Black Holes and Quantum Gravity

Aurélien Barrau
ALTHOUGH BLACK HOLES were first imagined in the late eighteenth century, it was not until Karl Schwarzchild devised a solution to Einstein’s field equations in 1915 that they were accurately described. Despite Schwarzchild’s pioneering work, black holes were still widely thought to be purely theoretical, and so devoid of physical meaning. This view persisted until recent decades, an accumulation of observational evidence removing any lingering doubts about their existence. Beyond their obvious interest as astrophysical phenomena, black holes may, in time, come to be considered a laboratory for new physics. It is conceivable that black holes could be used to study quantum gravity; and a complete and consistent theory of quantum gravity remains the most elusive goal in theoretical physics…

Reflections on a Revolution

John Iliopoulos, reply by Sheldon Lee Glashow
In response to “The Yang–Mills Model” (Vol. 5, No. 2).

To the editors:

It is an honor and a pleasure to comment on Sheldon Lee Glashow’s magisterial essay on the Yang–Mills model. His essay is all the more special for describing one of the greatest chapters in the history of physics, the story of a beautiful mathematical concept transformed into a theory for all seasons, the Standard Model of particle physics. It was of course Glashow who played a central role in the electroweak part of the story by coming up with his masterpiece, the SU(2) × U(1) gauge theory. And with the added masterstroke from Steven Weinberg, his high-school buddy, as he reminds us, we now have not just a theory of all relevant particle interactions at today’s energies, but also a theory of the origin of the masses of elementary particles. Simply, mass turned into a dynamical variable, the knowledge of which enables us to unambiguously predict the associated Higgs boson decay into the relevant particles. And we know with certainty that the W and Z bosons, predicted by Glashow, and the third-generation fermions receive their masses from the Higgs mechanism….

The Sun Diver: Combining solar sails with the Oberth effect

Coryn A.L. Bailer-Jones
A highly reflective sail provides a way to propel a spacecraft out of the solar system using solar radiation pressure. The closer the spacecraft is to the Sun when it starts its outward journey, the larger the radiation pressure and so the larger the final velocity. For a spacecraft starting on the Earth’s orbit, closer proximity can be achieved via a retrograde impulse from a rocket engine. The sail is then deployed at the closest approach to the Sun. Employing the so-called Oberth effect, a second, prograde, impulse at closest approach will raise the final velocity further. Here I investigate how a fixed total impulse (Δv) can best be distributed in this procedure to maximize the sail’s velocity at infinity. Once Δv exceeds a threshold that depends on the lightness number of the sail (a measure of its sun-induced acceleration), the best strategy is to use all of the Δv in the retrograde impulse to dive as close as possible to the Sun. Below the threshold the best strategy is to use all of the Δv in the prograde impulse and thus not to dive at all. Although larger velocities can be achieved with multi-stage impulsive transfers, this study shows some interesting and perhaps counter-intuitive consequences of combining impulses with solar sails.

Click to access 2009.12659.pdf

Dark Energy: is it ‘just’ Einstein’s Cosmological Constant Λ?

Ofer Lahav
The Cosmological Constant Lambda, a concept introduced by Einstein in 1917, has been with us ever since in different variants and incarnations, including the broader concept of Dark Energy. Current observations are consistent with a value of Lambda corresponding to about present-epoch 70% of the critical density of the Universe. This is causing the speeding up (acceleration) of the expansion of the Universe over the past 6 billion years, a discovery recognised by the 2011 Nobel Prize in Physics. Coupled with the flatness of the Universe and the amount of 30% matter (5% baryonic and 25% Cold Dark Matter), this forms the so-called Lambda-CDM standard model, which has survived many observational tests over about 30 years. However, there are currently indications of inconsistencies (`tensions’ ) within Lambda-CDM on different values of the Hubble Constant and the clumpiness factor. Also, time variation of Dark Energy and slight deviations from General Relativity are not ruled out yet. Several grand projects are underway to test Lambda-CDM further and to estimate the cosmological parameters to sub-percent level. If Lambda-CDM will remain the standard model, then the ball is back in the theoreticians’ court, to explain the physical meaning of Lambda. Is Lambda an alteration to the geometry of the Universe, or the energy of the vacuum? Or maybe it is something different, that manifests a yet unknown higher-level theory?


Click to access 2009.10177.pdf

The thermodynamics of clocks

G J Milburn
All clocks, classical or quantum, are open non equilibrium irreversible systems subject to the constraints of thermodynamics. Using examples I show that these constraints necessarily limit the performance of clocks and that good clocks require large energy dissipation. For periodic clocks, operating on a limit cycle, this is a consequence of phase diffusion. It is also true for non periodic clocks (for example, radio carbon dating) but due to telegraph noise not to phase diffusion. In this case a key role is played by accurate measurements that decrease entropy, thereby raising the free energy of the clock, and requires access to a low entropy reservoir. In the quantum case, for which thermal noise is replaced by quantum noise (spontaneous emission or tunnelling), measurement plays an essential role for both periodic and non periodic clocks. The paper concludes with a discussion of the Tolman relations and Rovelli’s thermal time hypothesis in terms of clock thermodynamics.

Click to access 2007.02217.pdf

It’s Still Not Too Late To Fight COVID-19 Like A Scientist

1.) We need a national mask mandate. Sorry to all those with a medical reason why you can’t wear a mask; the fact of the matter is that the risks of both transmitting and acquiring the novel coronavirus skyrocket without a mask. While N95 and surgical masks are (and should be) reserved for hospital settings, both hybrid masks and two-layer cotton masks offer outstanding protection in three vital ways:

  • they efficiently filter large droplets,
  • they efficiently filter aerosols,
  • and they effectively reduce the distance your droplets and aerosols travel, protecting others.

While masks, scarves, bandanas and gaiters all vary in quality and effectiveness, any face covering is significantly better than no face covering.

Face shields, though a popular alternative, offer little-to-no protection to the wearer compared to masks.

Contrariwise, masks with built-in valves or vent protect the wearer, but do not protect others. As a rule of thumb, if you cannot blow out a candle while wearing your covering, it’s likely to be effective. Those who wear masks are not only far less likely to infect others, but their COVID-19 infections are much more statistically likely to be mild, rather than serious or worse.

2.) Remain socially distant, and have it enforced. When you leave your house, or come into contact with anyone who doesn’t directly live with you in your household, you should keep a minimum of 6 feet (~2 meters) away from every other person. What constitutes 6 feet?

  • The width of a passenger car.
  • The length of two typical shopping carts.
  • Or if two people both held their arms out fully extended, leave approximately a full foot (30 cm) gap between your mutual fingertips.

The reason is simple: the virus is spread by airborne particles that are expelled by respiratory activity. This includes breathing, laughing, singing, talking, shouting, playing musical instruments, coughing, and sneezing, among others. With an effective mask, the viral load launched into the air can be greatly reduced, as can the distance that droplets and aerosols travel. Without a mask, they can easily travel up to 26+ feet. A recent meta-study found that not only were 6 foot (2 meter) distances effective in reducing viral transmission, but that every additional 3 feet (1 meter) reduced transmission and susceptibility even further. Speaking up when someone’s standing too close should be normalized, and leaving enough distance between people for safety’s sake should be mandatory.

3.) Do not gather indoors with people who live outside your household. The top determinant in whether someone catching the novel coronavirus SARS-CoV-2 is as straightforward as it gets: exposure. The greater your exposure, the greater your odds of catching COVID-19, and the greater your exposure, the greater the likelihood you’ll have a serious, severe, or even deadly case of it.

When you’re indoors, aerosol particles remain in the air and — if anyone who’s been in that space is infected with SARS-CoV-2 — continue to increase the viral load a person in that space is exposed to. Spaces where eating or drinking occurs (like bars or restaurants), where singing or raised-voice speaking is common (like classrooms or churches), or where strained, heavy breathing occurs (like gyms) only exacerbate this effect.

If we cared about taking public health precautions seriously and reducing infection rates, there would be a moratorium on gathering in-person in spaces such as this until the virus was under control. While the virus isn’t under control right now in the United States, there’s a remarkably simple way to get there.

4.) Enforce a 4-to-6 week “stay at home” plan. This is the enormous step that could lead us to victory over the virus, but that requires national coordination and enormous levels of societal compliance to be successful. The reason the virus has continued to spread throughout the population over the past six months — and will continue to do so as long as the current conditions do not change substantially — is that far too many of us are engaging in non-essential contacts far too frequently.

We can combat that by having a nationwide shelter-in-place order. We can prepare for this by:

  • setting up infrastructure to provide essentials (like food and medicine) delivered or picked-up without person-to-person contact,
  • paying all Americans to stay home and not (need to) work during that time,
  • and to enforce fines and other punishments for those who violate the order.

The few essential contacts that cannot be eliminated (mostly for healthcare reasons) will ensure that some amount of virus will remain in our population, but shelter-in-place (i.e., stay-at-home) orders are one of the most successful public health interventions a society can take to combat a pandemic such as COVID-19.

5.) Reopen according to science-based guidelines. Even though we’ve been combating the novel coronavirus in the United States for 6 months, we still aren’t following the recommendations of scientists. We don’t have universal contact tracing. We don’t have rapid, widespread, mass testing. Because we don’t know who’s positive and who’s been exposed, we aren’t isolating or quarantining appropriately. That means that asymptomatic carriers, presymptomatic carriers, and people who have active, symptomatic infections are all out in public, with the potential to infect each and every one of us.

If we cared about stopping this pandemic and preventing hundreds of thousands of further deaths, we would engage in all three of these interventions:

  • universal contact tracing,
  • widespread, rapid, mass testing,
  • and science-based isolation and quarantine practices for the infected and exposed.

Where infections start to rise, a local stay-at-home order could squash an outbreak with this information before it spreads to other communities.

A network epidemiology map of the connections between a set of households if only essential connections are allowed to occur. Note both the large number of isolated households that do not have connections with any other households, and the relatively small size of the largest connected network. GOODREAU SM ET AL., ON BEHALF OF THE STATNET DEVELOPMENT TEAM (2020)

These five interventions, taken together, could in principle take us from tens of thousands of new cases per day to merely hundreds in just a few weeks. Just as many countries with large populations and widely varying population densities have already safely reopened by taking exactly these steps, recovering from catastrophically large initial infection rates, the United States (and the United Kingdom, and other countries with similar circumstances) could completely turn the tide in the fight against COVID-19. It simply requires following the best scientific advice that the experts have to offer.

Without it, the disease will continue to ravage individuals, families, communities and the nation as a whole. The economic impacts will be severe and drawn-out, and our best hope of defeating it will come from a vaccine. Even then, we have to be scientifically responsible on that front, too: vaccines need to be proven safe and effective, and realistically not a single one is close to the finish line yet. A rushed vaccine not only might not work — giving people false hopes of protection — it could have unacceptably dangerous side effects.

The best part about this science-based solution is that it’s always available to us: we can choose to adopt it at any time. The social and economic impacts of the pandemic continue to be disastrous, but they can be remedied by making the conditions safe for everyone: workers, customers, teachers, and students. By enforcing mask mandates, distancing requirements, moratoriums on risky activities and businesses, a 4-to-6 week coordinated shelter-in-place order, and only reopen with science-based guidelines in place (that include contact tracing, widespread testing, and isolation/quarantines for the infected and exposed), we can beat the virus as a society.

Many of us, understandably, are beyond fatigued at the impact that the novel coronavirus has had on our lives. The idea of having to isolate ourselves further is truly frightening, but it’s an action that can have a greater impact on stopping this virus in its tracks than anything we’ve done to date. Until a safe and effective vaccine is widely available, this is the scientific path to a safely reopened America. We can choose it at any time. The sooner we do, the more lives we’ll have saved.