
Juan Ramón Muñoz de Nova, Katrine Golubkov, Victor I. Kolobov, Jeff Steinhauer
arXiv:1809.00913 [gr-qc]

While the measurement is not very exact owing to the noise in the system, the result agrees with Hawking’s prediction, at least to the precision that the experiment allows to identify a temperature to begin with.

The authors also point out in the paper that they see no evidence of a black hole firewall. A black hole firewall would have been conflict with Hawking’s prediction according to which radiation from the black hole does not carry information.

In 2012, a group of researchers from UCSB argued that preserving information would necessitate a barrier of highly energetic particles – the “firewall” – at the black hole horizon. Their argument is wrong: I demonstrated that it is very well possible to preserve information without creating a firewall. The original proof contains a mistake. Nevertheless, the firewall issue has arguably attracted a lot of attention. The new experiment shows that the fluid black holes obey Hawking’s prediction, and no firewall appears.

Of course the fluid black hole does not reproduce the mathematics of real black hole entirely. Most importantly, the emission of radiation does not reduce the mass of the black hole, as it should if the radiation would carry away energy. This is the lack of “backreaction” (which this blog is named after). Note, however, that Hawking’s calculation also neglects backreaction. So for what the premises of Hawking’s calculation are concerned, fluid analogies should work fine.

The fluid analogies for black holes also differ from real black holes also because they have a different symmetry (it’s a linear system, a line basically, rather than a sphere) and they have a finite size. You may complain that’s a rather unrealistic case, and I would agree. But I think that makes them more, not less, interesting. That’s because these fluids really simulate lower-dimensional black holes in a box. And this is exactly the case for which string theorists claim they can calculate what happens using what’s known as the AdS/CFT correspondence.

Now, if the string theory calculations were correct then the information should leak out of the black hole. If you want to avoid a black hole firewall – because that hasn’t been observed – you need to break the entanglement across the horizon. But this isn’t compatible with the earlier results of Steinhauer’s group.

So, this result documents that black holes in a box do not behave like string theorists think they should. Of course the current measurement results have large uncertainties and will have to be independently reproduced before the case can be considered settled. But I have little doubt the results of the Steinhauer group will hold up. And I’ll be curious to hear what string theorists say about this.

Will you come to outer space? [Music Video]

I’ve done it again. This times I layered up to nine copies of myself. I have also squeaked out a high D, drawn a space ship that looks like a crossover of shark and saucer, and bought a new lipstick. But really the biggest improvement comes from me finally replacing my crappy camcorder with a mid-tier camera, which is why you can now enjoy all my wrinkles and pimples in unprecedented clarity.

Book Review: “Making Sense of Science” by Cornelia Dean

Making Sense of Science: Separating Substance from Spin
By Cornelia Dean
Belknap Press (March 13, 2017)

It’s not easy, being a science journalist. On one hand, science journalists rely on good relations with scientists. On the other hand, their next article may be critical of those scientists’ work. On the one hand they want to get the details right. On the other hand they have tight deadlines and an editor who scraps that one paragraph which took a full day to write. That’s four hands already, and I wasn’t even counting the hands they need to write.

Like most scientists, I used to think if I see a bogus headline it’s the writers’ fault. But the more science writers I got to know, the better my opinion of them has become. Unlike scientists, journalists strongly adhere to professional guidelines. They want to get things right and they want the reader to know the truth. If they get something wrong, the misinformation almost always came from scientists themselves.

The amount of misinformation about research in my own discipline is so high that no one who doesn’t work in the field has a chance to figure out what’s going on. Naturally this makes me wonder how much I can trust the news I read about other research areas. Cornelia Dean’s book “Making Sense of Science” tells the reader what to look out for.

Cornelia Dean has been a science writer for the New York Times for 30 years and she knows her job. The book begins with a general introduction, explaining what science is, how it works, and why it matters. She then moves on to conflicts of interest, checking sources, difficulties in assessing uncertainty and risk, scientific evidence in court, pitfalls of statistical analysis and analytical modeling, overconfident scientists, and misconduct.

The book is full with examples, proceeds swiftly, and reads well. The chapters end with bullet-point lists of items to recall which is helpful if you, like I, tend to sometimes switch books half through and then forgot what you read already.

“Making Sense of Science” also offers quick summaries of topics that are frequently front-page news: climate change, genetically modified crops, organic food, and cancer risk. While I have found those summaries well-done they seem somewhat randomly selected. I guess they are mostly there because the author is familiar with those topics.

The biggest shortcoming of the book is its lacking criticism of the scientific disciplines and of journalism itself. While the author acknowledges that she and her colleagues often operate under time pressure and shit happens, she doesn’t assess how much of a problem it is or which outlets are more likely to suffer from it. She also doesn’t mention that even scientists who do not take money from the industry have agendas to push, and that both the scientists as well as the writers profit from big headlines.

In summary, I have found the book to be very useful especially for what the discussion of risk-assessment is concerned, but it presents a suspiciously clean and sanitized picture of journalism.

I’m now older than my father has ever been

Old photo.

My father died a few weeks shy of his 42nd birthday. Went to bed one night, didn’t wake up the next morning. The death certificate says heart failure. Family gossip says it was a history of clinical depression that led to obesity and heavy drinking. They tell me I take after him. They may not be entirely wrong.

I’ve had troubles with my blood pressure ever since I was a teenager. I also have fainting episodes. One time I infamously passed out on a plane as it was approaching the runway. The pilot had to cancel take-off and call an ambulance. Paramedics carried me off the plane, wheeled me away, and then kept me in the hospital for a week. While noteworthy for the trouble I had getting hold of a bag that traveled without me, this was neither the first nor the last time my blood pressure suddenly gave in for no particular reason. I’ve been on the receiving end of epinephrine shots more than once.

Besides being a constant reminder that life is short, having a close relative who died young from heart failure has also added a high-risk stamp to my medical documents. This blessed me with countless extra exams thanks to which I now know exactly that some of my heart valves don’t properly close and the right chambers are enlarged. I also have a heart arrhythmia.

My doctors say I’m healthy, which really means they don’t know what’s wrong with me. Maybe I just have a fickle vagus nerve that pulls the plug every once in a while. Whatever the cause of my indisposition, I’ve spent most of my life in the awareness that I may not wake up tomorrow.

Today I woke up to find I reached the end of my subconscious life-expectation. In two weeks I’ll turn 42. I have checked off almost all boxes on my to-do list for life. Plant a tree, have a child, write a book. The only unchecked item is visiting New Zealand. But besides this, folks, I feel like I’m done here.

And what the heck do I do now with the rest of my life?

I didn’t really think about this until a few people asked what I plan on doing now that my book has been published. My current contract will run out next year, and then what? Will I write another book? Apply for another grant? Do something entirely different? To which my answer was, I have no idea. Ask me anything about quantum gravity and I may have a smarter reply.

I worry about the future, of course, constantly. Oh yes, I am a great worrier. But the future I worry about is not mine, it’s that of mankind. I’m just a blip in the symphony, a wheel in the machinery, a node in a giant information-processing network. Science, to me, is our collective attempt to accurately understand the laws of nature. It’s not about me, it’s not about you, it’s about us; it’s about whether the human race will last or whether we’re just too dumb to figure out how the world works.

Some days I am optimistic, but today I fear we are too dumb. Interactions of humans in large groups have consequences that we do not intuitively grasp, a failure that underlies not only twitter witch-hunts and viral fake news, but is also the reason why science works so inefficiently. I’m not sure we can fix this. Scientists have known for decades that the pressure to work on topics that produce results quickly and that are well-cited supports the widespread use of bad methodologies. But they do nothing about it except for the occasional halfhearted complaint.

Unsurprisingly, taxpayers who are financing research-bubbles with zero return on investment have taken cue. Some of them conclude, not entirely incorrectly, that much of the scientific enterprise is corrupt and conclusions cannot be trusted. If we carry on like this, science skeptics are bound to become more numerous. And that’s how it will end, the great human civilization: Not with a bang and not with a whimper, but with everyone yelling at each other that someone else was responsible to do something about it.

And if not even scientists can learn that social feedback influences their decisions, how can we expect the same of people who have not been trained to objectively evaluate evidence? Most scientists still believe their enterprise is governed by an invisible hand that will miraculously set things right should they go astray. They believe science self-corrects. Hahaha. It does not, of course. Someone, somewhere, has to actually do the correcting. Someone has to stand up and say: “This isn’t good science. We shouldn’t do this. Stop it.” Hence my book.

I used to think old people must hate all younger people because who wouldn’t rather be young. Now that I’ve reached a certain age myself I find the opposite is true. Not only am I relieved that my hyperactive brain is slowing down, making it much easier for me to focus on one thing at a time. I also love young people. They give me hope, hope that I lost in my own generation. Kids, I know you inherit a mess. I am sorry. Now hand me the wine.

But getting older also has an awkward side, which is that younger people ask me for advice. Worse, I get invited to speak about my experience as a woman in science. I am supposed to be a role model now, you see, I am supposed to encourage young women to follow my footsteps. If only I had something encouraging to say; if only those footsteps would lead elsewhere than nowhere. I decline these invitations. My advice, ladies, is to find your own way. And keep in mind, life is short.

Today’s advice to myself is to come up with an idea how I’ll make a living next year. But after two weeks of travel, 4 lectures and 2 interviews, with a paper and an essay and two blogposts squeezed in between, I am only tired. I have also quite possibly had a glass of wine too much.

Maybe I’ll make a plan tomorrow, first thing when I wake up. If I wake up.

Superfluid dark matter passes another check: strong gravitational lensing

[image: kamere.com]

Physicists still haven’t figured out what dark matter is made of, if anything. The idea that it’s made of particles that interact so weakly we haven’t yet measured them works well to explain some of the observational evidence. Notably the motions of galaxies bound to clusters and the features of the cosmic microwave background fit with theories of particle dark matter straight-forwardly. The galaxies themselves, not so much.

Astronomers have found that galaxies have regularities that are difficult to accommodate in theories of particle dark matter, for example the Tully-Fisher relation and the Radial Acceleration Relation. These observed patterns in the measurements don’t follow all that easily from the simple models of particle dark matter. Thrifty theorists have to invoke additional effects that are assigned to various astrophysical processes, notably stellar feedback. While these processes arguably exist, it isn’t clear that they actually act in galaxies in amounts necessary to explain the observations.

In the past 20 years or so, astrophysicists have improved computer simulations for galaxy formation until everything fit with the data, sometimes adapting the models to new observations. These computer simulations now contain about a dozen or so parameters (there are various simulations and not all of them list the parameters, so it’s hard to tell exactly) and the results agree well with observation.

But I find it somewhat hard to swallow that regularities that seem to be generic in galaxies follow from the theory only after much fiddling. Indeed, the very fact that it took astrophysicists so long to get galaxies right tells me that the patters in our observations are not generic to particle dark matter. It signals that the theories are missing something important.

One of the proposals for the missing piece has long been that gravity must be modified. But I, as many theorists, have not been particularly convinced by this idea, the reason being that it’s hard to change anything about Einstein’s theory of general relativity without running into conflict with the many high precision measurements that are in excellent agreement with the theory. On the other hand, modified gravity works dramatically well for galaxies and explains the observed regularities.

For a long time I’ve been rather agnostic about the whole issue. Then, three years ago, I read a paper in which Berezhiani and Khoury proposed that dark matter is a superfluid. The reason I even paid attention to this had nothing to do with dark matter; at the time I was working on superfluid condensates that can mimic gravitational effects and I was looking for inspiration. But I have since become a big fan of superfluid dark matter – because it makes so much sense!

You see, the superfluid that Berezhiani and Khoury proposed at isn’t just any superfluid. It has an interaction with normal matter and this interaction creates a force. This force looks like modified gravity. Indeed, I think, it is justified to call it modified gravity because the pull acting on galaxies it now no longer that of general relativity alone.

However, to get the stuff to condense, you need sufficient pressure, and the pressure comes from the gravitational attraction of the matter itself. Only if you have matter sufficiently clumped together will the fluid become a superfluid and generate the additional force. If the matter isn’t sufficiently clumped, or is just too warm, it’ll not condense.

This simple idea works remarkably well to explain why the observations that we assign to dark matter seem to fall into two categories: Those that fit better to particle dark matter and those that fit better to modified gravity. It’s because the dark matter is a fluid with two phases. In galaxies it’s condensed. In galaxy clusters, most of it isn’t condensed because the average potential isn’t deep enough. And in the early universe it’s too warm for condensation. On scales of the solar system, finally, it doesn’t make sense to even speak of the superfluid’s force, it would be like talking about van der Waals forces inside a proton. The theory just isn’t applicable there.

I was pretty excited about this until it occurred to me there’s a problem with this idea. The problem is that we know at least since the 170817 gravitational wave event with an optical counterpart that gravitational waves travel to good precision at the same speed as light. This by itself is easy to explain with the superfluid idea: Light just doesn’t interact with the superfluid. There could be various reason for this, but regardless of what the reason, it’s simple to accommodate this in the model.

This has the consequence however that light which travels through the superfluid region of galaxies will not respond to the bulk of what we usually refer to as dark matter. The superfluid does have mass and therefore also has a gravitational pull. Light notices that and will bend around it. But most of the dark matter that we infer from the motion of normal matter is a “phantom matter” or an “impostor field”. It’s really due to the additional force from the superfluid. And light will not respond to this.

As a result, the amount of dark matter inferred from lensing on galaxies should not match the amount of dark matter inferred from the motion of stars. My student, Tobias Mistele, and I hence sent out to have a look at strong gravitational lensing. We just completed our paper on this and it’s now available on the arXiv.

Strong lensing with superfluid dark matter
Sabine Hossenfelder, Tobias Mistele
arXiv:1809.00840 [astro-ph.GA]

It turns out that the observations from strong gravitational lenses are not hard to accommodate with superfluid dark matter. The reason is, loosely speaking, that the amount of superfluid can be adjusted or, somewhat more technically, that the additional fields require additional initial conditions and those allow us to always find solutions that fit the data.

This finding hence exemplifies why criticisms on modified gravity that insist on there only being one way to fit a galaxy are ill-founded. If you modify gravity by introducing additional fields – and that’s how almost all modifications of gravity work – the additional fields will have additional degrees of freedom and generally require additional initial conditions. There will hence usually be several solutions for galaxies. Indeed, some galaxies may by some statistical fluke not have attracted enough of the fluid for it to condense to begin with, though we have found no evidence of that.

We have been able to fit all lenses in our sample – 65 in total – except for one. The one outlier is a near-miss. It could be off for a variety of reasons, either because the measurement is imprecise, or because our model is overly simplistic. We assume, for example, that the distribution of the superfluid is spherically symmetric and time-independent, which almost certainly isn’t the case. Actually it’s remarkable it works at all.

Of course that doesn’t mean that the model is off the hook; it could still run into conflict with data that we haven’t checked so far. That observations based on the passage of light should show an apparent lack of dark matter might have other observable consequences, for example for gravitational redshift. Also, we have only looked at one particular sample of galaxies and those have no detailed data on the motion of stars. Galaxies for which there is more data will be more of a challenge to fit.

In summary: So far so good. Suggestions for what data to look at next are highly welcome.

---

Further reading: My Aeon essay “The Superfluid Universe”, and my recent SciAm article with Stacy McGaugh “Is dark matter real?”

Science has a problem, and we must talk about it

Bad stock photos of my job.
A physicist is excited to
have found a complicated way
of writing the number 2.

When Senator Rand Paul last year proposed that non-experts participate in review panels which award competitive research grants, my first reaction was to laugh. I have reviewed my share of research proposals, and I can tell you that without experience in the respective discipline you can’t even judge whether the proposal is feasible, not to mention promising.

I nodded to myself when I read that Jeffrey Mervis, reporting for Science Magazine, referred to Sen Paul’s bill as an “attack on peer review,” and Sean Gallagher from the American Association for the Advancement of Science called it “as blatant a political interference into the scientific process as it gets.”

But while Sen Paul’s cure is worse than the disease (and has, to date, luckily not passed the Senate), I am afraid his diagnosis is right. The current system is indeed “baking in bias,” as he put it, and it’s correct that “part of the problem is the old adage publish or perish.” And, yes, “We do have silly research going on.” Let me tell you.

For the past 15 years, I have worked in the foundations of physics, a field which has not seen progress for decades. What happened 40 years ago is that theorists in my discipline became convinced the laws of nature must be mathematically beautiful in specific ways. By these standards, which are still used today, a good theory should be simple, and have symmetries, and it should not have numbers that are much larger or smaller than one, the latter referred to as “naturalness.”

Based on such arguments from beauty, they predicted that protons should be able to decay. Experiments have looked for this since the 1980s, but so far not a single proton has been caught in the act. This has ruled out many symmetry-based theories. But it is easy to amend these theories so that they evade experimental constraints, hence papers continue to be written about them.

Theorists also predicted that we should be able to detect dark matter particles, such as axions or weakly interacting massive particles (WIMPs). These hypothetical particles have been searched for in dozens of experiments with increasing sensitivity – unsuccessfully. In reaction, theorists now write papers about hypothetical particles that are even harder to detect.

The same criteria of symmetry and naturalness led many particle physicists to believe that the Large Hadron Collider (LHC) should see new particles besides the Higgs-boson, for example supersymmetric particles or dark matter candidates. But none were seen. The LHC data is not yet fully analyzed, but it’s clear already that if something hides in the data, it’s not what particle physicists thought it would be.

You can read the full story in my book “Lost in Math: How Beauty Leads Physics Astray.”

Most of my colleagues blame the lack of progress on the maturity of the field. Our theories work extremely well already, so testing new ideas is difficult, not to mention expensive. The easy things have been done, they say, we must expect a slowdown.

True. But this doesn’t explain the stunning profusion of blundered predictions. It’s not like we predicted one particle that wasn’t there. We predicted hundreds of particles, and fields, and new symmetries, and tiny black holes, and extra-dimensions (in various shapes, and sizes, and widths), none of which were there.

This production of fantastic ideas has been going on for so long it has become accepted procedure. In the foundations of physics we now have a generation of researchers who make career studying things that probably don’t exist. And instead of discarding methods that don’t work, they write increasingly more papers of decreasing relevance. Instead of developing theories that better describe observations, they develop theories that are harder to falsify. Instead of taking risks, they stick to ideas that are popular with their peers.

Of course I am not the first to figure beauty doesn’t equal truth. Indeed, most physicists would surely agree that using aesthetic criteria to select theories is not good scientific practice. They do it anyway. Because all their colleagues do it. And because they all do it, this research will get cited, will get published, and then it will be approved by review panels which take citations and publications as a measure of quality. “Baked in bias” is a pretty good summary.

This acceptance of bad scientific practice to the benefit of productivity is certainly not specific to my discipline. Look for example at psychologists whose shaky statistical analyses now make headlines. The most prominent victim is Amy Cuddy’s “Power Posing” hypothesis, but the problem has been known for a long time. As Jessica Utts, President of the American Statistical Association, pointed out in 2016 “statisticians and other scientists have been writing on the topic for decades.”

Commenting on this “False Positive Psychology,” Joseph Simmons, Leif Nelson, and Uri Simonsohn, wrote “Everyone knew it was wrong.” But I don’t think so. Not only have I myself spoken to psychologists who thought their methods were fine because it’s what they were taught to do. It also doesn’t make sense. Had psychologists known their results were likely statistical artifacts, they’d also have known other groups could use the same methods to refute their results.

Or look at Brian Wansink, the Cornell Professor with the bottomless soup bowl experiment. He recently drew unwanted attention to himself with a blogpost in which he advised a student to try harder getting results out of data because it “cost us a lot of time and our own money to collect.” Had Wansink been aware that massaging data until it delivers is not sound statistical procedure, he’d probably not have blogged about it.

What is going on here? In two words: “communal reinforcement,” more commonly known as group-think. The headlines may say “research shows” but it doesn’t: researchers show. Scientists, like all of us, are affected by their peers’ opinions. If everyone does it, they think it’s probably ok. They also like to be liked, not to mention that they like having an income. This biases their judgement, but the current organization of the academic system does not offer protection. Instead, it makes the problem worse by rewarding those who work on popular topics.

This problem cannot be solved by appointing non-experts to review panels – that merely creates incentives for research that’s easy to comprehend. We can impose controls on statistical analyses, and enforce requirements for reproducibility, and propose better criteria for theory development, but this is curing the symptoms, not the disease. What we need is to finally recognize that scientists are human, and that we don’t do enough to protect scientists’ ability to make objective judgements.

We will never get rid of social biases entirely, but simple changes would help. For starters, every scientist should know how being part of a group can affect their opinion. Grants should not be awarded based on popularity. Researchers who leave fields of declining promise need encouragement, not punishment because their productivity may dwindle while they retrain. And we should generally require scientists to name both advantages and shortcomings of their hypotheses.

Most importantly, we should not sweep the problem under the rug. As science denialists become louder both in America and in Europe, many of my colleagues publicly cheer for their profession. I approve. On the flipside, they want no public discussion about our problems because they are afraid of funding cuts. I disagree. The problems with the current organization of research are obvious – so obvious even Sen Paul sees them. It is pretending the problem doesn’t exist, not acknowledging it and looking for a solution, that breeds mistrust.

Tl;dr: Academic freedom risks becoming a farce if we continue to reward researchers for working on what is popular. Denying the problem doesn’t help.

Dear Dr B: What does the universe expand into?

“When the universe expands, into what is it expanding? In what medium is it expanding? Is the universe like a bubble in a higher dimension something?
[Anonymous], Indiana, USA”

[image: luftballon-profi.at]

This is a very good question and one, I should add, I get frequently. It is, I believe, to no small part caused by the common illustrations of a curved universe: it’s a rubber-sheet with a bowling-ball on it, it’s an inflating balloon, or – in the rarer case that someone tries to illustrate negative curvature, it’s a potato chip (because really I have no idea what a saddle looks like).

But in each of these cases what the illustration actually shows is a two-dimensional surface embedded in a non-curved (“flat”) three-dimensional space. That’s good because you can draw it, but it’s bad because it raises the impression that to speak of curvature you need to put the surface into a larger space. That, however, isn’t so: Curvature is a property of the surface itself.

To get an idea of how this works, consider the simplest example of a curved surface, a ball. On the ball’s surface the angles of triangles will not add up to 180 degrees. You can calculate the curvature from measuring all the angles in all triangles that you could draw onto the ball. This is a measurement which can be done entirely on the surface itself. Or by ants crawling on the surface, if you wish, to use another common analogy.

Curvature, hence, is an intrinsic property of the surface – you do not need the embedding space to define it and to measure it. Also note that the curvature is a local property; it can change from one place to the next, just that a ball has constant curvature.

General relativity uses the same notion of local, intrinsic curvature, just that in this case we aren’t dealing with two dimensions of space and ants crawling on it, but with three dimensions of space, one dimension of time, and humans crawling around in it. So the math is more complicated and all the properties of space-time are collected in something called the curvature-tensor, but that is still an entirely internal construct. We can measure it by tracking the motion of particles, and it’s this curvature that creates the effect we usually refer to as gravity.

Now, what cosmologists mean when they speak of the expansion of the universe is a trend of certain measurement results that, using Einstein’s equations, can be interpreted as being due to an increasing distance between galaxies. Again, this expansion is an entirely internal notion. It is defined and measured in our universe. You do not have to embed this four dimensional space-time into anything else to quantify it. You do not need a medium and you do not need a larger space. Einstein’s theory is entirely self-contained with a four-dimensional, internally curved space-time.

While you do not have to embed space-time in a higher-dimensional flat space, you can. Indeed it can be mathematically proved that you can embed any curved four dimensional space-time into a ten dimensional flat space-time. The reason physicists don’t normally do this is that these additional dimensions are superfluous and they don’t aid the math either.

Black hole embedding diagram.
Only the surface itself has physical meaning.
The surrounding space is for visual purposes.
[Image source: Quora]  

We do, however, on occasion use what is called an “embedding diagram”, which
can be useful to visualize the extrinsic curvature of certain slices of space-time. This is, for example, what gives rise to the idea that when matter collapses to a black hole, space develops a long throat with a bubble that eventually pinches off. But please keep in mind that these are merely visual aids. They have their uses as such, but one has to be very careful in interpreting them because they depend on the chosen embedding.

Now you ask what does the universe expand into? It doesn’t expand into anything, it just expands. That the universe expands is a statement about what happens inside the universe, supported by measurements inside the universe. It’s an entirely internal notion that does not require us to speak of an outside of the universe or a medium into which it is embedded.

Thanks for an interesting question!

Away Note

I’ll be traveling the next two weeks. First I am in Santa Fe, giving both a colloquium and a public lecture, and then I am in Oslo, giving two talks, one at the Kavli Symposium and one at the public library.

Later in September I’ll be in London at HowTheLightGetsIn. The first week of October, I’ll be in NYC and afterwards in Lexington, Kentucky. The week after that I’ll be at the international book fair in Frankfurt, and in early November I’ll be in Berlin (details to come).

I have been advised that giving talks about my book is private business, so please note that the next two weeks I am officially on vacation for the first time since 2008 (which was our two-years-late honeymoon trip).

Vacation or not, it is foreseeable that I will be offline for extended periods, so please prepare for a slow time on this blog.

Roger Penrose still looks for evidence of universal rebirth

Roger Penrose really should have won a Nobel Prize for something. Though I’m not sure for what. Maybe Penrose-tilings. Or Penrose diagrams. Or the Penrose process. Or the Penrose-Hawking singularity theorems. Or maybe just because there are so many things named after Penrose.

And at 87 years he’s still at it. Penrose has a reputation for saying rude things about string theory, has his own interpretation of quantum mechanics, and he doesn’t like inflation, the idea that the early universe underwent a rapid phase of exponential expansion. Instead, he has his own theory called “conformal cyclic cosmology” (CCC).

According to Penrose’s conformal cyclic cosmology, the universe goes through an infinite series of “aeons,” each of which starts with a phase resembling a big bang, then forming galactic structures as usual, then cooling down as stars die. In the end the only thing that’s left are evaporating black holes and thinly dispersed radiation. Penrose then conjectures a slight change to particle physics that allows him to attach the end of one aeon to the beginning of another, and everything starts anew with the next bang.

This match between one aeon’s end and another’s beginning necessitates the introduction of a new field – the “erebon” – that makes up dark matter, and that decays throughout the coming aeon. We previously met the erobons because Penrose argued their decay should create noise in gravitational wave interferometers. (Not sure what happened to that.)

If Penrose’s CCC hypothesis is correct, we should also be able to see some left-over information from the previous aeon in the cosmic microwave background around us. To that end, Penrose has previously looked for low-variance rings in the CMB, that he argued should be caused by collisions between supermassive black holes in the aeon prior to ours. The search for that, however, turned out to be inconclusive. In a recent paper with Daniel An and Krzysztof Meissner he has now suggested to look instead for a different signal.

The new signal that Penrose et al are looking for are points in the CMB at the places where in the previous aeon supermassive black holes evaporated. He and collaborators called these “Hawking Points” in memory of the late Stephen Hawking. The idea is that when you glue together the end of the previous aeon with the beginning of ours, you squeeze together the radiation emitted by those black holes and that makes a blurry point at which the CMB temperature is slightly increased.

Penrose estimates the total number of such Hawking Points which should be in the total cosmic microwave background is about a million. The analysis in the paper, covering about 1/3 of the sky, finds tentative evidence for about 20. What’s with the rest remains somewhat unclear, presumably too weak to be observed.

They look for these features by generating fake “normal” CMBs, following standard procedure, and then trying to find Hawking Points in these simulations. They have now done about 5000 of such simulations, but none of them, they claim, has features similar to the actually observed CMB. This makes their detection highly statistically significant, with a chance of less than 1/5000 that the Hawking Points which they find in the CMB are due to random chance.

In the paper, the authors also address an issue that I am guessing was raised by someone else somewhere, which is that in CCC there shouldn’t be a CMB polarization signal like the one BICEP was looking for. This signal still hasn’t been confirmed, but Penrose et al claim pre-emptively that in CCC there should also be a polarization, and it should go with the Hawking Peaks because:

“primordial magnetic fields might arise in CCC as coming [...] from galactic clusters in the previous aeon […] and such primordial magnetic fields could certainly produce B-modes […] On the basis that such a galactic cluster ought to have contained a supermassive black hole which could well have swallowed several others, we might expect concentric rings centred on that location”

Quite a collection of mights and coulds and oughts.

Like Penrose, I am not a big fan of inflation, but I don’t find conformal cyclic cosmology well-motivated either. Penrose simply postulates that the known particles have a so-far unobserved property (so the physics becomes asymptotically conformally invariant) because he wants to get rid of all gravitational degrees of freedom. I don’t see what’s wrong with that, but I also can’t see any good reason for why that should be correct. Furthermore, I can’t figure out what happens with the initial conditions or the past hypothesis, which leaves me feeling somewhat uneasy.

But really I’m just a cranky ex-particle-physicist with an identity crisis, so I’ll leave the last words to Penrose himself:

“Of course, the theory is “crazy”, but I strongly believe (in view of observational facts that seem to be coming to light) that we have to take it seriously.”

Guest Post: Tam Hunt questions Carlo Rovelli about the Nature of Time

Tam Hunt.

[Tam Hunt, photo on the right, is a renewable energy lawyer in Hawaii and an “affiliate” in the Department of Psychological and Brain Sciences at UC Santa Barbara. (Scare quotes his, not mine, make of this what you wish.) He has also published some papers about philosophy and likes to interview physicists. The below is an email interview he conducted with Carlo Rovelli about the Nature of Time. Carlo Rovelli is director of the quantum gravity group at Marseille University in France and author of several popular science books.]

TH:Let me start by asking why discussions about the nature of time should matter to the layperson?

CR: There is no reason it “should” matter. People have the right to be ignorant, if they wish to be. But many people prefer not to be ignorant. Should the fact that the Earth is not flat matter for normal people? Well, the fact that Earth is a sphere does not matter during most of our daily lives, but we like to know.

TH: Are there real-world impacts with respect to the nature of time that we should be concerned with?

CR: There is already technology that has been strongly impacted by the strange nature of time: the GPS in our cars and telephones, for instance.

Carlo Rovelli.

TH: What inspired you to make physics and the examination of the nature of time a major focus of your life's work?

CR: My work on quantum gravity has brought me to study time. It turns out that in order to solve the problem of quantum gravity, namely understanding the quantum aspects of gravity, we have to reconsider the nature of space and time. But I have always been curious about the elementary structure of reality, since my adolescence. So, I have probably been fascinated by the problem of quantum gravity precisely because it required rethinking the nature of space and time.

TH: Your work and your new book continue and extend the view that the apparent passage of time is largely an illusion because there is no passage of time at the fundamental level of reality. Your new book is beautifully and clearly written -- even lyrical at times -- and you argue that the world described by modern physics is a “windswept landscape almost devoid of all trace of temporality.” (P. 11). How does this view of time pass the “common sense” test since everywhere we look in our normal waking consciousness there is nothing but a passage of time from moment to moment to moment?

CR: Thanks. No, I do not argue that the passage of time is an illusion. “Illusion” may be a misleading word. It makes it seem that there is something wrong about our common-sense views on time. There is nothing wrong with it. What is wrong is to think that this view must hold for the entire universe, or that it is valid at all scales and in all situations. It is like the flat Earth: Earth is almost perfectly flat at the scale of most of our daily life, so, there is nothing wrong in considering it flat when we build a house, say. But on larger scales the Earth just happens not to be flat. So with time: as soon as we look a bit farther than our myopic eyes allow, we see that it works differently from what we thought.

This view passes the “common sense” test in the same way in which the fact that the Earth rotates passes the “common sense” view that the Earth does not move and the Sun moves down at sunset. That is, “common sense” is often wrong. What we experience in our “normal waking consciousness” is not the elementary structure of reality: it is a complex construction that depends on the physics of the world but also on the functioning of our brain. We have difficulty in disentangling one from the other.

“Time” is an example of this confusion; we mistake for an elementary fact about physics what is really a complex construct due to our brain. It is a bit like colors: we see the world in combinations of three basic colors. If we question physics as to why the colors we experience are combination of three basic colors, we do not find any answer. The explanation is not in physics, it is in biology: we have three kinds of receptors in our eyes, sensible to three and only three frequency windows, out of the infinite possibilities. If we think that the three-dimensional structure of colors is a feature of reality external to us, we confuse ourselves.

There is something similar with time. Our “common sense” feeling of the passage of time is more about ourselves than the physical nature of the external world. It regards both, of course, but in a complex, stratified manner. Common sense should not be taken at face value, if we want to understand the world.

TH: But is the flat Earth example, or similar examples of perspectival truth, applicable here? It seems to me that this kind of perspectival view of truth (that the Earth seems flat at the human scale but is clearly spherical when we zoom out to a larger perspective) isn’t the case with the nature of time because no matter what scale/perspective we use to examine time there is always a progression of time from now to now to now. When we look at the astronomical scale there is always a progression of time. When we look at the microscopic scale there is always a progression of time.

CR: What indicates that our intuition of time is wrong is not microscopes or telescopes. It’s clocks. Just take two identical clocks indicating the same time and move them around. When they meet again, if they are sufficiently precise, they do not indicate the same time anymore. This demolishes a piece of our intuition of time: time does not pass at the same “rate” for all the clocks. Other aspects of our common-sense intuition of time are demolished by other physics observations.

TH: In the quote from your book I mentioned above, what are the “traces” of temporality that are still left over in the windswept landscape “almost devoid of all traces of temporality,” a “world without time,” that has been created by modern physics?

CR: Change. It is important not to confuse “time” and “change.” We tend to confuse these two important notions because in our experience we can merge them: we can order all the change we experience along a universal one-dimensional oriented line that we call “time.” But change is far more general than time. We can have “change,” namely “happenings,” without any possibility of ordering sequences of these happenings along a single time variable.

There is a mistaken idea that it is impossible to describe or to conceive change unless there exists a single flowing time variable. But this is wrong. The world is change, but it is not [fundamentally] ordered along a single timeline. Often people fall into the mistake that a world without time is a world without change: a sort of frozen eternal immobility. It is in fact the opposite: a frozen eternal immobility would be a world where nothing changes and time passes. Reality is the contrary: change is ubiquitous but if we try to order change by labeling happenings with a time variable, we find that, contrary to intuition, we can do this only locally, not globally.

TH: Isn’t there a contradiction in your language when you suggest that the common-sense notion of the passage of time, at the human level, is not actually an illusion (just a part of the larger whole), but that in actuality we live in a “world without time”? That is, if time is fundamentally an illusion isn’t it still an illusion at the human scale?

CR: What I say is not “we live in a world without time.” What I say is “we live in a world without time at the fundamental level.” There is no time in the basic laws of physics. This does not imply that there is no time in our daily life. There are no cats in the fundamental equations of the world, but there are cats in my neighborhood. Nice ones. The mistake is not using the notion of time [at our human scale]. It is to assume that this notion is universal, that it is a basic structure of reality. There are no micro-cats at the Planck scale, and there is no time at the Planck scale.

TH: You argue that time emerges: “Somehow, our time must emerge around us, at least for us and at our scale.” As such, how do you reconcile the notion of emergence of time itself with the fact that the definition of emergence necessarily includes change over time? That is, how is it coherent to argue that time itself emerges over time?

CR: The notion of “emergence” does not always include change over time. For instance we say that if you look at how humans are distributed on the surface of the Earth, there are some general patterns that “emerge” by looking at a very large scale. You do not see them at the small scale, you see them looking at the large scale. Here “emergence” is related to the scale at which something is described. Many concepts we use in science emerge at some scale. They have no meaning at smaller scales.

TH: But this kind of scale emergence is a function solely of an outside conscious observer, in time, making an observation (in time) after contemplating new data. So aren’t we still confronted with the problem of explaining how time emerges in time?

CR: There is no external observer in the universe, but there are internal observers that interact with one another. In the course of this interaction, the temporal structure that they ascribe to the rest may differ. I think that you are constantly misunderstanding the argument of my book, because you are not paying attention to the main point: the book does not deny the reality of change: it simply confronts the fact that the full complexity of the time of our experience does not extend to the entire reality. Read the book again!

TH: I agree that common sense can be a faulty guide to the nature of reality but isn’t there also a risk of unmooring ourselves from empiricism when we allow largely mathematical arguments to dictate our views on the nature of reality?

CR: It is not “largely mathematical arguments” that tell us that our common sense idea of time is wrong. It is simple brute facts. Just separate two accurate clocks and bring them back together and this shows that our intuition about time is wrong. When the GPS global positioning system was first mounted, some people doubted the “delicate mathematical arguments” indicating that time on the GPS satellites runs faster than at sea level: the result was that the GPS did not work [when it was first set up]. A brute fact. We have direct facts of evidence against the common-sense notion of time.

Empiricism does not mean taking what we see with the naked eye as the ultimate reality. If it was so, we would not believe that there are atoms or galaxies, or the planet Uranus. Empiricism is to take seriously the delicate experience we gather with accurate instruments. The idea that we risk unmooring “ourselves from empiricism when we allow largely mathematical arguments to dictate our views on the nature of reality” is the same argument used against Galileo when we observed with the telescope, or used by Mach to argue against the real existence of atoms. Empiricism is to base our knowledge of reality on experience, and experience includes looking into a telescope, looking into an electronic microscope, where we actually can see the atoms, and reading accurate clocks. That is, using instruments.

TH: I’m using “empiricism” a little differently than you are here; I’m using the term to refer to all methods of data gathering, whether directly with our senses or indirectly with instruments (but still mediated through our senses because ultimately all data comes through our human senses). So what I’m getting at is that human direct experience, and the constant passage of time in our experience, is as much data as are data from experiments like the 1971 Hafele-Keating experiment using clocks traveling opposite directions on airplanes circling the globe. And we cannot discount either category of experience. Does this clarification of “empiricism” change your response at all?

CR: We do not discount any category of experience. There is no contradiction between the complex structure of time and our simple human experience of it. The contradiction appears only if we extrapolate our experience and assume it captures a universal aspect of reality. In our daily experience, the Earth is flat and we take it to be flat when we build a house or plan a city; there is no contradiction between this and the round Earth. The contradiction comes if we extrapolate our common-sense view of the flat Earth beyond the small region where it works well. So, we are not discounting our daily experience of time, we are just understanding that it is an approximation to a more complicated reality.

TH: There have been, since Lorentz developed his version of relativity, which Einstein adapted into his Special Theory of Relativity in 1905, interpretations of relativity that don’t render time an illusion. Isn’t the Lorentz interpretation still valid since it’s empirically equivalent to Special Relativity?

CR: I think you refer here to the so called neo-Lorentzian interpretations of Special Relativity. There is a similar case in the history of science: after Copernicus developed his systems in which all planets turn around the Sun and the Earth moves, there were objections similar to those you mention: “the delicate mathematical arguments” of Copernicus cannot weight as much as our direct experience that the Earth does not move.

So, Tycho Brahe developed his own system, where the Earth is at the center of the universe and does not move, the Sun goes around the Earth and all the other planets rotate around the Sun. Nice, but totally useless for science and for understanding the world: a contorted and useless attempt to save the common sense-view of a motionless Earth, in the face of overwhelming opposite evidence.

If Tycho had his way, science would not have developed. The neo-Lorentzian interpretations of Special Relativity do the same. They hang on to the wrong extrapolation of a piece of common sense.

There is an even better example: the Moon and the Sun in the sky are clearly small. When in antiquity astronomers like Aristarchus come out with an estimate of the size of the Moon and the Sun, it was a surprise, because it turned out that the Moon is big and the Sun even bigger than the Earth itself. This was definitely the result of “largely mathematical arguments.” Indeed it was a delicate calculation using geometry, based on angles under which we see these objects. Would you say that the fact that the Sun is larger than the Earth should not be believed because it is based on a “largely mathematical argument“ and contradicts our direct experience?

TH: But in terms of alternative interpretations of the Lorentz transformations, shouldn’t we view these alternatives, if they’re empirically equivalent as they are, in the same light as the various different interpretations of quantum theory (Copenhagen, Many Worlds, Bohmian, etc.)? All physics theories have two elements: 1) the mathematical formalisms; 2) an interpretive structure that maps those formalisms onto the real world. In the case of alternatives to Special Relativity, some have argued that we don’t need to adopt the Einstein interpretation of the formalisms (the Lorentz transformations) in order to use those formalisms. And since Lorentz’s version of relativity and Einstein’s Special Relativity are thought to be empirically equivalent, doesn’t a choice between these interpretations come down to a question of aesthetics and other considerations like explanatory power?

CR: It is not just a question of aesthetics, because science is not static, it is dynamic. Science is not just models. It is a true continuous process of better understanding reality. A better version of a theory is fertile: it takes us ahead; a bad version takes no part. The Lorentzian interpretation of special relativity assumes the existence of entities that are unobservable and undetectable (a preferred frame). It is contorted, implausible, and in fact it has been very sterile.

On the other hand, realizing that the geometrical structure of spacetime is altered has led to general relativity, to the prediction of black holes, gravitational waves, the expansion of the universe. Science is not just mathematical models and numerical predictions: it is developing increasingly effective conceptual tools for making sense and better understanding the world. When Copernicus, Galileo and Newton realized that the Earth is a celestial body like the ones we see in the sky, they did not just give us a better mathematical model for more accurate predictions: they understood that man can walk on the moon. And man did.

TH: But doesn’t the “inertial frame” that is the core of Einstein’s Special Relativity (instead of Lorentz’s preferred frame) constitute worse “sins”? As Einstein himself states in his 1938 book The Evolution of Physics, inertial frames don’t actually exist because there are always interfering forces; moreover, inertial frames are defined tautologically (p. 221). Einstein’s solution, once he accepted these issues, was to create the general theory of relativity and avoid focusing on fictional inertial frames. We also have the cosmic frame formed by the Cosmic Microwave Background that is a very good candidate for a universal preferred frame now, which wasn’t known in Einstein’s time. When we add the numerous difficulties that the Einstein view of time results in (stemming from special not general relativity), the problems in explaining the human experience of time, etc., might it be the case that the sins of Lorentzian relativity are outweighed by Special Relativity’s sins?

CR: I do not know what you are talking about. Special Relativity works perfectly well, is very heavily empirically supported, there are no contradictions with it in its domain of validity, and has no internal inconsistency whatsoever. If you cannot digest it, you should simply study more physics.

TH: You argue that “the temporal structure of the world is not that of presentism,” (p. 145) but isn’t there still substantial space in the scientific and philosophical debate for “presentism,” given different possible interpretations of the relevant data?

CR: There is a tiny minority of thinkers who try to hold on to presentism, in the contemporary debate about time. I myself think that presentism is de facto dead.

TH: I’m surprised you state this degree of certainty here when in your book you acknowledge that the nature of time is one of physics’ last remaining large questions. Andrew Jaffe, in a review of your book for Nature, writes that the issues you discuss “are very much alive in modern physics.”

CR: The debate on the nature of time is very much alive, but it is not a single debate about a single issue, it is a constellation of different issues, and presentism is just a rather small side of it. Examples are the question of the source of the low initial entropy, the source of our sense of flow, the relation between causality and entropy. The non-viability of presentism is accepted by almost all relativists.

TH: Physicist Lee Smolin (another loop quantum gravity theorist, as you know) argued views quite different than yours, in his book, Time Reborn, for example. In an interview with Smolin I did in 2013, he stated that “the experience we have of time flowing from moment into moment is not an illusion but one of the deepest clues we have as to the nature of reality.” Is Smolin part of the tiny minority you refer to?

CR: Yes, he is. Lee Smolin is a dear friend for me. We have collaborated repeatedly in the past. He is a very creative scientists and I have much respect of his ideas. But we disagree on this. And he is definitely in the minority on this issue.

TH: I’ve also been influenced by Nobel Prize winner Ilya Prigogine’s work and particularly his 1997 book, The End of Certainty: Time, Chaos and the New Laws of Nature, which opposes the eternalist view of time as well as reversibility in physics. Prigogine states in his book that reversible physics and the notion of time as an illusion are “impossible for me to accept” He argues that whereas many theories of modern physics include a reversible t term, this is an empirical mistake because in reality the vast majority of physical processes are irreversible. How do you respond to Prigogine and his colleagues’ arguments that physics theories should be modified to include irreversibility?

CR: That he is wrong, if this is what he writes. There is no contradiction between the reversibility of the laws that we have and the irreversibility of the phenomena. All phenomena we see follow the laws we have, as far as we can see. The surprise is that these laws allow also other phenomena that we do not see. So, something may be missing in our understanding --and I discuss this at length in my book-- but something missing does not mean something wrong.

I do not share the common “block universe” eternalist view of time either. What I argue in the book is that the presentist versus eternalist alternative is a fake alternative. The universe is neither evolving in a single time, nor static without change. Temporality is just more complex than either of these naïve alternatives.

TH: You argue that “the world is made of events, not things” in part II of your book. Alfred North Whitehead also made events a fundamental feature of his ontology, and I’m partial to his “process philosophy.” If events—happenings in time—are the fundamental “atoms” of spacetime (as Whitehead argues), shouldn’t this accentuate the importance of the passage of time in our ontology, rather than downgrade it as you seem to otherwise suggest?

CR: “Time” is a stratified notion. The existence of change, by itself, does not imply that there is a unique global time in the universe. Happenings reveal change, and change is ubiquitous, but nothing states that this change should be organized along the single universal uniform flow that we commonly call time. The question of the nature of time cannot be reduced to a simple “time is real”, “time is not real.” It is the effort of understanding the many different layers giving rise to the complex phenomenon that we call the passage of time.