Valley of stability

From Wikipedia, the free encyclopedia

 https://en.wikipedia.org/wiki/Valley_of_stability

In nuclear physics, the valley of stability (also called the belt of stability, nuclear valley, energy valley, or beta stability valley) is a characterization of the stability of nuclides to radioactivity based on their binding energy. Nuclides are composed of protons and neutrons. The shape of the valley refers to the profile of binding energy as a function of the numbers of neutrons and protons, with the lowest part of the valley corresponding to the region of most stable nuclei. The line of stable nuclides down the center of the valley of stability is known as the line of beta stability. The sides of the valley correspond to increasing instability to beta decay (β⁻ or β⁺). The decay of a nuclide becomes more energetically favorable the further it is from the line of beta stability. The boundaries of the valley correspond to the nuclear drip lines, where nuclides become so unstable they emit single protons or single neutrons. Regions of instability within the valley at high atomic number also include radioactive decay by alpha radiation or spontaneous fission. The shape of the valley is roughly an elongated paraboloid corresponding to the nuclide binding energies as a function of neutron and atomic numbers.

The nuclides within the valley of stability encompass the entire table of nuclides. The chart of those nuclides is known as a Segrè chart, after the physicist Emilio Segrè. The Segrè chart may be considered a map of the nuclear valley. The region of proton and neutron combinations outside of the valley of stability is referred to as the sea of instability.

Scientists have long searched for long-lived heavy isotopes outside of the valley of stability, hypothesized by Glenn T. Seaborg in the late 1960s. These relatively stable nuclides are expected to have particular configurations of "magic" atomic and neutron numbers, and form a so-called island of stability.

Description

All atomic nuclei are composed of protons and neutrons bound together by the nuclear force. There are 286 primordial nuclides that occur naturally on earth, each corresponding to a unique number of protons, called the atomic number, Z, and a unique number of neutrons, called the neutron number, N. The mass number, A, of a nuclide is the sum of atomic and neutron numbers, A = Z + N. Not all nuclides are stable, however. According to Byrne, stable nuclides are defined as those having a half-life greater than 10¹⁸ years, and there are many combinations of protons and neutrons that form nuclides that are unstable. A common example of an unstable nuclide is carbon-14 that decays by beta decay into nitrogen-14 with a half-life of about 5,730 years

¹⁴
₆C
→ ¹⁴
₇N
+
e⁻
+
ν
_e

In this form of decay, the original element becomes a new chemical element in a process known as nuclear transmutation and a beta particle and an electron antineutrino are emitted. An essential property of this and all nuclide decays is that the total energy of the decay product is less than that of the original nuclide. The difference between the initial and final nuclide binding energies is carried away by the kinetic energies of the decay products, often the beta particle and its associated neutrino.

The concept of the valley of stability is a way of organizing all of the nuclides according to binding energy as a function of neutron and proton numbers. Most stable nuclides have roughly equal numbers of protons and neutrons, so the line for which Z = N forms a rough initial line defining stable nuclides. The greater the number of protons, the more neutrons are required to stabilize a nuclide, however, so nuclides with larger values for Z require an even larger number of neutrons, N > Z, to be stable. The valley of stability is formed by the negative of binding energy, the binding energy being the energy required to break apart the nuclide into its proton and neutron components. The stable nuclides have high binding energy, and these nuclides lie along the bottom of the valley of stability. Nuclides with weaker binding energy have combinations of N and Z that lie off of the line of stability and further up the sides of the valley of stability. Unstable nuclides can be formed in nuclear reactors or supernovas, for example. Such nuclides often decay in sequences of reactions called decay chains that take the resulting nuclides sequentially down the slopes of the valley of stability. The sequence of decays take nuclides toward greater binding energies, and the nuclides terminating the chain are stable. The valley of stability provides both a conceptual approach for how to organize the myriad stable and unstable nuclides into a coherent picture and an intuitive way to understand how and why sequences of radioactive decay occur.

• 

  Chart of nuclides (isotopes) by binding energy, depicting the valley of stability. The diagonal line corresponds to equal numbers of neutrons and protons. Dark blue squares represent nuclides with the greatest binding energy, hence they correspond to the most stable nuclides. The binding energy is greatest along the floor of the valley of stability.

• 

  Chart of nuclides by half life. Black squares represent nuclides with the longest half lives hence they correspond to the most stable nuclides. The most stable, long-lived nuclides lie along the floor of the valley of stability. Nuclides with more than 20 protons must have more neutrons than protons to be stable.

• 

  Chart of nuclides by type of decay. Black squares are stable nuclides. Nuclides with excessive neutrons or protons are unstable to β⁻ (light blue) or β⁺ (green) decay, respectively. At high atomic number, alpha emission (orange) or spontaneous fission (dark blue) become common decay modes.

The role of neutrons

The protons and neutrons that comprise an atomic nucleus behave almost identically within the nucleus. The approximate symmetry of isospin treats these particles as identical, but in a different quantum state. This symmetry is only approximate, however, and the nuclear force that binds nucleons together is a complicated function depending on nucleon type, spin state, electric charge, momentum, etc. and with contributions from non-central forces. The nuclear force is not a fundamental force of nature, but a consequence of the residual effects of the strong force that surround the nucleons. One consequence of these complications is that although deuterium, a bound state of a proton (p) and a neutron (n) is stable, exotic nuclides such as diproton or dineutron are unbound. The nuclear force is not sufficiently strong to form either p-p or n-n bound states, or equivalently, the nuclear force does not form a potential well deep enough to bind these identical nucleons.

Stable nuclides require approximately equal numbers of protons and neutrons. The stable nuclide carbon-12 (¹²C) is composed of six neutrons and six protons, for example. Protons have a positive charge, hence within a nuclide with many protons there are large repulsive forces between protons arising from the Coulomb force. By acting to separate protons from one another, the neutrons within a nuclide play an essential role in stabilizing nuclides. With increasing atomic number, even greater numbers of neutrons are required to obtain stability. The heaviest stable element, lead (Pb), has many more neutrons than protons. The stable nuclide ²⁰⁶Pb has Z = 82 and N = 124, for example. For this reason, the valley of stability does not follow the line Z = N for A larger than 40 (Z = 20 is the element calcium). Neutron number increases along the line of beta stability at a faster rate than atomic number.

The line of beta stability follows a particular curve of neutron–proton ratio, corresponding to the most stable nuclides. On one side of the valley of stability, this ratio is small, corresponding to an excess of protons over neutrons in the nuclides. These nuclides tend to be unstable to β⁺ decay or electron capture, since such decay converts a proton to a neutron. The decay serves to move the nuclides toward a more stable neutron-proton ratio. On the other side of the valley of stability, this ratio is large, corresponding to an excess of neutrons over protons in the nuclides. These nuclides tend to be unstable to β⁻ decay, since such decay converts neutrons to protons. On this side of the valley of stability, β⁻ decay also serves to move nuclides toward a more stable neutron-proton ratio.

Neutrons, protons, and binding energy

See also: Semi-empirical mass formula

The mass of an atomic nucleus is given by

${\displaystyle m=Z{m}_p+N{m}_n-\frac{E_B}{c^2}}$

where ${\displaystyle m_p}$ and ${\displaystyle m_n}$ are the rest mass of a proton and a neutron, respectively, and ${\displaystyle E_B}$ is the total binding energy of the nucleus. The mass–energy equivalence is used here. The binding energy is subtracted from the sum of the proton and neutron masses because the mass of the nucleus is less than that sum. This property, called the mass defect, is necessary for a stable nucleus; within a nucleus, the nuclides are trapped by a potential well. A semi-empirical mass formula states that the binding energy will take the form

${\displaystyle E_B=a_{V}A-a_S{A}^{2/3}-a_C\frac{Z^2}{A^{1/3}}-a_A\frac{(A-2Z{)}^2}{A}\pm \delta (A,Z)}$

The difference between the mass of a nucleus and the sum of the masses of the neutrons and protons that comprise it is known as the mass defect. E_B is often divided by the mass number to obtain binding energy per nucleon for comparisons of binding energies between nuclides. Each of the terms in this formula has a theoretical basis. The coefficients ${\displaystyle a_V}$, ${\displaystyle a_S}$, ${\displaystyle a_C}$, ${\displaystyle a_A}$ and a coefficient that appears in the formula for ${\displaystyle \delta (A,Z)}$ are determined empirically.

The binding energy expression gives a quantitative estimate for the neutron-proton ratio. The energy is a quadratic expression in Z that is minimized when the neutron-proton ratio is ${\displaystyle N/Z\approx 1+\frac{a_C}{2a_A}{A}^{2/3}}$. This equation for the neutron-proton ratio shows that in stable nuclides the number of neutrons is greater than the number of protons by a factor that scales as ${\displaystyle A^{2/3}}$.

The negative of binding energy per nucleon for the stable nuclides located along the bottom of the valley of stability. Iron-56 is about the most stable nuclide, and it is about the lowest point within the valley of stability.

The figure at right shows the average binding energy per nucleon as a function of atomic mass number along the line of beta stability, that is, along the bottom of the valley of stability. For very small atomic mass number (H, He, Li), binding energy per nucleon is small, and this energy increases rapidly with atomic mass number. Nickel-62 (28 protons, 34 neutrons) has the highest mean binding energy of all nuclides, while iron-58 (26 protons, 32 neutrons) and iron-56 (26 protons, 30 neutrons) are a close second and third. These nuclides lie at the very bottom of the valley of stability. From this bottom, the average binding energy per nucleon slowly decreases with increasing atomic mass number. The heavy nuclide ²³⁸U is not stable, but is slow to decay with a half-life of 4.5 billion years. It has relatively small binding energy per nucleon.

For β⁻ decay, nuclear reactions have the generic form

^A
_ZX
→ ^A
_Z+1X′
+
e⁻
+
ν
_e

where A and Z are the mass number and atomic number of the decaying nucleus, and X and X′ are the initial and final nuclides, respectively. For β⁺ decay, the generic form is

^A
_ZX
→ ^A
_Z−1X′
+
e⁺
+
ν
_e

These reactions correspond to the decay of a neutron to a proton, or the decay of a proton to a neutron, within the nucleus, respectively. These reactions begin on one side or the other of the valley of stability, and the directions of the reactions are to move the initial nuclides down the valley walls towards a region of greater stability, that is, toward greater binding energy.

The negative of binding energy per nucleon for nuclides with atomic mass number 125 plotted as a function of atomic number. The profile of binding energy across the valley of stability is roughly a parabola. Tellurium-125 is stable, while antimony-125 is unstable to β− decay.

The figure at right shows the average binding energy per nucleon across the valley of stability for nuclides with mass number A = 125. At the bottom of this curve is tellurium (₅₂Te), which is stable. Nuclides to the left of ₅₂Te are unstable with an excess of neutrons, while those on the right are unstable with an excess of protons. A nuclide on the left therefore undergoes β⁻ decay, which converts a neutron to a proton, hence shifts the nuclide to the right and toward greater stability. A nuclide on the right similarly undergoes β⁺ decay, which shifts the nuclide to the left and toward greater stability.

Heavy nuclides are susceptible to α decay, and these nuclear reactions have the generic form,

^A
_ZX
→ ^A-4
_Z-2X′
+ ⁴
₂He

As in β decay, the decay product X′ has greater binding energy and it is closer to the middle of the valley of stability. The α particle carries away two neutrons and two protons, leaving a lighter nuclide. Since heavy nuclides have many more neutrons than protons, α decay increases a nuclide's neutron-proton ratio.

Proton and neutron drip lines

Main articles: Nuclear drip line, proton emission, and neutron emission

The boundaries of the valley of stability, that is, the upper limits of the valley walls, are the neutron drip line on the neutron-rich side, and the proton drip line on the proton-rich side. The nucleon drip lines are at the extremes of the neutron-proton ratio. At neutron–proton ratios beyond the drip lines, no nuclei can exist. The location of the neutron drip line is not well known for most of the Segrè chart, whereas the proton and alpha drip lines have been measured for a wide range of elements. Drip lines are defined for protons, neutrons, and alpha particles, and these all play important roles in nuclear physics.

The difference in binding energy between neighboring nuclides increases as the sides of the valley of stability are ascended, and correspondingly the nuclide half-lives decrease, as indicated in the figure above. If one were to add nucleons one at a time to a given nuclide, the process will eventually lead to a newly formed nuclide that is so unstable that it promptly decays by emitting a proton (or neutron). Colloquially speaking, the nucleon has 'leaked' or 'dripped' out of the nucleus, hence giving rise to the term "drip line".

Proton emission is not seen in naturally occurring nuclides. Proton emitters can be produced via nuclear reactions, usually utilizing linear particle accelerators (linac). Although prompt (i.e. not beta-delayed) proton emission was observed from an isomer in cobalt-53 as early as 1969, no other proton-emitting states were found until 1981, when the proton radioactive ground states of lutetium-151 and thulium-147 were observed at experiments at the GSI in West Germany. Research in the field flourished after this breakthrough, and to date more than 25 nuclides have been found to exhibit proton emission. The study of proton emission has aided the understanding of nuclear deformation, masses and structure, and it is an example of quantum tunneling.

Two examples of nuclides that emit neutrons are beryllium-13 (mean life 2.7×10⁻²¹ s) and helium-5 (7×10⁻²² s). Since only a neutron is lost in this process, the atom does not gain or lose any protons, and so it does not become an atom of a different element. Instead, the atom will become a new isotope of the original element, such as beryllium-13 becoming beryllium-12 after emitting one of its neutrons.

In nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event. Prompt neutrons emerge from the fission of an unstable fissionable or fissile heavy nucleus almost instantaneously. Delayed neutron decay can occur within the same context, emitted after beta decay of one of the fission products. Delayed neutron decay can occur at times from a few milliseconds to a few minutes. The U.S. Nuclear Regulatory Commission defines a prompt neutron as a neutron emerging from fission within 10⁻¹⁴ seconds. 

Island of stability

Main article: Island of stability

The island of stability is a region outside the valley of stability where it is predicted that a set of heavy isotopes with near magic numbers of protons and neutrons will locally reverse the trend of decreasing stability in elements heavier than uranium. The hypothesis for the island of stability is based upon the nuclear shell model, which implies that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger electron shells in atoms. In both cases, shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells.

A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126. These configurations imply that the most stable spherical isotopes would be flerovium-298, unbinilium-304 and unbihexium-310. Of particular note is ²⁹⁸Fl, which would be "doubly magic" (both its proton number of 114 and neutron number of 184 are thought to be magic). This doubly magic configuration is the most likely to have a very long half-life. The next lighter doubly magic spherical nucleus is lead-208, the heaviest known stable nucleus and most stable heavy metal.

Discussion

The valley of stability can be helpful in interpreting and understanding properties of nuclear decay processes such as decay chains and nuclear fission.

The uranium-238 series is a series of α (N and Z less 2) and β− decays (N less 1, Z plus 1) to nuclides that are successively deeper into the valley of stability. The series terminates at lead-206, a stable nuclide at the bottom of the valley of stability.

Radioactive decay often proceeds via a sequence of steps known as a decay chain. For example, ²³⁸U decays to ²³⁴Th which decays to ^234mPa and so on, eventually reaching ²⁰⁶Pb:

${\displaystyle \begin{array}{l}\\ {}_{92}{}^{238}\text{U}\underset{4.5\times {10}^9\text{y}}{\overset{\alpha }{\to }}{}_{90}{}^{234}\text{Th}\underset{24\text{d}}{\overset{{\beta }^{-}}{\to }}{}_{91}{}^{234\text{m}}\text{Pa}\underset{1\text{min}}{\overset{{\beta }^{-}}{\to }}{}_{92}{}^{234}\text{U}\underset{2.4\times {10}^5\text{y}}{\overset{\alpha }{\to }}{}_{90}{}^{230}\text{Th}\underset{7.7\times {10}^4\text{y}}{\overset{\alpha }{\to }}\\ {}_{88}{}^{226}\text{Ra}\underset{1600y}{\overset{\alpha }{\to }}{}_{86}{}^{222}\text{Rn}\underset{3.8\text{d}}{\overset{\alpha }{\to }}{}_{84}{}^{218}\text{Po}\underset{3\text{min}}{\overset{\alpha }{\to }}{}_{82}{}^{214}\text{Pb}\underset{27\text{min}}{\overset{{\beta }^{-}}{\to }}{}_{83}{}^{214}\text{Bi}\underset{20\text{min}}{\overset{{\beta }^{-}}{\to }}\\ {}_{84}{}^{214}\text{Po}\underset{164\mu \text{s}}{\overset{\alpha }{\to }}{}_{82}{}^{210}\text{Pb}\underset{22\text{y}}{\overset{{\beta }^{-}}{\to }}{}_{83}{}^{210}\text{Bi}\underset{5\text{d}}{\overset{{\beta }^{-}}{\to }}{}_{84}{}^{210}\text{Po}\underset{138\text{d}}{\overset{\alpha }{\to }}{}_{82}{}^{206}\text{Pb}\\ \end{array}}$

With each step of this sequence of reactions, energy is released and the decay products move further down the valley of stability towards the line of beta stability. ²⁰⁶Pb is stable and lies on the line of beta stability.

Nuclear fission seen with a uranium-235 nucleus

The fission processes that occur within nuclear reactors are accompanied by the release of neutrons that sustain the chain reaction. Fission occurs when a heavy nuclide such as uranium-235 absorbs a neutron and breaks into lighter components such as barium or krypton, usually with the release of additional neutrons. Like all nuclides with a high atomic number, these uranium nuclei require many neutrons to bolster their stability, so they have a large neutron-proton ratio (N/Z). The nuclei resulting from a fission (fission products) inherit a similar N/Z, but have atomic numbers that are approximately half that of uranium. Isotopes with the atomic number of the fission products and an N/Z near that of uranium or other fissionable nuclei have too many neutrons to be stable; this neutron excess is why multiple free neutrons but no free protons are usually emitted in the fission process, and it is also why many fission product nuclei undergo a long chain of β⁻ decays, each of which converts a nucleus N/Z to (N − 1)/(Z + 1), where N and Z are, respectively, the numbers of neutrons and protons contained in the nucleus.

When fission reactions are sustained at a given rate, such as in a liquid-cooled or solid fuel nuclear reactor, the nuclear fuel in the system produces many antineutrinos for each fission that has occurred. These antineutrinos come from the decay of fission products that, as their nuclei progress down a β⁻ decay chain toward the valley of stability, emit an antineutrino along with each β⁻ particle. In 1956, Reines and Cowan exploited the (anticipated) intense flux of antineutrinos from a nuclear reactor in the design of an experiment to detect and confirm the existence of these elusive particles.

Evil demon

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Evil_demon 

 

The evil demon, also known as Descartes' demon, malicious demon and evil genius, is an epistemological concept that features prominently in Cartesian philosophy. In the first of his 1641 Meditations on First Philosophy, Descartes imagines that an evil demon, of "utmost power and cunning has employed all his energies in order to deceive me." This evil demon is imagined to present a complete illusion of an external world, so that Descartes can say, "I shall think that the sky, the air, the earth, colours, shapes, sounds and all external things are merely the delusions of dreams which he has devised to ensnare my judgement. I shall consider myself as not having hands or eyes, or flesh, or blood or senses, but as falsely believing that I have all these things."

Some Cartesian scholars opine that the demon is also omnipotent, and thus capable of altering mathematics and the fundamentals of logic, though omnipotence of the evil demon would be contrary to Descartes' hypothesis, as he rebuked accusations of the evil demon having omnipotence.

It is one of several methods of systematic doubt that Descartes employs in the Meditations.

In context

Prior to the Meditations proper, Descartes gives a synopsis of each Meditation and says of Meditation One that "reasons are provided which give us possible grounds for doubt about all things, especially material things" and that whilst the usefulness of such extensive doubt may not be immediately apparent, "its greatest benefit lies in

• freeing us from all our preconceived opinions, and
• providing the easiest route by which the mind may be led away from the senses.

The eventual result of this doubt is to

• make it impossible for us to have any further doubts about what we subsequently discover to be true."

Descartes offers some standard reasons for doubting the reliability of the senses culminating in the dream argument and then extends this with the deceiving God argument. Descartes refers to "the long-standing opinion that there is an omnipotent God who made me the kind of creature that I am" and suggests that this God may have "brought it about that there is no earth, no sky, no extended thing, no shape, no size, no place, while at the same time ensuring that all these things appear to me to exist just as they do now". Furthermore, this God may have "brought it about that I too go wrong every time I add two and three or count the sides of a square, or in some even simpler matter, if that is imaginable".

After the deceiving God argument Descartes concludes that he is "compelled to admit that there is not one of my former beliefs about which a doubt may not properly be raised".

It is only after arriving at this conclusion that Descartes introduces the evil demon.

Although Descartes has provided arguments for doubting all his former beliefs he notes that "my habitual opinions keep coming back". It is to deal with this problem that Descartes decides he must do more than just acknowledge that the beliefs are open to doubt and must deceive himself, "by pretending for a time that these former opinions are utterly false and imaginary" and that he shall do this "until the weight of preconceived opinion is counter-balanced and the distorting influence of habit no longer prevents my judgement from perceiving things correctly".

It is to achieve this state of denial that Descartes says he will suppose that "some malicious demon of the utmost power and cunning has employed all his energies in order to deceive me".

The evil demon is also mentioned at the beginning of Meditation Two. Descartes says that if there is "a deceiver of supreme power and cunning who is deliberately and constantly deceiving me" then he himself must undoubtedly exist for the deceiver can "never bring it about that I am nothing so long as I think that I am something". A little later he says, "But what shall I now say that I am, when I am supposing that there is some supremely powerful and, if it is permissible to say so, malicious deceiver, who is deliberately trying to trick me in every way he can?"

The deceiving god

Some writers, e.g. Williams and Musgrave, make no distinction between the deceiving God and evil demon arguments and regard anything said about the deceiving God as being equivalent to saying something about the evil demon.

Other writers acknowledge that Descartes makes mention of both but then claim they are 'epistemologically equivalent'. Kenny says, "the two hypotheses do not differ in any respect of epistemological importance... The content of the two hypotheses is the same..." Newman says, "Descartes' official position is that the Evil Genius Doubt is merely one among multiple hypotheses that can motivate the more general hyperbolic doubt... Even so, I regularly speak in terms of the evil genius... as a kind of mnemonic for the more general doubt about our cognitive nature."

If they are epistemologically equivalent then the question arises as to why Descartes temporarily shifted from the deceiving God to the evil demon. It is tempting to think it is because there is a relevant theological difference. In Meditation Three Descartes is going to establish not only that there is a God but that God is not a deceiver. When Descartes first introduces the evil demon he says, "I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon, had employed his whole energies in deceiving me." Kenny says, "The hypothesis of the evil genius is substituted for that of the deceitful God simply because it is less offensive and less patently incoherent." However, at least in Meditation One, Descartes doesn't have a problem in postulating a deceiving God and he rejects the objection that such deception is inconsistent with God's supreme goodness. He says, "if it were inconsistent with his goodness to have created me such that I am deceived all the time, it would seem equally foreign to his goodness to allow me to be deceived even occasionally; yet this last assertion cannot be made." This is consistent with what he writes in the Principles where he says, "we have been told that God who created us can do all that he desires, and we do not yet know whether he may not have willed to create us in such a way that we shall always be deceived even in the things that we think ourselves to know best."

Other writers insist that it is important to maintain the distinction between the deceiving God and the evil demon. Gouhier (quoted by Kenny) argues that the deceiving God is an intellectual scruple that will disappear when metaphysics demonstrates its falsity whilst the evil demon is a methodological procedure designed to make a certain experiment and it ceases with that experiment. He says, "Neither the purpose nor the content of the two hypotheses allow us to regard the one as a variant of the other."

Vendler argues that literary form of the Meditations is heavily influenced by St. Ignatius of Loyola's Spiritual Exercises to which Descartes will have been exposed during his training at the Jesuit college of La Fleche. As such, "The demon in the First Meditation is not evoked to serve as an epistomological menace, but as a psychological device: following Loyola's advice age contra! (go against!), it provides a counterweight to our inordinate inclination to trust the senses." He adds, "the 'demon-argument' is not an argument at all. Descartes does not need another argument at this stage: the dream argument has already shown the unreliability of the senses and the deceiver-God argument the uncertainty of mathematics. For one thing, the demon does not even touch mathematics or geometry. Why should he? He is evoked by Descartes to cure his inordinate attachment to the senses; he does not complain (and would not) of a similar attachment to mathematics or geometry." Hatfield takes a similar line saying, "Descartes adopts a common practice from the spiritual exercises upon which his metaphysical meditations are modelled, devising a program for training the will to keep the old beliefs at bay" adding, "It seems likely that he chose to call his hypothetical deceiver a "malicious demon" in order to avoid having the meditator concentrate extensively on the thought that God could be a deceiver, a proposition he considered false and one he intended to refute later."

Omnipotence

Further information: Theodicy and Dystheism

Among the accusations of blasphemy made against Descartes by Protestants was that he was positing an omnipotent malevolent God. Voetius accused Descartes of blasphemy in 1643. Jacques Triglandius and Jacobus Revius, theologians at Leiden University, made similar accusations in 1647, accusing Descartes of "hold[ing] God to be a deceiver", a position that they stated to be "contrary to the glory of God". Descartes was threatened with having his views condemned by a synod, but this was prevented by the intercession of the Prince of Orange (at the request of the French Ambassador Servien).

The accusations referenced a passage in the First Meditation where Descartes stated that he supposed not an optimal God but rather an evil demon "summe potens & callidus" (translated as "most highly powerful and cunning"). The accusers identified Descartes' concept of a deus deceptor with his concept of an evil demon, stating that only an omnipotent God is "summe potens" and that describing the evil demon as such thus demonstrated the identity. Descartes' response to the accusations was that in that passage he had been expressly distinguishing between "the supremely good God, the source of truth, on the one hand, and the malicious demon on the other". He did not directly rebut the charge of implying that the evil demon was omnipotent, but asserted that simply describing something with "some attribute that in reality belongs only to God" does not mean that that something is being held to actually be a supreme God.

According to Janowski, "The alleged distinction between the respective powers of God and the evil genius that escaped the attention of the two theologians also escaped the attention of a host of distinguished Cartesian scholars (Alquié, Beck, Brehier, Chevalier, Frankfurt, Gilson, Kenny, Laporte, Kemp-Smith, Wilson), who, only seldom interested in interpreting Descartes' philosophy through the prism of doctrinal orthodoxy, also insist on the omnipotence of the evil genius." He further claims that the reason for this is that there is progression through the First Meditation, leading to the introduction of the concept of the evil genius "which crowns the process begun at the outset of the Meditations."

However, it is not quite so straightforward. For example, Wilson notes that "Gouhier has shown, the hypothesis of the malign spirit takes over from that of the Deceiving God from the end of the First Meditation to the beginning of the Third—where the latter figure is resubstituted without comment or explanation. As Gouhier has also noted, the summary of 'doubts' in the concluding passage ... does not include mention of mathematical propositions—which are not again brought into discussion until the Third Meditation." She adds in the accompanying footnote that, even if one has to concede that the text doesn't reveal any sharp distinction between the power hypothetically ascribed to the 'malignant spirit' and that genuinely attributable to God, "Gouhier's observation is essentially accurate, and useful in understanding the rhetoric and organization of the first three Meditations. It may also have some deeper significance, because of the association ... of the possibility of deception in mathematics with the doctrine of the creation of the eternal truths."

Similarly, Kenny who does say that the evil genius is substituted for that of the deceitful God "simply because it is less offensive and less patently incoherent", for "The content of the two hypotheses is the same, namely that an omnipotent deceiver is trying to deceive", goes on to note that, "If the two hypotheses differ at all, it is the first that is more skeptical than the second. God ... may have made him go wrong in mathematics ... the evil genius merely reinforces the doubt that the external world may be a dream." When Kenny says that the evil genius is simply a substitute for the deceitful God, he is not trying to establish that, therefore, the evil genius was omnipotent, instead he is challenging the view that the evil genius somehow progressed on from God and is rejecting the view that "the evil genius is to serve a more radically skeptical purpose than the hypothesis of the deceitful God."

According to Janowski, the fact that the demon is not said to challenge mathematics, implies either that the evil demon is not omnipotent or that Descartes retracted Universal Doubt. Janowski notes that in the Principles of Philosophy (I, 15) Descartes states that Universal Doubt applies even to "the demonstration of mathematics", and so concludes that either Descartes' Meditation is flawed, lacking a reason for doubting mathematics, or that the charges of blasphemy were well placed, and Descartes was supposing an omnipotent evil demon.

However, this is only a problem if one assumes that Descartes was withdrawing the notion of a deceitful God and replacing it with the evil demon. More recent commentators take the argument to have reached its conclusion with the deceitful God. When Descartes says, "I will suppose therefore that not God, who is supremely good and the source of truth, but rather some malicious demon..." he is not rejecting the notion of a deceitful God on the grounds that God is not a deceiver for this is something he is not entitled to rely on, because, as he says at the beginning of Meditation three, he doesn't "yet even know for sure whether there is a God at all". Instead, he is introducing an aid to the meditator who finds that, despite the arguments presented, "habitual opinions keep coming back". Kenny says, "The purpose of taking seriously the hypothesis of the evil genius is to counterbalance natural credulity and keep in mind the doubts raised by the supposition of the deceitful God." When the role of the demon is understood this way the issue of the demon's omnipotence becomes unimportant.

The brain in a vat

Main article: Brain in a vat

In 1968, James Cornman and Keith Lehrer suggested something they called the braino machine that "operates by influencing the brain of a subject who wears a special cap, called a "braino cap." When the braino cap is placed on a subject's head, the operator of the braino can affect his brain so as to produce any hallucination in the subject that the operator wishes. The braino is a hallucination-producing machine. The hallucinations produced by it may be as complete, systematic, and coherent as the operator of the braino desires to make them." The braino argument was intended to show that, even if it is sometimes possible to tell when we are hallucinating, it is not possible to know that we are not hallucinating. If the braino is operated by an evil being, whom Cornman and Lehrer call Dr. O, then it would be possible for Dr. O to create in me experiences that are identical to the ones I am having now. If that were the case, then the experiences thus created would not constitute knowledge, for the source of those experiences would be the machine and not the world. However, since they are indistinguishable from my current experiences, it follows that my current experiences are also insufficient to generate knowledge.

In 1973, in the introduction to his book Thought, Gilbert Harman said, "it might be suggested that you have not the slightest reason to believe that you are in the surroundings you suppose you are in ... various hypotheses could explain how things look and feel. You might be sound asleep and dreaming or a playful brain surgeon might be giving you these experiences by stimulating your cortex in a special way. You might really be stretched out on a table in his laboratory with wires running into your head from a large computer. Perhaps you have always been on that table. Perhaps you are quite a different person from what you seem..."

Such scenarios had been used many times in science fiction but in philosophy it is now routine to refer to being like a 'brain in a vat' after Hilary Putnam produced an argument which, ironically, purported to show that "the supposition that we are actually brains in a vat, although it violates no physical law, and is perfectly consistent with everything we have experienced, cannot possibly be true. It cannot possibly be true, because it is, in a certain way, self-refuting."

Putnam's argument notwithstanding, the brain in a vat scenario is usually presented as a sceptical argument and in many ways equivalent to Descartes' deceiving God and evil demon.

One crucial difference that prevents such scenarios being a direct substitute for the deceiving God and evil demon is that they generally presuppose that we have heads or bodies whereas it is important for Descartes to argue that he can doubt the existence of his body and that he can only be sure he is a 'thinking thing'. Harman's version of the story does, however, add the final thought that having a brain "might be just part of the myth you are being given".

Internet censorship circumvention

From Wikipedia, the free encyclopedia

Internet censorship circumvention is the use of various methods and tools to bypass internet censorship.

Various techniques and methods are used to bypass Internet censorship, and have differing ease of use, speed, security, and risks. Some methods, such as the use of alternate DNS servers, evade blocking by using an alternate address or address lookup system to access the site. Techniques using website mirrors or archive sites rely on other copies of the site being available at different locations. Additionally, there are solutions that rely on gaining access to an Internet connection that is not subject to filtering, often in a different jurisdiction not subject to the same censorship laws, using technologies such as proxying, virtual private networks, or anonymization networks.

An arms race has developed between censors and developers of circumvention software, resulting in more sophisticated blocking techniques by censors and the development of harder-to-detect tools by researchers. Estimates of adoption of circumvention tools vary substantially and are disputed. Barriers to adoption can include usability issues, difficulty finding reliable and trustworthy information about circumvention, lack of desire to access censored content, and risks from breaking the law.

Circumvention methods

There are many methods available that may allow the circumvention of Internet filtering, which can widely vary in terms of implementation difficulty, effectiveness, and resistance to detection.

Alternate names and addresses

Filters may block specific domain names, either using DNS hijacking or URL filtering. Sites are sometimes accessible through alternate names and addresses that may not be blocked.

Some websites may offer the same content at multiple pages or domain names. For example, the English Wikipedia is available at Main Page, and there is also a mobile-formatted version at Wikipedia, the free encyclopedia.

If DNS resolution is disrupted but the site is not blocked in other ways, it may be possible to access a site directly through its IP address or modifying the host file. Using alternative DNS servers, or public recursive name servers (especially when used through an encrypted DNS client), may bypass DNS-based blocking.

Censors may block specific IP addresses. Depending on how the filtering is implemented, it may be possible to use different forms of the IP address, such as by specifying the address in a different base. For example, the following URLs all access the same site, although not all browsers will recognize all forms: http://1.1.1.1/ (dotted decimal), http://16843009/ Archived 7 July 2020 at the Wayback Machine (decimal), http://0001.0001.0001.0001/ Archived 7 July 2020 at the Wayback Machine (dotted octal), http://0x01010101/ (hexadecimal), and http://0x01.0x01.0x01.0x01/ (dotted hexadecimal).

Blockchain technology is an attempt to decentralize namespaces outside the control of a single entity. Decentralized namespaces enable censorship resistant domains. The BitDNS discussion began in 2010 with a desire to achieve names that are decentralized, secure and human readable.

Mirrors, caches, and copies

Cached pages: Some search engines keep copies of previously indexed webpages, or cached pages, which are often hosted by search engines and may not be blocked. For example, Google allows the retrieval of cached pages by entering "cache:some-url" as a search request.

Mirror and archive sites: Copies of web sites or pages may be available at mirror or archive sites such as the Internet Archive's Wayback Machine or Archive.today. The Docker Registry Image Repository is a centralized storage, application stateless, and node scalable HTTP public service and has a performance bottleneck in the multinational upload and download scenario. Decentralized Docker Registry avoids this centralization drawback. DDR uses a network-structured P2P network to store and query mirror manifest file and blob routing, while each node serves as an independent mirror repository to provide mirror upload and download for the entire network.

RSS aggregators: RSS aggregators such as Feedly may be able to receive and pass on RSS feeds that are blocked when accessed directly.

Alternative platforms

Decentralized Hosting: Content creators may publish to an alternative platform which is willing to host ones content. Napster was the first peer to peer platform but got technically closed due to centralized bootstrapping vulnerabilities. Gnutella was the first sustainable hosting by decentralization. Freenet's model is that "true freedom requires true anonymity." Later BitTorrent was developed to allocate resources with high performance and fairness. ZeroNet was the first DHT to support dynamic and updateable webpages. YaCy is the leading distributed search.

Anonymity Networks: The anonymity Tor Onion and I2P provides leads to more willingness to host content that would otherwise be censored. However hosting implementation and location may bring issues, and the content is still hosted by a single entity which can be controlled.

Federated: Being semi-decentralised, federated platforms such as Nextcloud and IRC make it easier for users to find an instance where they are welcomed.

Providers with a different policy: DuckDuckGo indexes results Google has delisted. However nothing by design keeps it so.

See: Darknets

Platform beguilement

Code-words: Users can use code-words which only the in-group are aware of the intended meaning. This is especially effective if the code-word is a common term.

Word connotations: Users may put a common word into a context to be given a banned meaning. This relies on the censor being unwilling to ban such a common term.

Link relaying: Users can link to a page which then contains a second link to a banned website which promotes the intended message. This linking to a page having a link prevents platforms from banning the direct link, and only requires an extra link click.

Proxying

Web proxies: Proxy websites are configured to allow users to load external web pages through the proxy server, permitting the user to load the page as if it is coming from the proxy server and not the (blocked) source. However, depending on how the proxy is configured, a censor may be able to determine the pages loaded and/or determine that the user is using a proxy server.

For example, the mobile Opera Mini browser uses a proxy-based approach employing encryption and compression in order to speed up downloads. This has the side effect of allowing it to circumvent several approaches to Internet censorship. In 2009 this led the government of China to ban all but a special Chinese version of the browser.

Domain fronting: Circumvention software can implement a technique called domain fronting, where the destination of a connection is hidden by passing the initial requests through a content delivery network or other popular site which censors may be unwilling to block. This technique was used by messaging applications including Signal and Telegram. Tor's meek uses Microsoft's Azure cloud. However, large cloud providers such as Amazon Web Services and Google Cloud no longer permit its use. Website owners can use a free account to use a Cloudflare domain for fronting.

SSH tunneling: By establishing an SSH tunnel, a user can forward all their traffic over an encrypted channel, so both outgoing requests for blocked sites and the response from those sites are hidden from the censors, for whom it appears as unreadable SSH traffic.

Virtual private network (VPN): Using a VPN, A user who experiences internet censorship can create a secure connection to a more permissive country, and browse the internet as if they were situated in that country. Some services are offered for a monthly fee; others are ad-supported. According to GlobalWebIndex in 2014, there were over 400 million people use virtual private networks to circumvent censorship or for increased level of privacy although this number is not verifiable. 

Tor: More advanced tools such as Tor route encrypted traffic through multiple servers to make the source and destination of traffic less traceable. It can in some cases be used to avoid censorship, especially when configured to use traffic obfuscation techniques.

Directions for Tor Pluggable Transports, which use traffic obfuscation techniques to increase censorship resistance

Traffic obfuscation

A censor may be able to detect and block use of circumvention tools through Deep Packet Inspection. There are efforts to make circumvention tools less detectable by randomizing the traffic, attempting to mimic a whitelisted protocol or tunneling traffic through a whitelisted site by using techniques including domain fronting or Meek. Tor and other circumvention tools have adopted multiple obfuscation techniques that users can use depending on the nature of their connection, which are sometimes called "Pluggable Transports".

Internet alternatives

Functionality that people may be after might overlap with non-internet services, such as traditional post, Bluetooth, or wakie-talkie. The following are some detailed examples:

Alternative data transport

Datacasting allows transmission of Web pages and other information via satellite broadcast channels bypassing the Internet entirely. This requires a satellite dish and suitable receiver hardware but provides a powerful means of avoiding censorship. Because the system is entirely receive only for the end user, a suitably air-gapped computer can be impossible to detect. 

Sneakernets

A sneakernet is the transfer of electronic information, especially computer files, by physically carrying data on storage media from one place to another. A sneakernet can move data regardless of network restrictions simply by not using the network at all. One example of a widely adopted sneakernet network is El Paquete Semanal in Cuba.

Adoption of circumvention tools

Circumvention tools have seen spikes in adoption in response to high-profile blocking attempts, however, studies measuring adoption of circumvention tools in countries with persistent and widespread censorship report mixed results.

In response to persistent censorship

Measures and estimates of circumvention tool adoption have reported widely divergent results. A 2010 study by Harvard University researchers estimated that very few users use censorship circumvention tools—likely less than 3% of users even in countries that consistently implement widespread censorship. Other studies have reported substantially larger estimates, but have been disputed.

In China, anecdotal reports suggest that adoption of circumvention tools is particularly high in certain communities, such as universities, and a survey by Freedom House found that users generally did not find circumvention tools to be difficult to use. Market research firm GlobalWebIndex has reported that there are over 35 million Twitter users and 63 million Facebook users in China (both services are blocked). However, these estimates have been disputed; Facebook's advertising platform estimates 1 million users in China, and other reports of Twitter adoption estimate 10 million users. Other studies have pointed out that efforts to block circumvention tools in China have reduced adoption of those tools; the Tor network previously had over 30,000 users connecting from China but as of 2014 had only approximately 3,000 Chinese users.

In Thailand, internet censorship has existed since 2002, and there is sporadic and inconsistent filtering. In a small-scale survey of 229 Thai internet users, a research group at the University of Washington found that 63% of surveyed users attempted to use circumvention tools, and 90% were successful in using those tools. Users often made on-the-spot decisions about use of circumvention tools based on limited or unreliable information, and had a variety of perceived threats, some more abstract and others more concrete based on personal experiences.

In response to blocking events

In response to the 2014 blocking of Twitter in Turkey, information about alternate DNS servers was widely shared, as using another DNS server such as Google Public DNS allowed users to access Twitter. The day after the block, the total number of posts made in Turkey was up 138%, according to Brandwatch, an internet measurement firm.

After an April 2018 ban on the Telegram messaging app in Iran, web searches for VPN and other circumvention software increased as much as 48× for some search terms, but there was evidence that users were downloading unsafe software. As many as a third of Iranian internet users used the Psiphon tool in the days immediately following the block, and in June 2018 as many as 3.5 million Iranian users continued to use the tool.

Anonymity, risks, and trust

Circumvention and anonymity are different. Circumvention systems are designed to bypass blocking, but they do not usually protect identities. Anonymous systems protect a user's identity. And while they can contribute to circumvention, that is not their primary function. It is important to understand that open public proxy sites do not provide anonymity and can view and record the location of computers making requests as well as the websites accessed.

In many jurisdictions accessing blocked content is a serious crime, particularly content that is considered child pornography, a threat to national security, or an incitement of violence. Thus it is important to understand the circumvention technologies and the protections they do or do not provide and to use only tools that are appropriate in a particular context. Great care must be taken to install, configure, and use circumvention tools properly. Individuals associated with high-profile rights organizations, dissident, protest, or reform groups should take extra precautions to protect their online identities.

Circumvention sites and tools should be provided and operated by trusted third parties located outside the censoring jurisdiction that do not collect identities and other personal information. Best are trusted family and friends personally known to the circumventor, but when family and friends are not available, sites and tools provided by individuals or organizations that are only known by their reputations or through the recommendations and endorsement of others may need to be used. Commercial circumvention services may provide anonymity while surfing the Internet, but could be compelled by law to make their records and users' personal information available to law enforcement.

Software

There are five general types of Internet censorship circumvention software:

CGI proxies use a script running on a web server to perform the proxying function. A CGI proxy client sends the requested url embedded within the data portion of an HTTP request to the CGI proxy server. The CGI proxy server pulls the ultimate destination information from the data embedded in the HTTP request, sends out its own HTTP request to the ultimate destination, and then returns the result to the proxy client. A CGI proxy tool's security can be trusted as far as the operator of the proxy server can be trusted. CGI proxy tools require no manual configuration of the browser or client software installation, but they do require that the user use an alternative, potentially confusing browser interface within the existing browser.

HTTP proxies send HTTP requests through an intermediate proxying server. A client connecting through a HTTP proxy sends exactly the same HTTP request to the proxy as it would send to the destination server unproxied. The HTTP proxy parses the HTTP request; sends its own HTTP request to the ultimate destination server; and then returns the response back to the proxy client. An HTTP proxy tool's security can be trusted as far as the operator of the proxy server can be trusted. HTTP proxy tools require either manual configuration of the browser or client side software that can configure the browser for the user. Once configured, an HTTP proxy tool allows the user transparently to use his normal browser interface.

Application proxies are similar to HTTP proxies, but support a wider range of online applications.

Peer-to-peer systems store content across a range of participating volunteer servers combined with technical techniques such as re-routing to reduce the amount of trust placed on volunteer servers or on social networks to establish trust relationships between server and client users. Peer-to-peer system can be trusted as far as the operators of the various servers can be trusted or to the extent that the architecture of the peer-to-peer system limits the amount of information available to any single server and the server operators can be trusted not to cooperate to combine the information they hold.

Re-routing systems send requests and responses through a series of proxying servers, encrypting the data again at each proxy, so that a given proxy knows at most either where the data came from or is going to, but not both. This decreases the amount of trust required of the individual proxy hosts.