Wormhole

A hypothetical "tunnel" connecting two different points in spacetime in such a way that a trip through the wormhole could take much less time than a journey between the same starting and ending points in normal space. The ends of a wormhole could, in theory, be intra-universe (i.e. both exist in the same universe) or inter-universe (exist in different universes, and thus serve as a connecting passage between the two).

Wormholes arise as solutions to the equations of Einstein's general theory of relativity. In fact, they crop up so readily in this context that some theorists are encouraged to think that real counterparts may eventually be found or fabricated and, perhaps, used for high-speed space travel and/or time travel. However, a known property of wormholes is that they are highly unstable and would probably collapse instantly if even the tiniest amount of matter, such as a single photon, attempted to pass through them. A possible way around this problem is the use of exotic matter to prevent the wormhole from pinching off.

A brief history of wormholes

The theory of wormholes goes back to 1916, shortly after Einstein published his general theory, when Ludwig Flamm, an obscure Austrian physicist, looked at the simplest possible solution of Einstein's field equations, known as the Schwarzschild solution (or Schwarzschild metric). This describes the gravitational field around a spherically-symmetric non-rotating mass. If the mass is sufficiently compact, the solution describes a particular form of the phenomenon now called a black hole – the Schwarzschild black hole. Flamm realized that Einstein's equations allowed a second solution, now known as a white hole, and that the two solutions, describing two different regions of (flat) spacetime were connected (mathematically) by a spacetime conduit.¹ Because the theory has nothing to say about where these regions of spacetime might be in the real world, the black hole "entrance" and white hole "exit" could be in different parts of the same universe or in entirely different universes.

In 1935, Einstein and Nathan Rosen further explored, it can be appreciated with hindsight, the theory of intra- or inter-universe connections in a paper² whose actual purpose was to try to explain fundamental particles, such as electrons, in terms of spacetime tunnels threaded by electric lines of force. Their work gave rise to the formal name Einstein-Rosen bridge for what the physicist John Wheeler would later call a "wormhole." (Wheeler also coined the terms "black hole" and "quantum foam".) Wheeler's 1955 paper³ discusses wormholes in terms of topological entities called geons and, incidentally, provides the first (now familiar) diagram of a wormhole as a tunnel connecting two openings in different regions of spacetime.

Traversable wormholes

Interest in so-called traversable wormholes gathered pace following the publication of a 1987 paper by Michael Morris, Kip Thorne, and Uri Yertsever (MTY) at the California Institute of Technology.^{4, 5} This paper stemmed from an inquiry to Thorne by Carl Sagan who was mulling over a way of conveying the heroine in his novel Contact across interstellar distances at trans-light speed. Thorne gave the problem to his Ph.D. students, Michael Morris and Uri Yertsever, who realized that such a journey might be possible if a wormhole could be held open long enough for a spacecraft (or any other object) to pass through. MTY concluded that to keep a wormhole open would require matter with a negative energy density and a large negative pressure – larger in magnitude than the energy density. Such hypothetical matter is called exotic matter.

Although the existence of exotic matter is speculative, a way is known of producing negative energy density: the Casimir effect. As a source for their wormhole, MTY turned to the quantum vacuum. "Empty space" at the smallest scale, it turns out, is not empty at all but seething with violent fluctuations in the very geometry of spacetime. At this level of nature, ultra-small wormholes are believed to continuously wink into and out of existence. MTY suggested that a sufficiently advanced civilization could expand one of these tiny wormholes to macroscopic size by adding energy. Then the wormhole could be stabilized using the Casimir effect by placing two charged superconducting spheres in the wormhole mouths. Finally, the mouths could be transported to widely-separated regions of space to provide a means of FTL communication and travel. For example, a mouth placed aboard a spaceship might be carried to some location many light-years away. Because this initial trip would be through normal spacetime, it would have to take place at sublight speeds. But during the trip and afterwards instantaneous communication and transport through the wormhole would be possible. The ship could even be supplied with fuel and provisions through the mouth it was carrying. Also, thanks to relativistic time-dilation, the journey need not take long, even as measured by Earth-based observers. For example, if a fast starship carrying a wormhole mouth were to travel to Vega, 25 light-years away, at 99.995% of the speed of light (giving a time-dilation factor of 100), shipboard clocks would measure the journey as taking just three months. But the wormhole stretching from the ship to Earth directly links the space and time between both mouths – the one on the ship and the one left behind on (or near) Earth. Therefore, as measured by Earthbound clocks too, the trip would have taken only three months – three months to establish a more-or-less instantaneous transport and communications link between here and Vega.

Star Trek's Deep Space 9 is located alongside a natural wormhole that leads to the other side of the Galaxy

Of course, the MTY scheme is not without technical difficulties, one of which is that the incredibly powerful forces needed to hold the wormhole mouths open might tear apart anything or anyone that tried to pass through. In an effort to design a more benign environment for travelers using a wormhole, Matt Visser of Washington University in St. Louis conceived an arrangement in which the spacetime region of a wormhole mouth is flat (and thus force-free) but framed by struts of exotic matter that contain a region of very sharp curvature.⁶ Visser envisages a cubic design, with flat-space wormhole connections on the square sides and cosmic strings as the edges. Each cube-face may connect to the face of another wormhole-mouth cube, or the six cube faces may connect to six different cube faces in six separated locations.

In 2011, Panagiota Kanti (University of Ioannina) and Burkhard Kleihaus (Universität Oldenburg) showed how it might be possible to construct traversable wormholes without using exotic matter by resorting to a form of string theory.¹⁰

Given that our technology is not yet up to the task of building a wormhole subway, the question arises of whether they might already exist. One possibility is that advanced races elsewhere in the Galaxy or beyond have already set up a network of wormholes that we could learn to use. Another is that wormholes might occur naturally. David Hochberg and Thomas Kephart of Vandebilt University have discovered that, in the earliest moments of the Universe, gravity itself may have given rise to regions of negative energy in which natural, self-stabilizing wormholes may have formed. Such wormholes, created in the Big Bang, might be around today, spanning small or vast distances in space.

References

1. Flamm, L. "Comments on Einstein's theory of gravity," Physikalische Zeitschrift, 17, 48 (1916).
2. Einstein, A., and Rosen, N. "The Particle Problem in the General Theory of Relativity", Physical Review, 48, 73 (1935)
3. Wheeler, J. A. "Geons," Physical Review, 97, 511–536 (1955).
4. Morris, M. S, Thorne, K. S., and Yurtsever, U. "Wormholes, time machines, and the weak energy condition," Phys. Rev. Letters, 61, 1446–1449 (1988).
5. Morris, M. S., and Thorne, K. S. "Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity", Am. J. Phys., 56, No. 5, 395–412 (1988).
6. Visser, M. "Wormholes, baby universes, and causality", Phys. Rev. D, 41, No. 4, 1116–1124 (1990).
7. Hochberg, D. and Visser, M. "Geometric structure of the generic static traversable wormhole throat", Phys. Rev. D, Phys. Rev D56, 4745 (1997).
8. Maccone, C. "Interstellar travel through magnetic wormholes", Journal of the British Interplanetary Society, 48, No. 11, 453–458 (1995).
9. Visser, M. (1995) Lorentzian Wormholes – From Einstein to Hawking, Woodbury, NY: AIP Press (1995).
10. Kantio, P. and Kleihaus, B., "Wormholes in dilatonic Einstein-Gauss-Bonnet theory." arXiv (2011).

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Active Galaxies and Quasars

Active galaxies are galaxies which have a small core of emission embedded in an otherwise typical galaxy. This core may be highly variable and very bright compared to the rest of the galaxy. Models of active galaxies concentrate on the possibility of a supermassive black hole which lies at the center of the galaxy. The dense central galaxy provides material which accretes onto the black hole releasing a large amount of gravitational energy. Part of the energy in this hot plasma is emitted as x-rays and gamma rays.
For "normal" galaxies, we can think of the total energy they emitas the sum of the emission from each of the stars found in the galaxy. Forthe "active" galaxies, this is not true. There is a great deal moreemitted energy than there should be... and this excess energy is found in theinfrared, radio, UV, and X-ray regions of the electromagnetic spectrum. Theenergy emitted by an active galaxy (or AGN) is anything but "normal". So what is happening in these galaxies to produce such an energetic output?
There are several types of active galaxies: Seyferts, quasars, andblazars. Most scientistsbelieve that, even though these types look very different to us, they arereally all the same thing viewed from different directions! Quasars are activegalaxies which are all very, very, very far away from us. Some of the quasarswe have seen so far are 12 billion light-years away! Blazars are verybright in the radio band, which results from looking directly down ajet which is emitting in synchrotron radiation. On the other hand, if the jet is not pointing toward youat all, and the dusty disk of material which lies in the plane of thegalaxy is in the way, you would see justwhat we see from the Seyferts. By measuring their redshifts, we findthat Seyferts are much closer to us thanquasars or blazars.
Active galaxies are intensely studied at all wavelengths. Since theycan change their behavior on short timescales, it is useful to studythem simultaneously at all energies. X-ray and gamma-ray observations haveproven to be important parts of this multiwavelength approach since manyhigh-energy quasars emit a large fraction of their power at such energies. X-rays can penetrate outward from very nearthe center of a galaxy. Since that is where the "engines" of AGN arelocated, X-rays provide scientists with unique insights into the physicalprocesses occurring there. In addition, gamma-ray observations alone canprovide valuable information on the nature of particle acceleration in thequasar jet, and clues as to how the particles interact with theirsurroundings.

A diagram of an active galaxy, showingthe primary components.

Seyfert Galaxies

Of the two types of Active Galactic Nuclei (AGN) which emit gamma rays, Seyfert galaxies are the low-energy gamma-ray sources. Seyfert galaxies typically emit most of their gamma rays up to energies of about100 keV and then fade as we observe them at higher energies. Early gamma-ray observations of Seyfert galaxies indicated that photons were detected up to MeV energies, but more sensitive observations have cast doubt on this possibility. At these low gamma-ray energies, the emission is usually a smooth continuation of the X-ray emission from such objects. This generally indicates that the physical processes creating the gamma rays are thermal processes similar to thoseresponsible for emission from galactic black hole sources. As a result, gamma-ray studies of the high-energy spectrum and variability can give scientists important information about the physical environment in the AGN.
Observations of Seyfert galaxies in gamma rays are also important for studies of the cosmic gamma-ray background. Even in regions of the sky wherethere are no point sources, a faint gamma-ray glow is detectable. It may be that this glow is the sum of many faint galaxies or perhaps a more exotic process. Studies of individual Seyfert galaxies can be combined with a model of how such objects are distributed in the Universe to compare to the diffuse gamma-ray background. In this way, astronomers not only learn about the interesting AGN phenomena, but learn more about the general nature of the Universe as a whole.

An artists concept of an active galactic nucleus

Quasars

One of the most remarkable trends in gamma-ray astronomy in recentyears has been the emergence of high-energy gamma-ray quasars as an important component of the gamma-ray sky. At gamma-ray energies, these active galaxies are bright; they are highly variable at all energies. Unlike the Seyfert type AGN, most of these sources arepreferentially detected at high energies, usually 100 MeV or more. In fact, they have been detected above 1 GeV, and some up to several TeV! Given the large distances to these objects and the strong emission of high-energy gamma rays, these are the most powerful particle accelerators in the Universe. Over 50 high-energy quasars are known at this time.Some appear as fuzzy stars that can be seen with large amateur telescopes. Many astronomers believe that Seyfert galaxies and high-energyquasars are basically the same type of objects, but we are simply viewingthem differently. Radio observations of AGN often show powerful jets, streamsof particles coming from the central source -- like water from a spigot.Charged particles are accelerated to nearly the speed of light in these jets. In the unified view of active galaxies,high-energy quasars are being viewed with the jet pointed towards uswhich allows us to see the resulting energetic radiation. With Seyfert galaxies, we are viewing from the side and do not see the very high-energyradiation which is traveling down the jet.

The region of the sky containing one of the high-energy quasars, PKS 0528+134, is shown at two different times using the EGRET instrument on the Compton Gamma-Ray Observatory. These active galaxies are highly variable, strongly emitting gamma-rays sometimes, disappearing at other times.

Blazars

The AGNs observed at higher energies form a subclass of AGNs known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that what we observe is primarily emission from the jet region. They arethus similar to quasars, but are not observed to be as luminous. The visible and gamma-ray emission from blazars is variable on timescales from minutes to days. Although theories exist as to thecauses of this variability, the sparse data do not yet allow any of theideas to be tested.To date more than 60 blazars have been detected by the EGRET experimentaboard the Compton Gamma-Ray Observatory. All these objects appear toemit most of their bolometricluminosity at gamma-ray energies and, inaddition, are strong extragalactic radio sources.

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Feeling "Weightless" When You Go "Over the Hump"

The phenomenon of "weightlessness" occurs when there is no force of support on your body. When your body is effectively in "free fall", accelerating downward at the acceleration of gravity, then you are not being supported. The sensation of apparent weight comes from the support that you feel from the floor, from the seat, etc. Different sensations of apparent weight can occur on a roller-coaster or in an aircraft because they can accelerate either upward or downward.
If you travel in a curved path in a vertical plane, then when you go over the top on such a path, there is necessarily a downward acceleration. Taking the example of the roller-coaster which is constrained to follow a track, then the condition for weightlessness is met when the downward acceleration of your seat is equal to the acceleration of gravity. Considering the path of the roller-coaster to be a segment of a circle so that it can be related to the centripetal acceleration, the condition for weightlessness is
The "weightlessness" you may feel in an aircraft occurs any time the aircraft is accelerating downward with acceleration 1g. It is possible to experience weightlessness for a considerable length of time by turning the nose of the craft upward and cutting power so that it travels in a ballistic trajectory. A ballistic trajectory is the common type of trajectory you get by throwing a rock or a baseball, neglecting air friction. At every point on the trajectory, the acceleration is equal to g downward since there is no support. A considerable amount of experimentation has been done with such ballistic trajectories to practice for orbital missions where you experience weightlessness all the time.

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The Nobel Prize in Physics 1930 Sir Venkata Raman

Chandrasekhara Venkata Raman was born at Trichinopoly in Southern India on November 7th, 1888. His father was a lecturer in mathematics and physics so that from the first he was immersed in an academic atmosphere. He entered Presidency College, Madras, in 1902, and in 1904 passed his B.A. examination, winning the first place and the gold medal in physics; in 1907 he gained his M.A. degree, obtaining the highest distinctions.

His earliest researches in optics and acoustics - the two fields of investigation to which he has dedicated his entire career - were carried out while he was a student.

Since at that time a scientific career did not appear to present the best possibilities, Raman joined the Indian Finance Department in 1907; though the duties of his office took most of his time, Raman found opportunities for carrying on experimental research in the laboratory of the Indian Association for the Cultivation of Science at Calcutta (of which he became Honorary Secretary in 1919).

In 1917 he was offered the newly endowed Palit Chair of Physics at Calcutta University, and decided to accept it. After 15 years at Calcutta he became Professor at the Indian Institute of Science at Bangalore (1933-1948), and since 1948 he is Director of the Raman Institute of Research at Bangalore, established and endowed by himself. He also founded the Indian Journal of Physics in 1926, of which he is the Editor. Raman sponsored the establishment of the Indian Academy of Sciences and has served as President since its inception. He also initiated the Proceedings of that academy, in which much of his work has been published, and is President of the Current Science Association, Bangalore, which publishes Current Science (India).

Some of Raman's early memoirs appeared as Bulletins of the Indian Associationfor the Cultivation of Science (Bull. 6 and 11, dealing with the "Maintenance of Vibrations"; Bull. 15, 1918, dealing with the theory of the musical instruments of the violin family). He contributed an article on the theory of musical instruments to the 8th Volume of the Handbuch der Physik, 1928. In 1922 he published his work on the "Molecular Diffraction of Light", the first of a series of investigations with his collaborators which ultimately led to his discovery, on the 28th of February, 1928, of the radiation effect which bears his name ("A new radiation", Indian J. Phys., 2 (1928) 387), and which gained him the 1930 Nobel Prize in Physics.

Other investigations carried out by Raman were: his experimental and theoretical studies on the diffraction of light by acoustic waves of ultrasonic and hypersonic frequencies (published 1934-1942), and those on the effects produced by X-rays on infrared vibrations in crystals exposed to ordinary light. In 1948 Raman, through studying the spectroscopic behaviour of crystals, approached in a new manner fundamental problems of crystal dynamics. His laboratory has been dealing with the structure and properties of diamond, the structure and optical behaviour of numerous iridescent substances (labradorite, pearly felspar, agate, opal, and pearls).

Among his other interests have been the optics of colloids, electrical and magnetic anisotropy, and the physiology of human vision.

Raman has been honoured with a large number of honorary doctorates and memberships of scientific societies. He was elected a Fellow of the Royal Society early in his career (1924), and was knighted in 1929.

From Nobel Lectures, Physics 1922-1941, Elsevier Publishing Company, Amsterdam, 1965

This autobiography/biography was written at the time of the award and first published in the book series Les Prix Nobel. It was later edited and republished in Nobel Lectures. To cite this document, always state the source as shown above.

Sir Venkata Raman died on November 21, 1970.

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How many string theories are there?

The bosonic string world sheet action

for a string propagating in flat 26-dimensional spacetime with coordinates X^m(s,t) can give rise to four different quantum mechanically consistent string theories, depending on the choice of boundary conditions used to solve the equations of motion. The choices are divided into two categories:
A. Are the strings open (with free ends) or closed (with ends joined together in a loop)?
B. Are the strings orientable (you can tell which direction you're traveling along the string) or unorientable (you can't tell which direction you're traveling along the string)?
   There are four different combinations of options, giving rise to the four bosonic string theories shown in the table below. Notice in the table that open string theories also contain closed strings. Why is this? Because an open string can sometimes join its two free ends and become a closed string and then break apart again into an open string. In pure closed string theory, the analog of that process does not occur.
   The bosonic string theories are all unstable because the lowest excitation mode, or the ground state, is a tachyon with M²=-1/a'. The massless particle spectrum always includes the graviton, so gravity is always a part of any bosonic string theory. The vector boson is similar to the photon of electromagnetism or the gauge fields of any Yang-Mills theory. The antisymmetric tensor field carries a force that is difficult to describe in this short space. The strings act as a source of this field.

Bosonic strings, d=26
Type	Oriented?	Details
Open (plus closed)	Yes	Scalar tachyon, massless antisymmetric tensor, graviton and dilaton
Open (plus closed)	No	Scalar tachyon, massless graviton and dilaton
Closed	Yes	Scalar tachyon, massless vector boson, antisymmetric tensor, graviton and dilaton
Closed	No	Scalar tachyon, massless graviton and dilaton

It's just as well that bosonic string theory is unstable, because it's not a realistic theory to begin with. The real world has stable matter made from fermions that satisfy the Pauli Exclusion Principle where two identical particles cannot be in the same quantum state at the same time.
    Adding fermions to string theory introduces a new set of negative norm states or ghosts, to add to the ghost states that come from the bosonic sector described on the previous page. String theorists learned that all of these bad ghost states decouple from the spectrum when two conditions are satisfied: the number of spacetime dimensions is 10, and theory is supersymmetric, so that there are equal numbers of bosons and fermions in the spectrum.
   Fermions have more complicated boundary conditions than bosons, so unraveling the different possible consistent superstring theories took researchers quite a bit of doing. The simplest way to examine a superstring theory is to go to what is called superspace. In superspace, in addition to the normal commuting coordinates X^m, a set of anticommuting coordinates q^A are added. In superstring theories index A runs from 1 to 2 (an additional spinor index is not shown). The anticommutation relations of the coordinates are

   The options of open vs closed, and oriented Vs unoriented boundary conditions are still present, but there are also choices involving fermions that distinguish one superstring theory from another. The superspace coordinates q¹ and q² behave like particles with spin 1/2 and zero mass, which can only spin two ways -- with the spin axis in the same or opposite direction as the momentum. This property is called handedness. So q¹ and q² can have either the same or the opposite handedness.
   The resulting consistent string theories can be described in terms of the massless particle spectrum and the resulting number of spacetime supersymmetry charges, denoted by the letter N in the table below. None of the theories below suffer from the tachyon problem that plagues bosonic string theories. All of the theories below contain gravity.

Superstrings, d=10
Type	Open or closed?	Oriented?	N	Details
I	Open (plus closed)	No	1	Graviton, no tachyon, SO(32) gauge symmetry, charges are attached to the ends of the strings
IIA	Closed	No	2	Graviton, no tachyon, only a U(1) gauge symmetry
IIB	Closed	Yes	2	Graviton, no tachyon, no gauge symmetry
Heterotic E8XE8	Closed	Yes	1	Graviton, no tachyon, E8XE8 gauge symmetry
Heterotic SO(32)	Closed	Yes	1	Graviton, no tachyon, SO(32) gauge symmetry

A supersymmetric theory has a fermionic partner for every bosonic particle. The superpartner of a graviton is called a gravitino and has spin 3/2. All of the theories above contain gravitons and gravitinos.
   For open superstrings, the choices turn out to be restricted by conditions too complicated to explain here. It turns out that the only consistent theory has unoriented strings, with q¹ and q² having the same handedness, with an SO(32) gauge symmetry included by attaching little charges to the ends of the open string. These charges are called Chan Paton factors. The resulting theory is called Type I.
    Closed string oscillations can be separated into modes that propagate around the string in different directions, sometimes called left movers and right movers. If q¹ and q² have opposite handedness, then they also have opposite momentum, and hence travel in opposite directions. Therefore they provide a way to tell which direction one is traveling around the string. Therefore these strings are oriented. This is called Type IIA superstring theory.
    Because q¹ and q² have opposite handedness, this theory winds up being too symmetric for real life. Every fermion has a partner of the opposite handedness, which is not what is observed in our world, where the neutrino comes in a left-handed version but not a right-handed version. The real world seems to be chiral, which means having a preferred handedness for massless fermions. But Type IIA superstring theory is a nonchiral theory. There is also no way to add a gauge symmetry to Type IIA superstrings, so here also the theory fails as a model of the real world.
    If q¹ and q² have the same handedness, and the string is oriented, then we get Type IIB superstring theory. This theory is chiral, and so there will be massless fermions that don't have partners of the opposite handedness, as is observed in our world today. However, there is no way to add a gauge symmetry to the Type IIB theory. So there isn't a way to include any of the known forces other than gravity.
    If q¹ and q² have the same handedness, but the string is unoriented, that turns out to just give the closed string part of the Type I theory.
    This seems to have exhausted all of the obvious options. But there's actually something crazy that can be done with a closed string that yields two more important superstring theories.
    The left-moving and right-moving modes of a string can be separated and treated as different theories. In 1984 it was realized that consistent string theories could be built by combining a bosonic string theory moving in one direction along the string, with a supersymmetric string theory with a single q¹ moving in the opposite direction. These theories are called heterotic superstring theories.
    That sounds crazy -- because bosonic strings live in 26 dimensions but supersymmetric string theories live in 10 dimensions. But the extra 16 dimensions of the bosonic side of the theory aren't really spacetime dimensions. Heterotic string theories are supersymmetric string theories living in ten spacetime dimensions.
    The two types of heterotic theories that are possible come from the two types of gauge symmetry that give rise to quantum mechanically consistent theories. The first is SO(32) and the second is the more exotic combination called E₈XE_8. The E₈XE₈ heterotic theory was previously regarded as the only string theory that could give realistic physics, until the mid-1990s, when additional possibilities based on the other theories were identified.

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