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Here’s a gedankenexperiment (that’s German for “thought experiment”) that ought to interest you.
A gedankenexperiment is a way that physicists ask questions about how something in our universe works, for the joy of working out it’s consequences. The experiments don’t need to be practical, although many do lead to advances in physics. Famous examples of gedankenexperiments that led to new ideas in physics include Schrödinger’s cat and Maxwell’s demon.
Blueberry Earth: The Delicious Thought Experiment That’s Roiling Planetary Scientists
“A roaring ocean of boiling jam, with the geysers of released air and steam likely ejecting at least a few berries into orbit.”
Sarah Zhang, The Atlantic, 8/2/1018
Sarah Zhang, in The Atlantic, 8/2/1018, writes
Can I offer you a thought experiment on what would happen if the Earth were replaced by “an equal volume of closely packed but uncompressed blueberries”? When Anders Sandberg saw this question, he could not let it go. The asker was one “billybodega,” who posted the scenario on Physics Stack Exchange. (Though the question was originally posed on Twitter by writer Sandra Newman.)
A moderator of the usually staid forum closed the discussion before Sandberg could reply. That didn’t matter. Sandberg, a researcher at Oxford’s Future of Humanity Institute, wrote a lengthy answer on his blog and then an even lengthier paper that he posted to arxiv.org, a repository for physics preprints that have not yet been peer reviewed. The result is a brilliant explanation of how planets form.
To begin: The 1.5 x 1025 pounds of “closely packed but uncompressed” berries will start to collapse onto themselves and crush the berries deeper than 11.4 meters – or 37 feet – into a pulp. “Enormous amounts of air will be pushing out from the pulp as bubbles and jets, producing spectacular geysers,” writes Sandberg. What’s more, this rapid shrinking will release a huge amount of gravitational energy—equal to, according to Sandberg’s calculations, the energy output of the sun over 20 minutes. It’s enough to make the pulp boil. Behold:
“The result is that blueberry earth will turn into a roaring ocean of boiling jam, with the geysers of released air and steam likely ejecting at least a few berries into orbit. As the planet evolves a thick atmosphere of released steam will add to the already considerable air from the berries. It is not inconceivable that the planet may heat up further due to a water vapour greenhouse effect, turning into a very odd Venusian world.”
Deep under the roiling jam waves, the pressure is high enough that even the warm jam will turn to ice. Blueberry Earth will have an ice core 4,000 miles wide, by Sandberg’s calculations. “The end result is a world that has a steam atmosphere covering an ocean of jam on top of warm blueberry granita,” he writes.
The process is not so different from the birth of a planet out of a disc of rotating debris. The coalescing, the emergence of an atmosphere, the formation of a dense core—all of these happened at one point to the real Earth. And it is currently happening elsewhere in the universe, as exoplanets are forming around other stars in other galaxies.
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Through science fiction, most people are familiar with the idea of warp drive. It is a fictional form of FTL (Faster Than Light travel) Its most popular use is in the science-fiction series Star Trek.
According to the laws of physic could warp drive potentially be possible?
Warp drive in science fiction
first mention, “Islands of Space,” by John W. Campbell. 1931
Warping space in general relativity
Warp drive in real physics
The Alcubierre drive is a speculative analysis of physics which shows that warp drive may in fact be possible. It is based on a solution of Einstein’s field equations in general relativity. It was first proposed by Mexican theoretical physicist Miguel Alcubierre.
In his analysis, a spacecraft could effectively achieve a kind of FTL travel if a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created.
Rather than exceeding the speed of light within a local reference frame, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel.
In this analysis, objects still cannot accelerate to the speed of light within normal spacetime; therefore it doesn’t violate the laws of General Relativity.
Instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination faster than light would in normal space.
“Space-time bubble is the closest that modern physics comes to the “warp drive” of science fiction. It can convey a starship at arbitrarily high speeds. Space-time contracts at the front of the bubble, reducing the distance to the destination, and expands at its rear, increasing the distance from the origin (arrows). The ship itself stands still relative to the space immediately around it; crew members do not experience any acceleration. Negative energy (blue) is required on the sides of the bubble.” – Ford and Roman
Although the metric proposed by Alcubierre is consistent with the Einstein field equations, it may not be physically meaningful. We are not certain that the mathematical solutions are possible in the real world. If so then this warp drive will not be possible.
Even if it is physically meaningful, that does not necessarily mean that a drive can be constructed. The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter. So if exotic matter with the correct properties can not exist, then the drive could not be constructed.
This section is from “NASA –Faster-than-Speed-of-Light Space Travel? ‘Will ‘Warp Bubbles’ Enable Dreams of Interstellar Voyages?’ ” DailyGalaxy. com
Posted on Sep 21, 2018
In general relativity, one often first specifies a plausible distribution of matter and energy, and then finds the geometry of the spacetime associated with it; but it is also possible to run the Einstein field equations in the other direction, first specifying a metric and then finding the energy-momentum tensor associated with it. This is what Alcubierre did in building his metric.
This practice means that the solution can violate various energy conditions and require exotic matter. The need for exotic matter leads to questions about whether it is actually possible…
Yet another problem according to Serguei Krasnikov is that it would be impossible to generate the bubble without being able to force the exotic matter to move at locally FTL speeds, which would require the existence of tachyons. Some methods have been suggested which would avoid the problem of tachyonic motion, but would probably generate a naked singularity at the front of the bubble.
Dr. White believes that advances he and others have made render warp speed less implausible. Among other things, he has redesigned the theoretical warp-traveling spacecraft — and in particular a ring around it that is key to its propulsion system — in a way that he believes will greatly reduce the energy requirements. But ”We’re not bolting this to a spacecraft,” he said of the technology.
Richard Obousy, a physicist who is president of Icarus Interstellar, a nonprofit group composed of volunteers collaborating on starship design, said “it is not airy-fairy, pie in the sky. We tend to overestimate what we can do on short time scales, but I think we massively underestimate what we can do on longer time scales.”
… Still, one of the most dubious is Dr. Alcubierre himself. He listed a number of concerns, starting with the vast amounts of exotic matter that would be needed. “The warp drive on this ground alone is impossible,” he said. “At speeds larger than the speed of light, the front of the warp bubble cannot be reached by any signal from within the ship,” he said. “This does not just mean we can’t turn it off; it is much worse. It means we can’t even turn it on in the first place.”
… As Caltech physicist Sean Carroll notes: “In short, it requires negative energy densities, which can’t be strictly disproven but are probably unrealistic; the total amount of energy is likely to be equivalent to the mass-energy of an astrophysical body; and the gravitational fields produced would likely rip any ship to shreds. My personal estimate of the likelihood we will ever be able to build a ‘warp drive’ is much less than 1%. And the chances it will happen in the next hundred years I would put at less than 0.01%.”
The IXS Enterprise is a theory fitting concept for a space warping ship. It’s designed by Mark Rademaker with NASA scientist Dr. Harold White and used in his presentations as an extra. Mike Okuda also brought input, and designed the Ship’s insignia.
2016 Massachusetts Science and Technology Curriculum Framework
Appendix VIII: Value of Crosscutting Concepts and Nature of Science in Curricula
ETS3. Technological Systems. 5.3-5-ETS3-1(MA). Use informational text to provide examples of improvements to existing technologies (innovations) and the development of new technologies (inventions). Recognize that technology is any modification of the natural or designed world done to fulfill human needs or wants.
9. Influence of Engineering, Technology, and Science on Society and the Natural World
In grades 9–12, students can describe how modern civilization depends on major technological systems, such as agriculture, health, water, energy, transportation, manufacturing, construction, and communications. Engineers continuously modify these systems to increase benefits while decreasing costs and risks. New technologies can have deep impacts on society and the environment, including some that were not anticipated.
SAT Subject Test: Physics
Quantum phenomena, such as photons and photoelectric effect
Atomic, such as the Rutherford and Bohr models, atomic energy levels, and atomic spectra. Nuclear and particle physics, such as radioactivity, nuclear reactions, and fundamental particles. Relativity, such as time dilation, length contraction, and mass-energy equivalence.
Enduring Understanding 1D: Classica mechanics can not describe all properties of objects.
Ask Ethan: If Gravity Attracts, How Can The ‘Dipole Repeller’ Push The Milky Way?
Ethan Siegel, Contributor. Feb 4, 2017
One of the most peculiar things about the Universe is how quickly the Milky Way appears to be moving. Despite having mapped out the cosmic masses nearby to unprecedented accuracy, there still doesn’t appear to be enough to cause the motion we actually experience. The idea of a “great attractor” doesn’t quite match up with what we see; what’s actually present isn’t quite “great” enough. But a new idea — that of a dipole repeller — might finally explain this longstanding conundrum. How would that work, and what it is, exactly? That’s what Darren Redfern wants to know:
What are the mechanics behind a dipole repeller? How can an area of space void of matter repulse galaxies to any meaningful extent (or at all?)?
If you were to look at all the galaxies accessible to us, you’d find, on average, that they were moving away from us at a specific rate: the Hubble rate. The farther away a galaxy is, the faster it appears to move away from us, and that’s a consequence of living in an expanding Universe governed by General Relativity. But that’s only on average. Each individual galaxy has an additional motion on top of that, known as peculiar velocity, and that’s due to the combined gravitational influence of every imperfection in the Universe on it.
The closest large galaxy to us, Andromeda, is actually moving towards us, thanks to the Milky Way’s gravitational pull. Galaxies in the closest giant cluster of galaxies — the Virgo cluster — get extra speeds of up to 2,000 km/s on top of the Hubble flow we see. And when we look at the Big Bang’s leftover glow, the Cosmic Microwave Background, we’re able to measure our own peculiar motion through the Universe.
This “cosmic dipole” we see is redshifted in one direction (meaning we’re moving away from it) and blueshifted in the other (meaning we’re moving towards it), and we can reconstruct the motion of the entire local group as a result. Us, Andromeda, Triangulum and everything else is moving at a speed of 631 km/s relative to the Hubble flow, and we know that gravitation must be the cause of this. When we look out at where the galaxies are located, we can map out their masses and how much of an attractive force they exert.
Thanks to the recent Cosmic Flows project, we’ve not only mapped out the nearby Universe to better precision than ever before, we discovered that the Milky Way lies on the outskirts of a giant collection of galaxies pulling us towards it: Laniakea. This is a significant contributor to our peculiar motion, but it isn’t enough to explain all of it on its own. Gravitational attraction is only half the story. The other half? It comes from gravitational repulsion. Let me explain.
Imagine you have a Universe where you have an equal number of masses evenly spaced everywhere you look. In all directions, at all locations, the Universe is filled with matter of even density. If you put an extra mass a certain distance to your left, you’ll be attracted towards your left, because of gravitational attraction.
But if you remove some of the mass that same distance to your right, you’ll also be attracted towards your left! In a perfectly uniform Universe, you’d be attracted to all directions equally, and that attractive force would cancel out. But if you remove some mass from one particular direction, it can’t attract you as strongly, and so you’re attracted preferentially in the other direction.
It’s not technically a gravitational repulsion, since gravitation is always attractive, but you’re less attracted to one direction than all the others, and so an underdense region effectively acts as a gravitational repeller. You can even imagine a situation where you have an overly dense region on one side of you with an underdense region on the other side. You’d experience the greatest magnitude of attraction and repulsion simultaneously. This is what the idea of the dipole repeller is.
It’s difficult to measure where an underdense region is, since regions of average density are fairly devoid of galaxies as well as the underdense ones. But a recently discovered cosmic void relatively nearby, and in the opposite direction to the large concentration of galaxies attracting us, seems to be responsible for roughly 50% of our peculiar motion, which is exactly the amount that was unaccounted for by the overdense regions alone.
Youtube video: The Dipole Repeller video, by Daniel Pomarède. produced as part of the following publication: “The Dipole Repeller” by Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène Courtois, Nature Astronomy 1, 0036 (2017).
At long last, this could be the solution to why our Sun, galaxy and local group all exhibit the motion that they do. Gravity is never repulsive, but a less attractive force in one direction than all the others behaves indistinguishably from a repulsion. We might distinguish between a pull in one direction and a push in the opposite direction, but in astrophysics, it’s all the same thing: forces and acceleration. It doesn’t have anything to do with dark energy or a mysterious fifth force; it’s simply having an excess of matter in one direction and a dearth of matter in nearly the exact opposite direction. The result? We move through the Universe in our own particular, peculiar fashion.
Reference: The dipole repeller, Yehuda Hoffman, Daniel Pomarède, R. Brent Tully & Hélène M. Courtois, Nature Astronomy 1, Article number: 0036 (2017).
Ethan Siegel, Contributor. Feb 4, 2017
Newton’s first law of motion: Inertia
Every object continues in it’s state of rest,
or of uniform velocity,
as long as no net force acts on it.
If at rest, objects require force to start moving
If moving, objects require a force to stop moving
An object at rest, stays at rest, unless accelerated by some external force
Elegantly illustrated by the leaves staying behind here (until gravity accelerates them!)
Animations showing Newton’s Law of Inertia
2 different definitions of mass
a measure of inertia
the quantity of matter
Don’t confuse mass with volume
Here are five cylinders of different metals:
they all are different volumes, yet all are of equal mass.
Lead, copper, brass, zinc, and aluminum.
How is this possible?
Somehow, more matter can be crammed into the same volume with denser materials
Less matter takes up the same volume in less-dense materials
Mass is not weight
Weight is how much a mass is pulled down by gravity.
This girl has the same mass on both worlds, yet her weight varies.
Mass is the quantity of matter in an object.
Weight is the force of gravity on an object.
One kilogram weighs (approximately) 10 Newtons
The gravity of Earth gives a downward acceleration,
g = 9.8 m/s2, to objects.
We often approximate this as g ≅ 10 m/s2
Because the object is being accelerated down, we feel this as “weight”.
Can we convert between mass and weight?
Strictly speaking – no. Why not?
The same mass will have different weights, when placed on different planets.
Can we convert between mass and weight, assuming that the object is here on Earth?
Oh, that’s different. Sure – for that one planet alone, we can convert between mass and weight. Here’s a conversion that’s valid only on Earth.
1 kg × g = 9.8 N (more exact)
1 kg × g ≅ 10N (approximation)
So these approximate conversions are useful.
1 kg is about 10 N
1/10 kg is about 1 N
100 kg is about a kN
Key Idea 5: Energy and matter interact through forces that result in changes in motion.
5.1 Explain and predict different patterns of motion of objects (e.g., linear and uniform circular motion, velocity and acceleration, momentum and inertia)
5.1i According to Newton’s First Law, the inertia of an object is directly proportional to its mass. An object remains at rest or moves with constant velocity, unless acted upon by an unbalanced force
Enduring Understanding 1C: Objects and systems have properties of inertial mass and gravitational mass that are experimentally verified to be the same. Inertial mass is the property of an object or a system that determines how its motion changes when it interacts with other objects or systems.
Astrophysicists may finally have discovered gravitational waves
In TechInsider, Dave Mosher writes:
Gravitational waves may have been detected for the first time, but we won’t know for sure until February 11, 2016 — when scientists will either confirm or dispel the rumors, sources close to the matter tell Tech Insider.
Detection of gravitational waves would be unprecedented. Whoever finds them is also likely to pick up a Nobel prize, since the phenomenon would confirm one of the last pieces of Albert Einstein’s famous 1915 theory of general relativity.
Confirming they exist would tell us we’re still on the right track to understanding how the universe works. Failing to find them after all these years might suggest we need to revisit our best explanation for gravity or rethink our most sensitive experiments, or that we simply haven’t looked long enough.
“Gravitational waves are ripples in the fabric of space-time, predicted by Einstein 100 years ago,” Szabi Marka, a physicist at Columbia University, told Tech Insider. “They can be created during the birth and collision of black holes, and can reach us from distant galaxies.”
Black holes are the densest, most gravitationally powerful objects in existence — so a rare yet violent collision of two should trigger a burst of gravitational waves that we could detect here on Earth.
Colliding neutron stars and huge exploding stars, called supernovas, are thought to generate detectable gravitational waves, too.
However, any sort of signal has eluded the planet’s brightest minds and the most advanced experiments for decades. Until now — maybe.
Columbia University in New York City is hosting a “major” event the morning of Thursday February 11, 2016, a source who is close to the matter, but asked not to be named, told Tech Insider.
Another source also confirmed the event but downplayed the significance of the event as anything “major.”
Regardless, several physicists and astronomers with expertise in gravitational wave science are scheduled to attend.
The topic? The latest data from the Laser Interferometer Gravitational-Wave Observatory (LIGO), a $1 billion experiment that has searched for signs of the phenomenon since 2002.
LIGO has two L-shaped detectors that are run and monitored by a collaboration of more than 1,000 researchers from 15 nations, and Marka is one of them.
Marka said that he and his colleagues have worked in the field for more than 15 years and that “these are very exciting and busy times for all of us.”
He also said that Advanced LIGO, an upgrade that went online in September 2015, finished a period of hunting for gravitational waves on January 12, 2016. (That was one day after we saw the first alluring rumors of detection.)
Both LIGO instruments are L-shaped arrays of lasers and mirrors that should be able to detect gravitational waves. Szabi Marka compared them to a pair of giant ears that can “hear” the spacetime ripples that result from black hole mergers, or some other catastrophic event in space. The closer a collision is to Earth, the “louder” the signal should be.
LIGO’s hearing is sensitive enough to detect mind-blowingly small disturbances of space, “much smaller than the size of the atoms the detector is built of,” he said. PhD Comics says LIGO’s level of sensitivity is “like being able to tell that a stick 1,000,000,000,000,000,000,000 meters long has shrunk by 5mm.”
Put another way, detecting a gravitational wave is like noticing the Milky Way — which is about 100,000 light-years wide — has stretched or shrunk by the width of a pencil eraser.
It would be no wonder why it has taken researchers so long to find gravitational waves; it’s terribly difficult work. (Even a truck driving on a nearby road can disturb LIGO, despite the instruments having state-of-the-art vibration-dampening equipment.)
It would also be no wonder why scientists might try to stay tight-lipped about the discovery yet “suck at keeping secrets just like everyone else,” as Jennifer Ouellette wrote at Gizmodo.
But at this point, there’s only one way to know for sure if the latest rumors are true: Wait until Thursday.
When Einstein’s General Relativity was first proposed, it was incredibly different from the concept of space and time that came before. Rather than being fixed, unchanging quantities that matter and energy traveled through, they are dependent quantities: dependent on one another, dependent on the matter and energy within them, and changeable over time. If all you have is a single mass, stationary in spacetime (or moving without any acceleration), your spacetime doesn’t change. But if you add a second mass, those two masses will move relative to one another, will accelerate one another, and will change the structure of your spacetime. In particular, because you have a massive particle moving through a gravitational field, the properties of General Relativity mean that your mass will get accelerated, and will emit a new type of radiation: gravitational radiation.
This gravitational radiation is unlike any other type of radiation we know. Sure, it travels through space at the speed of light, but it itself is a ripple in the fabric of space. It carries energy away from the accelerating masses, meaning that if the two masses orbit one another, that orbit will decay over time. And it’s that gravitational radiation — the waves that cause ripples through space — that carries the energy away. For a system like the Earth orbiting the Sun, the masses are so (relatively) small and the distances so large that the system will take more than 10^150 years to decay, or many, many times the current age of the Universe. (And many times the lifetime of even the longest-lived stars that are theoretically possible!) But for black holes or neutron stars that orbit each other, those orbital decays have already been observed.
We suspect there are even stronger systems out there that we simply haven’t been able to detect, like black holes that spiral into and merge with one another. These should exhibit characteristic signals, like an inspiral phase, a merger phase, and then a ringdown phase, all of which result in the emission of gravitational waves that Advanced LIGO should be able to detect. The way the Advanced LIGO system works is nothing short of brilliant, and it takes advantage of the unique radiation of these gravitational waves. In particular, it takes advantage of how they cause spacetime to respond.
Detection range of AdvLIGO