MATT STRASSLER
Of Particular Significance

Category: LHC Background Info

What Ocean Tides Teach Us About the Sun and Moon

The Moon has a four-week cycle; it is full every four weeks (actually every 29.5 days). But ocean tides exhibit a two-week cycle; they are large one week and then smaller the next.

Specifically, as in Fig. 1 below, ocean tides are stronger (“spring tides”) around New Moon and Full Moon than they are at First Quarter Moon and Last Quarter Moon (“neap tides”). The pattern is seen, roughly at least, all around the world (though the details are not simple, as they depend on the shape of the coastline and other factors.)

Figure 1: Tides in Anchorage, Alaska, USA, during October 2023; the blue line shows how the water rises and falls about twice a day (note the vertical columns are each two days wide!) The pattern of strong and weak tides on alternate weeks is clearly visible. New Moon occurred on October 14 and Full Moon on October 28th, just before the peak tides.

What’s behind this pattern? And what does it tell us about the Sun and Moon that we wouldn’t otherwise easily know? Perhaps surprisingly, it tells us that the average mass density of the Sun — its mass divided by its volume — is only a little smaller than the average mass density of the Moon. Here’s why.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON January 12, 2024

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Heads Up — Northern Lights Possible in Next 24 Hours

[Note added: the predicted storm has begun, as of about 1000 UTC, 5:00 AM NYC time; good for early birds on the west coast and those in Asia.] If you live in Canada, Europe or the northern half of the US, keep an eye to the north late tonight and possibly tomorrow night. A series of solar flares occurred on the Sun in the last couple of days, and when their repercussions reach Earth, they may cause quite a storm in the Earth’s magnetic field… resulting in Northern Lights (Aurora Borealis). [There will be Southern Lights too, though the nights are short right now down south.]

Something to watch: https://www.swpc.noaa.gov/products/ace-real-time-solar-wind , data from the ACE satellite, serves as an early-warning system; if its readings start suddenly going crazy, that typically means a CME (coronal mass emission, i.e. a swarm of particles blown out of the Sun’s outer atmosphere [its corona]) has hit the satellite. Usually that means the CME is an hour at most from hitting Earth, at which point auroras become more likely; the stronger the CME, the more southerly the northern lights will usually reach.

Here’s a picture of some of the ACE data as of 10:30 NYC (330 UTC) time, showing that something already happened a few hours ago, right at around 2300 UTC, enough to start a mild storm. The expectation is that something more dramatic may happen soon, and if it does, you should start making tea to keep you warm when you go out to look.

Readings from the ACE satellite as of 330 UTC Dec 1 (10:30 pm NYC Nov 30); note the sudden jump in the readings at 2300, typical of an arriving CME.

More generally, there’s a lot of data at https://www.swpc.noaa.gov/ , though it is not very user-friendly. A lot of its information is delayed by 3 hours, which is not so useful when you’re trying to catch a fleeting opportunity!

Good spotting! I myself am out of luck again, due to exhaustion tonight and bad weather tomorrow. But we are approaching solar maximum, so we should get multiple chances in 2024.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON November 30, 2023

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Is Einstein’s Theory of General Relativity Truly Elegant?

  • Quote: . . . the Higgs field exhibits the most inelegant of the known laws governing fields and particles. There’s an amusing tendency for those who tout beauty to ignore this, as though it were an inconvenient family member, and to focus instead on Einstein’s elegant theory of gravity. Yet even that theory has its issues.
  • Endnote: Einstein’s theory of gravity is amazingly elegant as long as one ignores the puzzle of “dark energy,” which would have been easier to do had it been exactly zero, and as long as gravity is a very weak force, as its weakness leads to extremely simple equations. In string theory, Einstein’s equations become much more complex, and the elegant simplicity of the math shifts to the level of the strings themselves . . . perhaps.

I’ll expound below upon the second bullet point, hoping to draw attention to general questions concerning aesthetics in theoretical physics.

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POSTED BY Matt Strassler

ON November 6, 2023

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A Half Century Since the Birth of QCD

This year marks a half-century since the discovery that a quantum field theory, now known as QCD (quantum chromodynamics), could be the underlying explanation for the strong nuclear force. That’s the force that holds quarks and gluons inside of protons and neutrons, and keeps protons and neutrons clumped together in atomic nuclei. This major step in theoretical physics occurred just a couple of years after it was discovered that a similar quantum field theory for the weak nuclear force (which includes W bosons, a Z boson and a Higgs boson) is mathematically consistent.

With these two breakthroughs came the sudden and unexpected triumph of quantum field theory, emerging as the basic mathematical and conceptual language for understanding the cosmos. It came after two decades in which most experts were convinced that quantum field theory was inconsistent, and only a stepping stone to something deeper.

This week I am in New York City attending two attached scientific meetings, both focused on QCD and other quantum field theories that share its key property, known as “confinement.” One meeting is hosted by New York University, and the other, the Annual Meeting of the Simons Collaboration on Confinement and QCD Strings, by the Simons Foundation. Many luminaries who have spent time on this subject are here together, ranging from David Gross, who co-invented the subject (and was a winner of the 2004 Nobel Prize), to brilliant graduate students who are hoping to reinvent it.

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POSTED BY Matt Strassler

ON November 2, 2023

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What [Really] Causes our Twice-Daily Ocean Tides?

More about tidal forces today (see also yesterday’s post) and the conceptual point underlying Earth’s ocean tides.

  • (Quote) Because gravity dwindles at greater distances, the Moon’s pull is stronger on the near side of the Earth and weaker on the far side than it is on the Earth’s center. This uneven pull stretches our planet’s oceans slightly, resulting in a small bulge of water, not much taller than a human, both on the Earth’s side facing the Moon and on the opposite side, too.
  • (Endnote) To explain why gravity leads to a water bulge on both sides of the Earth is too complex for a footnote, and I’d rather not repeat the most commonly heard explanations, which are misleading. One can see a hint of the cause as follows: if one drops a water balloon in constant gravity, it will fall as a sphere, whereas if it is pulled more strongly at the bottom than at the top, it will stretch into an oval as it falls.

Here I’ll explain this last observation more carefully.

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POSTED BY Matt Strassler

ON October 27, 2023

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The Impossible Commentary: Is Gravity a Force? Is it an Illusion?

[This is a tricky one… it’s easy to make confusing statements about Einstein’s theory of gravity (general relativity), and so I am especially hopeful of getting readers’ feedback on this subtle issue, to make sure what follows is 100% clear and correctly stated.]

  • (Quote) On Earth’s surface, we are roughly 4,000 miles from Earth’s center. But if you ascended another 22,000 miles, where you’d find the GOES weather satellites that monitor Earth’s weather patterns, you’d find your weight (but not your mass!) reduced to one-fortieth of what it is on Earth… And if you traveled out into deep space, far from any large object, you’d weigh virtually nothing. Yet all the while, your body’s mass—the difficulty I would face if I tried to speed you up or slow you down—would never change.
  • (Endnote) Confusingly, astronauts orbiting Earth inside nearby space stations appear to float as though weightless. From Newton’s perspective, they are not truly weightless; if they were, they’d coast, leaving the Earth’s vicinity and moving rapidly into deep space.
    Instead, they and their spaceship are pulled by gravity into a common orbit around the Earth. Since they travel on the same path as their container and as the camera which films them, they seem and feel weightless. (This subtle issue is turned on its head in Einstein’s view of gravity.)

Astronauts in a space station seem to float, as though they are weightless. Are they truly weightless? Or are they only apparently weightless?

The same issues arise for people in a freely falling elevator, accelerating downward with ever greater speed. They will feel weightless, too. But are they?

Newton would have said they are apparently weightless, subject to gravity but all falling together along with their vehicle. A naive (but instructive!) reading of Einstein might lead us to say that they are truly weightless… that the gravity that Newton claims is present is a pure illusion, a fictitious force. But a precise Einsteinian would say they are almost but not quite weightless — and the lack of perfect weightlessness is a clue, a smoking gun in fact, that they are indeed subject to gravity.

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POSTED BY Matt Strassler

ON October 26, 2023

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