gravity
esiegel | June 16, 2017“There is no such thing as perpetual tranquillity of mind while we live here; because life itself is but motion, and can never be without desire, nor without fear, no more than without sense.” -Thomas Hobbes
It’s hard to believe, but despite being at rest here on the surface of Earth, we’re actually hurtling through the Universe in a variety of impressive ways. The Earth spins on its axis, giving someone at the equator a speed of some 1700 km/hr. Yet at even faster speeds, the Earth orbits the Sun, the Sun moves through the Milky Way, and there’s a great cosmic motion that applied to the…
esiegel | May 30, 2017"If you're puzzled by what dark energy is, you're in good company." -Saul Perlmutter
We normally assume that the fundamental constants of the Universe are actually constant, but they don’t have to be that way. They could vary in space, in time, or with the energy density of the Universe, in principle. Before believing in such an extraordinary claim, however, you’d need some remarkable evidence. It’s arguable that exactly that sort of evidence is emerging: from the tensions in the expansion rate of the Universe.
Three different types of measurements, distant stars and galaxies, the large…
esiegel | March 29, 2017"If something doesn’t reach you on a personal level, let it go. It’s hard enough dealing with everything that does." -Judi Culbertson
There are two types of electric charge: positive and negative. Like charges repel; opposite charges attract. In gravitation, though, there’s only one kind of gravitational charge, more commonly known as mass. And everything we know of has a positive mass. But since there’s a counterpart to matter -- antimatter -- isn’t it possible that antimatter would have negative gravitational charge, and fall “up” in a gravitational field?
If there were some type of matter…
esiegel | March 7, 2017"If you haven't found something strange during the day, it hasn't been much of a day." -John Archibald Wheeler
Today, we take the existence of gravitational waves for granted. They were predicted by Einstein almost immediately following the first publication of general relativity, they were indirectly detected decades ago and they’ve been directly detected multiple times by the different LIGO observatories. Yet Einstein and his former student argued, back from the 1930s through the 1950s, that the waves were mere mathematical artifacts, and didn’t physically exist.
Richard Feynman, at…
esiegel | February 28, 2017“In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, as far as possible, relations between the manifold aspects of our experience.” -Niels Bohr
When it comes to galaxies, clusters of galaxies or the largest-scales on the Universe, dark matter is absolutely required. There’s no way to explain colliding galaxy clusters, correlations between different galaxies, the filamentary structure of the Universe or the fluctuations in the Cosmic Microwave Background without it. But it isn’t just dark matter that hopes to solve these…
esiegel | February 24, 2017"It's the gravity that shapes the large scale structure of the universe, even though it is the weakest of four categories of forces." -Stephen Hawking
Galaxies don’t just exist in isolation in our Universe, but are often found bound together as a part of even grander structures. Our own Milky Way is bound in a galactic group (our local group), nearby are larger groups and galaxy clusters, and on still larger scales, cosmic superclusters appear to encompass as many as 100,000 individual galaxies.
Outlined in light blue, giant collections of galaxies can be divided up into superclusters. But…
esiegel | February 14, 2017"'I'll follow him to the ends of the earth,' she sobbed. Yes, darling. But the earth doesn't have any ends." -Tom Robbins
We have some pretty good definitions of what it takes to be a planet, and one part of that definition is that a world needs to be massive enough to pull itself into hydrostatic equilibrium. In the absence of external forces and rotation, that means it will be a perfect sphere.
The line for a planet vs. a non-planet is mass-dependent, and making a thin, rigid body fails on that account. You can have a flat "thing" in space, but it wouldn't be a planet if you did. Image…
esiegel | February 4, 2017"For the first time, astronomers have outlined and named the network of galaxies that includes the Milky Way, adding a line to our cosmic address and further defining our place in the universe." -Douglas Quenqua
There’s been a longstanding puzzle in astrophysics that’s finally coming to a head. For nearly a century, we’ve known that our Universe is expanding, and that the distance to a galaxy determines its average apparent recessional speed from us. But on top of that is an additional motion -- a peculiar velocity -- caused by the local gravitational field of the Universe.
A two-dimensional…
esiegel | January 28, 2017"'Space-time' - that hideous hybrid whose very hyphen looks phoney." -Vladimir Nabokov
Sure, you know what space and time are. If you heard of Einstein and relativity, you might know that they’re not absolute quantities, but that how you experience distances and the ticking of clocks is dependent on your motion through the Universe. But did you also know that the addition of masses and gravitation to the theory didn’t just result in general relativity, but changed the way we viewed the Universe completely?
Quantum field theory calculations are normally done in flat space, but general…
esiegel | January 24, 2017"Work gives you meaning and purpose and life is empty without it." -Stephen Hawking
Throw a book into a black hole, and the information must somehow wind up inside. Same goes for a star, a planet, or even a single proton: that information must be maintained. But allow enough time to pass, and quantum theory and general relativity, combined, predict something troubling: that black hole will decay, and none of the information will come out in the decay products.
While Einstein's theory makes explicit predictions for a black hole's event horizon and the spacetime just outside, quantum…
esiegel | January 21, 2017"It was a strange lightness, a drifting feeling. Zero gravity. I understood that everything that once seemed solid and immovable might just float away." -Lisa Unger
If you want to experience zero gravity, all you need to do is go to space, turn your rocket off and feel that weightless sensation, right? Except, from up there in Earth’s orbit, the gravitational force on you is still almost as great as it is on Earth’s surface! You still accelerate down towards the center of the Earth, not quite at 9.8 m/s^2, but not by much less.
Astronauts, and fruit, aboard the International Space Station.…
esiegel | January 2, 2017“You may hate gravity, but gravity doesn’t care.” –Clayton Christensen
If all we had were galactic rotation curves -- like those measured by Vera Rubin -- we would know that something was wrong with our picture of the Universe, but we wouldn’t know how. Two equally good explanations, that there was either a flaw in the law of gravity or there was the existence of some unseen mass, could account for what we saw. But observations of galaxy clusters point to dark matter in a dramatic fashion.
The Coma cluster of galaxies, whose galaxies move far too quickly to be accounted for by gravitation…
esiegel | December 30, 2016“The joy of life consists in the exercise of one’s energies, continual growth, constant change, the enjoyment of every new experience. To stop means simply to die. The eternal mistake of mankind is to set up an attainable ideal.” -Aleister Crowley
As we learn more and more about the Universe, we'd like to describe it as simply as possible. While we have thousands or even millions of chemical configurations, they arise from less than 100 different atoms. Atoms themselves are made up of protons, neutrons and electrons. The great hope of unification and for fans of elegance in general is that…
esiegel | December 27, 2016"It's the first time the universe has spoken to us through gravitational waves, up to now we've been deaf to them." -Dave Reitze
No doubt about it: the greatest science advance of 2016 was the end of the century-long wait for the first direct detection of gravitational waves. Not only were we able to detect the inspiral and merger of two black holes from their emission of gravitational waves, we were able to do it more than once. The announcement was a 101-year-after-the-fact confirmation of one of Einstein’s greatest and most unique predictions.
The signal from LIGO of the first robust…
esiegel | December 20, 2016"Because philosophy arises from awe, a philosopher is bound in his way to be a lover of myths and poetic fables. Poets and philosophers are alike in being big with wonder." -Thomas Aquinas
Sure, you've studied a little physics. You know about Einstein and General Relativity, the quantum nature of the Universe and the fundamental particles and their interactions. You know about the Big Bang, and you've read up on the latest theories and ideas. So how did you wind up believing in many (or even all) of these scientific myths out there?
Light and ripples in space; as the light passes through non…
esiegel | December 16, 2016“First, you should check out my house. It’s, like, kinda lame, but way less lame than, like, your house.” -Lumpy Space Princess, Adventure Time
Billions of years ago, before the Universe contained clusters, galaxies, stars or even neutral atoms, everything was uniform. Almost perfectly uniform, where the densest regions weren’t even 0.01% denser than average. Over billions of years, those overdense regions have attracted more and more matter and grown under the influence of gravity.
Both simulations (red) and galaxy surveys (blue/purple) display the same large-scale clustering patterns.…
esiegel | December 6, 2016"The bedrock nature of space and time and the unification of cosmos and quantum are surely among science's great 'open frontiers.' These are parts of the intellectual map where we're still groping for the truth - where, in the fashion of ancient cartographers, we must still inscribe 'here be dragons.'" -Martin Rees
In 2015, LIGO collected data from a total of three candidate gravitational wave events, all of which were announced and released in 2016. These events verified the great prediction of Einstein: that decaying orbits should emit gravitational radiation with specific magnitudes and…
esiegel | November 25, 2016"I just think too many nice things have happened in string theory for it to be all wrong. Humans do not understand it very well, but I just don't believe there is a big cosmic conspiracy that created this incredible thing that has nothing to do with the real world." -Edward Witten
You don't have to be a fan of string theory to understand why it's such a promising area of scientific research. One of the holy grails of physics is for a quantum theory of gravitation: that describes gravity on the same footing as the other three forces, in very strong fields and at very tiny distances.…
esiegel | November 12, 2016"The whole edifice of modern physics is built up on the fundamental hypothesis of the atomic or molecular constitution of matter." -C. V. Raman
As far as humans have figured out, there are four fundamental forces in the Universe: the three of the Standard Model (electromagnetism, the weak and strong nuclear forces) and General Relativity, our theory of gravitation. It’s the great hope of many that a quantum theory of gravitation will someday emerge, and that all four forces could be unified into a single framework. Among others, this is the dream of string theory.
The four forces (or…
esiegel | November 1, 2016"All we know so far is what doesn't work." -Richard Feynman
The past month has seen a slew of papers out highlighting the tension between modified gravity and dark matter. Both recognize the same puzzles and problems with the Universe, and both ideas recognize that either one could be valid. In fact, if you look at the two greatest "crises" in gravity in the 19th century, it's arguable that dark matter (Neptune) solved one, the Uranus problem, while modifying gravity (with Einstein's general relativity) solved the other.
The X-ray (pink) and overall matter (blue) maps of various colliding…