United Nations Convention Against Corruption

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United Nations Convention Against Corruption

Signatories (yellow) and ratifiers (green) of the treaty; those who did not sign are in red.
Drafted	31 October 2003
Signed	9 December 2003
Location	Mérida and New York
Effective	14 December 2005
Condition	30 ratifications
Signatories	140
Parties	189
Depositary	Secretary-General of the United Nations
Languages	Arabic, Chinese, English, French, Russian and Spanish

The United Nations Convention Against Corruption (UNCAC) is the only legally binding international anti-corruption multilateral treaty. Negotiated by member states of the United Nations (UN) it has been adopted by the UN General Assembly in October 2003 and entered into force in December 2005. The treaty recognises the importance of both preventive and punitive measures, addresses the cross-border nature of corruption with provisions on international cooperation and on the return of the proceeds of corruption. The UN Office on Drugs and Crime (UNODC) in Vienna serves as Secretariat for the UNCAC. UNCAC's goal is to reduce various types of corruption that can occur across country borders, such as trading in influence and abuse of power, as well as corruption in the private sector, such as embezzlement and money laundering. Another goal of the UNCAC is to strengthen international law enforcement and judicial cooperation between countries by providing effective legal mechanisms for international asset recovery.

States Parties – countries that have ratified the Convention – are expected to cooperate in criminal matters and consider assisting each other in investigations of and proceedings in civil and administrative matters relating to corruption. The Convention further calls for the participation of civil society and non-governmental organisations in accountability processes and underlines the importance of citizens’ access to information.

Signatures, ratifications and entry into force

UNCAC was adopted by the United Nations General Assembly on 31 October 2003 by Resolution 58/4. It was opened for signature in Mérida, Yucatán, Mexico, from 9 to 11 December 2003 and thereafter at UN headquarters in New York City. It was signed by 140 countries. As of December 2021, there are 189 parties, which includes 181 UN member states, the Cook Islands, Niue, the Holy See, the State of Palestine, and the European Union.

As of December 2021, the 9 UN member states that have not ratified the convention are (asterisk indicates that the state has signed the convention):

•  Andorra
•  Barbados*
•  Eritrea
•  Monaco
•  North Korea
•  Saint Kitts and Nevis
•  Saint Vincent and the Grenadines
•  San Marino
•  Syria*

Measures and provisions of the Convention

UNCAC covers five main areas that includes both mandatory and non-mandatory provisions:

• Preventive measures,
• Criminalization and law enforcement,
• International cooperation,
• Asset recovery, and
• Technical assistance and information exchange.

General provisions (Chapter I, Articles 1–4)

United Nations Convention Against Corruption, 2003

The opening Articles of UNCAC include a statement of purpose (Article1), which covers both the promotion of integrity and accountability within each country and the support of international cooperation and technical assistance between States Parties. They also include definitions of critical terms used in the instrument. Some of these are similar to those used in other instruments, and in particular the United Nations Convention against Transnational Organized Crime (UNTOC), but those defining "public official", "foreign public official", and "official of a public international organization" are new and are important for determining the scope of application of UNCAC in these areas. UNCAC does not provide a definition of corruption. In accordance with Article 2 of the UN Charter, Article 4 of UNCAC provides for the protection of national sovereignty of the States Parties.

Preventive measures (Chapter II, Articles 5–14)

UNCAC recognizes the importance of the prevention in both the public and private sectors. Chapter II includes preventive policies, such as the establishment of anti-corruption bodies and enhanced transparency in the financing of election campaigns and political parties. Anti-corruption bodies should implement anti-corruption policies, disseminate knowledge and must be independent, adequately resourced and have properly trained staff.

Countries that sign the convention must assure safeguards their public services are subject to safeguards that promote efficiency, transparency and recruitment based on merit. Once recruited, public servants should be bound by codes of conduct, requirements for financial and other disclosures, and appropriate disciplinary measures. Transparency and accountability in the management of public finances must also be promoted, and specific requirements are established for the prevention of corruption in the particularly critical areas of the public sector, such as the judiciary and public procurement. Preventing corruption also requires an effort from all members of society at large. For these reasons, UNCAC calls on countries to promote actively the involvement of civil society, and to raise public awareness of corruption and what can be done about it. The requirements made for the public sector also apply to the private sector – it too is expected to adopt transparent procedures and codes of conduct.

Criminalization and law enforcement (Chapter III, Articles 15–44)

Chapter III calls for parties to establish or maintain a series of specific criminal offences including not only long-established crimes such as bribery and embezzlement, but also conducts not previously criminalized in many states, such as trading in influence and other abuses of official functions. The broad range of ways in which corruption has manifested itself in different countries and the novelty of some of the offences pose serious legislative and constitutional challenges, a fact reflected in the decision of the Ad Hoc Committee to make some of the provisions either optional ("...shall consider adopting...") or subject to domestic constitutional or other fundamental requirements ("...subject to its constitution and the fundamental principles of its legal system..."). Specific acts that parties must criminalize include

• active bribery of a national, international or foreign public officials
• passive bribery of a national public official
• embezzlement of public funds

Other mandatory crimes include obstruction of justice, and the concealment, conversion or transfer of criminal proceeds (money laundering). Sanctions extend to those who participate in and may extend to those who attempt to commit corruption offences. UNCAC thus goes beyond previous instruments of this kind that request parties to criminalize only basic forms of corruption. Parties are encouraged – but not required – to criminalize, inter alia, passive bribery of foreign and international public officials, trading in influence, abuse of function, illicit enrichment, private sector bribery and embezzlement, and the concealment of illicit assets.

Furthermore, parties are required to simplify rules pertaining to evidence of corrupt behavior by, inter alia, ensuring that obstacles that may arise from the application of bank secrecy laws are overcome. This is especially important, as corrupt acts are frequently very difficult to prove in court. Particularly important is also the introduction of the liability of legal persons. In the area of law enforcement, UNCAC calls for better cooperation between national and international bodies and with civil society. There is a provision for the protection of witnesses, victims, expert witnesses and whistle blowers to ensure that law enforcement is truly effective.

Russia ratified the convention in 2006, but failed to include article 20, which criminalizes "illicit enrichment". In March 2013, the Communist Party of the Russian Federation submitted a petition with 115,000 signatures to the State Duma in favour of doing so. In 2015, however, no such law was yet in effect in Russia.

International cooperation (Chapter IV, Articles 43–49)

Under Chapter IV of UNCAC, States Parties are obliged to assist one another in every aspect of the fight against corruption, including prevention, investigation, and the prosecution of offenders. Cooperation takes the form of extradition, mutual legal assistance, transfer of sentences persons and criminal proceedings, and law enforcement cooperation. Cooperation in civil and administrative matters is also encouraged. Based on Chapter IV, UNCAC itself can be used as a basis for extradition, mutual legal assistance and law enforcement with respect to corruption-related offences. "Dual criminality", which is a requirement that the relevant offence shall be criminalized in both the requesting and requested country, is considered fulfilled irrespective of whether the same terminology or category of offense is used in both jurisdictions. In case of a request for assistance involving non-coercive measures, States Parties are required to provide assistance even when dual criminality is absent subject only to the basic concepts of their legal systems. Chapter IV also contains other innovative provisions designed to facilitate international cooperation. For example, States Parties that use UNCAC as a basis for extradition shall not consider corruption-related offences as political ones; assistance can also be provided in relation to offences for which legal persons can be held responsible; and bank secrecy cannot be cited as a ground to refuse a request for assistance. In order to ensure speedy and efficient cooperation, each State Party is required to designate a central authority responsible for receiving MLA requests. Overall, Chapter IV provides a broad and flexible platform for international cooperation. However, its provisions do not exhaust all international cooperation issues covered by UNCAC, thus the purposes of UNCAC and provisions of other chapters also need to be taken into consideration.

Asset recovery (Chapter V, Articles 51–59)

Main article: International asset recovery

The agreement on asset recovery is considered a major breakthrough and many observers claim that it is one of the reasons why so many developing countries have signed UNCAC. Asset recovery is indeed a very important issue for many developing countries where high-level corruption has plundered the national wealth. Reaching an agreement on this Chapter involved intensive negotiations, as the legitimate interests of countries wishing to recover illicit assets had to be reconciled with the legal and procedural safeguards of the countries from which assistance will be sought. Generally, in the course of the negotiations, countries seeking to recover assets sought to establish presumptions that would make clear their ownership of the assets and give priority for return over other means of disposal. Countries from which the return was likely to be sought, on the other hand, had concerns about the language that might have compromised basic human rights and procedural protections associated with criminal liability and the freezing, seizure, forfeiture and return of such assets.

Chapter V of UNCAC establishes asset recovery as a "fundamental principle" of the convention. The provisions on asset recovery lay a framework, in both civil and criminal law, for tracing, freezing, forfeiting and returning funds obtained through corrupt activities. The requesting state will in most cases receive the recovered funds as long as it can prove ownership. In some cases, the funds may be returned directly to individual victims.

If no other arrangement is in place, States Parties may use the Convention itself as a legal basis. Article 54(1)(a) of UNCAC provides that: "Each State Party (shall)... take such measures as may be necessary to permit its competent authorities to give effect to an order of confiscation issued by a court of another state party" Indeed, Article 54(2)(a) of UNCAC also provides for the provisional freezing or seizing of property where there are sufficient grounds for taking such actions in advance of a formal request being received.

Recognizing that recovering assets once transferred and concealed is an exceedingly costly, complex and an all-too-often unsuccessful process, this Chapter also incorporates elements intended to prevent illicit transfers and generate records that can be used where illicit transfers eventually have to be traced, frozen, seized and confiscated (Article 52). The identification of experts who can assist developing countries in this process is also included as a form of technical assistance (Article 60(5)).

Technical assistance and information exchange (Chapter VI, Articles 60–62)

Chapter VI of UNCAC is dedicated to technical assistance, meaning support offered to developing and transition countries in the implementation of UNCAC. The provisions cover training, material and human resources, research, and information sharing. UNCAC also calls for cooperation through international and regional organizations (many of which already have established anti-corruption programmes), research efforts, and the contribution of financial resources both directly to developing countries and countries with economies in transition, and to the UNODC.

Mechanisms for implementation (Chapter VII, Articles 63–64)

Chapter VII deals with international implementation through the CoSP and the UN Secretariat.

Final provisions (Chapter VIII, Articles 65 – 71)

The final provisions are similar to those found in other UN treaties. Key provisions ensure that UNCAC requirements are to be interpreted as minimum standards, which States Parties are free to exceed with measures "more strict or severe" than those set out in specific provisions; and the two Articles governing signature, ratification and the coming into force of the convention.

Conference of the States Parties

Pursuant to article 63 of UNCAC, a Conference of the States Parties (CoSP) to UNCAC was established to improve the capacity of and cooperation between States Parties to achieve the objectives set forth in UNCAC, and to promote and review its implementation. UNODC acts as the secretariat to the CoSP. At its different sessions, besides regularly calling States Parties and signatories to adapt their laws and regulations to bring them into conformity with the provisions of UNCAC the CoSP has adopted resolutions and has mandated UNODC to implement them, including through the development of technical assistance projects.

The CoSP has established a number of subsidiary bodies to further the implementation of specific aspects of UNCAC. The Implementation Review Group, which focuses on the implementation review mechanism and technical assistance, the Working Group on Asset Recovery, the Working Group on Prevention, as well as expert group meetings on international cooperation meet regularly in the intersessional period.

• The first session of the CoSP took place on 10–14 December 2006 at the Dead Sea, Jordan. In its resolution 1/1, States Parties agreed that it was necessary to establish an appropriate and effective mechanism to assist in the review of the implementation of UNCAC. An inter-governmental working group was established to start working on the design of such a mechanism. Two other working groups were set up to promote coordination of activities related to technical assistance and asset recovery, respectively.
• The second session was held in Bali, Indonesia, from 28 January to 1 February 2008. As to the mechanism for the review of implementation, the States Parties decided to take into account a balanced geographical approach, to avoid any adversarial or punitive elements, to establish clear guidelines for every aspect of the mechanism and to promote universal adherence to UNCAC and the constructive collaboration in preventive measures, asset recovery, international cooperation and other areas. The CoSP also requested donors and receiving countries to strengthen coordination and enhance technical assistance for the implementation of UNCAC, and dealt with the issue of bribery of officials of public international organizations.
• The third session of the CoSP took place in Doha, Qatar, from 9 to 13 November 2009. The CoSP adopted the landmark Resolution 3/1 on the review of the implementation of UNCAC, containing the terms of reference of the Implementation Review Mechanism (IRM). In view of the establishment of the IRM, and considering that the identification of needs and the delivery of technical assistance to facilitate the successful and consistent implementation of UNCAC are at the core of the mechanism, the CoSP decided to abolish the Working Group on Technical Assistance and to fold its mandate into the work of the Implementation Review Group. For the first time, the CoSP also adopted a resolution on preventive measures, in which it established the Open-Ended Intergovernmental Working Group on Prevention to further explore good practices in this field. The CoSP was preceded and accompanied by numerous side events, such as the last Global Forum for Fighting Corruption and Safeguarding Integrity (in cooperation with businesses) and a Youth Forum.
• The fourth session of the CoSP took place in Marrakech, Morocco, from 24 to 28 October 2011. The Conference considered the progress made in the IRM and recognized the importance of addressing technical assistance needs in the Review Mechanism. It also reiterated its support for the Working Groups on Asset Recovery and Prevention and established Open-Ended Intergovernmental Expert Group Meetings on International Cooperation to advise and assist the CoSP with respect to extradition and mutual legal assistance.

Other sessions of the CoSP took place in Panama in 2013, the Russian Federation in 2015, Austria in 2017 and United Arab Emirates in 2019.

Implementation of the UNCAC and monitoring mechanism

In accordance with Article 63(7) of UNCAC, "the Conference shall establish, if it deems necessary, any appropriate mechanism or body to assist in the effective implementation of the Convention". At its first session, the CoSP established an open-ended intergovernmental expert group to make recommendations to the Conference on the appropriate mechanism. A voluntary "Pilot Review Programme", which was limited in scope, was initiated to offer adequate opportunity to test possible methods to review the implementation of UNCAC, with the overall objective to evaluate efficiency and effectiveness of the tested mechanism(s) and to provide to the CoSP information on lessons learnt and experience acquired, thus enabling the CoSP to make informed decisions on the establishment of an appropriate mechanism for reviewing the implementation of UNCAC.

The CoSP at its third session, held in Doha in November 2009, adopted Resolution 3/1 on the review of the implementation of the convention, containing the terms of reference of an Implementation Review Mechanism (IRM). It established a review mechanism aimed at assisting countries to meet the objectives of UNCAC through a peer review process. The IRM is intended to further enhance the potential of the UNCAC, by providing the means for countries to assess their level of implementation through the use of a comprehensive self-assessment checklist, the identification of potential gaps and the development of action plans to strengthen the implementation of UNCAC domestically. UNODC serves as the secretariat to the review mechanism.

The Terms of Reference contain procedures and processes for the peer review of the States Parties implementation of the UNCAC, including the formation of an oversight body called the Implementation Review Group (IRG).

In July 2010 the IRG met for the first time in Vienna and adopted the guidelines for governmental experts and the UNCAC secretariat – the UN Office on Drugs and Crime (UNODC) – in the conduct of a country review. The mechanism consists of a multi-stage peer review which involves the review of each State Party by two peers – one from the same UN region and one from another one. To cover all States Parties, the review process is divided into two five-year cycles where countries are randomly selected to be reviewed in each year of the cycle. The first cycle started in 2010 and covers Chapter III and IV of the convention. The second cycle was launched in November 2015 and is currently underway, covering Chapters II and V, reviewing corruption prevention measures and asset recovery. While the second cycle is scheduled to end in 2021, the process is facing substantial delays, more than three years into the second cycle, only 20 of the 184 countries had completed the review process by May 2019.

It has yet to be decided if and how the review mechanism will continue after the end of the five years foreseen for the second cycle, but if the first cycle is a guide, then the reviews will continue beyond the five years.

A country review process follows these phases:

1. Self-assessment: UNODC informs the State Party that it is under review. The State Party identifies a focal point to coordinate the country's participation in the review and then fills out a standardised self-assessment checklist.
2. Peer review: two reviewer States Parties – decided by lots – provide experts to form a review team. The team conducts a desk review of the completed self-assessment checklist. It may require further information from the focal point and direct dialogue through conference calls, or a country visit if agreed by the country reviewed.
3. Country review report and executive summary: with the assistance of UNODC, the expert review team prepares a country review report (80–300 pages). The report is sent to the focal point for approval. In cases of disagreement, the reviewers and the contact point engage in dialogue to arrive at a consensual final report, which is published in full only with the agreement of the country under review. The expert review team produces an executive summary of this report (7–12 pages), which is automatically published on the UNODC website.

UNCAC Coalition

The UNCAC Coalition, established in 2006, is a global network of over 350 civil society organisations (CSOs) in over 100 countries, committed to promoting the ratification, implementation and monitoring of the UNCAC. The Coalition engages in joint action around common positions on the UNCAC, facilitates the exchange of information among members, and supports national civil society efforts to promote the UNCAC. Coalition members share views via the Coalition website and a mailing list and ad hoc working groups. The Coalition supports civil society organisations to engage in and contribute to the UNCAC review process, including through technical support.

The Coalition, directly and through its members, advocates for greater transparency and space for civil society participation in all UNCAC fora – the Conference of States Parties, the meetings of the Implementation Review Group, working groups and the review process on the national level. Furthermore, the Coalition seeks to advance discussions on key issues covered by the convention, including:

• Access to Information
• Asset Recovery
• Beneficial Company Ownership Transparency
• Protection of Whistleblowers and Anti-Corruption Activists

It aims to mobilize broad civil society support for UNCAC and to facilitate strong civil society action at national, regional and international levels in support of UNCAC. The Coalition is open to all organizations and individuals committed to these goals. The breadth of UNCAC means that its framework is relevant for a wide range of CSOs, including groups working in the areas of human rights, labour rights, governance, economic development, environment and private sector accountability.

Challenges

Ratification of UNCAC, while essential, is only the first step. Fully implementing its provisions presents significant challenges for the international community as well as individual States parties, particularly in relation to the innovative areas of UNCAC. For this reason, countries have often needed policy guidance and technical assistance to ensure the effective implementation of UNCAC. The results of the first years of IRM have shown that many developing countries have identified technical assistance needs. The provision of technical assistance, as foreseen in UNCAC, is crucial to ensure the full and effective incorporation of the provisions of UNCAC into domestic legal systems and, above all, into the reality of daily life.

Emulsion polymerization

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Emulsion polymerization is a type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified (with surfactants) in a continuous phase of water. Water-soluble polymers, such as certain polyvinyl alcohols or hydroxyethyl celluloses, can also be used to act as emulsifiers/stabilizers. The name "emulsion polymerization" is a misnomer that arises from a historical misconception. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. These latex particles are typically 100 nm in size, and are made of many individual polymer chains. The particles are prevented from coagulating with each other because each particle is surrounded by the surfactant ('soap'); the charge on the surfactant repels other particles electrostatically. When water-soluble polymers are used as stabilizers instead of soap, the repulsion between particles arises because these water-soluble polymers form a 'hairy layer' around a particle that repels other particles, because pushing particles together would involve compressing these chains.

Emulsion polymerization is used to make several commercially important polymers. Many of these polymers are used as solid materials and must be isolated from the aqueous dispersion after polymerization. In other cases the dispersion itself is the end product. A dispersion resulting from emulsion polymerization is often called a latex (especially if derived from a synthetic rubber) or an emulsion (even though "emulsion" strictly speaking refers to a dispersion of an immiscible liquid in water). These emulsions find applications in adhesives, paints, paper coating and textile coatings. They are often preferred over solvent-based products in these applications due to the absence of volatile organic compounds (VOCs) in them.

IUPAC definition

Emulsion polymerization: Polymerization whereby monomer(s), initiator, dispersion
medium, and possibly colloid stabilizer constitute initially an inhomogeneous system
resulting in particles of colloidal dimensions containing the formed polymer.

Note: With the exception of mini-emulsion polymerization, the term “emulsion polymerization”
does not mean that polymerization occurs in the droplets of a monomer emulsion.

Batch emulsion polymerization: Emulsion polymerization in which all the ingredients are
placed in a reactor prior to reaction.

Advantages of emulsion polymerization include:

• High molecular weight polymers can be made at fast polymerization rates. By contrast, in bulk and solution free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate.
• The continuous water phase is an excellent conductor of heat, enabling fast polymerization rates without loss of temperature control.
• Since polymer molecules are contained within the particles, the viscosity of the reaction medium remains close to that of water and is not dependent on molecular weight.
• The final product can be used as is and does not generally need to be altered or processed.

Disadvantages of emulsion polymerization include:

• Surfactants and other polymerization adjuvants remain in the polymer or are difficult to remove
• For dry (isolated) polymers, water removal is an energy-intensive process
• Emulsion polymerizations are usually designed to operate at high conversion of monomer to polymer. This can result in significant chain transfer to polymer.
• Can not be used for condensation, ionic, or Ziegler-Natta polymerization, although some exceptions are known.

History

The early history of emulsion polymerization is connected with the field of synthetic rubber. The idea of using an emulsified monomer in an aqueous suspension or emulsion was first conceived at Bayer, before World War I, in an attempt to prepare synthetic rubber. The impetus for this development was the observation that natural rubber is produced at room temperature in dispersed particles stabilized by colloidal polymers, so the industrial chemists tried to duplicate these conditions. The Bayer workers used naturally occurring polymers such as gelatin, ovalbumin, and starch to stabilize their dispersion. By today's definition these were not true emulsion polymerizations, but suspension polymerizations.

The first "true" emulsion polymerizations, which used a surfactant and polymerization initiator, were conducted in the 1920s to polymerize isoprene. Over the next twenty years, through the end of World War II, efficient methods for production of several forms of synthetic rubber by emulsion polymerization were developed, but relatively few publications in the scientific literature appeared: most disclosures were confined to patents or were kept secret due to wartime needs.

After World War II, emulsion polymerization was extended to production of plastics. Manufacture of dispersions to be used in latex paints and other products sold as liquid dispersions commenced. Ever more sophisticated processes were devised to prepare products that replaced solvent-based materials. Ironically, synthetic rubber manufacture turned more and more away from emulsion polymerization as new organometallic catalysts were developed that allowed much better control of polymer architecture.

Theoretical overview

The first successful theory to explain the distinct features of emulsion polymerization was developed by Smith and Ewart, and Harkins in the 1940s, based on their studies of polystyrene. Smith and Ewart arbitrarily divided the mechanism of emulsion polymerization into three stages or intervals. Subsequently, it has been recognized that not all monomers or systems undergo these particular three intervals. Nevertheless, the Smith-Ewart description is a useful starting point to analyze emulsion polymerizations.

Schematic of emulsion polymerization

The Smith-Ewart-Harkins theory for the mechanism of free-radical emulsion polymerization is summarized by the following steps:

• A monomer is dispersed or emulsified in a solution of surfactant and water, forming relatively large droplets in water.
• Excess surfactant creates micelles in the water.
• Small amounts of monomer diffuse through the water to the micelle.
• A water-soluble initiator is introduced into the water phase where it reacts with monomer in the micelles. (This characteristic differs from suspension polymerization where an oil-soluble initiator dissolves in the monomer, followed by polymer formation in the monomer droplets themselves.) This is considered Smith-Ewart interval 1.
• The total surface area of the micelles is much greater than the total surface area of the fewer, larger monomer droplets; therefore the initiator typically reacts in the micelle and not the monomer droplet.
• Monomer in the micelle quickly polymerizes and the growing chain terminates. At this point the monomer-swollen micelle has turned into a polymer particle. When both monomer droplets and polymer particles are present in the system, this is considered Smith-Ewart interval 2.
• More monomer from the droplets diffuses to the growing particle, where more initiators will eventually react.
• Eventually the free monomer droplets disappear and all remaining monomer is located in the particles. This is considered Smith-Ewart interval 3.
• Depending on the particular product and monomer, additional monomer and initiator may be continuously and slowly added to maintain their levels in the system as the particles grow.
• The final product is a dispersion of polymer particles in water. It can also be known as a polymer colloid, a latex, or commonly and inaccurately as an 'emulsion'.

Smith-Ewart theory does not predict the specific polymerization behavior when the monomer is somewhat water-soluble, like methyl methacrylate or vinyl acetate. In these cases homogeneous nucleation occurs: particles are formed without the presence or need for surfactant micelles.

High molecular weights are developed in emulsion polymerization because the concentration of growing chains within each polymer particle is very low. In conventional radical polymerization, the concentration of growing chains is higher, which leads to termination by coupling, which ultimately results in shorter polymer chains. The original Smith-Ewart-Hawkins mechanism required each particle to contain either zero or one growing chain. Improved understanding of emulsion polymerization has relaxed that criterion to include more than one growing chain per particle, however, the number of growing chains per particle is still considered to be very low.

Because of the complex chemistry that occurs during an emulsion polymerization, including polymerization kinetics and particle formation kinetics, quantitative understanding of the mechanism of emulsion polymerization has required extensive computer simulation. Robert Gilbert has summarized a recent theory.

More detailed treatment of Smith-Ewart theory

Interval 1

When radicals generated in the aqueous phase encounter the monomer within the micelle, they initiate polymerization. The conversion of monomer to polymer within the micelle lowers the monomer concentration and generates a monomer concentration gradient. Consequently, the monomer from monomer droplets and uninitiated micelles begin to diffuse to the growing, polymer-containing, particles. Those micelles that did not encounter a radical during the earlier stage of conversion begin to disappear, losing their monomer and surfactant to the growing particles. The theory predicts that after the end of this interval, the number of growing polymer particles remains constant.

Interval 2

This interval is also known as steady state reaction stage. Throughout this stage, monomer droplets act as reservoirs supplying monomer to the growing polymer particles by diffusion through the water. While at steady state, the ratio of free radicals per particle can be divided into three cases. When the number of free radicals per particle is less than 1⁄2, this is called Case 1. When the number of free radicals per particle equals 1⁄2, this is called Case 2. And when there is greater than 1⁄2 radical per particle, this is called Case 3. Smith-Ewart theory predicts that Case 2 is the predominant scenario for the following reasons. A monomer-swollen particle that has been struck by a radical contains one growing chain. Because only one radical (at the end of the growing polymer chain) is present, the chain cannot terminate, and it will continue to grow until a second initiator radical enters the particle. As the rate of termination is much greater than the rate of propagation, and because the polymer particles are extremely small, chain growth is terminated immediately after the entrance of the second initiator radical. The particle then remains dormant until a third initiator radical enters, initiating the growth of a second chain. Consequently, the polymer particles in this case either have zero radicals (dormant state), or 1 radical (polymer growing state) and a very short period of 2 radicals (terminating state) which can be ignored for the free radicals per particle calculation. At any given time, a micelle contains either one growing chain or no growing chains (assumed to be equally probable). Thus, on average, there is around 1/2 radical per particle, leading to the Case 2 scenario. The polymerization rate in this stage can be expressed by

$${\displaystyle R_{p}=k_{p}[\mathrm {M} ][\mathrm {P} ^{\bullet }]}$$

where ${\textstyle k_{p}}$is the homogeneous propagation rate constant for polymerization within the particles and ${\displaystyle [\mathrm {M} ]}$ is the equilibrium monomer concentration within a particle. ${\textstyle [\mathrm {P} ^{\bullet }]}$ represents the overall concentration of polymerizing radicals in the reaction. For Case 2, where the average number of free radicals per micelle are ${\displaystyle 1/2}$, ${\textstyle [\mathrm {P} ^{\bullet }]}$ can be calculated in following expression:

$${\displaystyle [\mathrm {P} ^{\bullet }]={\frac {N_{\mathrm {micelles} }}{2N_{\mathrm {A} }}}}$$

where ${\displaystyle N_{\mathrm {micelles} }}$is number concentration of micelles (number of micelles per unit volume), and ${\displaystyle N_{\mathrm {A} }}$ is the Avogadro constant (6.02×10²³ mol⁻¹). Consequently, the rate of polymerization is then

$${\displaystyle R_{p}=k_{p}[\mathrm {M} ]{\frac {N_{\mathrm {micelles} }}{2N_{\mathrm {A} }}}.}$$

Interval 3

Separate monomer droplets disappear as the reaction continues. Polymer particles in this stage may be sufficiently large enough that they contain more than 1 radical per particle.

Process considerations

Emulsion polymerizations have been used in batch, semi-batch, and continuous processes. The choice depends on the properties desired in the final polymer or dispersion and on the economics of the product. Modern process control schemes have enabled the development of complex reaction processes, with ingredients such as initiator, monomer, and surfactant added at the beginning, during, or at the end of the reaction.

Early styrene-butadiene rubber (SBR) recipes are examples of true batch processes: all ingredients added at the same time to the reactor. Semi-batch recipes usually include a programmed feed of monomer to the reactor. This enables a starve-fed reaction to ensure a good distribution of monomers into the polymer backbone chain. Continuous processes have been used to manufacture various grades of synthetic rubber.

Some polymerizations are stopped before all the monomer has reacted. This minimizes chain transfer to polymer. In such cases the monomer must be removed or stripped from the dispersion.

Colloidal stability is a factor in design of an emulsion polymerization process. For dry or isolated products, the polymer dispersion must be isolated, or converted into solid form. This can be accomplished by simple heating of the dispersion until all water evaporates. More commonly, the dispersion is destabilized (sometimes called "broken") by addition of a multivalent cation. Alternatively, acidification will destabilize a dispersion with a carboxylic acid surfactant. These techniques may be employed in combination with application of shear to increase the rate of destabilization. After isolation of the polymer, it is usually washed, dried, and packaged.

By contrast, products sold as a dispersion are designed with a high degree of colloidal stability. Colloidal properties such as particle size, particle size distribution, and viscosity are of critical importance to the performance of these dispersions.

Living polymerization processes that are carried out via emulsion polymerization such as iodine-transfer polymerization and RAFT have been developed.

Components

Monomers

Typical monomers are those that undergo radical polymerization, are liquid or gaseous at reaction conditions, and are poorly soluble in water. Solid monomers are difficult to disperse in water. If monomer solubility is too high, particle formation may not occur and the reaction kinetics reduce to that of solution polymerization.

Ethene and other simple olefins must be polymerized at very high pressures (up to 800 bar).

Comonomers

Copolymerization is common in emulsion polymerization. The same rules and comonomer pairs that exist in radical polymerization operate in emulsion polymerization. However, copolymerization kinetics are greatly influenced by the aqueous solubility of the monomers. Monomers with greater aqueous solubility will tend to partition in the aqueous phase and not in the polymer particle. They will not get incorporated as readily in the polymer chain as monomers with lower aqueous solubility. This can be avoided by a programmed addition of monomer using a semi-batch process.

Ethene and other alkenes are used as minor comonomers in emulsion polymerization, notably in vinyl acetate copolymers.

Small amounts of acrylic acid or other ionizable monomers are sometimes used to confer colloidal stability to a dispersion.

Initiators

Both thermal and redox generation of free radicals have been used in emulsion polymerization. Persulfate salts are commonly used in both initiation modes. The persulfate ion readily breaks up into sulfate radical ions above about 50 °C, providing a thermal source of initiation. Redox initiation takes place when an oxidant such as a persulfate salt, a reducing agent such as glucose, Rongalite, or sulfite, and a redox catalyst such as an iron compound are all included in the polymerization recipe. Redox recipes are not limited by temperature and are used for polymerizations that take place below 50 °C.

Although organic peroxides and hydroperoxides are used in emulsion polymerization, initiators are usually water soluble and partition into the water phase. This enables the particle generation behavior described in the theory section. In redox initiation, either the oxidant or the reducing agent (or both) must be water-soluble, but one component can be water-insoluble.

Surfactants

Selection of the correct surfactant is critical to the development of any emulsion polymerization process. The surfactant must enable a fast rate of polymerization, minimize coagulum or fouling in the reactor and other process equipment, prevent an unacceptably high viscosity during polymerization (which leads to poor heat transfer), and maintain or even improve properties in the final product such as tensile strength, gloss, and water absorption.

Anionic, nonionic, and cationic surfactants have been used, although anionic surfactants are by far most prevalent. Surfactants with a low critical micelle concentration (CMC) are favored; the polymerization rate shows a dramatic increase when the surfactant level is above the CMC, and minimization of the surfactant is preferred for economic reasons and the (usually) adverse effect of surfactant on the physical properties of the resulting polymer. Mixtures of surfactants are often used, including mixtures of anionic with nonionic surfactants. Mixtures of cationic and anionic surfactants form insoluble salts and are not useful.

Examples of surfactants commonly used in emulsion polymerization include fatty acids, sodium lauryl sulfate, and alpha-olefin sulfonate.

Non-surfactant stabilizers

Some grades of polyvinyl alcohol and other water-soluble polymers can promote emulsion polymerization even though they do not typically form micelles and do not act as surfactants (for example, they do not lower surface tension). It is believed that growing polymer chains graft onto these water-soluble polymers, which stabilize the resulting particles.

Dispersions prepared with such stabilizers typically exhibit excellent colloidal stability (for example, dry powders may be mixed into the dispersion without causing coagulation). However, they often result in products that are very water sensitive due to the presence of the water-soluble polymer.

Other ingredients

Other ingredients found in emulsion polymerization include chain transfer agents, buffering agents, and inert salts. Preservatives are added to products sold as liquid dispersions to retard bacterial growth. These are usually added after polymerization, however.

Applications

Polymers produced by emulsion polymerization can roughly be divided into three categories.

• Synthetic rubber
  • Some grades of styrene-butadiene (SBR)
  • Some grades of Polybutadiene
  • Polychloroprene (Neoprene)
  • Nitrile rubber
  • Acrylic rubber
  • Fluoroelastomer (FKM)
• Plastics
  • Some grades of PVC
  • Some grades of polystyrene
  • Some grades of PMMA
  • Acrylonitrile-butadiene-styrene terpolymer (ABS)
  • Polyvinylidene fluoride
  • Polyvinyl fluoride
  • PTFE
• Dispersions (i.e. polymers sold as aqueous dispersions)
  • polyvinyl acetate
  • polyvinyl acetate copolymers
  • polyacrylates
  • Styrene-butadiene
  • VAE (vinyl acetate – ethylene copolymers)

Wheeler's delayed-choice experiment

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Wheeler%27s_delayed-choice_experiment 

 

John Wheeler, 1995

Wheeler's delayed-choice experiment is actually several thought experiments in quantum physics, proposed by John Archibald Wheeler, with the most prominent among them appearing in 1978 and 1984. These experiments are attempts to decide whether light somehow "senses" the experimental apparatus in the double-slit experiment it will travel through and adjusts its behavior to fit by assuming the appropriate determinate state for it, or whether light remains in an indeterminate state, exhibiting both wave-like and particle-like behavior until measured.

The common intention of these several types of experiments is to first do something that, according to some hidden-variable models, would make each photon "decide" whether it was going to behave as a particle or behave as a wave, and then, before the photon had time to reach the detection device, create another change in the system that would make it seem that the photon had "chosen" to behave in the opposite way. Some interpreters of these experiments contend that a photon either is a wave or is a particle, and that it cannot be both at the same time. Wheeler's intent was to investigate the time-related conditions under which a photon makes this transition between alleged states of being. His work has been productive of many revealing experiments. He may not have anticipated the possibility that other researchers would tend toward the conclusion that a photon retains both its "wave nature" and "particle nature" until the time it ends its life, e.g., by being absorbed by an electron, which acquires its energy and therefore rises to a higher-energy orbital in its atom.

This line of experimentation proved very difficult to carry out when it was first conceived. Nevertheless, it has proven very valuable over the years since it has led researchers to provide "increasingly sophisticated demonstrations of the wave–particle duality of single quanta". As one experimenter explains, "Wave and particle behavior can coexist simultaneously."

Introduction

"Wheeler's delayed-choice experiment" refers to a series of thought experiments in quantum physics, the first being proposed by him in 1978. Another prominent version was proposed in 1983. All of these experiments try to get at the same fundamental issues in quantum physics. Many of them are discussed in Wheeler's 1978 article "The 'Past' and the 'Delayed-Choice' Double-Slit Experiment", which has been reproduced in A. R. Marlow's Mathematical Foundations of Quantum Theory, pp. 9–48.

According to the complementarity principle, the 'particle-like' (like exact location) or 'wave-like' (like frequency or amplitude) properties of a photon can be measured, but not both at the same time. What characteristic is measured depends on whether experimenters use a device intended to observe particles or to observe waves. When this statement is applied very strictly, one could argue that by determining the detector type one could force the photon to become manifest only as a particle or only as a wave. Detection of a photon is generally a destructive process (see quantum nondemolition measurement for non-destructive measurements). For example, a photon can be detected as the consequences of being absorbed by an electron in a photomultiplier that accepts its energy, which is then used to trigger the cascade of events that produces a "click" from that device. In the case of the double-slit experiment, a photon appears as a highly localized point in space and time on a screen. The build up of the photons on the screen gives an indication on whether the photon must have traveled through the slits as a wave or could have traveled as a particle. The photon is said to have traveled as a wave if the build up results in the typical interference pattern of waves (see double-slit experiment#Interference of individual particles for an animation showing the build up). However, if one of the slits is closed or two orthogonal polarizers are placed in front of the slits (making the photons passing through different slits distinguishable) then no interference pattern will appear, and the build up can be explained as the result of the photon traveling as a particle.

Quantum mechanics predicts that the photon always travels as a wave, however one can only see this prediction by detecting the photon as a particle. Thus, the question arises: Could the photon decide to travel as a wave or a particle depending on the experimental setup? And if yes, when does the photon decide whether it is going to travel as a wave or as a particle? Suppose that a traditional double-slit experiment is prepared so that either of the slits can be blocked. If both slits are open and a series of photons are emitted by the laser then an interference pattern will quickly emerge on the detection screen. The interference pattern can only be explained as a consequence of wave phenomena, so experimenters can conclude that each photon "decides" to travel as a wave as soon as it is emitted. If only one slit is available then there will be no interference pattern, so experimenters may conclude that each photon "decides" to travel as a particle as soon as it is emitted, even if travel as a wave also correctly predicts the distribution of the photons in the single slit experiment.

Simple interferometer

One way to investigate the question of when a photon decides whether to act as a wave or a particle in an experiment is to use the interferometer method. Here is a simple schematic diagram of an interferometer in two configurations:

Open and closed

If a single photon is emitted into the entry port of the apparatus at the lower-left corner, it immediately encounters a beam-splitter. Because of the equal probabilities for transmission or reflection the photon will either continue straight ahead, be reflected by the mirror at the lower-right corner, and be detected by the detector at the top of the apparatus, or it will be reflected by the beam-splitter, strike the mirror in the upper-left corner, and emerge into the detector at the right edge of the apparatus. Observing that photons show up in equal numbers at the two detectors, experimenters generally say that each photon has behaved as a particle from the time of its emission to the time of its detection, has traveled by either one path or the other, and further affirm that its wave nature has not been exhibited.

If the apparatus is changed so that a second beam splitter is placed in the upper-right corner, then part of the beams from each path will travel to the right, where they will combine to exhibit interference on a detection screen. Experimenters must explain these phenomena as consequences of the wave nature of light. Each photon must have traveled by both paths as a wave, because if each photon traveled as a particle along just one path then the many photons sent during the experiment would not produce an interference pattern.

Since nothing else has changed from experimental configuration to experimental configuration, and since in the first case the photon is said to "decide" to travel as a particle and in the second case it is said to "decide" to travel as a wave, Wheeler wanted to know whether, experimentally, a time could be determined at which the photon made its "decision." Would it be possible to let a photon pass through the region of the first beam-splitter while there was no beam-splitter in the second position, thus causing it to "decide" to travel, and then quickly let the second beam-splitter pop up into its path? Having presumably traveled as a particle up to that moment, would the beam splitter let it pass through and manifest itself as would a particle were that second beam splitter not to be there? Or, would it behave as though the second beam-splitter had always been there? Would it manifest interference effects? And if it did manifest interference effects then to have done so it must have gone back in time and changed its "decision" about traveling as a particle to traveling as a wave. Note that Wheeler wanted to investigate several hypothetical statements by obtaining objective data.

Albert Einstein did not like these possible consequences of quantum mechanics. However, when experiments were finally devised that permitted both the double-slit version and the interferometer version of the experiment, it was conclusively shown that a photon could begin its life in an experimental configuration that would call for it to demonstrate its particle nature, end up in an experimental configuration that would call for it to demonstrate its wave nature, and that in these experiments it would always show its wave characteristics by interfering with itself. Furthermore, if the experiment was begun with the second beam-splitter in place but it was removed while the photon was in flight, then the photon would inevitably show up in a detector and not show any sign of interference effects. So the presence or absence of the second beam-splitter would always determine "wave or particle" manifestation. Many experimenters reached an interpretation of the experimental results that said that the change in final conditions would retroactively determine what the photon had "decided" to be as it was entering the first beam-splitter. As mentioned above, Wheeler rejected this interpretation.

Cosmic interferometer

Double quasar known as QSO 0957+561, also known as the "Twin Quasar", which lies just under 9 billion light-years from Earth. 

 

Wheeler's plan

In an attempt to avoid destroying normal ideas of cause and effect, some theoreticians suggested that information about whether there was or was not a second beam-splitter installed could somehow be transmitted from the end point of the experimental device back to the photon as it was just entering that experimental device, thus permitting it to make the proper "decision." So Wheeler proposed a cosmic version of his experiment. In that thought experiment he asks what would happen if a quasar or other galaxy millions or billions of light years away from Earth passes its light around an intervening galaxy or cluster of galaxies that would act as a gravitational lens. A photon heading exactly towards Earth would encounter the distortion of space in the vicinity of the intervening massive galaxy. At that point it would have to "decide" whether to go by one way around the lensing galaxy, traveling as a particle, or go both ways around by traveling as a wave. When the photon arrived at an astronomical observatory at Earth, what would happen? Due to the gravitational lensing, telescopes in the observatory see two images of the same quasar, one to the left of the lensing galaxy and one to the right of it. If the photon has traveled as a particle and comes into the barrel of a telescope aimed at the left quasar image it must have decided to travel as a particle all those millions of years, or so say some experimenters. That telescope is pointing the wrong way to pick up anything from the other quasar image. If the photon traveled as a particle and went the other way around, then it will only be picked up by the telescope pointing at the right "quasar." So millions of years ago the photon decided to travel in its guise of particle and randomly chose the other path. But the experimenters now decide to try something else. They direct the output of the two telescopes into a beam-splitter, as diagrammed, and discover that one output is very bright (indicating positive interference) and that the other output is essentially zero, indicating that the incoming wavefunction pairs have self-cancelled.

Paths separated and paths converged via beam-splitter

Wheeler then plays the devil's advocate and suggests that perhaps for those experimental results to be obtained would mean that at the instant astronomers inserted their beam-splitter, photons that had left the quasar some millions of years ago retroactively decided to travel as waves, and that when the astronomers decided to pull their beam splitter out again that decision was telegraphed back through time to photons that were leaving some millions of years plus some minutes in the past, so that photons retroactively decided to travel as particles.

Several ways of implementing Wheeler's basic idea have been made into real experiments and they support the conclusion that Wheeler anticipated — that what is done at the exit port of the experimental device before the photon is detected will determine whether it displays interference phenomena or not. Retrocausality is a mirage.

Double-slit version

Wheeler's double-slit apparatus.

A second kind of experiment resembles the ordinary double-slit experiment. The schematic diagram of this experiment shows that a lens on the far side of the double slits makes the path from each slit diverge slightly from the other after they cross each other fairly near to that lens. The result is that at the two wavefunctions for each photon will be in superposition within a fairly short distance from the double slits, and if a detection screen is provided within the region wherein the wavefunctions are in superposition then interference patterns will be seen. There is no way by which any given photon could have been determined to have arrived from one or the other of the double slits. However, if the detection screen is removed the wavefunctions on each path will superimpose on regions of lower and lower amplitudes, and their combined probability values will be much less than the unreinforced probability values at the center of each path. When telescopes are aimed to intercept the center of the two paths, there will be equal probabilities of nearly 50% that a photon will show up in one of them. When a photon is detected by telescope 1, researchers may associate that photon with the wavefunction that emerged from the lower slit. When one is detected in telescope 2, researchers may associate that photon with the wavefunction that emerged from the upper slit. The explanation that supports this interpretation of experimental results is that a photon has emerged from one of the slits, and that is the end of the matter. A photon must have started at the laser, passed through one of the slits, and arrived by a single straight-line path at the corresponding telescope.

The retrocausal explanation, which Wheeler does not accept, says that with the detection screen in place, interference must be manifested. For interference to be manifested, a light wave must have emerged from each of the two slits. Therefore, a single photon upon coming into the double-slit diaphragm must have "decided" that it needs to go through both slits to be able to interfere with itself on the detection screen (shouldn’t the detection screen be placed in front of the double slits?). For no interference to be manifested, a single photon coming into the double-slit diaphragm must have "decided" to go by only one slit because that would make it show up at the camera in the appropriate single telescope.

In this thought experiment the telescopes are always present, but the experiment can start with the detection screen being present but then being removed just after the photon leaves the double-slit diaphragm, or the experiment can start with the detection screen being absent and then being inserted just after the photon leaves the diaphragm. Some theorists argue that inserting or removing the screen in the midst of the experiment can force a photon to retroactively decide to go through the double-slits as a particle when it had previously transited it as a wave, or vice versa. Wheeler does not accept this interpretation.

The double slit experiment, like the other six idealized experiments (microscope, split beam, tilt-teeth, radiation pattern, one-photon polarization, and polarization of paired photons), imposes a choice between complementary modes of observation. In each experiment we have found a way to delay that choice of type of phenomenon to be looked for up to the very final stage of development of the phenomenon, and it depends on whichever type of detection device we then fix upon. That delay makes no difference in the experimental predictions. On this score everything we find was foreshadowed in that solitary and pregnant sentence of Bohr, "...it...can make no difference, as regards observable effects obtainable by a definite experimental arrangement, whether our plans for constructing or handling the instruments are fixed beforehand or whether we prefer to postpone the completion of our planning until a later moment when the particle is already on its way from one instrument to another."

Bohmian interpretation

One of the easiest ways of "making sense" of the delayed-choice paradox is to examine it using Bohmian mechanics. The surprising implications of the original delayed-choice experiment led Wheeler to the conclusion that "no phenomenon is a phenomenon until it is an observed phenomenon", which is a very radical position. Wheeler famously said that the "past has no existence except as recorded in the present", and that the Universe does not "exist, out there independent of all acts of observation".

However Bohm et al. (1985, Nature vol. 315, pp294–97) have shown that the Bohmian interpretation gives a straightforward account of the behaviour of the particle under the delayed-choice set up, without resorting to such a radical explanation. A detailed discussion is available in the open-source article by Basil Hiley and Callaghan, while many of the quantum paradoxes including delayed choice are summarized in Chapter 7 of the Book A Physicist's View of Matter and Mind (PVMM) using both Bohmian and standard interpretations.

In Bohm's quantum mechanics, the particle obeys classical mechanics except that its movement takes place under the additional influence of its quantum potential. A photon or an electron has a definite trajectory and passes through one or the other of the two slits and not both, just as it is in the case of a classical particle. The past is determined and stays what it was up to the moment T₁ when the experimental configuration for detecting it as a wave was changed to that of detecting a particle at the arrival time T₂. At T₁, when the experimental set up was changed, Bohm's quantum potential changes as needed, and the particle moves classically under the new quantum potential till T₂ when it is detected as a particle. Thus Bohmian mechanics restores the conventional view of the world and its past. The past is out there as an objective history unalterable retroactively by delayed choice, contrary to the radical view of Wheeler.

The "quantum potential" Q(r,T) is often taken to act instantly. But in fact, the change of the experimental set up at T₁ takes a finite time dT. The initial potential. Q(r,T<T₁) changes slowly over the time interval dT to become the new quantum potential Q(r,T>T₁). The book PVMM referred to above makes the important observation (sec. 6.7.1) that the quantum potential contains information about the boundary conditions defining the system, and hence any change of the experimental set up is immediately recognized by the quantum potential, and determines the dynamics of the Bohmian particle.

Experimental details

John Wheeler's original discussion of the possibility of a delayed choice quantum appeared in an essay entitled "Law Without Law," which was published in a book he and Wojciech Hubert Zurek edited called Quantum Theory and Measurement, pp 182–213. He introduced his remarks by reprising the argument between Albert Einstein, who wanted a comprehensible reality, and Niels Bohr, who thought that Einstein's concept of reality was too restricted. Wheeler indicates that Einstein and Bohr explored the consequences of the laboratory experiment that will be discussed below, one in which light can find its way from one corner of a rectangular array of semi-silvered and fully silvered mirrors to the other corner, and then can be made to reveal itself not only as having gone halfway around the perimeter by a single path and then exited, but also as having gone both ways around the perimeter and then to have "made a choice" as to whether to exit by one port or the other. Not only does this result hold for beams of light, but also for single photons of light. Wheeler remarked:

The experiment in the form an interferometer, discussed by Einstein and Bohr, could theoretically be used to investigate whether a photon sometimes sets off along a single path, always follows two paths but sometimes only makes use of one, or whether something else would turn up. However, it was easier to say, "We will, during random runs of the experiment, insert the second half-silvered mirror just before the photon is timed to get there," than it was to figure out a way to make such a rapid substitution. The speed of light is just too fast to permit a mechanical device to do this job, at least within the confines of a laboratory. Much ingenuity was needed to get around this problem.

After several supporting experiments were published, Jacques et al. claimed that an experiment of theirs follows fully the original scheme proposed by Wheeler. Their complicated experiment is based on the Mach–Zehnder interferometer, involving a triggered diamond N–V colour centre photon generator, polarization, and an electro-optical modulator acting as a switchable beam splitter. Measuring in a closed configuration showed interference, while measuring in an open configuration allowed the path of the particle to be determined, which made interference impossible.

In such experiments, Einstein originally argued, it is unreasonable for a single photon to travel simultaneously two routes. Remove the half-silvered mirror at the [upper right], and one will find that the one counter goes off, or the other. Thus the photon has traveled only one route. It travels only one route. but it travels both routes: it travels both routes, but it travels only one route. What nonsense! How obvious it is that quantum theory is inconsistent!

Interferometer in the lab

The Wheeler version of the interferometer experiment could not be performed in a laboratory until recently because of the practical difficulty of inserting or removing the second beam-splitter in the brief time interval between the photon's entering the first beam-splitter and its arrival at the location provided for the second beam-splitter. This realization of the experiment is done by extending the lengths of both paths by inserting long lengths of fiber optic cable. So doing makes the time interval involved with transits through the apparatus much longer. A high-speed switchable device on one path, composed of a high-voltage switch, a Pockels cell, and a Glan–Thompson prism, makes it possible to divert that path away from its ordinary destination so that path effectively comes to a dead end. With the detour in operation, nothing can reach either detector by way of that path, so there can be no interference. With it switched off the path resumes its ordinary mode of action and passes through the second beam-splitter, making interference reappear. This arrangement does not actually insert and remove the second beam-splitter, but it does make it possible to switch from a state in which interference appears to a state in which interference cannot appear, and do so in the interval between light entering the first beam-splitter and light exiting the second beam-splitter. If photons had "decided" to enter the first beam-splitter as either waves or a particles, they must have been directed to undo that decision and to go through the system in their other guise, and they must have done so without any physical process being relayed to the entering photons or the first beam-splitter because that kind of transmission would be too slow even at the speed of light. Wheeler's interpretation of the physical results would be that in one configuration of the two experiments a single copy of the wavefunction of an entering photon is received, with 50% probability, at one or the other detectors, and that under the other configuration two copies of the wave function, traveling over different paths, arrive at both detectors, are out of phase with each other, and therefore exhibit interference. In one detector the wave functions will be in phase with each other, and the result will be that the photon has 100% probability of showing up in that detector. In the other detector the wave functions will be 180° out of phase, will cancel each other exactly, and there will be a 0% probability of their related photons showing up in that detector.

Interferometer in the cosmos

The cosmic experiment envisioned by Wheeler could be described either as analogous to the interferometer experiment or as analogous to a double-slit experiment. The important thing is that by a third kind of device, a massive stellar object acting as a gravitational lens, photons from a source can arrive by two pathways. Depending on how phase differences between wavefunction pairs are arranged, correspondingly different kinds of interference phenomena can be observed. Whether to merge the incoming wavefunctions or not, and how to merge the incoming wavefunctions can be controlled by experimenters. There are none of the phase differences introduced into the wavefunctions by the experimental apparatus as there are in the laboratory interferometer experiments, so despite there being no double-slit device near the light source, the cosmic experiment is closer to the double-slit experiment. However, Wheeler planned for the experiment to merge the incoming wavefunctions by use of a beam splitter.

The main difficulty in performing this experiment is that the experimenter has no control over or knowledge of when each photon began its trip toward earth, and the experimenter does not know the lengths of each of the two paths between the distant quasar. Therefore, it is possible that the two copies of one wavefunction might well arrive at different times. Matching them in time so that they could interact would require using some kind of delay device on the first to arrive. Before that task could be done, it would be necessary to find a way to calculate the time delay.

One suggestion for synchronizing inputs from the two ends of this cosmic experimental apparatus lies in the characteristics of quasars and the possibility of identifying identical events of some signal characteristic. Information from the Twin Quasars that Wheeler used as the basis of his speculation reach earth approximately 14 months apart. Finding a way to keep a quantum of light in some kind of loop for over a year would not be easy.

Double-slits in lab and cosmos

Replace beam splitter by registering projected telescope images on a common detection screen.

Wheeler's version of the double-slit experiment is arranged so that the same photon that emerges from two slits can be detected in two ways. The first way lets the two paths come together, lets the two copies of the wavefunction overlap, and shows interference. The second way moves farther away from the photon source to a position where the distance between the two copies of the wavefunction is too great to show interference effects. The technical problem in the laboratory is how to insert a detector screen at a point appropriate to observe interference effects or to remove that screen to reveal the photon detectors that can be restricted to receiving photons from the narrow regions of space where the slits are found. One way to accomplish that task would be to use the recently developed electrically switchable mirrors and simply change directions of the two paths from the slits by switching a mirror on or off. As of early 2014 no such experiment has been announced.

The cosmic experiment described by Wheeler has other problems, but directing wavefunction copies to one place or another long after the photon involved has presumably "decided" whether to be a wave or a particle requires no great speed at all. One has about a billion years to get the job done.

The cosmic version of the interferometer experiment could easily be adapted to function as a cosmic double-slit device as indicated in the illustration. Wheeler appears not to have considered this possibility. It has, however, been discussed by other writers.

Current experiments of interest

The first real experiment to follow Wheeler's intention for a double-slit apparatus to be subjected to end-game determination of detection method is the one by Walborn et al.

Researchers with access to radio telescopes originally designed for SETI research have explicated the practical difficulties of conducting the interstellar Wheeler experiment.

A recent experiment by Manning et al. confirms the standard predictions of standard quantum mechanics with an atom of Helium.

Conclusions

Ma, Zeilinger et al. have summarized what can be known as a result of experiments that have arisen from Wheeler's proposals. They say:

Any explanation of what goes on in a specific individual observation of one photon has to take into account the whole experimental apparatus of the complete quantum state consisting of both photons, and it can only make sense after all information concerning complementary variables has been recorded. Our results demonstrate that the viewpoint that the system photon behaves either definitely as a wave or definitely as a particle would require faster-than-light communication. Because this would be in strong tension with the special theory of relativity, we believe that such a viewpoint should be given up entirely.