Quantum Physics




big bang
This is an artist's concept of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development. Credit: NASA


The universe may have existed forever, according to a new model that applies quantum correction terms to complement Einstein's theory of general relativity. The model may also account for dark matter and dark energy, resolving multiple problems at once.

 

The widely accepted age of the universe, as estimated by general relativity, is 13.8 billion years. In the beginning, everything in existence is thought to have occupied a single infinitely dense point, or singularity. Only after this point began to expand in a "Big Bang" did the universe officially begin.

Although the Big Bang singularity arises directly and unavoidably from the mathematics of general relativity, some scientists see it as problematic because the math can explain only what happened immediately after—not at or before—the singularity.

"The Big Bang singularity is the most serious problem of general relativity because the laws of physics appear to break down there," Ahmed Farag Ali at Benha University and the Zewail City of Science and Technology, both in Egypt, told Phys.org.

Ali and coauthor Saurya Das at the University of Lethbridge in Alberta, Canada, have shown in a paper published in Physics Letters B that the Big Bang singularity can be resolved by their new model in which the universe has no beginning and no end.

Old ideas revisited

The physicists emphasize that their quantum correction terms are not applied ad hoc in an attempt to specifically eliminate the Big Bang singularity. Their work is based on ideas by the theoretical physicist David Bohm, who is also known for his contributions to the philosophy of physics. Starting in the 1950s, Bohm explored replacing classical geodesics (the shortest path between two points on a curved surface) with quantum trajectories.

In their paper, Ali and Das applied these Bohmian trajectories to an equation developed in the 1950s by physicist Amal Kumar Raychaudhuri at Presidency University in Kolkata, India. Raychaudhuri was also Das's teacher when he was an undergraduate student of that institution in the '90s.

Using the quantum-corrected Raychaudhuri equation, Ali and Das derived quantum-corrected Friedmann equations, which describe the expansion and evolution of universe (including the Big Bang) within the context of general relativity. Although it's not a true theory of quantum gravity, the model does contain elements from both quantum theory and general relativity. Ali and Das also expect their results to hold even if and when a full theory of quantum gravity is formulated.

No singularities nor dark stuff

In addition to not predicting a Big Bang singularity, the new model does not predict a "big crunch" singularity, either. In general relativity, one possible fate of the universe is that it starts to shrink until it collapses in on itself in a big crunch and becomes an infinitely dense point once again.

Ali and Das explain in their paper that their model avoids singularities because of a key difference between classical geodesics and Bohmian trajectories. Classical geodesics eventually cross each other, and the points at which they converge are singularities. In contrast, Bohmian trajectories never cross each other, so singularities do not appear in the equations.

In cosmological terms, the scientists explain that the quantum corrections can be thought of as a cosmological constant term (without the need for dark energy) and a radiation term. These terms keep the universe at a finite size, and therefore give it an infinite age. The terms also make predictions that agree closely with current observations of the cosmological constant and density of the universe.

New gravity particle

In physical terms, the model describes the universe as being filled with a quantum fluid. The scientists propose that this fluid might be composed of gravitons—hypothetical massless particles that mediate the force of gravity. If they exist, gravitons are thought to play a key role in a theory of quantum gravity.

In a related paper, Das and another collaborator, Rajat Bhaduri of McMaster University, Canada, have lent further credence to this model. They show that gravitons can form a Bose-Einstein condensate (named after Einstein and another Indian physicist, Satyendranath Bose) at temperatures that were present in the universe at all epochs.

Motivated by the model's potential to resolve the Big Bang singularity and account for dark matter and dark energy, the physicists plan to analyze their model more rigorously in the future. Their future work includes redoing their study while taking into account small inhomogeneous and anisotropic perturbations, but they do not expect small perturbations to significantly affect the results.

"It is satisfying to note that such straightforward corrections can potentially resolve so many issues at once," Das said.



Did the universe originate from a hyper-dimensional black hole?

 

Did the universe originate from a hyper-dimensional black hole?


Lately there's been news of a radical new theory proposing that the universe began from a hyper-dimensional black hole. Most of the reports seem to stem from an article posted a while back on the Nature blog, which references the original paper. So let's have a little reality check.
 

No one is abandoning the big bang model. The original paper hasn't even been peer reviewed yet and the paper doesn't present a radical new theory to overturn the big bang. What the paper is actually about is higher-dimensional gravitational theory.

The standard theory of gravity (general relativity) describes our universe as a geometry of three-dimensional space with one dimension of time. This is sometimes called 3 + 1 space, and it gives a very accurate description of the universe we observe. But theorists like to play around with alternative models to see how they differ from regular general relativity. They may look at 2 + 1 space, a kind of flatland with time, or 2 + 2, with two time dimensions. There isn't necessarily anything "real" about these models, and there certainly isn't any experimental evidence to support anything other than 3 + 1 gravity, but alternative models are useful because they help us gain a deeper understanding of general relativity. In this particular paper, the authors were exploring 4 + 1 gravity. That is, a five-dimensional universe with 4 spatial dimensions and 1 time.

Back in 2000, another team of authors proposed a model where our regular 3 + 1 gravity could be treated as a brane within a larger 4 + 1 universe. It is similar to the way a 2 + 1 universe could be imagined as a 2-dimensional surface (the brane) within our 3-dimensional space. In the 2000 paper, the authors showed that a particular 4 + 1 universe with a 3 + 1 brane could give rise to the type of gravity we actually see.

The new paper takes this model one step further. In it, the authors show that 4 + 1 gravity allows for the existence of black holes. So if a 4 + 1 universe had large stars, some of those stars could collapse into a 4-dimensional "hyper black hole". Like black holes in regular general relativity, these hyper black holes would have a central "singularity" of extremely dense and hot matter/energy. The authors then went on to show that a hyper black hole with the right conditions could not only create a three-dimensional brane, but the new brane would look very similar to the early universe we actually observe.

In other words, if we imagine a five-dimensional 4 + 1 universe, and if such a universe could create stars that collapse into hyper black holes, and if a particular hyper black hole had the right energy, then it might be possible for for such a hyper black hole to produce a 3 + 1 brane-universe with a beginning that looks like a big bang. That's a lot of ifs.

Just to be clear, this is good theoretical work. The model is interesting, and it shows a curious connection between the universe we observe and higher-dimensional gravity. It could also address some of the issues in cosmology, but it also predicts the universe is flat, which as I mentioned yesterday may not be the case. The authors note this problem, and are careful not to make broad claims. They also outline possible ways that such a model could be tested. This is what good theoreticians do.

But currently there is no experimental evidence to support higher-dimensions, much less hyper black holes. So don't toss the big bang just yet.





Source: Phys.org



Slowing down light

 

Scientists have slowed down light inside a vacuum for the first time. By changing the shape of the individual particles in a light beam, they have now proved that light speed in free space is not a constant.

 

By building a liquid crystal device to alter the shape of individual photons, the physicists worked at the quantum level.

 

Light travels at an amazing -186,282 miles an hour. Ordinarily, some barrier is needed to slow it down even by a tiny margin. Water and glass work best to illustrate this. But a team of researchers from the Glasgow and Heriot-Watt universities proved that the issue could be approached differently, by using individual photons instead of whole beams.

 

Not only did they manage to slow down light by using a special ‘mask,’ they managed to keep it moving slowly even after the device was no longer acting upon the photons.

 

Professor Miles Padgett, Dr Jacquiline Romero, Dr Daniel Giovannini and their teammates working from Glasgow’s laboratory demonstrated their finding by building a sort of ‘race track’ of two photons. They set out to demonstrate how a single photon’s quantum characteristics of both wave and particle can be used to build a special software-controlled device that could alter these functions at will.

 

The results of their study were published in the journal Science Express on January 23.

 

The dual racecourse culminated at a light sensor about three feet away, to measure the resulting speeds of two groups of photons. The first group was fired off through empty space unhindered, while the second traveled through the device shaping it into a curved beam. Unlike working with beams from the outset, shaping light in this way proved much more efficient. Individual particles offer greater flexibility.

 

The software-controlled ‘mask’ slowed down individual photons – an event the researchers compare to a cycling race. If before, we were inclined to measure the average speed of an entire group, we can now make more precise calculations by measuring the speed of a single racer. The group formation always makes it difficult to do that.

 

The first group of beams reached their destination quicker by a measly 0.001 percent, but that is already a massive implication for physics and the future prospects of working with light.

 

Image from gla.ac.uk
Image from gla.ac.uk
 

More amazing was that the light continued traveling slower after the mask was removed. Ordinarily, once a beam is no longer acted on by water or glass, it returns to normal speed. Not in this case.

 

As Padgett explains to the BBC, the mask “looks a little bit like a bull’s-eye target.”

 

"And that mask patterns the light beam, and we show that it's the patterning of the light beam that slows it down.”

 

It’s not clear to the layman how exactly the mask does what it does, “but once the pattern has been imposed – even now the light is no longer in the mask, it’s just the propagating in free space – the speed is still slow.”

 

This is all thanks to the photons being able to shape-shift into waves and particles, a quality called wave-particle duality. If you alter the shape of the wave, you create a slower, curvier photon.

 

"We've achieved this slowing effect with some subtle but widely-known optical principles. This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum,” co-author Romero explains in the University of Glasgow press release.

 

"Although we measure the effect for a single photon, it applies to bright light beams too. The effect is biggest when the lenses used to create the beam are large and when the distance over which the light is focused is small, meaning the effect only applies at short range.”

 

Padgett says the team are “confident” their observations are correct, and is excited for the implications this might have for physics.

 

"The results give us a new way to think about the properties of light and we're keen to continue exploring the potential of this discovery in future applications. We expect that the effect will be applicable to any wave theory, so a similar slowing could well be created in sound waves, for example," Padgett adds.

 

Source: Russia Today



What Is Quantum Physics?

 

Quantum physics is the study of the behavior of matter and energy at the molecular, atomic, nuclear, and even smaller microscopic levels. In the early 20th century, it was discovered that the laws that govern macroscopic objects do not function the same in such small realms.

What Does Quantum Mean?

"Quantum" comes from the Latin meaning "how much." It refers to the discrete units of matter and energy that are predicted by and observed in quantum physics. Even space and time, which appear to be extremely continuous, have smallest possible values.

 

Who Developed Quantum Mechanics?

As scientists gained the technology to measure with greater precision, strange phenomena was observed. The birth of quantum physics is attributed to Max Planck's 1900 paper on blackbody radiation. Development of the field was done by Max Planck,Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schroedinger, and many others. Ironically, Albert Einstein had serious theoretical issues with quantum mechanics and tried for many years to disprove or modify it.

 

What's Special About Quantum Physics?

 

In the realm of quantum physics, observing something actually influences the physical processes taking place. Light waves act like particles and particles act like waves (calledwave particle duality). Matter can go from one spot to another without moving through the intervening space (called quantum tunnelling). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities. Fortunately, it breaks down when dealing with large objects, as demonstrated by the Schrödinger's Cat thought experiment.

 

 

What is Quantum Entanglement?

 

One of the key concepts is quantum entanglement, which describes a situation where multiple particles are associated in such a way that measuring the quantum state of one particle also places constraints on the measurements of the other particles. This is best exemplified by the EPR Paradox. Though originally a thought experiment, this has now been confirmed experimentally through tests of something known as Bell's Theorem.

 

Quantum Optics

Quantum optics is a branch of quantum physics that focuses primarily on the behavior of light, or photons. At the level of quantum optics, the behavior of individual photons has a bearing on the outcoming light, as opposed to classical optics, which was developed by Sir Isaac Newton. Lasers are one application that has come out of the study of quantum optics.

Quantum Electrodynamics (QED)

Quantum electrodynamics (QED) is the study of how electrons and photons interact. It was developed in the late 1940s by Richard Feynman, Julian Schwinger, Sinitro Tomonage, and others. The predictions of QED regarding the scattering of photons and electrons are accurate to eleven decimal places.

Unified Field Theory

Unified field theory is a collection of research paths that are trying to reconcile quantum physics with Einstein's theory of general relativity, often by trying to consolidate thefundamental forces of physics. Some types of unified theories include (with some overlap):

 

Other Names:

Quantum physics is sometimes called quantum mechanics or quantum field theory. It also has various subfields, as discussed above, which are sometimes used interchangeably with quantum physics, though quantum physics is actually the broader term for all of these disciplines.


Major Figures in Quantum Physics


Major Findings - Experiments, Thought Experiments & Basic Explanations

 

 

 

 

 

 



Double Slit Experiment


One of the most bizarre experiments in physics is the so called "double slit experiment". It keeps me baffled, everytime I witness it's strange outcomes. One seriously has to consider if we MAKE OUR OWN REALITY. This has profound implications, which I can't even begin to describe here. Beneath this article are three excellent videos. My favourite is the (lengthy one) of professor Richard Feynman. That man is hilarious and gets his points across. You just got to love the guy.

For a layman in physics, it's better to begin with watching the video of Dr. Quantum (how corny sounding) and after that, read this article. Then you have a much better understanding of what I'm trying to explain here. 



 

Young's Double Slit Experiment

                                                                                                                     - Distributed for non-commercial use through Wikipedia.
                                A depiction of Young's double slit experiment. Distributed for non-commercial use through Wikipedia.
 
Thomas Young performed his famous double slit experiment which seemed to prove that light was a wave. This experiment had profound implications, determining most of nineteenth century physics and resulting in several attempts to discover the ether, or the medium of light propagation. Though the experiment is most notable with light, the fact is that this sort of experiment can be performed with any type of wave, such as water. For the moment, however, we'll focus on the behavior of light.

What Was the Experiment?

 
In the early 1800's (1801 to 1805, depending on the source), Thomas Young conducted his experiment. He allowed light to pass through a slit in a barrier so it expanded out in wave fronts from that slit as a light source (Huygens' Principle). That light, in turn, passed through pair of slits in another barrier (carefully placed the right distance from the original slit). Each slit, in turn, diffracted the light as if they were also individual sources of light. The light impacted an observation screen. This is shown to the right.

When a single slit was open, it merely impacted the observation screen with greater intensity at the center and then faded as you moved away from the center. There are two possible results of this experiment (click on the image to see another image showing both interpretations):

Particle interpretation: If light exists as particles, the intensity of both slits will be the sum of the intensity from the individual slits.

Wave interpretation: If light exists as waves, the light waves will have interference under the principle of superposition, creating bands of light (constructive interference) and dark (destructive interference).

 
When the experiment was conducted, the light waves did indeed show these interference patterns. A third image that you can view is a graph of the intensity in terms of position, which matches with the predictions from interference .


Impact of Young's Experiment

At the time, this seemed to conclusively prove that light traveled in waves, causing a revitalization in Huygen's wave theory of light, which included an invisible medium,ether , through which the waves propagated. Several experiments throughout the 1800s, most notably the famed Michelson-Morley experiment , attempted to detect the ether or its effects directly.

They all failed and a century later Einstein's work in the photoelectric effect and relativity resulted in the ether no longer being necessary to explain the behavior of light. Again a particle theory of light took dominance.

Expanding the Double Slit Experiment

Still, once the photon theory of light came about, saying the light moved only in discrete quanta, the question became how these results were possible. Over the years, physicists have taken this basic experiment and explored it in a number of ways...


 - Distributed for non-commercial use through Wikipedia.
 
A depiction of Thomas Young's double slit experiment, with particle and wave predictions. Distributed for non-commercial use through Wikipedia.

In the early 1900s, the question remained how light - which was now recognized to travel in particle-like "bundles" of quantized energy, called photons - could also exhibit the behavior of waves. Certainly, a bunch of water atoms (particles) when acting together form waves. Maybe this was something similar.

One Photon at a Time

It became possible to have a light source that was set up so that it emitted one photon at a time. This would be, literally, like hurling microscopic ball bearings through the slits. By setting up a screen that was sensitive enough to detect a single photon, you could determine whether there were or were not interference patterns in this case.

 

One way to do this is to have a sensitive film set up and run the experiment over a period of time, then look at the film to see what the pattern of light on the screen is. Just such an experiment was performed and, in fact, it matched Young's version identically - alternating light and dark bands, seemingly resulting from wave interference.

This result both confirms and bewilders the wave theory. In this case, photons are being emitted individually. There is literally no way for wave interference to take place, because each photon can only go through a single slit at a time. But the wave interference is observed. How is this possible? Well, the attempt to answer that question has spawned many intriguing interpretations of quantum physics, from the Copenhagen interpretation to the many-worlds interpretation.

It Gets Even Stranger

Now assume that you conduct the same experiment, with one change. You place a detector that can tell whether or not the photon passes through a given slit. If we know the photon passes through one slit, then it cannot pass through the other slit to interfere with itself.

 

It turns out that when you add the detector, the bands disappear! You perform the exact same experiment, but only add a simple measurement at an earlier phase, and the result of the experiment changes drastically.

Something about the act of measuring which slit is used removed the wave element completely. At this point, the photons acted exactly as we'd expect a particle to behave. The very uncertainty in position is related, somehow, to the manifestation of wave effects.

More Particles

Over the years, the experiment has been conducted in a number of different ways. In 1961, Claus Jonsson performed the experiment with electrons, and it conformed with Young's behavior, creating interference patterns on the observation screen. Jonsson's version of the experiment was voted "the most beautiful experiment" byPhysics World readers in 2002.

In 1974, technology became able to perform the experiment by releasing a single electron at a time. Again, the interference patterns showed up. But when a detector is placed at the slit, the interference once again disappears. The experiment was again performed in 1989 by a Japanese team that was able to use much more refined equipment.

Once the observers looked at the results when performing the experiment, the particles behaved like particles. But when the observers left the room for an hour while leaving the detection equipment on, but not recording any data, when they came back they were baffled. When they looked at the screen, the particles again behaved like a wave and left an interference pattern behind. This means that consiousness is involved and that we probably create our own reality!

The experiment has been performed with photons, electrons, and atoms, and each time the same result becomes obvious - something about measuring the position of the particle at the slit removes the wave behavior. Many theories exist to explain why, but so far much of it is still conjecture.


  


 

 




 

String Theory

String theory is a mathematical theory that tries to explain certain phenomena which is not currently explainable under the standard model of quantum physics.

The Basics of String Theory

At its core, string theory uses a model of one-dimensional strings in place of the particles of quantum physics. These strings, the size of the Planck length (i.e. 10 -35m) vibrate at specific resonant frequencies. (NOTE: Some recent versions of string theory have predicted that the strings could have a longer length, up to nearly a millimeter in size, which would mean they're in the realm that experiments could detect them.) The formulas that result from string theory predict more than four dimensions (10 or 11 in the most common variants, though on version requires 26 dimensions), but the extra dimensions are "curled up" within the Planck length.

 

In addition to the strings, string theory contains another type of fundamental object called a brane, which can have many more dimensions. In some "braneworld scenarios," our universe is actually "stuck" inside of a 3-dimensional brane (called a 3-brane).

String theory was initially developed in the 1970s in an attempt to explain some inconsistencies with the energy behavior of hadrons and other fundamental particles of physics.

 

As with much of quantum physics, the mathematics that applies to string theory cannot be uniquely solved. Physicists must apply perturbation theory to obtain a series of approximated solutions. Such solutions, of course, include assumptions which may or may not be true.

The driving hope behind this work is that it will result in a "theory of everything," including a solution to the problem of quantum gravity, to reconcile quantum physics with general relativity, thus reconciling the fundamental forces of physics.

Variants of String Theory

Bosonic String Theory : The first string theory, which focused only on bosons.

Superstring Theory: This variant of string theory (short for "supersymmetric string theory") incorporates fermions and supersymmetry. There are five independent superstring theories:


M-Theory : A superstring theory, proposed in 1995, which attempts to consolidate the Type I, Type IIA, Type IIB, Type HO, and Type HE models as variants of the same fundamental physical model.

Research in String Theory

At present, string theory has not successfully made any prediction which is not also explained through an alternative theory. It is neither specifically proven nor falsified, though it has mathematical features which give it great appeal to many physicists.

A number of proposed experiments might have the possibility of displaying "string effects." The energy required for many such experiments is not currently obtainable, although some are in the realm of possibility in the near future, such as possible observations from black holes.

Only time will tell if string theory will be able to take a dominant place in science, beyond inspiring the hearts and minds of many physicists.

      



Source: Physics About



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