What If Earth Was Near the Center of the Milky Way?

 

Quantum Field Theory

In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity and quantum mechanics,but not general relativity's description of gravity. QFT is used in particle physics to construct physical models of subatomic particles and in condensed matter physics to construct models of quasiparticles.

QFT treats particles as excited states (also called quanta) of their underlying quantum fields, which are more fundamental than the particles. Interactions between particles are described by interaction terms in the Lagrangian involving their corresponding quantum fields. Each interaction can be visually represented by Feynman diagrams according to perturbation theory in quantum mechanics. 

 

 

Feynman diagram

 

In theoretical physics, a Feynman diagram is a pictorial representation of the mathematical expressions describing the behavior and interaction of subatomic particles. The scheme is named after American physicist Richard Feynman, who introduced the diagrams in 1948. The interaction of subatomic particles can be complex and difficult to understand; Feynman diagrams give a simple visualization of what would otherwise be an arcane and abstract formula.

 

see more

 The First Quantum Field Theory- By Space Time

  

Why Everything You Thought You Knew About Quantum Physics is Different - with Philip Ball

 

 

Quantum Decoherence

 

Quantum decoherence is the loss of quantum coherence. 

Quantum decoherence is the interaction of pure quantum states (like isolated particles) with macroscopic objects.

In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. 

As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics.

If a quantum system were perfectly isolated, it would maintain coherence indefinitely, but it would be impossible to manipulate or investigate it. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics.

Top 10 Unsolved Mysteries of the Strange Universe

 

Our world is shaped by all sorts of unseen forces that we don’t fully understand. So let’s take a look at some of the unsolved mysteries that plague the minds of physicists. From dark matter to the multiverse, it’s time to delve into a world in which truth is stranger than fiction.

1. Dark Matter

Planets, stars, asteroids, galaxies – the things that we can actually see – make up less than 5% of the total universe. Scientists think another ~25% is a strange substance called dark matter: we can’t see it, we don’t understand it, but we’re pretty sure it’s out there because everything moves to its gravitational tune.

Scientists believe that dark matter acts like a spider’s web, holding fast-moving galaxies together. And there’s so much of this stuff that it bends the appearance of space, so that when astronomers observe distant galaxies, they often appear distorted.

We have plenty of evidence that dark matter exists, but as for what it is, that remains a mystery. Some think dark matter is composed of an undiscovered particle or particles, others believe it’s an undiscovered property of gravity. Whatever the truth, dark matter is a real puzzle, and it’s proved hugely tricky to pin down.

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2. Dark Energy 

 

So if dark matter makes up 25% of the universe and normal matter makes up 5%, what about the other 70%?

We think that the remainder is entirely ‘dark energy’, powerful enough to tear the entire universe asunder. Whilst dark matter appears to mesh galaxies together, dark energy seems to want to push everything apart.

We all know that the universe is expanding, but it’s expanding more and more quickly than it should be, and scientists think that dark energy is the culprit.

But where’s dark energy coming from? Some believe that it’s produced from collisions between quantum particles, but no-one knows for sure.

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3. Quantum Entanglement – Spooky Action

Famously dubbed ‘spooky action at a distance’ by a dubious Albert Einstein, quantum entanglement is the phenomenon by which two particles in totally different parts of the universe can be linked to one another, mirroring the behaviour and state of their partner.

Quantum entanglement is a bit of a nuisance for classical physics, because it breaks some fundamental laws that we previously thought unbreakable. For particles to be connected across such vast distances, they must be sending signals to one another that travel faster than the speed of light: a feat previously considered impossible. What’s more, objects are only supposed to be affected by their surroundings; the notion of a particle being affected by something happening on the other side of the universe is just...strange.

Nonetheless, studies suggest that quantum entanglement does indeed exist. And even though we don’t understand it, we could still potentially use it. Because of its spooky characteristics, entanglement could eventually become the bedrock of next-generation computing and communications. So watch this space.

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4. Antimatter – The Evil Twins

Imagine yourself in opposite land. Black is white, up is down and...matter is antimatter?

It sounds crazy, but the sub-atomic particles that make up everything around us – electrons, protons and neutrons – all have evil twins. Antimatter particles are the same mass as normal particles, but the opposite electric charge.

And because of this, antimatter wipes out normal matter on contact. Poof! Both are destroyed in an instant. So antimatter has the potential to destroy us and everything we love. But fear not! There’s very little antimatter roaming around in the cosmos.

What’s more, antimatter could even prove useful. When antimatter and matter meet and destroy one another, it releases energy. In a PET scanner, anti-electrons are created and their annihilation in the body allows doctors to create sophisticated images. What’s more, scientists hope to one day use the energy released by antimatter/matter interactions to power spacecraft. So perhaps antimatter isn’t quite so evil after all.

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5. The Fermi Paradox – Little Green Men

The universe is really big; like, really REALLY big. In the grand scheme of things, human beings are just small fry. And yet, we currently seem to be the only ones at the party.

The Fermi Paradox refers to the contradiction between the high probability of extraterrestrial life and the apparent lack of evidence that such life exists.

We’ve now identified a handful of potentially habitable ‘Earth-like’ planets, but we’re still yet to see any signs of intelligent life from ‘out there’. So why the radio silence? There are numerous theories, ranging from the possibility that intelligent life is exceptionally rare or short-lived, to the notion that alien species are purposefully avoiding detection.

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6. Black Holes – Massive Monsters

A constant staple of sci-fi thrillers, black holes are violent, vastly destructive and invisible.

Black holes are regions of space in which the force of gravity is so powerful that everything around is drawn in. Not even light can escape, which is why we can’t see any of this going on.

Experts think there could be up to 100 million black holes in our galaxy alone, and these monsters can grow to become billions of times more massive than the sun. What’s more, at the centre of most galaxies, including our own, lurks a super-massive black hole.

But we don’t know what happens when objects pass through the centre. They might become ‘spaghettified’: stretched apart into long strings of matter; they could even be transported through a short-cut to a different part of our universe. Spooky.

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7. Space Roar – Silent Scream

In space, no—one can hear you scream. Right? Space is a vacuum, so there shouldn’t be any noise. And yet...you guessed it, there is.

The entire universe is alive with sound. And space roar isn’t just everyday sound; it’s actually these odd radio signals that we’ve detected throughout space. You know radio waves – we use them for communications: TV, cell phones, radios. Well, it looks like space is full of them, kicking out a noise that’s loud enough to drown out other signals – which is quite the nuisance for scientists trying to explore the cosmos.

So where’s the roar coming from? Some think that it’s leftover radiation from early stars, others believe that it’s gasses swirling around galaxy clusters, or else galaxies themselves. But for now, the roaring universe remains another unsolved (and noisy) mystery.

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8. Cosmic Rays – Ghostly Visitors

Space can be an intense place. But we’re totally shielded down here on Earth, aren’t we? Um...about that...

Cosmic rays are high energy particles that come from outer space, and regularly bombard Earth. Generally, these particles are completely harmless – our atmosphere kindly protects us. But there are some exceptions.

Up high in the stratosphere, cosmic rays can affect both human beings and electronics. Astronauts and aircraft crew are exposed to higher levels of radiation than the average person because of the presence of cosmic rays – although still not enough to be a major risk.

But electronics are the real potential victims here. Very rarely, a cosmic ray particle with enough energy can go straight into an electronic system, causing serious damage. The high energy particles can disrupt electronic data, leading to system crashes. And in an increasingly digital world, that’s not good news.

We’re only just beginning to learn about the potential impact that cosmic rays could have, and the race is on to find a solution.

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9. The Multiverse – The Doppelganger

Want to feel small? Well, here goes: humanity is but a tiny speck on a planet, within a galaxy, that itself makes up just a tiny, infinitesimal fraction of the universe. In fact, the universe is so vast, we’ve explored far less than 0.1% of it.

And yet, it’s entirely possible that our universe is just one of many others. The multiverse theory suggests that the cosmos contains multiple universes. Indeed, some scientists believe that there are an infinite number of universes; which means an infinite number of civilisations, histories, and versions of you.

However, the multiverse theory is still highly controversial, and we’re not likely to be charting parallel universes anytime soon. Sorry, guys.

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10. The Big Crunch – The End of the World as We Know It?

All good things must come to an end, even the universe itself. ‘But how’ you ask? Well, there are lots mind-blowing ideas out there.

In the past, the deliciously named ‘Big Crunch’ suggests a scenario in which the universe’s expansion – which has been going on since the Big Bang – tapers off and instead gives way to the force of gravity. As a result, everything – planets, galaxies, clusters – is drawn together into a single, dense point of mass, until everything is wiped out. Don't get too worried though: this is all many billions of years away.

These days, the Big Crunch is by no means the only theory out there concerning our inevitable demise. Other ideas include ‘the Big Freeze’, ‘the Big Bounce’ and ‘the Big Rip’. So rest assured, even if we don’t know how the universe ends, we know it’s going to be a pretty big event.

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For centuries, human beings have looked up at the stars and contemplated the universe and our place in it. From the dawn of time, we’ve always wanted to explore and make sense of the world, and yet, so many mysteries still remain.

But amidst all the lingering uncertainties, one thing is for sure: the universe is so much stranger and more complex than we could ever have imagined.

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Observable Universe

The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs (93 billion light-years, or 8.8×10²⁶ metres)

	Diameter 8.8×1026 m Volume 4×1080 m3 Mass (ordinary matter) 1.5×1053 kg Density (of total energy) 9.9×10−27 kg/m3 (equivalent to 6 protons per cubic meter of space) Age 13.799±0.021 billion years Average temperature 2.72548 K   Contents: Ordinary (baryonic *) matter (5%) Dark matter (27%) Dark energy (68%)  More Info (*) The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason they are sometimes called triquarks. These particles make up most of the mass of the visible matter in the universe and compose the nucleus of every atom.       The observable Universe might be 46 billion light years in all directions from our point of view, but there's certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that. Over time, we'll be able to see more of it, eventually revealing approximately 2.3 times as much matter as we can presently view. Frédéric MICHEL and Andrew Z. Colvin, annotated by E. Siegel

Shape of the Universe

 

 General relativity describes how spacetime is curved and bent by mass and energy (gravity). The topology or geometry of the universe includes both local geometry in the observable universe and global geometry. Cosmologists often work with a given space-like slice of spacetime called the comoving coordinates. The section of spacetime which can be observed is the backward light cone, which delimits the cosmological horizon. The cosmological horizon (also called the particle horizon or the light horizon) is the maximum distance from which particles can have traveled to the observer in the age of the universe. This horizon represents the boundary between the observable and the unobservable regions of the universe. The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model.

An important parameter determining the future evolution of the universe theory is the density parameter, Omega (Ω), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether Ω is equal to, less than, or greater than 1. These are called, respectively, the flat, open and closed universes.

 

The Arrow of Time

 

I put the water in a mug and place the mug in the microwave. It takes 2 minutes on high to bring it to a proper boil. The microwave beeps and I drop the tea bag in, Earl Grey of course, listening for the sound of bubbling as the tea bag creates additional nucleation sites within the water.

The tea piping hot, I pull it out and set it on the counter to steep.

Suppose I play that back in reverse now.

I take the piping hot tea from the counter. I place it in the microwave and pull out the tea bag. The microwave beeps and the clock runs up to 2 minutes while the tea inside cools from boiling to room temperature. The microwave radiation inside removes the heat and stores it as electricity. The water now room temperature I take out the cup and the water leaps up into the tap.

Why is it that it makes sense in one direction but not the other?

It all has to do with cause and effect.

The microwave is the cause of the water heating. The water heating is the effect. If you reverse them, it makes no sense.

The reason why things make sense in a particular order from cause to effect has to do with the 2nd law of thermodynamics which says that entropy, the level of disorder in a system, must always increase. Thus, a cause, it seems, must have lower entropy than its effect.

Suppose I were to drop the mug of tea on the floor. The mug smashes into pieces. The tea sprays out over the floor. The entropy of the mug and the tea has increased by being allowed to smash and spill. One must be the cause of the other.

Thermodynamics was developed in the late 18th century and boils down into a set of laws. The two main laws are these:

1. Energy cannot be created or destroyed.
2. Entropy must always increase with time.

Law #1 is an absolute law of physics. It is strictly obeyed, even in quantum mechanics.

Law #2 however is a “de facto” law of physics. It is not strictly obeyed, but only obeyed on average. Now, averages in thermodynamics tend to be pretty solid because, if every particle is making bets and there are trillions of trillions of particles, then the law of large numbers says that the average is as good as law. But, it is still not a strict law of physics.

Instead, the laws of physics that are strict are all “time reversible” meaning that causes and effects are actually interchangeable. Theoretically, in a closed system, it is possible for the cup to leap up and reassemble itself. It is just not probable.

For any physics enthusiast, this is all common knowledge, but what isn’t commonly known is why it must be so. Why does entropy increase in one direction and not the other? If all physical processes are time reversible, then it should allow some processes to increase in entropy in one direction while others increase in entropy in the reverse time direction. Yet, we do not see this actually happen. All physical processes increase in entropy in only one direction.

The overarching question is: why?

This problem is called the Problem of the Arrow of Time.

Quite a few solutions have been proposed to the dilemma:

1. The universe simply began in a low entropy state. This is ad hoc and hardly explains why entropy increase is always observed.
2. The expansion of the universe forces the thermodynamic arrow of time in one direction. Why this should be so is unknown, since the thermodynamic arrow applies to all systems no matter how small.
3. Physical laws are incomplete and should reflect irreversible processes, things that are not time symmetric. Current physical laws depend on this symmetry, so it would be a major breakthrough to find that time symmetry is not respected (this would be called a Charge-Parity-Time or CPT violation and would be as impactful as showing that Lorentz symmetry is violated).
4. Quantum decoherence — this interaction of pure quantum states (like isolated particles) with macroscopic objects — causes the arrow of time. To me this is just moving the question from entropy to decoherence. Why decohere in one direction and not the other when quantum mechanics is time reversible?

The most powerful resolution to the problem, however, focuses not on a defect of physical law but on us, or more properly, any information-based system.

This is the information or memory-based arrow of time resolution which states that the information content of a system, i.e., its memory increases with increasing entropy. Thus, all information storage systems experience time as flowing towards increasing entropy despite time being reversible.

Like the Silence from Doctor Who, any processes that evolve towards lower entropy are not remembered.

In 1950 Erwin Schroedinger, one of the founders of quantum mechanics, described this point of view using statistical mechanics. Suppose you have a system in non-equilibrium, such as a crystal that has gained energy and is in the process of melting into a liquid. Now, before it has reached an equilibrium, you divide it into four isolated pieces.

Schroedinger showed that each of these systems will pick a time direction and evolve to equilibrium in that direction. The evolution of the closed and isolated system will define a direction of time for itself. Therefore, the time t of Einstein, Newton, and even Boltzmann is different from the phenomenological, that is, observed, arrow of time which is t or -t. Unlike physical time, phenomenological time, the time we experience, is an entropy gradient, a path of increase that is defined through spacetime.

The information theory of time combines the entropy gradient arrow of Schroedinger with the information theory of Shannon.

Claude Shannon was just another researcher at Bell Labs working on the telephone system. Given that the phone system is intimately related to the transmission of information, he started thinking about how to characterize information in terms of entropy. He developed a theory that entropy is related to the number of bits, binary 0’s and 1’s, that a system can carry. This has to do with the number of different configurations a system can have. A low entropy system can only have a few configurations, so only a few bits. A high entropy system can have many configurations, so many bits.

This relationship suggests that the flow of time we perceive and the entropy gradient are related to the direction information grows. More importantly, it also suggests that while entropy growth adds memories, entropy reduction takes them away.

This theory was made more secure using quantum information theory in 2009 in a stunning paper by Italian researcher Lorenzo Maccone. Maccone showed that phenomenological time can proceed in either time direction as Schroedinger indicated and as time reversibility dictates. Quantum mechanics, however, says that processes that decrease entropy cannot leave any trace that they ever happened. That is, entropy decreasing processes actually remove the fact that they happened as if time were running in reverse.

Maccone works out his theory using the mathematics of quantum mechanics, using wavefunctions in Hilbert spaces (infinite dimensional spaces that guarantee a 2-way Fourier transform exists). To illustrate the idea, Maccone offers the following thought experiment:

Alice is in a lab that is perfectly isolated so that she evolves as a complete quantum state.

Bob sends her some energy in the form of light.

Alice detects the energy with detectors that heat up when exposed to the light. She is not making any measurements of Bob’s light modes because the energy has been converted into entropy, as thermodynamic heat.

Bob wants his energy back, i.e., wants to time reverse the energy process. In order to do so, he must remove the entropy. To do that, he has to remove any correlation between the energy he sent her and her instruments, including any lab notes she made. If he had that power, he could reduce her lab back to a lower entropy state and recover the energy. But, the result would be that Alice would have no memory of the event, lab notes empty, detectors cool.

Thus, in the process of reversing entropy, a system must remove any correlations with its having happened. No correlations, no information, no memory.

In Schroedinger’s example, therefore, those of the four closed systems that run in reverse time cannot be studied by physics because it is as if their evolution did not happen at all. So rather than seeing the systems as if they were running in reverse, we would not be aware of them. Our memory and the states of any other apparatus we used to study them would disappear.

Indeed, closed systems can run in reverse time all the “time” in our universe but we have no way of detecting them because they abscond with their information content. It would be like trying to see the future.

An additional, and more strange conclusion, is that we would be able to “remember” those processes in the past and lose those memories in the future. That is, a process reversed from our own phenomenological direction removes information content (quantum correlations) about its existence, which suggests that in the past that information existed and decreased.

Could this mean that we can remember the future?

Well, yes and no, as macroscopic beings we are highly correlated with one another and one of the requirements for systems to be moving in opposite phenomenological directions is that they have to be almost perfectly isolated from one another.

Ignoring that fact, let’s go back to the cup of tea:

While the tea cup can leap up from the floor and reassemble itself in my hand, quantum mechanics does not allow me to perceive it because my brain is running in the other phenomenological direction. Instead, I would perceive the smashed cup and remember its having fallen. Then I would gradually lose my memory of its falling as it leapt into the air until I’m holding the cup with no memory that it fell at all.

If you’ve seen the movie Memento, you’ll know what I’m talking about.

Maccone concludes that the universe itself may be in a zero-entropy state but we perceive its evolution from low to high entropy because of the trick of information systems.

There may, in fact, be another universe overlapping ours but isolated evolving backwards in time from us that we cannot perceive. Perhaps there are entire alien worlds out there, beyond the observable universe, evolving backwards.

 Source:

Brown, Harvey R., and Jos Uffink. “The origins of time-asymmetry in thermodynamics: The minus first law.” Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 32.4 (2001): 525–538.

Maccone, Lorenzo. “Quantum solution to the arrow-of-time dilemma.” Physical review letters 103.8 (2009): 080401.

 

 

 

Timeline of the universe.

Timeline of the universe. A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of "inflation" produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP (*) was emitted about 375,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe.

 

 (*)
The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP), was an uncrewed spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) – the radiant heat remaining from the Big Bang

What Is a Black Hole?

 

 

 

 

 

 

 

 

 

 

 

 

An artist's drawing a black hole named Cygnus X-1. It formed when a large star caved in. This black hole pulls matter from blue star beside it.

Credits: NASA/CXC/M.Weiss

 

 

 

An artist's drawing shows the current view of the Milky Way galaxy. Scientific evidence shows that in the middle of the Milky Way is a supermassive black hole.

Credits: NASA/JPL-Caltech

 

 

This image of the center of the Milky Way galaxy was taken by the Chandra X-ray Observatory.

Credits: NASA/CXC/MIT/F.K. Baganoff et al.

 

 

Sagittarius A* is the black hole at the center of the Milky Way galaxy.

Credits: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.

Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

How Big Are Black Holes?

 
Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object.

Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

How Do Black Holes Form?

 
Scientists think the smallest black holes formed when the universe began.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists think supermassive black holes were made at the same time as the galaxy they are in.

If Black Holes Are "Black," How Do Scientists Know They Are There?

 
A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth?

 
Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.

Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

How Is NASA Studying Black Holes?
NASA is using satellites and telescopes that are traveling in space to learn more about black holes. These spacecraft help scientists answer questions about the universe.

Extrasolar planet

 

Extrasolar planet, also called exoplanet, any planetary body that is outside the solar system and that usually orbits a star other than the Sun. Extrasolar planets were first discovered in 1992. More than 4,000 are known, and about 6,000 await further confirmation.

Because planets are much fainter than the stars they orbit, extrasolar planets are extremely difficult to detect directly. By far the most successful technique for finding and studying extrasolar planets has been the radial velocity method, which measures the motion of host stars in response to gravitational tugs by their planets. Swiss astronomers Michel Mayor and Didier Queloz discovered the first planet using this technique, 51 Pegasi b, in 1995. (Mayor and Queloz won the 2019 Nobel Prize in Physics for their discovery.) Radial velocity measurements determine the sizes and shapes of the orbits of extrasolar planets as well as the lower limits of the masses of these planets. (They provide only lower limits on planetary mass because they measure just the portion of the star’s motion toward and away from Earth.)

A complementary technique is transit photometry, which measures drops in starlight caused by those planets whose orbits are oriented in space such that they periodically pass between their stars and the telescope; transit observations reveal the sizes of planets as well as their orbital periods. Radial velocity data can be combined with transit measurements to yield precise planetary masses as well as densities of transiting planets and thereby limit the possible materials of which the planets are composed. Spectroscopic studies that rely on variations in the depth of the transit with wavelength have been used to identify gases such as hydrogen, sodium, and methane in the upper atmospheres of some close-in giant planets. The first detected transiting planet was HD 209458b in 1999. Both radial velocity and transit techniques are most sensitive to large planets orbiting close to their stars.

Three other techniques that have detected extrasolar planets are pulsation timing, microlensing, and direct imaging. Pulsation timing measures the change in distance between the signal source and the telescope by using the arrival times of signals that are emitted periodically by the source. When the source is a pulsar (a rotating, magnetized neutron star), current technology can detect motions in response to a planet whose mass is as small as that of Earth’s Moon, whereas only giant planets can be detected around pulsating normal stars. The first extrasolar planets to be discovered were found in 1992 around the pulsar PSR 1257+12 by using this method. Microlensing relies upon measurements of the gravitational bending of light (predicted by Albert Einstein’s general theory of relativity) from a more distant source by an intervening star and its planets. This technique is most sensitive to massive planets orbiting hundreds of millions of kilometres from their star and has also been used to discover a population of free-floating giant planets that do not orbit any star. Direct imaging can be done by using starlight reflected off the planet or thermal infrared radiation emitted by the planet. Imaging works best for planets orbiting those stars that are nearest to the Sun, with infrared imaging being especially sensitive to young massive planets that orbit far from their star.

Physical properties

Between 5 and 10 percent of stars surveyed have planets at least 100 times as massive as Earth with orbital periods of a few Earth years or less. Almost 1 percent of stars have such giant planets in very close orbits, with orbital periods of less than one week. Some of these planets seem to be distended in size as a result of heating by their stars. More than 20 percent of stars have somewhat smaller nearby planets, with sizes of several to a few tens of Earth masses and with orbital periods of less than three months.

 

The most massive planets that transit their stars are made primarily of the two lightest elements, hydrogen and helium, as are the Sun and its two largest planets, Jupiter and Saturn. The term Jupiters is often used to describe these worlds, and the term hot Jupiters is applied to those massive planets orbiting very near their stars. Similarly, the terms Neptunes and hot Neptunes refer to planets less than about 10 percent of Jupiter’s mass, and the term super-Earths refers to those planets that may well be rocky bodies only a few times as massive as Earth. The divisions between these various classes are not well defined, and these terms may well overemphasize the similarities with particular objects in the solar system. However, the lowest-mass transiting planets contain larger fractions of heavier elements than do transiting giant planets. An analogous relationship between planetary mass and composition exists within the solar system.

Nevertheless, many of the mentioned properties of extrasolar planets are in sharp contrast to those in the solar system. Jupiter, which takes nearly 12 years to travel around the Sun, has the shortest orbital period of any large planet (more massive than Earth) in the solar system. Even the closest planet to the Sun, Mercury, requires 88 days to complete an orbit. Within the solar system, the planets, especially the larger ones, travel on nearly circular paths about the Sun. Most extrasolar giant planets with orbital periods longer than two weeks have elongated orbits. Models of planetary formation suggest that giant extrasolar planets detected very near their stars formed at greater distances and migrated inward as a result of gravitational interactions with remnants of the circumstellar disks from which they accumulated. The free-floating giant planets had a different history in that they were probably formed in circumstellar disks but were ejected from their solar systems through gravitational interactions.

Stars that contain a larger fraction of heavy elements (i.e., any element aside from hydrogen and helium) are more likely to possess detectable gas giant planets. More massive stars are more likely to host planets more massive than Saturn, but this correlation may not exist for smaller planets. Many extrasolar planets orbit stars that are members of binary star systems, and it is common for stars with one detectable planet to have others. The planets detected so far around stars other than the Sun have masses from nearly twice to thousands of times that of Earth. All appear to be too massive to support life like that of Earth, but this too is the result of detection biases and does not indicate that planets like Earth are uncommon.

Directions for future research

Research in the field of extrasolar planets is advancing rapidly as new technologies enable the detection of smaller and more distant planets as well as the characterization of previously detected planets. Almost all the extrasolar planetary systems known appear very different from the solar system, but planets like those within the solar system would with current technology be very difficult to find around other stars. Thus, as most of those stars surveyed do not have detectable planets, it is still not known whether the solar system is normal or unusual.

The U.S. National Aeronautics and Space Administration’s Kepler mission, launched on March 6, 2009, used transit photometry from space to achieve unprecedented sensitivity for small planets with orbital periods of up to two years and found that about 1 in 4 Sun-like stars had a planet analogous to Earth. In 2010 the Kepler team announced its first discoveries: four gas giant planets somewhat larger than Jupiter and one planet slightly larger than Neptune that is more enriched in heavy elements; all five orbit very close to their stars. In 2011 the Kepler team announced that they had discovered a planet, Kepler-22b, that was the first to be found in the habitable zone of a star like the Sun. They also discovered the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f (with radii 0.87 and 1.03 times the radius of Earth, respectively). By the end of its mission in 2018, Kepler had discovered 2,741 planets, about two-thirds of all known extrasolar planets. Thousands more candidate planets awaited confirmation.

Other projects have also studied transits to discover extrasolar planets. The most notable such discovery has been the TRAPPIST-1 system. Both the TRAPPIST telescope on Earth and the Spitzer Space Telescope were used to discover seven Earth-sized planets in this system, three of which are in the habitable zone. The Transiting Exoplanet Survey Satellite (TESS), launched on April 18, 2018, is designed to study more than 200,000 stars in an effort to detect hundreds of Earth-sized planets.

 

Quantum Systems - Weird Features

 The quantum systems have some weird features:

- Wave–particle duality 

Wave–particle duality is the concept in quantum mechanics that every particle or quantum entity    may be described as either a particle or a wave. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects.

- Superposition  

The feature of a quantum system whereby it exists in several separate quantum states at the same time. Each electron, until it is measured, will have a finite chance of being in either state. 

Superposition is a system that has two different states that can define it and it's possible for it to exist in both. For example, in physical terms, an electron has two possible quantum states: spin up and spin down.

Quantum superposition arises because, at the quantum scale, particles behave like waves. Similar to the way in which multiple waves can overlap each other to form a single new wave, quantum particles can exist in multiple overlapping states at the same time.

 

- Quantum nonlocality 

The concepts of entanglement and nonlocality are now recognized as defining features of quantum theory. Distant observers sharing a quantum system prepared in an entangled state, can establish strong correlations, which could provably not been achieved in any theory satisfying a natural constraint of locality.

- Quantum Tunnelling 

 
Quantum tunnelling or tunneling (US) is the quantum mechanical phenomenon where a wavefunction can propagate through a potential barrier. Some authors also identify the mere penetration of the wavefunction into the barrier, without transmission on the other side as a tunneling effect.

For more info please whatch the attached video

What is the difference between Quantum Physics, Quantum Theory, Quantum Mechanics, and Quantum Field Theory

 

Quantum mechanics (QM – also known as quantum physics, or quantum theory) is a branch of physics which deals with physical phenomena at microscopic scales, where the action is on the order of the Planck constant. Quantum mechanics departs from classical mechanics primarily at the quantum realm of atomic and subatomic length scales. Quantum mechanics provides a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. Quantum mechanics is the non-relativistic limit of Quantum Field Theory (QFT), a theory that was developed later that combined Quantum Mechanics with Relativity.Quantum field theory (QFT) is a theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics and quasiparticles in condensed matter physics, by treating a particle as an excited state of an underlying physical field.

The major way this framework differs from quantum mechanics is that not merely the particles, but also the fields are quantized.You need a quantum field theory to successfully describe the interactions between not merely particles and particle or particles and fields, but between fields and fields as well.

 

A Brief History of Quantum Mechanics

 

Entanglement

 

Even more surprising than superposition, quantum theory predicts that entities may have correlated fates. That is, the result of a measurement on one photon or atom leads instantaneously to a correlated result when an entangled photon or atom is measured.

For a more intuitive grasp of what we mean by “correlated results,” imagine that two coins could be entangled (there is no known way of doing this with coins, of course). Imagine one is tossing a coin. Careful records show it comes up “heads” about half the time and “tails” half the time, but any one result is unpredictable. Tossing another coin has similar, random results, but surprisingly, the records of the coin tosses show a correlation! When one coin comes up heads, the other coin comes up tails and vice versa. We say that the state of the two coins is entangled. Before the measurement (the toss), the outcome is unknown, but we know the outcomes will be correlated. As soon as either coin is tossed (measured), the fate of tossing the other coin is sealed. We cannot predict in advance what an individual coin will do, but their results will be correlated: once one is tossed, there is no uncertainty about the other.

This imaginary coin tossing is only to give the reader a sense of entanglement. Although one might come up with a classical explanation for these results, multitudes of ingenious experiments have confirmed the existence of entanglement and ruled out any possible classical explanation. Over several decades, physicists have continually refined these experiments to remove loopholes in measurement accuracy or subtle assumptions. All have confirmed the predictions of quantum mechanics.

With actual particles any measurement collapses uncertainty in the state. A real experiment would manufacture entangled particles, say by bringing particles together and entangling them or by creating them with entangled properties. For instance, we can “downconvert” one higher energy photon into two lower energy photons which leave in directions not entirely predictable. Careful experiments show that the directions are actually a superposition, not merely a random, unknown direction. However, since the momentum of the higher energy photon is conserved, the directions of the two lower energy photons are entangled. Measuring one causes both photons to collapse into one of the measurement bases. However, once entangled, the photons can be separated by any distance, at any two points in the universe; yet measuring one will result in a perfectly correlated measurement for the other.

Even though measurement brings about a synchronous collapse regardless of the separation, entanglement doesn’t let us transmit information. We cannot force the result of a measurement any more than we can force the outcome of tossing a fair coin (without interference).

 

Source:www.sciencedirect.com

 

Many Worlds Interpretation (MWI)

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse^.This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic. Many-worlds is also called the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957.

The many-worlds theory is the most straightforward approach to understanding quantum mechanics. It accepts the reality of the wave function. In fact, it says that there is one wave function, and only one, for the entire Universe. Further, it states that when an event happens in our world, the other possibilities contained in the wave function do not go away. Instead, new worlds are created, in which each possibility is a reality. Don’t worry about those extra worlds, we can’t see them, and if the many-worlds theory is true, we won’t notice the difference. The many other worlds are parallel to our own, but so hidden from it that they “might as well be populated by ghosts”.

The many-worlds interpretation is distinct from the  multiverse hypothesis, which envisions other universes, born in separate Big Bangs, that have always been physically disconnected from our own.

Sean Carroll explains: what is the many-worlds interpretation:

 

What's Higgs Boson

 

Why has the Higgs been the subject of so much hype, funding, and (mis)information? For two reasons. One, it was the last hold-out particle remaining hidden during the quest to check the accuracy of the Standard Model of Physics. This meant its discovery would validate more than a generation of scientific publication. Two, the Higgs is the particle which gives other particles their mass, making it both centrally important and seemingly magical. We tend to think of mass as an intrinsic property of all things, yet physicists believe that without the Higgs boson, mass fundamentally doesn’t exist.

So what's the Higgs boson, and why are people spending billions of dollars to find that god-danged subatomic particle?

First, a little context: The Higgs particle, and its associated field, were hypothesized back in the 1960s by British physicist Peter Higgs and others to fill a weird gap in the Standard Model, one of physics' most successful theories. The model as it stood had no mechanism to explain why some particles are massless (such as the photon, which is the quantum bit for light and other types of electromagnetic radiation), while other particles have varying degrees of mass (such as the W and Z bosons, which play a part in the weak nuclear force). By rights, all particles should be without mass and zipping around freely.

The Higgs mechanism sets up a field that interacts with particles to endow them with mass, and the Higgs boson is the particle associated with that field — just as photons are associated with an electromagnetic field. For more than four decades, physicists have assumed that the Higgs field existed, but found no experimental evidence for it. It requires a super-powerful particle smasher such as the Large Hadron Collider to produce energies high enough to knock a Higgs boson into existence under controlled conditions.

But the heavy particles created in a collider exist for just an instant before they decay into lighter particles. The LHC's physicists have been looking for particular patterns in the spray of particles that match what they'd expect to see from the decay of the Higgs boson. They've collected data for roughly a quadrillion proton-on-proton collisions, and on Wednesday they'll announce the status of the Higgs search based on those conclusions.

 

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