Category: gravity

Scientists Admit, Embarrassingly, We Don&rsquo…

Scientists Admit, Embarrassingly, We Don’t Know How Strong The Force Of Gravity Is

“The gravitational constant of the Universe, G, was the first constant to ever be measured. Yet more than 350 years after we first determined its value, it is truly embarrassing how poorly known, compared to all the other constants, our knowledge of this one is. We use this constant in a whole slew of measurements and calculations, from gravitational waves to pulsar timing to the expansion of the Universe. Yet our ability to determine it is rooted in small-scale measurements made right here on Earth. The tiniest sources of uncertainty, from the density of materials to seismic vibrations across the globe, can weave their way into our attempts to determine it. Until we can do better, there will be an inherent, uncomfortably large uncertainty anywhere the gravitational phenomenon is important. It’s 2018, and we still don’t know how strong gravity actually is.”

Of all the fundamental constants in the Universe, such as Planck’s constant, the speed of light, or the mass of the electron, only one of them can lay claim to being the first one to be identified and measured to any degree of accuracy. That is G, the gravitational constant, first determined decades before Newton’s work: in the mid-17th century. Yet even today, scientists performing the experiments can’t agree on whether it’s 6.672 or 6.676 (or somewhere in between) x 10^-11 N/kg^2/m^2. Experiments are coming out all the time, claiming precisions to just a few parts-per-million, yet they disagree with one another at the level of nearly a part in a thousand, making G the least well-determined fundamental constant of all.

It’s 2018, and we still don’t know how strong gravity actually is. For those of us trying to understand the Universe at a fundamental level, it’s maddening.

One Simple Reason Why Touching The Sun Is So H…

One Simple Reason Why Touching The Sun Is So Hard

“It doesn’t simply take a suite of clever instruments to measure the Sun up close, although the Parker Solar Probe has those. It isn’t enough to have a thick, carbon-composite shield to withstand the incredible radiation and temperatures present in close proximity to the Sun, although the Parker Solar Probe has those, too. It also requires an incredibly complex, intricate plan to insert yourself into a stable orbit that’s capable of bringing you closer to the Sun than anything else ever has before.”

If there’s one law of physics that most people know, it’s Newton’s first law. Objects at rest remain at rest, and objects in motion remain in uniform motion, unless they’re acted on by an outside force. This applies not just to straight-line motions, but to orbiting motions as well. It isn’t just momentum that’s conserved in physics, but angular (or rotational) momentum, too. In order to touch the Sun, the Parker Solar Probe has to somehow get rid of a tremendous amount of angular momentum, and rockets alone aren’t powerful enough to do it. The trick? You have to use the other planets in the Solar System, and give up your angular momentum to them. The Parker Solar Probe will pass close by Venus a record seven times in order to do this, coming within less than 4 million miles of the Sun when they’re all over.

We’ve never touched the Sun before, but thanks to new technology, an ambitious mission, and an incredible flight plan, we’re about to accomplish what was once unthinkable.

Could We Create A Bottomless Pit On Earth?

Could We Create A Bottomless Pit On Earth?

“A round-trip journey, from the North Pole to just shy of the South Pole and back to the North Pole again, all through the Earth’s center, should take just a whisker under 90 minutes. Under ideal conditions:

* creating a vacuum,
* straight through the Earth’s rotational axis,
* starting with no tangential velocity,
* devoid of any type of air resistance and subject only to gravitational forces,

you’d wind up right back where you started just 90 minutes later: roughly the same time it takes the international space station to orbit the Earth. So long as you brought an oxygen supply with you, you’d be no worse for the wear.”

From tourist traps to Alice in Wonderland to modern entertainment like Gravity Falls, bottomless pits are tropes that hardly seem physically possible. Sure, you can always envision a thought experiment, but that doesn’t mean you could actually build one. Despite the engineering challenges and the enormous expense that would be associated with such a project, this one turns out to be physically plausible with not-too-distant-future technology. There are a number of obstacles we’d have to overcome, including the Earth’s rotation, drilling a shaft clear through the planet, and stabilizing a passenger against the heat and radioactivity of the natural interior of our world. But if we could do it, and not get stuck at the center, we’d come back to where we started just 90 minutes later.

Here’s the story behind how to create and successfully use a bottomless pit here on Earth!

Why Do All The Planets Orbit In The Same Plane…

Why Do All The Planets Orbit In The Same Plane?

“So why are all the planets in the same plane? Because they form from an asymmetric cloud of gas, which collapses in the shortest direction first; the matter goes “splat” and sticks together; it contracts inwards but winds up spinning around the center, with planets forming from imperfections in that young disk of matter; they all wind up orbiting in the same plane, separated only by a few degrees — at most — from one another.”

When we look out not only at our own solar systems, but at the solar systems we’ve found around other stars, we find they have a remarkable feature in common: their planets all appear to rotate in the same plane. They might be off by a handful of degrees, but as far as we can tell, they all align with one another. This isn’t some mere coincidence, but seems to be a consequence of how solar systems form in the first place. Just as spiral galaxies orbit in the same, single plane, so do solar systems. Remarkably, it seems to be the same process at play: large structures collapse, which they do faster in one direction, and then angular momentum takes over, forming a disk. Over time, imperfections in the disk fragment, causing clumps to form and grow over time. When all is said and done, the survivors are all left in the same, single plane.

Here’s the remarkable story – with some remarkable, real images of what we’ve seen in action – of how all the planets came to orbit in the same plane!

Black Holes Must Have Singularities, Says Eins…

Black Holes Must Have Singularities, Says Einstein’s Relativity

“The thing is, there’s a speed limit to how fast these force-carriers can go: the speed of light. If you want an interaction to work by having an interior particle exert an outward force on an exterior particle, there needs to be some way for a particle to travel along that outward path. If the spacetime containing your particles is below the density threshold necessary to create a black hole, that’s no problem: moving at the speed of light will allow you to take that outward trajectory.

But what if your spacetime crosses that threshold? What if you create an event horizon, and have a region of space where gravity is so intense that even if you moved at the speed of light, you couldn’t escape?”

Usually, when physicists first start teaching about black holes, the attitude they’re met with is skepticism. People can accept that as you compress a large mass into a smaller and smaller volume, it gets harder to escape its gravitational pull. As you go from a star to a white dwarf to a neutron star, you have to move closer to the speed of light to leave it’s surface. If you go even denser, you’ll create an event horizon: a region of space where the gravitational pull is so strong that nothing, not even light can escape. People are okay with that, but when you go to the next step and declare that anything that crosses the event horizon eventually falls into a central singularity, suddenly they’re not okay. Why, they reason, couldn’t there be some denser, exotic, degenerate form of matter than what we presently know? Why couldn’t that lie inside a black hole, rather than a singular point or ring? It’s a good question and an interesting bit of intuition, but there’s an answer for that.

If you wanted to hold anything up against collapse to a singularity inside a black hole, the force-carriers governing the interaction would have to travel faster than light, which is a no-go. Find out the full story on why black holes must have singularities!

The Scientific Failure Of The Original Elegant…

The Scientific Failure Of The Original Elegant Universe

“Kepler’s idea was nothing short of brilliant, and each of the ratios for the planetary radii were predicted exactly by his model. The problem came when you compared them with observations. While the ratios for Mercury to Venus, Venus to Earth, and Earth to Mars lined up pretty well, the final two worlds failed to adhere to Kepler’s predicted ratios. In particular, it orbit of Mars, and its failure to conform to a circle of any type, was the downfall of Kepler’s model. Even though Kepler continued to work on it, even publishing a second edition more than 20 years later, his most remarkable contribution came from doing what most scientists can never bring themselves to do: abandoning their most cherished hypothesis.”

Call it what you will: beauty, elegance, simplicity, or reductionism, one of the biggest dreams of any fundamental science is to describe as much as possible with as little as possible. From F = ma to dreams of string theory, this powerful idea has been a guiding force in the formulation of scientific theories since ancient times. But while advances in mathematics often lead to brilliant new ideas in physics, they (perhaps even more) often lead to beautiful ideas that have nothing to do with our physical Universe. This isn’t a failing on our part, but rather a truth of the Universe that we must accept, whether we like it or not. But cheer up! This is nothing new, and was an uncomfortable truth faced by none other than Johannes Kepler. Kepler, famous now for his three laws of planetary motion and discovering the elliptical nature of planetary orbits, began with a different idea based on mathematics and the music of the spheres. His idea was beautiful, elegant, and a complete failure. That doesn’t mean it isn’t valuable, however, as it was a step that led us, eventually, to a greater scientific truth. 

Come learn about the scientific failure of the original elegant Universe, and learn not to accept your prejudices about what the next great breakthroughs ought to be!

Ask Ethan: How Does Spinning Affect The Shape …

Ask Ethan: How Does Spinning Affect The Shape Of Pulsars?

“[S]ome pulsars have incredible spin rates. How much does this distort the object, and does it shed material this way or is gravity still able to bind all of the material to the object?”

If you spin too quickly, the matter on the outskirts of your surface will fly off. If you’re in hydrostatic equilibrium, your shape will simply distort until your equatorial bulge and your polar flattening result in the most stable, lowest-energy configuration. For our Earth, this means the best place to launch a rocket is near the equator, and our planet’s polar diameter is a little more than 20 km shorter than its equatorial diameter. But what about for the fastest-rotating natural object we know of: a neutron star. While most neutron stars rotate a few times a second, the fastest one makes 766 rotations in that span, meaning that a neutron on the surface moves at about 16% the speed of light. Much faster, and could it escape? Or, perhaps, is the pulsar’s shape highly distorted, either due to that rotation or to the incredibly strong magnetic fields inside? Neutron star matter is very different from anything we’re used to, so don’t bet on any of those.

Other than the first few fractions-of-a-second, changes to neutron stars are slow and mostly inconsequential. Come find out how bad it is on this edition of Ask Ethan!

Ask Ethan: Why don’t comets orbit the same way…

Ask Ethan: Why don’t comets orbit the same way planets do?

“Why [do] comets orbit the Sun in a parabolic path, unlike planets which orbit in an elliptical one? Where do comets get the energy to travel such a long distance, from the Oort cloud to the Sun & back? Also, how could interstellar comets/asteroids come out of their parent star [system] and visit other ones?”

When we see comets in our Solar System, they can be either periodic, passing near the Sun and then extending very far away, to return many years later, or they could be a one-shot deal. But comets are driven by the same gravitational laws that drive the planets, which simply make fast-moving, nearly-circular ellipses around the Sun. So what makes these orbits so different, particularly if they’re obeying the same laws? Believe it or not, most of the would-be comets out there are moving in exactly the same nearly-circular paths, only they’re far more tenuously held by the Sun. Gravitational interactions might make small changes in their orbits, but if you’re already moving very slowly, a small change can have a very big effect!

Why don’t comets orbit the same way as the planets? Find out on this edition of Ask Ethan!

Richard Feynman And John Wheeler Revolutionized Time, Reality,…

Richard Feynman And John Wheeler Revolutionized Time, Reality, And Our Quantum Universe

“Yet at their core, these two were practically tailor-made to collaborate with one another. Wheeler’s wild ideas always contained components that were spectacularly wrong and unworkable, but often contained a kernel of deep truth that would pave the road to an understanding that was otherwise unachievable. The idea of a path integral, the essential tool used to calculate physical observables in quantum field theory, came about from Wheeler’s insistence on a sum over histories, but it was Feynman who worked out the details correctly, and applied them properly to our physical Universe.

Feynman’s ability to connect the wild ideas to the physical Universe, never far afield from what could be measured, was the perfect complement to Wheeler’s imagination. Together and separately, they took on gravitation, the quantum nature of reality, and even space and time itself. And as much as any physicist ever did, they not only took these ideas on; they won.”

In popular culture, Richard Feynman is revered as a non-conformist/genius, whose bongo-playing, carefree antics are as notable as his groundbreaking physics research. John Wheeler, renowned for his contributions to General Relativity, gravitation, and information theory, has no similar stories from his personal lives. Yet professionally, these two complemented one another in ways that were unimaginable to an outside onlooker: Wheeler’s imagination ran wildly into the speculative and unworkable, while Feynman was always dragging things back to observable and measurable quantities. In the end, both are remembered as towering figures in physics in the 20th century, on par with names like Bohr, Dirac, Pauli, and Heisenberg. In a sweeping new book, Paul Halpern takes an in-depth look at the scientific and personal lives of these two physicists, who first met in 1939 and spent the next five decades revolutionizing our conception of the Universe.

Get the full book review here, and if you’re at all interested in picking up a copy for yourself (or the physics enthusiast in your life), follow the links and go get a copy of The Quantum Labyrinth! 

The Bullet Cluster Proves Dark Matter Exists, But Not For The…

The Bullet Cluster Proves Dark Matter Exists, But Not For The Reason Most Physicists Think

“When your cluster is undisturbed, the gravitational effects are located where the matter is distributed. It’s only after a collision or interaction has taken place that we see what appears to be a non-local effect. This indicates that something happens during the collision process to separate normal matter from where we see the gravitational effects. Adding dark matter makes this work, but non-local gravity would make differing before-and-after predictions that can’t both match up, simultaneously, with what we observe.

Interestingly, this argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes. This is a fundamental issue that must be addressed, if alternatives to dark matter are to be taken seriously as complete theories, rather than ideas in their infancy. That time is not yet at hand.”

Recently, a paper came out challenging alternative theories to dark matter and claiming that many of them were invalid. The basis for that argument? That those theories predict different arrival times for gravitational waves and light waves from a neutron star merger, when we saw them arrive practically simultaneously. One of those theories, MOG, claims to survive, but it’s already been discredited for another reason that’s discussed far less frequently: the Bullet Cluster. When the apparent effects of gravitation are well-separated in space from where we see the matter, you require non-locality to save your theory. MOG is a non-local theory of gravity, so you might think everything is fine. But if gravitational effects aren’t where the matter is located, we’d expect to see these non-local effects in clusters that are in a pre-merger state, and those don’t exist.

Can a theory like MOG survive in this context? I don’t believe so. The Bullet Cluster proves dark matter exists, but not for the reason most physicists think!