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).

Read More

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.

Read More

Newton's Three Laws of Motion

For Animated version See Newton's three law of motion: Animated



Let us begin our explanation of how Newton changed our understanding of the Universe by enumerating his Three Lawsof Motion.

Newton's First Law of Motion:

I. Every object in a state of uniform motion tends to remain in thatstate of motion unless an external force is applied to it.

This we recognize as essentially Galileo's concept of inertia, and this is oftentermed simply the "Law of Inertia".

Newton's Second Law of Motion:

II. The relationship between an object's mass m, its accelerationa, and the applied force F isF = ma.Acceleration and force are vectors (as indicated by their symbols beingdisplayed in slant bold font); in this law the direction of the forcevector is the same as the direction of the acceleration vector.

This is the most powerful of Newton's three Laws, because it allows quantitativecalculations of dynamics: how do velocities change when forces are applied.Notice the fundamental difference between Newton's 2nd Law and the dynamics ofAristotle: according to Newton, a force causes only a change invelocity (an acceleration); it does not maintain the velocity as Aristotleheld.
This is sometimes summarized by saying that under Newton, F =ma, but under Aristotle F = mv, where v is the velocity.Thus, according to Aristotle there is only a velocity if there is a force, butaccording to Newton an object with a certain velocity maintains thatvelocity unless a force acts on it to cause an acceleration (that is,a change in the velocity). As we have noted earlier in conjunction with thediscussion of Galileo, Aristotle's view seems to bemore in accord with common sense, but that is because of a failure toappreciate the role played by frictional forces. Once account is taken of all forcesacting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that arefound to be in accord with the observations.

Newton's Third Law of Motion:

III. For every action there is an equal and opposite reaction.

This law is exemplified by what happens if we step off a boat onto the bank ofa lake: as we move in the direction of the shore, the boat tends to move inthe opposite direction (leaving us facedown in the water, if we aren'tcareful!).

Read More