The transactional interpretation makes no experimental predictions different from standard quantum mechanics. But it does remove some of the puzzling and perhaps unnecessary assumptions that are part of other Interpretations of quantum mechanics. In particular, it denies that conscious observers are needed to cause the "collapse of the wave function" (without which there is no actual "outcome" in the measurement process).

The transactional interpretation adds nothing ad hoc to the standard theory, such as "hidden variables or additional terms to the Schrōdinger equation to force a collapse. It is explicitly indeterministic and non-local. Cramer is exploring the radical possibility of sending information between entangled particles faster than the speed of light, as well as causal relations that go backwards in time (retrocausality). And, like Schrōdinger and the decoherence advocates, Cramer denies the existence of particles!

The core physics in the transactional interpretation is a way of looking at photon emissions and absorptions as an exchange of advanced and retarded waves that is based on the 1945 Wheeler-Feynman Absorber Theory of radiation, which was abandoned by Feynman, who went on to develop the Path Integral formulation of quantum mechanics and later, with Julian Schwinger and Sin-Itiro Tomonaga, the theory of Quantum Electrodynamics (QED).

While QED is a powerful theory that allows precise calculations of physical observables such as the motions of photons and electrons and the emission and absorption of a photon by an electron, the transactional interpretation is simply a way of looking at the emission and absorption of photons based on the Wheeler-Feynman attempt to describe the exchange of energy in the classical electromagnetic field as a time-symmetric process.

Wheeler-Feynman proposed adding advanced field potentials (which look like never-seen-in-nature incoming spherical waves converging on light sources) to the normal outgoing spherical waves (with retarded potentials) of classical electrodynamics. Wheeler and Feynman's goal was to symmetrize electrodynamics with respect to time. One view of the advanced-potential incoming waves is that they are going backwards in time. There is nothing inherent in electromagnetic theory that explains the time asymmetry we see in radiation propagation (forward-in-time outgoing waves only).

Cramer's transactional interpretation describes an electron as sending out probabilistic "offer waves" (OW) to potential absorbers. He adds what he calls "confirmation waves" (CW) incoming to an emitter from the many possible absorbers of an emitted photon. An offer wave is not an actual photon emission, and a confirmation wave is not an actual absorption or "detection" of a photon. But Cramer did see the two waves as connecting events in four-dimensional spacetime. Eventually, one advanced-potential confirmation wave indeterministically "handshakes" with the retarded-potential offer wave and produces an actual absorption.

This "handshake" completes the transaction, but perhaps not at a single point in spacetime. Cramer sees the transaction as "atemporal" in that it takes place all along the four-dimensional spacetime vector between the emission and absorption events. Because it happens over the extended space of a worldline of a photon between emission and absorption, Cramer says it is "explicitly nonlocal," but this linear space is tiny compared to the huge space of nonlocal behavior of two entangled particles in the EPR experiment, for example.

In the transactional interpretation the collapse of the state vector is interpreted as the completion of the transaction started by the OW and the CW exchanged between emitter and - absorber. The emergence of the transaction from the SV [state vector or wave function] does not occur at some particular location in space or at some particular instant of time, but rather forms along the entire four-vector that connects the emission locus with the absorption locus (or loci in the case of multiple correlated particles). The transaction employs both retarded and advanced waves, which propagate, respectively, along positive and negative lightlike (or timelike) four-vectors. Since the sum of these four-vectors can span spacelike and negative timelike and lightlike intervals, the "influence" of the transaction in enforcing the correlations of the quantum event is explicitly both nonlocal and atemporal.

(The transactional interpretation of quantum mechanics, Rev. Mod. Phys., Vol. 58, No. 3, July 1986, p.667)

Since the transaction is atemporal, forming along the entire interval separating emission locus from absorption locus "at once, " it makes no difference to the outcome or the transactional description if separated experiments occur "simultaneously" or in any time sequence. There is likewise no issue of which of the separated measurements occurs first and precipitates the SV collapse, since in the transactional interpretation both measurements participate equally and symmetrically in the formation of the transaction. Furthermore, the paths across which the correlation enforcing exchange takes place are lightlike four-vectors and remain so under any Lorentz transformation. Therefore the outcome and the transactional description of any correlation experiment is the same independent of the inertial reference frame from which it is viewed, as it must be if quantum mechanics and relativity are to be compatible theories.

(The transactional interpretation of quantum mechanics, Rev. Mod. Phys., Vol. 58, No. 3, July 1986, pp.668-9)

If there is a special frame of reference (not a preferred frame in the relativistic sense), surely it is the one in which the origin of the two entangled particles is at rest. Assuming that Alice and Bob are also at rest in this special frame and equidistant from the origin, we arrive at the simple picture in which any measurement that causes the two-particle wave function to collapse makes both particles appear "simultaneously" at determinate places with fully correlated properties (just those that are needed to conserve energy, momentum, angular momentum, and spin).

In the two-particle case (instead of just one particle making an appearance), when either particle is measured, we know instantly those properties of the other particle that satisfy the conservation laws, including its location equidistant from, but on the opposite side of, the source, and its other properties such as spin.

We cannot measure just one particle in a two-particle wave function. As Schrödinger told Einstein in 1935, entanglement means that the particles cannot be represented as the product of single-particle wave functions.

Cramer says the transactional interpretation sheds light on the collapse of the state vector, identifying the collapse with as absorber's "handshake" with the emitter that completes the transaction. Is this in conflict with his view of transactions as time symmetric and fully reversible? Standard quantum mechanics insists that something thermodynamically irreversible must happen in a measurement. Cramer seems skeptical about irreversibility.

the Copenhagen interpretation implicitly associates with quantum events a time directionality that, while appropriate to macroscopic observers, is quite alien to and inconsistent with the even-handedness with which microphysics deals with the flow of time. Somehow the thermodynamic irreversibility of the macroscopic observer is intruding into the description of a fully reversible microscopic process. (p.651)

Wigner and others have suggested that the process of collapse should involve a special role for consciousness (Wigner, 1962), for permanent recording of experimental results (Schrödinger, 1935) or for entry of the system into the domain of thermodynamic irreversibility (Heisenberg, 1960). (p.654)

Where precisely is the border between macrophysics and microphysics and the border at which irreversibility begins? (p.683)

In the information interpretation, the collapse is when information about an event (it may not be a measurement) is irreversibly recorded in the universe. It need not be a measurement by an observer. Indeed, information must be recorded (for example, by a measuring instrument) before it can be seen by an observer.

Despite his description of the transactional "handshake" as atemporal, Cramer says the collapse occurs when the emitter accepts the confirmation wave from an absorber. It is the absorber that precipitates the collapse, he says,

In the transactional interpretation the collapse, i.e., the development of the transaction, is atemporal and thus avoids the contradictions and inconsistencies implicit in any time-localized SV collapse.

Furthermore, the transactional description does not need to invoke arbitrary collapse triggers, such as consciousness, etc., because it is the absorber rather than the observer which precipitates the collapse of the SV, and this can occur atemporally and nonlocally across any sort of interval between elements of the measuring apparatus.

(The transactional interpretation of quantum mechanics, Rev. Mod. Phys., Vol. 58, No. 3, July 1986, p.668)

Cramer is quite critical of the need for a "conscious observer."

This "consciousness" interpretation, while it is a reasonable working hypothesis for an observer who does not wish to find himself dissolved into the state vector of the system he is measuring, does beg a number of questions. Did the SV of the universe remain uncollapsed until the first consciousness evolved? Where is the borderline between consciousness and unconsciousness? Will "smart" measuring instruments eventually achieve the abihty to collapse SV's, and how will one know when they do? And so on.

Schrodinger (1935) suggested an alternative to the consciousness interpretation, which he called the principle of state distinction and which asserts, "states of a microscopic system which could be told apart by macroscopic observation are distinct from each other whether observed or not. " In other words, the SV collapses as soon as some macroscopic record of the result of a measurement is made, whether a conscious observer looks at that record or not. Heisenberg (1960) and others have suggested a variant of this position which asserts that as soon as the quantum measurement passes from the domain of reversible processes into the domain of thermodynamic irreversibility the SV collapses.

The latter two "collapse triggers" are more appealing to most physicists than the former because they avoid giving some special significance to consciousness and because, as pointed out by Weisskopf (1959,1980), they correspond more closely to the operating assumptions that practicing physicists use in thinking about how quantum measurements are done. However, these models also beg the question of borders: Where precisely is the border between macrophysics and microphysics and the border at which irreversibility begins?

(The transactional interpretation of quantum mechanics, Rev. Mod. Phys., Vol. 58, No. 3, July 1986, p.683)

The answer to Cramer's question about the border between microphysics and macrophysics is found in an analysis of the "quantum-to-classical transition" and in Heisenberg and von Neumann's speculations about the "cut" between quantum events and an observer's information, knowledge, or conscious awareness. Below the cut everything is governed by the wave function. Above the cut, Heisenberg and Bohr insisted a classical description must be used.

Decoherence theorists claim that the quantum-to-classical transition is caused by environmental interactions, but the information interpretation claims it is when a macroscopic object contains such a large number of atoms that independent quantum events that they can be averaged over, that their random phases cancel out, and that there is statistical determinism.

Heisenberg, von Neumann, Wigner, and many others puzzled over the location of the "cut," perhaps none more than John Bell, who drew a diagram of possible places for what he called the "shifty split." We can now edit Bell's diagram to point to the location of "cut" as the moment when irreversible information enters the universe.

In her 2012 book, The Transactional Interpretation of Quantum Mechanics, Ruth Kastner proposes to regard the outgoing offer wave and many incoming confirmation waves as "possible" transactions, only one of which indeterministically becomes "actual."

Kastner is a possibilist who argues that OWs and CWs are possibilities that are "real." She says that they are less real than actual empirically measurable events, but more real than an idea or concept in a person's mind. She suggests the alternate term "potentia," Aristotle's term that she found Heisenberg had cited. For Kastner, the possibilities are physically real as compared to merely conceptually possible ideas that are consistent with physical law (for example, David Lewis' "possible worlds." But she says the "possibilities" described by offer and confirmation waves are "sub-empirical" and pre-spatiotemporal (i.e., they have not shown up as actual in spacetime). She calls these "incipient transactions."

The subtitle of Kastner's book is "The Reality of Possibility." She says that her main thesis is that "it is perfectly reasonable to be realist about the subject matter of quantum theory" (p.28). And she calls for a new metaphysical category to describe "not quite actual...possibilities."