NEBULA

                                           

                                        NEBULA

        

                                                 If we translate the Latin word ‘nebula’, it would simply mean ‘cloud’.A nebula is an interstellar cloud of dust,ionized gasses.The main gases are hydrogen and helium gas, and plasma.It is form when portion of interstellar medium collapse due to gravitational force(attraction) by the particle in that medium.Many of  you think that outer space is vacuum but it aint true it is fill with medium called ether which may contain many gasses and other dust particles,and this all known as ISM(interstellar medium

                        There are many nebula till found and some of most prominent are Eagle Nebula,Horse head Nebula,Red Rectangle Nebula,Crab Nebula and many more.

                        As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths.The more masses clump together, the greater their gravitational attraction will be to other bodies and particles in their vicinity. As the particles clump further to form larger and more massive structures, they attract more dust and gas. The pressure inside then gets so high that nuclear fusion ensues. This results in the emission of high-energy electromagnetic radiation, which in turn ionizes the outer layers of gas.Ionized gas is plasma, and so plasma and electromagnetic radiation are now added to the mix. This now becomes the earliest stages of star formation.
                                   Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce,other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun.

                                                                           (HELIX NEBULA)

                                                  Planetary Nebula.

                               Planetary nebulae form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs.Nebula is just not the starting point of the formation of heavy masses,ironically it may be end point.Stars that evolve into red giants can lose their outer layers during pulsations in their outer layers, known as their atmospheres releasing hydrogen and helium into 9.7:0.3 ratio.It is this released matter that forms what is known as a planetary nebula.

       

                                                                     (CAT EYE NEBULA)

THEORY OF RELATIVITY.

RELATIVITY

 

RELATIVITY IS THE WORD WHICH CAN MAKE US TO BLAME SOMEONE WRONG OR TO APPRECIATE FOR SOMEONE'S GOOD.BUT HOW?

DO YOU REALLY THINK THAT DR.ALBERT EINSTEIN WHEN GAVE THE PAPERS ON THE THEORY OF RELATIVITY WAS THINKING OF THIS PAPERS ON THE VERY STARTING OF HIS LIFE,NO THE ANSWER IS NO(in my opinion). AT THE EARLY STAGE OF HIS LIFE HE WAS SO MUCH DISTRESSED BECAUSE OF THE INCOMPLETE EDUCATION,NO JOB,NO MONEY,HIS PARENTS THOUGHT THAT HE WAS A TOTAL WASTE. THIS ALL THING MADE HIM FEEL VERY SORRY FOR HIMSELF . HE STARTED THINKING WHY THEY ALL CALL ME BAD? 

ONE DAY, TRAVELING ON THE BUS HE LOOKS AT VERY FAMOUS WATCH TOWER AND HE STARES AT THE WATCH AND NOTICED THAT IF BUS CAN REACH THE SPEED OF THE LIGHT THEN HE WOULD NOT BE ABLE TO SEE THE HANDS OF THE WATCH MOVING. THIS GIVE HIM THE IDEA OF RELATIVITY BUT AS WE KNOW  THAT IDEA CAN NOT MAKE A DIFFERENCE ALONE,THERE MUST BE SOMETHING ELSE AND EINSTEIN HAD THAT THING AND WE CALL IT SADNESS. THE FAILURE OF HIS MAKE OTHERS TO POINT FINGER ON HIM AND THAT POINTY FINGER MAKE HIM TO THINK WHY HE IS SO WORTHLESS. THEN ONCE FROM THE IDEA OF RELATIVITY HE STARTED BELIEVING THAT I AM BAD BECAUSE THERE ARE SOME GOOD PEOPLE ALSO,THIS ALL THING ARE IN RELATIVE WITH EACH OTHER.

HE MIGHT BE WRONG BUT I DO ALSO HAVE SAME BELIEVE THAT IF THERE ARE BAD PEOPLE THAT ONLY BECAUSE  THAT THERE SOME GOOD PEOPLE ALSO.THIS UNIVERSE MUST BE BALANCED TO RUN SMOOTHLY.IF AN ATOM ALSO NEED EQUAL NUMBER OF ELECTRON PROTON TO BE BALANCE AND STABLE AND WE ARE HERE TALKING ABOUT THE WHOLE UNIVERSE.

THEORY OF RELATIVITY

THEORY OF RELATIUVITY IS FIRST PAPER PROPES BY ONE OF THE GREATEST MIND OF ALL TIME,DR.ALBERT EINSTEIN.

ACCORDING TO HIS THEORY,MOTION AND REST ARE DEPENDED TO EACH OTHER OR WE CAN SAY RELATED TO EACH OTHER.IF A BODY IS MOVING THEN ITS NOT SEEN TO BE MOVING UNTIL THERE IS NO  OTHER BODY ON REST POSITION AND  VICE-VERSE IS ALSO TRUE. 

THE THEORY OF RELATYIVITY GENERALLY ENCOMPASSES TWO THEORIS NAMELY GANEREAL THEORY OF RELATIVITY AND SPECIAL THEORY OF RELATIVITY.

Einstein sought to explain situations in which Newtonian physics might fail to deal successfully with phenomena, and in so doing proposed revolutionary changes in human concepts of time, space, and gravity.

The special theory of relativity was based on two main postulates: first, that the Speed of light is constant for all observers; and second, that observers moving at constant speeds should be subject to the same physical laws. Following this logic, Einstein theorized that time must change according to the speed of a moving object relative to the frame of reference of an observer. Scientists have tested this theory through experimentation - proving, for example, that an atomic clock ticks more slowly when traveling at a high speed than it does when it is not moving. The essence of Einstein's paper was that both space and time are relative (rather than absolute), which was said to hold true in a special case, the absence of a gravitational field. Relativity was a stunning concept at the time; scientists all over the world debated the veracity of Einstein's famous equation, E=mc2, which implied that matter and energy were equivalent and, more specifically, that a single particle of matter could be converted into a huge quantity of energy. However, since the special theory of relativity only held true in the absence of a gravitational field, Einstein strove for 11 more years to work gravity into his equations and discover how relativity might work generally as well.

 

 According to the Theory of General Relativity, matter causes space to curve. It is posited that gravitation is not a force, as understood by Newtonian physics, but a curved field (an area of space under the influence of a force) in the space-time continuum that is actually created by the presence of mass. According to Einstein, that theory could be tested by measuring the deflection of starlight traveling near the sun; he correctly asserted that light deflection would be twice that expected by Newton's laws. This theory also explained why the light from stars in a strong gravitational field was closer to the red end of the spectrum than those in a weaker one.

 Proofs of theory of relativity.

1.
The latest test of Einstein's theory of relativity,, looks specifically at time dilation, a piece of the theory that predicts that two identical clocks resting at different heights or moving at different speeds will tick at different rates. Time dilation is most commonly thought of in terms of the twin paradox: If one twin goes asteroid-hopping on a rocket moving at extremely high speeds, he'll have aged less than his earthbound sibling when he gets home. Now, however, physicist Chin-Wen Chou and his colleagues at the National Institute of Standards and Technology have shown that time dilation can be observed even without a far-flung, fast-moving trip.

PROOFS.
Using super-sensitive optical clocks, they measured changes in the clocks' tick rates at speeds of less than 25 miles per hour and at differences in altitude of about a foot. The optical clocks, each powered by a single aluminum ion, are nearly 40 times as accurate as the international-standard cesium-powered atomic clocks, giving researchers the ability to look at minute differences in tick rates. Sitting still at the same height, the clocks had the same tick rate. To move one clock, the researchers simply started one of the ions oscillating at a speed of their choosing. "It can be as slow as you sitting on a swing, swinging back and forth, or as fast as a bullet train," Chou says. When he set the ion moving at 15 meters per second (a little under 50 miles per hour), Chou found that that clock ticked at a measurably slower rate than the stationary clock. The same thing happened when the clocks were at slightly different heights. When Chou and his team used hydraulic jacks to lift one clock just over a foot, the lower clock's tick rate was ever-so-slightly lower than that of the higher clock. Because optical clocks allowed them to measure carefully enough, the researchers could see that Einstein's predictions played out even in everyday circumstances like the height of a footstool and the speed of a car on a residential street. 

2.

Sending communications to and from the Viking lander on Mars in 1979, scientists showed that signals traveling between Earth and Mars took slightly longer when they passed the Sun, due to the curvature in space-time caused by the massive star.
PROOFS.
  As the spacecraft Cassini was heading towards Saturn in 2002, scientists again measured the effect of solar gravity, looking at how the round-trip time of a radio signal changed when it went near the sun. Although the Cassini test showed the same result as that of the Viking, it was 50 times as accurate—within 20 parts per million, thanks to a better communication system that could filter out interference from the solar corona

3.

Like all falsifiable scientific theories, relativity makes predictions that can be tested by experiment. In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation.The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 1881 and 1938 were critical to its validation. These are the Michelson–Morley experiment, the Kennedy–Thorndike experiment, and the Ives–Stilwell experiment. Einstein derived the Lorentz transformations from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.

Maxwell's equations – the foundation of classical electromagnetism – describe light as a wave which moves with a characteristic velocity. The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond. This hypothetical medium was called the luminiferous aether, at rest relative to the "fixed stars" and through which the Earth moves. Fresnel's partial ether dragging hypothesis ruled out the measurement of first-order (v/c) effects, and although observations of second-order effects (v2/c2) were possible in principle, Maxwell thought they were too small to be detected with then-current technology.

The Michelson–Morley experiment was designed to detect second order effects of the "aether wind" – the motion of the aether relative to the earth. Michelson designed an instrument called the Michelson interferometer to accomplish this. The apparatus was more than accurate enough to detect the expected effects, but he obtained a null result when the first experiment was conducted in 1881,and again in 1887.Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community.In an attempt to salvage the aether paradigm, Fitzgerald and Lorentz independently created an ad hoc hypothesis in which the length of material bodies changes according to their motion through the aether.This was the origin of FitzGerald–Lorentz contraction, and their hypothesis had no theoretical basis. The interpretation of the null result of the Michelson–Morley experiment is that the round-trip travel time for light is isotropic (independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity.

General relativity has also been confirmed many times, the classic experiments being the perihelion precession of Mercury's orbit, the deflection of light by the Sun, and the gravitational redshift of light. Other tests confirmed the equivalence principle and frame dragging.

Dark Energy, Dark Matter

                                     

             Dark Energy, Dark Matter

 In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.


The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies, most recently by the WMAP spacecraft, indicate that the universe is close to flat. For the shape of the universe to be flat, the mass/energy density of the universe must be equal to a certain critical density. The total amount of matter in the universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%. The WMAP five tear analysis estimate a universe made up of 74% dark energy, 22% dark matter, and 4% ordinary matter. More recently, the WMAP seven year analysis gave an estimate of 72.8% dark energy, 22.7% dark matter and 4.6% ordinary matter.Recently release information based on the work done by the Plank spacecraft on the distribution of the universe, gave a more accurate estimate of 68.3% of dark energy, 26.8% of dark matter and 4.9% of ordinary matter. 

what is dark energy?

More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10¹²⁰ times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.

what is dark matter?

By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~70% dark energy, ~25% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHO's". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or weakly intracting massive partical.
 

TIME DILATION

                                                     TIME DILATION


In the theory of relativity, time dilation is an actual difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses.

An accurate clock at rest with respect to one observer may be measured to tick at a different rate when compared to a second observer's own equally accurate clocks. This effect arises neither from technical aspects of the clocks nor from the fact that signals need time to propagate, but from the nature of spacetime itself.


When two observers are in relative uniform motion and uninfluenced by any gravitational mass, the point of view of each will be that the other's (moving) clock is ticking at a slower rate than the local clock. The faster the relative velocity, the greater the magnitude of time dilation. This case is sometimes called special relativistic  time dilation.
For instance, two rocket ships (A and B) speeding past one another in space would experience time dilation. If they somehow had a clear view into each other's ships, each crew would see the others' clocks and movement as going too slowly. That is, inside the frame of reference  of Ship A, everything is moving normally, but everything over on Ship B appears to be moving slower (and vice versa).
From a local perspective, time registered by clocks that are at rest with respect to the local frame of reference (and far from any gravitational mass) always appears to pass at the same rate. In other words, if a new ship, Ship C, travels alongside Ship A, it is "at rest" relative to Ship A. From the point of view of Ship A, new Ship C's time would appear normal too.
A question arises: If Ship A and Ship B both think each other's time is moving slower, who will have aged more if they decided to meet up? With a more sophisticated understanding of relative velocity time dilation, this seeming twin paradox turns out not to be a paradox at all (the resolution of the paradox involves a jump in time, as a result of the accelerated observer turning around). Similarly, understanding the twin paradox would help explain why astronauts on the ISS age slower (e.g. 0.007 seconds behind for every 6 months) even though they are experiencing relative velocity time dilation.

 A comparison of muon lifetimes at different speeds is possible. In the laboratory, slow muons are produced, and in the atmosphere very fast moving muons are introduced by cosmic rays. Taking the muon lifetime at rest as the laboratory value of 2.22 μs, the lifetime of a cosmic ray produced muon traveling at 98% of the speed of light is about five times longer, in agreement with observations.In this experiment the "clock" is the time taken by processes leading to muon decay, and these processes take place in the moving muon at its own "clock rate", which is much slower than the laboratory clock.

People have a strong intuitive notion about what "time" is.  It works very well in every-day use, but unfortunately, it's wrong.  And because it works so well, it's going to be difficult to substitute a different intuition. 

We can, at least, pinpoint exactly where intution fails you.  You have the notion that there's a single "time" that's the same for everybody, some universal clock ticking away that we can all agree on, and all actual clocks are their best representation of that real clock. 

This is the part that's false.  It turns out that in reality, every "frame of reference" has its own little clock.  A "frame of reference" is a bunch of objects all moving together in space.  They all have the same clock, because they're moving together.  Viewed within that reference frame, all of the laws of physics remain exactly as we expect them to be, and we all agree about what it means for two things to be happening "at the same time".

(I need to clarify one thing at this point: I'm talking about inertial frames of reference.  That is, those that are moving at a constant speed, or not moving at all, as opposed to one that is accelerating. Or put another way, one that is experiencing no force.  There will be no force anywhere in this.  In physics we need to be very precise in our terminology, or we'll be led astray.  So imagine we're talking about spaceships in outer space.)

The problem is that what's true within a reference frame doesn't apply between reference frames.  That is, if your reference frame is moving, and I take a look at it, your clock will be moving differently from mine.  In fact, it will appear to be moving slower.

The interesting part is that if you look at my clock, you'll think that my clock is moving slower.

Which of us is right?  Both of us!  That's the unintuitive part.

Einstein gave us a way to mathematically determine exactly how much slower the other clock is moving, and it says that both of us think that the other clock is slower.  But when the difference in speed is small, the difference in time is small, so you're used to thinking that the clocks are the same.  But when the speed gets very close to the speed of light (more like 90%; even 25% of the speed of light has only a very small correction factor) you can notice that the times are different.

Neither one of us thinks our clock is running fast, or slow.  There's no "force" slowing the clock down.  It's just time, running, and it seems the same to us. Any physics experiment we do within our reference frame will behave exactly as if we were stopped.

How is it possible that we can BOTH see the clocks going slower in the other reference frame?  What you have to give up is the idea that there's a universal, "correct" reference frame.  You imagine that you can jump into the other frame and say, "Hey, your clock is going slow!" but you can only do that by accelerating.  And when you accelerate, the whole thing falls down, because this only applies to inertial reference frames that aren't accelerating.  When you accelerate, now YOU are the one who's doing something special, and your "reference frame" acts weird.

And yes, this has been confirmed by experiment.  They send atomic clocks out on airplanes and space ships, and we can see that they do come back as if they ran slower.  Why is it the clocks on the planes that ran slow, and not the ones on the ground?  Because the airplane is the one that accelerated away, and then decelerated to come back.  The numbers are so small that only the most precise clocks can measure it, but it's very real.

So... if you want to make this intuitive, the best thing you can do is to realize that time is always different in somebody else's frame of reference, and that neither one is wrong, but you can't compare the two without accelerating to join the other frame.  And it's the one who accelerates who changes to join the other frame of reference.



               

        



where Δt is the time interval between two co-local events (i.e. happening at the same place) for an observer in some inertial frame (e.g. ticks on his clock), this is known as the proper time, Δt' is the time interval between those same events, as measured by another observer, inertially moving with velocity v with respect to the former observer, v is the relative velocity between the observer and the moving clock, c is the speed of light, and

is the Lorentz form. Thus the duration of the clock cycle of a moving clock is found to be increased: it is measured to be "running slow". The range of such variances in ordinary life, where v ≪ c, even considering space travel, are not great enough to produce easily detectable time dilation effects and such vanishingly small effects can be safely ignored. It is only when an object approaches speeds on the order of 30,000 km/s (1/10 the speed of light) that time dilation becomes important.

TIME TRAVEL

                            TIME TRAVEL

Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space.

Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period

We have seen lots of movie about time traveling and got lots of idea of  time traveling,but does it says true?how do we know what is true about time traveling.many people actually every people think that time traveling is rubbish but some human like us believe that this is possible.how this is possible?

Some theories, most notably special  and general relativity, suggest that suitable geometries of spacetime, or specific types of motion in space, might allow time travel into the past and future if these geometries or motions are possible.In technical papers, physicist  generally avoid the commonplace language of "moving" or "traveling" through time ("movement" normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of close timlike curve, which are wordlines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, but the physical plausibility of these solutions is uncertain.

Relativity predicts that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (according to relativity there is no single objective answer to how much time has really passed between the departure and the return, but there is an objective answer to how much proper time has been experienced by both the Earth and the traveler, i.e., how much each has aged. On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of causality. The classic example of a problem involving causality is the grandfather paradox, what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, by appealing either to the notion of branching parallel universe.

However, the theory of general relativity does suggest a scientific basis for the possibility of backwards time travel in certain unusual scenarios, although arguments from semi classic gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semi classical arguments led Hawking to formulate the  chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations of general relativity, although it would be impossible to travel through a wormhole unless it were what is known as a traversable wormhole.
A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less than the stationary end, as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around.This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backwards in time as seen by other observers outside. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine,in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time.
According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy. More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.
In 1993, Matt Visser argued that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is possible.

                                         

                                           

                                                                     (WORM HOLES)

Time dilation is permitted by Albert Einstein's special and general theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well. For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame, as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a black hole .
It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter.For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future.
There is a great deal of experimental evidence supporting the validity of equations for velocity-based time dilation in special relativityand gravitational time dilation in general relativity. However, with current technologies it is only possible to cause a human traveller to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev.

 

Uncertainty principle

In quantum mechanics, the uncertainty principle is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, such as position x and momentum p, can be known simultaneously. For instance, the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa.The original heuristic argument that such a limit should exist was given by Werner Heisenberg in 1927, after whom it is sometimes named the Heisenberg principle. A more formal inequality relating the standard deviation of position σx and the standard deviation of momentum σp was derived by Earle Hesse Kennard and independently by Hermann Wey

where ħ is the reduced Planck constant.

Historically, the uncertainty principle has been confused with a somewhat similar effect in physics, called the observer effect, which notes that measurements of certain systems cannot be made without affecting the systems. Heisenberg offered such an observer effect at the quantum level as a physical "explanation" of quantum uncertainty.It has since become clear, however, that the uncertainty principle is inherent in the properties of all wave-like systems, and that it arises in quantum mechanics simply due to the matter wave nature of all quantum objects. Thus, the uncertainty principle actually states a fundamental property of quantum systems, and is not a statement about the observational success of current technology.It must be emphasized that measurement does not mean only a process in which a physicist-observer takes part, but rather any interaction between classical and quantum objects regardless of any observer.

Since the uncertainty principle is such a basic result in quantum mechanics, typical experiments in quantum mechanics routinely observe aspects of it. Certain experiments, however, may deliberately test a particular form of the uncertainty principle as part of their main research program. These include, for example, tests of number-phase uncertainty relations in superconducting or quantum optics systems. Applications are for developing extremely low noise technology such as that required in gravitational-wave interferometers.