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Re: I just realised how to travel back in time!!
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| 07 Jun 2016 02:09 AM |
Because of gravity, if you drop something, it falls down, instead of up. Well, everybody knows that! But, what does this really mean? What is gravity?
Spiral galaxy Gravity has played a big part in making the universe the way it is. Gravity is what makes pieces of matter clump together into planets, moons, and stars. Gravity is what makes the planets orbit the stars--like Earth orbits our star, the Sun. Gravity is what makes the stars clump together in huge, swirling galaxies.
Albert Einstein A great scientist, Albert Einstein, who lived in the 20th century, had a new idea about gravity. He thought that gravity is what happens when space itself is curved or warped around a mass, such as a star or a planet. Thus, a star or planet would cause kind of a dip in space so that any other object that came too near would tend to fall into the dip.
Quite a number of experiments show that Einstein was right about this idea and a lot of others. But there are questions for which even Einstein had no answers.
Model of an atom. For example, if gravity is a force that causes all matter to be attracted to all other matter, why are atoms mostly empty space inside? (There is really hardly any actual matter in an atom!) How are the forces that hold atoms together different from gravity? Is it possible that all the forces we see at work in nature are really different sides of the same basic force or structure? A delicately patterned fern leaf. Shell of a pearly nautilus. An exquisitely detailed flower.
Could some of the same laws of nature be at work in the designs of all things in the pictures above?
These are big questions that scientists and ordinary people like us have wondered about for a long time. For a long time we haven't known how to go about finding the answers, other than trying to work things out on paper.
But now NASA has a special program, called Fundamental Physics
. . . for seeking answers to these and other mysteries of the universe. Fundamental Physics hopes to do two things:
To discover and explore fundamental physical laws governing matter, space, and time.
To discover and understand the basic rules nature uses to build the complex and beautiful structures we see around us.
International Space Station Over the years, scientists and engineers have developed new technologies and instruments that will help us understand nature. Now we can take these new instruments into space and do experiments where the forces of gravity are very, very small (like when the Space Shuttle or the International Space Station are orbiting Earth in "free fall"). This way, scientists can do very delicate experiments to see what single atoms do under special conditions.
NASA hopes these experiments will help us understand our universe and ourselves. NASA also hopes the experiments will help develop technologies that will benefit people in their everyday lives. |
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Nextly
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| Joined: 11 Feb 2012 |
| Total Posts: 38639 |
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| 07 Jun 2016 02:10 AM |
that doesn't make sense due to these reasons:
It is not possible to simultaneously determine the position and momentum of anything with infinite precision. This is known as the Heisenberg Uncertainty Principle. The uncertainty x in position and the uncertainty p in momentum are related by: x p ≥ h / 2 This can also be written (by doing a little calculus) as: E t ≥ h / 2 which means that you cannot simultaneously know the energy of a particle, and also how long it has had that energy, with infinite precision. Classically, I can (in principle) measure x for a particle to any accuracy I choose. I can also, simultaneously, measure its velocity (and thus its momentum, since p = mv) to whatever accuracy I choose by timing it between two points. Thus, x p = 0 is allowed classically. But quantum mechanics has no real “particles”, only probability waves. To determine the frequency of any wave to infinite precision, I would need to count an infinite number of wave crests as they pass by. But an infinite number of wave crests implies that the wave stretches from plus infinity to minus infinity – in other words, x is infinite because the wave is everywhere. If I constrain the wave to be a “packet” that is confined within some space x (like the wave splash from a rock falling in the water), then I cannot have an infinite number of wave crests to count. The smaller I make x then the fewer crests there are, and as x goes to zero there aren’t any wave crests left at all, so the frequency becomes completely unknown. Since frequency is related to both E and p in quantum mechanics (by the de Broglie and Einstein equations), this means that p and x cannot simultaneously be made zero. The size of Planck’s constant tells us how much uncertainty the universe can tolerate, that is, how far away it is from a classical Newtonian universe in which h = 0. The E t ≥ h / 2version of the Uncertainty Principle means that conservation of momentum and conservation of energy can be “suspended” by quantum particles. Strange as it may seem, uncertainty applies even to empty space – how can you know there is nothing there during a given time t when it is impossible to know the energy of anything there to better than E? In fact, it’s not only possible for particles with energy less than E to appear from nothing, a essentially infinite number of them leap in and out of existence every second. (Physicists consider “empty” space to be much closer to a furiously boiling cauldron of quantum soup than to anything empty.) The critical restriction on such particles is that whatever energy they possess (mainly their rest mass, but also other types of energy), the absolute maximum amount of time they can exist is given by the Uncertainty Principle. Particles whose energy is “borrowed” from uncertainty and cannot exist longer than t are called virtual particles. The only difference between, say, an electron and a virtual electron, is that the electron possesses its mass-energy and can last forever, but the virtual electron has borrowed its mass-energy from the Uncertainty Principle, and at the end of t it must vanish, like Cinderella’s carriage at midnight. If you run through the numbers, it turns out that a virtual electron cannot exist for longer than about 10-21 sec. That may sound impossibly short, but the particles which mediate the strong and weak nuclear forces have lifetimes roughly 200 times shorter even than this. Note – the Uncertainty Principle does not apply to charge, lepton number, or baryon number. Those are still conserved at all times. Summary of Important Ideas in Nuclear Physics 1) The nuclei of atoms are made up of protons and neutrons. As a group, they are called nucleons (nuclear physics terminology) or baryons (particle physics terminology). Their physical properties when they are free in space are: Nucleon Charge Mass Half Life Proton +1 e 1.6725 x 10-27 kg infinite Neutron 0 1.6748 x 10-27 kg 10.6 minutes Since like charges repel, the electrostatic repulsive force between the protons in a nucleus is enormous. Nuclei are held together against this repulsion by the strong nuclear force (or strong force, for short). The strong force is one of the four fundamental forces in the Universe. It operates only between baryons, i.e., protons and neutrons. Electrons, photons, and neutrinos are not affected by it. 2) The chemical properties of an element are determined solely by the number of protons in its nucleus. The proton number is the same as the position of the element within the periodic table. Atoms with the same number of protons, but different numbers of neutrons, have exactly the same chemical properties and differ only in mass. Nuclei of the same element (same number of protons) but with differing numbers of neutrons are called isotopes. 3) The strong force, unlike gravity and electromagnetism, is extremely short ranged. You can think of a free neutron passing a nucleus almost like a golf ball on a putting green: either you hit the cup exactly and go in, or you roll past it unaffected. This means that a nucleus can only grow so large, because the strong force is so shortranged that it more-or-less acts like a chain linking each nucleon to the next – and like a chain, its strength does not change as you add more links. But the electromagnetic force has an infinite range, so all the protons in a nucleus participate in repelling each other. This repulsion rapidly becomes more powerful as you add more protons, so sooner or later the electromagnetic force must overwhelm the “chain-link” attraction of the strong force, and disrupt the nucleus. The heaviest stable element is bismuth, which has 83 protons. 4) Stable nuclei consist of roughly equal numbers of protons and neutrons. Without the presence of neutrons to provide additional strong force, not even two protons can be held together against their electrostatic repulsion, never mind 83. Neutrons, on the other hand, are not stable. Left alone, they will decay into a proton, an electron, and a neutrino. But inside a nucleus, formidably complicated rules of quantum mechanics allow the strong force to “forbid” the neutron’s decay – provided there are enough protons around. So, a nucleus cannot have too few neutrons, or the protons will electrostatically fly apart. But it also cannot have too many neutrons, because eventually the strong force can no longer “forbid” their decay. 5) Actually, it is naive to think of a proton-neutron pair as consisting of a proton, separately, and a neutron, separately. Like everything in quantum mechanics, such a pairing is a probabilistic entity. It is closer to reality to imagine the proton as a black ball, and the neutron as a white ball, and then to visualize them continuously swapping identities so fast that they just blur into two grey balls. There is certainly a proton and a neutron there – but if you try to capture one of them, it’s a 50/50 proposition which you’ll catch. 6) One of the consequences of Item (2) in the Quantum Physics Summary is that quantum particles have a certain probability, albeit usually hyper-small, of appearing anywhere in the Universe. This is the cause behind one type of natural radioactivity, known as -decay. If a nucleus is very large, or has an excess of protons or neutrons, then the strong force can just barely hold it together. Classically, of course, bowling balls do not roll up hills regardless of whether they are barely sloping or look like Mt. Everest. Quantum mechanically, however, the probability that a quantum particle will leap to the other side of an energy “hill” becomes much larger if the hill is small. In the case of a radioactive nucleus such as uranium, the energy needed to liberate protons or neutrons is small enough that, sooner or later, some will quantum-mechanically tunnel through the strong-force barrier and appear outside the nucleus. Then, they speed away, and this is what we call radioactivity. 7) There are three forms of natural radioactivity, known as - - and -decay, respectively. An -particle consists of two protons and two neutrons. (This is a helium nucleus.) -decay proceeds via the mechanism described in Item (6). A -particle is a high-energy electron. In some barely-bound nuclei, a second, much weaker nuclear force known as the weak force can compete with the strong force and cause a neutron to decay even though it is in the presence of protons. As noted in Item (4), the decaying neutron gives off an electron, and this is the -particle. A -particle is a high-energy photon, also called a -ray. -rays are exactly the nuclear equivalent of spectral lines in atoms: if the protons and neutrons shift between energy levels within the nucleus, they must either give up or absorb a photon. But, since the strong force between protons and neutrons is much stronger than the electromagnetic force between a proton and an electron, nuclear energy levels are much further apart, and thus the photons given off are very energetic. As a rule, -emitters are excited nuclei that have been created in the aftermath of either -decay or -decay. 8) The three forms of radioactivity have very different abilities to penetrate matter. -particles are very easy to stop (a sheet of cardboard will do it); -particles are harder to stop (you need a sheet of aluminum); and -rays are the most difficult to stop (they can penetrate over an inch of lead). If you consider microscopically what is happening, this is easy to understand. Matter consists of charged particles: positively charged nuclei and negatively charged electrons. -particles are heavy, thus relatively slow-moving, and have two electric charges. So they are pummeled by attractive and repulsive forces as they move through matter, and quickly lose their energy. -particles travel much faster and have only one charge, thus they can penetrate matter more easily. -rays have no charge at all and are moving at the speed of light, thus they are very penetrating. They more-or-less have to hit a nucleus head-on to be stopped. 9) Since the chemical properties of an element (i.e., its position in the periodic table) are determined solely by the number of protons in its nucleus, - and -decay change the chemical identity of the decaying nucleus. -decay subtracts two protons, so the element moves down two notches in the periodic table. -decay changes a neutron into a proton, so the element moves up one notch in the table. 10) Radioactivity is characterized by the half-life of the nucleus. Definition: if I have x atoms of any radioactive element at some time, then one half-life later I will have ½ x of those atoms remaining. And one half-life later I have a half of the half, or one-quarter, of the original atoms left. And so forth. The half-life is a statistical concept. It is completely impossible to determine whether any given atom will decay one microsecond from now or in a billion years. You can only say that it has a 50-50 chance of doing so in the time of one half-life.
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| 07 Jun 2016 02:11 AM |
the power to turn back tiiiiiimeeeeeeee
does spider habe de puss puss |
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| 07 Jun 2016 02:12 AM |
w hat
and i have crippling depression |
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| 07 Jun 2016 02:14 AM |
wow, u literally blew my mind
mainly because Edwards-Casimir quantum vacuum drive A hypothetical drive exploiting the peculiarities of quantum mechanics by restricting allowed wavelengths of virtual photons on one side of the drive (the bow of the ship); the pressure generated from the unrestricted virtual photons toward the aft generates a net force and propels the drive. See Casimir effect.
Ehrenfest paradox (Ehernfest, 1909) The special relativistic "paradox" involving a rapidly rotating disc. Since any radial segment of the disc is perpendicular to the direction of motion, there should be no length contraction of the radius; however, since the circumference of the disc is parallel to the direction of motion, it should contract. Einstein field equation The cornerstone of Einstein's general theory of relativity, relating the gravitational tensor G to the stress-energy tensor T by the simple equation G = 8 pi T. Einstein-Podolsky-Rosen effect; EPR effect Consider the following quantum mechanical thought-experiment: Take a particle which is at rest and has spin zero. It spontaneously decays into two fermions (spin 1/2 particles), which stream away in opposite directions at high speed. Due to the law of conservation of spin, we know that one is a spin +1/2 and the other is spin -1/2. Which one is which? According to quantum mechanics, neither takes on a definite state until it is observed (the wavefunction is collapsed). The EPR effect demonstrates that if one of the particles is detected, and its spin is then measured, then the other particle -- no matter where it is in the Universe -- instantaneously is forced to choose as well and take on the role of the other particle. This illustrates that certain kinds of quantum information travel instantaneously; not everything is limited by the speed of light.
However, it can be easily demonstrated that this effect does not make faster-than-light communication or travel possible.
electric constant See permeability of free space. Eotvos law of capillarity (Baron L. von Eotvos; c. 1870) The surface tension gamma of a liquid is related to its temperature T, the liquid's critical temperature, T*, and its density rho by gamma ~= 2.12 (T* - T)/rho3/2. EPR effect See Einstein-Podolsky-Rosen effect. epsilon_0 See permittivity of free space. equivalence principle The basic postulate of A. Einstein's general theory of relativity, which posits that an acceleration is fundamentally indistinguishable from a gravitational field. In other words, if you are in an elevator which is utterly sealed and protected from the outside, so that you cannot "peek outside," then if you feel a force (weight), it is fundamentally impossible for you to say whether the elevator is present in a gravitational field, or whether the elevator has rockets attached to it and is accelerating "upward." Although that in practical situations -- say, sitting in a closed room -- it would be possible to determine whether the acceleration felt was due to uniform thrust or due to gravitation (say, by measuring the gradient of the field; if nonzero, it would indicate a gravitational field rather than thrust); however, such differences could be made arbitrarily small. The idea behind the equivalence principle is that it acts around the vicinity of a point, rather than over macroscopic distances. It would be impossible to say whether or not a given (arbitrary) acceleration field was caused by thrust or gravitation by the use of physics alone.
The equivalence principle predicts interesting general relativistic effects because not only are the two indistinguishable to human observers, but also to the Universe as well -- any effect that takes place when an observer is accelerating should also take place in a gravitational field, and vice versa.
See weak equivalence principle.
ergosphere The region around a rotating black hole, between the event horizon and the static limit, where rotational energy can be extracted from the black hole. event horizon The radius that a spherical mass must be compressed to in order to transform it into a black hole, or the radius at which time and space switch responsibilities. Once inside the event horizon, it is fundamentally impossible to escape to the outside. Furthermore, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a "point of no return." The radius of the event horizon, r, for generalized black holes (in geometrized units) is
r = m + (m2 - q2 - s/m2)1/2, where m is the mass of the hole, q is its electric charge, and s is its angular momentum.
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| 07 Jun 2016 02:19 AM |
yes indeed...
Time travel is one of my favorite topics! I wrote some time travel stories in junior high school that used a machine of my own invention to travel backwards in time, and I have continued to study this fascinating concept as the years have gone by.
We all travel in time. During the last year, I've moved forward one year and so have you. Another way to say that is that we travel in time at the rate of 1 hour per hour.
But the question is, can we travel in time faster or slower than "1 hour per hour"? Or can we actually travel backward in time, going back, say 2 hours per hour, or 10 or 100 years per hour?
It is mind-boggling to think about time travel. What if you went back in time and prevented your father and mother from meeting? You would prevent yourself from ever having been born! But then if you hadn't been born, you could not have gone back in time to prevent them from meeting. Albert Einstein
The great 20th century scientist Albert Einstein developed a theory called Special Relativity. The ideas of Special Relativity are very hard to imagine because they aren't about what we experience in everyday life, but scientists have confirmed them. This theory says that space and time are really aspects of the same thing—space-time. There's a speed limit of 300,000 kilometers per second (or 186,000 miles per second) for anything that travels through space-time, and light always travels the speed limit through empty space.
Special Relativity also says that a surprising thing happens when you move through space-time, especially when your speed relative to other objects is close to the speed of light. Time goes slower for you than for the people you left behind. You won't notice this effect until you return to those stationary people.
Say you were 15 years old when you left Earth in a spacecraft traveling at about 99.5% of the speed of light (which is much faster than we can achieve now), and celebrated only five birthdays during your space voyage. When you get home at the age of 20, you would find that all your classmates were 65 years old, retired, and enjoying their grandchildren! Because time passed more slowly for you, you will have experienced only five years of life, while your classmates will have experienced a full 50 years.
Time traveler
So, if your journey began in 2003, it would have taken you only 5 years to travel to the year 2053, whereas it would have taken all of your friends 50 years. In a sense, this means you have been time traveling. This is a way of going to the future at a rate faster than 1 hour per hour.
Time travel of a sort also occurs for objects in gravitational fields. Einstein had another remarkable theory called General Relativity, which predicts that time passes more slowly for objects in gravitational fields (like here on Earth) than for objects far from such fields. So there are all kinds of space and time distortions near black holes, where the gravity can be very intense.
In the past few years, some scientists have used those distortions in space-time to think of possible ways time machines could work. Some like the idea of "worm holes," which may be shortcuts through space-time. This and other ideas are wonderfully interesting, but we don't know at this point whether they are possible for real objects. Still the ideas are based on good, solid science. In all time travel theories allowed by real science, there is no way a traveler can go back in time to before the time machine was built.
I am confident time travel into the future is possible, but we would need to develop some very advanced technology to do it. We could travel 10,000 years into the future and age only 1 year during that journey. However, such a trip would consume an extraordinary amount of energy. Time travel to the past is more difficult. We do not understand the science as well.
Actually, scientists and engineers who plan and operate some space missions must account for the time distortions that occur because of both General and Special Relativity. These effects are far too small to matter in most human terms or even over a human lifetime. However, very tiny fractions of a second do matter for the precise work necessary to fly spacecraft throughout the solar system. |
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| 07 Jun 2016 02:20 AM |
I THINK THIS IS COPYPASTA
I REALLY CANT FREAKING TELL
does spider habe de puss puss |
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| 07 Jun 2016 02:23 AM |
i only got one problem with time travel,
To better understand what we're dealing with here, consider the famous grandfather paradox. You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster, but just what happens when you pull the trigger?
Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop.
On the other hand, we have to consider the idea of a consistent causal loop. While equally thought-provoking, this theoretical model of time travel is paradox free. According to physicist Paul Davies, such a loop might play out like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.
Then there's the post-selected model of time travel, which involves distorted probability close to any paradoxical situation [source: Sanders]. What does this mean? Well, put yourself in the shoes of the time-traveling assassin again. This time travel model would make your grandfather virtually death proof. You can pull the trigger, but the laser will malfunction. Perhaps a bird will poop at just the right moment, but some quantum fluctuation will occur to prevent a paradoxical situation from taking place.
But then there's another possibility: The future or past you travel into might just be a parallel universe. Think of it as a separate sandbox: You can build or destroy all the castles you want in it, but it doesn't affect your home sandbox in the slightest. So if the past you travel into exists in a separate timeline, killing your grandfather in cold blood is no big whoop. Of course, this might mean that every time jaunt would land you in a new parallel universe and you might never return to your original sandbox.
Confused yet? Welcome to the world of time travel.
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| 07 Jun 2016 02:23 AM |
This can be disproved by the nature of gravity. Gravity can only slow down time, even in a blackhole the strongest things in the universe time still somewhat exists. So it cannot go back, it can only slow things down.
"Ride the winds and fly, far, far away...Dandelion" |
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| 07 Jun 2016 02:24 AM |
"Spiral galaxy Gravity has played a big part in making the universe the way it is. Gravity is what makes pieces of matter clump together into planets, moons, and stars. Gravity is what makes the planets orbit the stars--like Earth orbits our star, the Sun. Gravity is what makes the stars clump together in huge, swirling galaxies.
Albert Einstein A great scientist, Albert Einstein, who lived in the 20th century, had a new idea about gravity. He thought that gravity is what happens when space itself is curved or warped around a mass, such as a star or a planet. Thus, a star or planet would cause kind of a dip in space so that any other object that came too near would tend to fall into the dip.
Quite a number of experiments show that Einstein was right about this idea and a lot of others. But there are questions for which even Einstein had no answers.
Model of an atom. For example, if gravity is a force that causes all matter to be attracted to all other matter, why are atoms mostly empty space inside? (There is really hardly any actual matter in an atom!) How are the forces that hold atoms together different from gravity? Is it possible that all the forces we see at work in nature are really different sides of the same basic force or structure? A delicately patterned fern leaf. Shell of a pearly nautilus. An exquisitely detailed flower.
Could some of the same laws of nature be at work in the designs of all things in the pictures above?
These are big questions that scientists and ordinary people like us have wondered about for a long time. For a long time we haven't known how to go about finding the answers, other than trying to work things out on paper.
But now NASA has a special program, called Fundamental Physics
. . . for seeking answers to these and other mysteries of the universe. Fundamental Physics hopes to do two things:
To discover and explore fundamental physical laws governing matter, space, and time.
To discover and understand the basic rules nature uses to build the complex and beautiful structures we see around us.
International Space Station Over the years, scientists and engineers have developed new technologies and instruments that will help us understand nature. Now we can take these new instruments into space and do experiments where the forces of gravity are very, very small (like when the Space Shuttle or the International Space Station are orbiting Earth in "free fall"). This way, scientists can do very delicate experiments to see what single atoms do under special conditions.
NASA hopes these experiments will help us understand our universe and ourselves. NASA also hopes the experiments will help develop technologies that will benefit people in their everyday lives. Nextly is online. Nextly"
I thought nerds were suppose to be smart, but you sir, you nerd is a special kind of stupid :3 Anyways if your theory was possible then we would have already found this out. Also it is not possible due to these reasons:
It is not possible to simultaneously determine the position and momentum of anything with infinite precision. This is known as the Heisenberg Uncertainty Principle. The uncertainty x in position and the uncertainty p in momentum are related by: x p ≥ h / 2 This can also be written (by doing a little calculus) as: E t ≥ h / 2 which means that you cannot simultaneously know the energy of a particle, and also how long it has had that energy, with infinite precision. Classically, I can (in principle) measure x for a particle to any accuracy I choose. I can also, simultaneously, measure its velocity (and thus its momentum, since p = mv) to whatever accuracy I choose by timing it between two points. Thus, x p = 0 is allowed classically. But quantum mechanics has no real “particles”, only probability waves. To determine the frequency of any wave to infinite precision, I would need to count an infinite number of wave crests as they pass by. But an infinite number of wave crests implies that the wave stretches from plus infinity to minus infinity – in other words, x is infinite because the wave is everywhere. If I constrain the wave to be a “packet” that is confined within some space x (like the wave splash from a rock falling in the water), then I cannot have an infinite number of wave crests to count. The smaller I make x then the fewer crests there are, and as x goes to zero there aren’t any wave crests left at all, so the frequency becomes completely unknown. Since frequency is related to both E and p in quantum mechanics (by the de Broglie and Einstein equations), this means that p and x cannot simultaneously be made zero. The size of Planck’s constant tells us how much uncertainty the universe can tolerate, that is, how far away it is from a classical Newtonian universe in which h = 0. The E t ≥ h / 2version of the Uncertainty Principle means that conservation of momentum and conservation of energy can be “suspended” by quantum particles. Strange as it may seem, uncertainty applies even to empty space – how can you know there is nothing there during a given time t when it is impossible to know the energy of anything there to better than E? In fact, it’s not only possible for particles with energy less than E to appear from nothing, a essentially infinite number of them leap in and out of existence every second. (Physicists consider “empty” space to be much closer to a furiously boiling cauldron of quantum soup than to anything empty.) The critical restriction on such particles is that whatever energy they possess (mainly their rest mass, but also other types of energy), the absolute maximum amount of time they can exist is given by the Uncertainty Principle. Particles whose energy is “borrowed” from uncertainty and cannot exist longer than t are called virtual particles. The only difference between, say, an electron and a virtual electron, is that the electron possesses its mass-energy and can last forever, but the virtual electron has borrowed its mass-energy from the Uncertainty Principle, and at the end of t it must vanish, like Cinderella’s carriage at midnight. If you run through the numbers, it turns out that a virtual electron cannot exist for longer than about 10-21 sec. That may sound impossibly short, but the particles which mediate the strong and weak nuclear forces have lifetimes roughly 200 times shorter even than this. Note – the Uncertainty Principle does not apply to charge, lepton number, or baryon number. Those are still conserved at all times. Summary of Important Ideas in Nuclear Physics 1) The nuclei of atoms are made up of protons and neutrons. As a group, they are called nucleons (nuclear physics terminology) or baryons (particle physics terminology). Their physical properties when they are free in space are: Nucleon Charge Mass Half Life Proton +1 e 1.6725 x 10-27 kg infinite Neutron 0 1.6748 x 10-27 kg 10.6 minutes Since like charges repel, the electrostatic repulsive force between the protons in a nucleus is enormous. Nuclei are held together against this repulsion by the strong nuclear force (or strong force, for short). The strong force is one of the four fundamental forces in the Universe. It operates only between baryons, i.e., protons and neutrons. Electrons, photons, and neutrinos are not affected by it. 2) The chemical properties of an element are determined solely by the number of protons in its nucleus. The proton number is the same as the position of the element within the periodic table. Atoms with the same number of protons, but different numbers of neutrons, have exactly the same chemical properties and differ only in mass. Nuclei of the same element (same number of protons) but with differing numbers of neutrons are called isotopes. 3) The strong force, unlike gravity and electromagnetism, is extremely short ranged. You can think of a free neutron passing a nucleus almost like a golf ball on a putting green: either you hit the cup exactly and go in, or you roll past it unaffected. This means that a nucleus can only grow so large, because the strong force is so shortranged that it more-or-less acts like a chain linking each nucleon to the next – and like a chain, its strength does not change as you add more links. But the electromagnetic force has an infinite range, so all the protons in a nucleus participate in repelling each other. This repulsion rapidly becomes more powerful as you add more protons, so sooner or later the electromagnetic force must overwhelm the “chain-link” attraction of the strong force, and disrupt the nucleus. The heaviest stable element is bismuth, which has 83 protons. 4) Stable nuclei consist of roughly equal numbers of protons and neutrons. Without the presence of neutrons to provide additional strong force, not even two protons can be held together against their electrostatic repulsion, never mind 83. Neutrons, on the other hand, are not stable. Left alone, they will decay into a proton, an electron, and a neutrino. But inside a nucleus, formidably complicated rules of quantum mechanics allow the strong force to “forbid” the neutron’s decay – provided there are enough protons around. So, a nucleus cannot have too few neutrons, or the protons will electrostatically fly apart. But it also cannot have too many neutrons, because eventually the strong force can no longer “forbid” their decay. 5) Actually, it is naive to think of a proton-neutron pair as consisting of a proton, separately, and a neutron, separately. Like everything in quantum mechanics, such a pairing is a probabilistic entity. It is closer to reality to imagine the proton as a black ball, and the neutron as a white ball, and then to visualize them continuously swapping identities so fast that they just blur into two grey balls. There is certainly a proton and a neutron there – but if you try to capture one of them, it’s a 50/50 proposition which you’ll catch. 6) One of the consequences of Item (2) in the Quantum Physics Summary is that quantum particles have a certain probability, albeit usually hyper-small, of appearing anywhere in the Universe. This is the cause behind one type of natural radioactivity, known as -decay. If a nucleus is very large, or has an excess of protons or neutrons, then the strong force can just barely hold it together. Classically, of course, bowling balls do not roll up hills regardless of whether they are barely sloping or look like Mt. Everest. Quantum mechanically, however, the probability that a quantum particle will leap to the other side of an energy “hill” becomes much larger if the hill is small. In the case of a radioactive nucleus such as uranium, the energy needed to liberate protons or neutrons is small enough that, sooner or later, some will quantum-mechanically tunnel through the strong-force barrier and appear outside the nucleus. Then, they speed away, and this is what we call radioactivity. 7) There are three forms of natural radioactivity, known as - - and -decay, respectively. An -particle consists of two protons and two neutrons. (This is a helium nucleus.) -decay proceeds via the mechanism described in Item (6). A -particle is a high-energy electron. In some barely-bound nuclei, a second, much weaker nuclear force known as the weak force can compete with the strong force and cause a neutron to decay even though it is in the presence of protons. As noted in Item (4), the decaying neutron gives off an electron, and this is the -particle. A -particle is a high-energy photon, also called a -ray. -rays are exactly the nuclear equivalent of spectral lines in atoms: if the protons and neutrons shift between energy levels within the nucleus, they must either give up or absorb a photon. But, since the strong force between protons and neutrons is much stronger than the electromagnetic force between a proton and an electron, nuclear energy levels are much further apart, and thus the photons given off are very energetic. As a rule, -emitters are excited nuclei that have been created in the aftermath of either -decay or -decay. 8) The three forms of radioactivity have very different abilities to penetrate matter. -particles are very easy to stop (a sheet of cardboard will do it); -particles are harder to stop (you need a sheet of aluminum); and -rays are the most difficult to stop (they can penetrate over an inch of lead). If you consider microscopically what is happening, this is easy to understand. Matter consists of charged particles: positively charged nuclei and negatively charged electrons. -particles are heavy, thus relatively slow-moving, and have two electric charges. So they are pummeled by attractive and repulsive forces as they move through matter, and quickly lose their energy. -particles travel much faster and have only one charge, thus they can penetrate matter more easily. -rays have no charge at all and are moving at the speed of light, thus they are very penetrating. They more-or-less have to hit a nucleus head-on to be stopped. 9) Since the chemical properties of an element (i.e., its position in the periodic table) are determined solely by the number of protons in its nucleus, - and -decay change the chemical identity of the decaying nucleus. -decay subtracts two protons, so the element moves down two notches in the periodic table. -decay changes a neutron into a proton, so the element moves up one notch in the table. 10) Radioactivity is characterized by the half-life of the nucleus. Definition: if I have x atoms of any radioactive element at some time, then one half-life later I will have ½ x of those atoms remaining. And one half-life later I have a half of the half, or one-quarter, of the original atoms left. And so forth. The half-life is a statistical concept. It is completely impossible to determine whether any given atom will decay one microsecond from now or in a billion years. You can only say that it has a 50-50 chance of doing so in the time of one half-life. 11) ITS LITERALLY JUST IMPOSSIBLE 12) Common Sense
Also not to mention the Edwards-Casimir quantum vacuum drive A hypothetical drive exploiting the peculiarities of quantum mechanics by restricting allowed wavelengths of virtual photons on one side of the drive (the bow of the ship); the pressure generated from the unrestricted virtual photons toward the aft generates a net force and propels the drive. See Casimir effect.
Ehrenfest paradox (Ehernfest, 1909) The special relativistic "paradox" involving a rapidly rotating disc. Since any radial segment of the disc is perpendicular to the direction of motion, there should be no length contraction of the radius; however, since the circumference of the disc is parallel to the direction of motion, it should contract. Einstein field equation The cornerstone of Einstein's general theory of relativity, relating the gravitational tensor G to the stress-energy tensor T by the simple equation G = 8 pi T. Einstein-Podolsky-Rosen effect; EPR effect Consider the following quantum mechanical thought-experiment: Take a particle which is at rest and has spin zero. It spontaneously decays into two fermions (spin 1/2 particles), which stream away in opposite directions at high speed. Due to the law of conservation of spin, we know that one is a spin +1/2 and the other is spin -1/2. Which one is which? According to quantum mechanics, neither takes on a definite state until it is observed (the wavefunction is collapsed). The EPR effect demonstrates that if one of the particles is detected, and its spin is then measured, then the other particle -- no matter where it is in the Universe -- instantaneously is forced to choose as well and take on the role of the other particle. This illustrates that certain kinds of quantum information travel instantaneously; not everything is limited by the speed of light.
However, it can be easily demonstrated that this effect does not make faster-than-light communication or travel possible.
electric constant See permeability of free space. Eotvos law of capillarity (Baron L. von Eotvos; c. 1870) The surface tension gamma of a liquid is related to its temperature T, the liquid's critical temperature, T*, and its density rho by gamma ~= 2.12 (T* - T)/rho3/2. EPR effect See Einstein-Podolsky-Rosen effect. epsilon_0 See permittivity of free space. equivalence principle The basic postulate of A. Einstein's general theory of relativity, which posits that an acceleration is fundamentally indistinguishable from a gravitational field. In other words, if you are in an elevator which is utterly sealed and protected from the outside, so that you cannot "peek outside," then if you feel a force (weight), it is fundamentally impossible for you to say whether the elevator is present in a gravitational field, or whether the elevator has rockets attached to it and is accelerating "upward." Although that in practical situations -- say, sitting in a closed room -- it would be possible to determine whether the acceleration felt was due to uniform thrust or due to gravitation (say, by measuring the gradient of the field; if nonzero, it would indicate a gravitational field rather than thrust); however, such differences could be made arbitrarily small. The idea behind the equivalence principle is that it acts around the vicinity of a point, rather than over macroscopic distances. It would be impossible to say whether or not a given (arbitrary) acceleration field was caused by thrust or gravitation by the use of physics alone.
The equivalence principle predicts interesting general relativistic effects because not only are the two indistinguishable to human observers, but also to the Universe as well -- any effect that takes place when an observer is accelerating should also take place in a gravitational field, and vice versa.
See weak equivalence principle.
ergosphere The region around a rotating black hole, between the event horizon and the static limit, where rotational energy can be extracted from the black hole. event horizon The radius that a spherical mass must be compressed to in order to transform it into a black hole, or the radius at which time and space switch responsibilities. Once inside the event horizon, it is fundamentally impossible to escape to the outside. Furthermore, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a "point of no return." The radius of the event horizon, r, for generalized black holes (in geometrized units) is
r = m + (m2 - q2 - s/m2)1/2, where m is the mass of the hole, q is its electric charge, and s is its angular momentum.
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