**Articles & Reviews**

###### Authored by RSF Research Staff

# Firewalls or Cool Horizons?

Physicists attempt to cool down a heated debate by suggesting quantum entanglement occurs through spacetime wormholes.

The theoretical physics of black holes abounds with paradoxes, such as the loss of information behind the point of no return – the event horizon – and within the singularity (the theoretical object at the center of black holes, where all the mass is thought to be compressed into a point of zero-dimension and infinite density – stay tuned for our upcoming article on *Planck Stars*, a solution to singularities and information loss). In the investigation of the effects of quantum behaviors around the event horizon of black holes, a team of physicists have proposed another possible paradoxical situation – the multiple entanglement of particles emitted from the event horizon (known as Hawking Radiation), which would cause violations in the known dynamics of quantum entanglement. The research team’s resolution has been to propose a thermal curtain of high temperature particles and radiation surrounding the event horizon – a so called “firewall” (which led Stephen Hawking to make his most recent proclamation on black holes being metastable, see our article Hawking Goes Grey). In an interesting turn of events, renowned physicists Leonard Susskind and Juan Maldacena not only demonstrated flaws in the firewall paradox argument – but also demonstrated resolutions involving black holes connected by a wormhole, and most interestingly of all perhaps – how quantum entanglement itself may be the result of spacetime connections through wormholes.

At some point we have all heard the term quantum entanglement, but what exactly is entanglement? When two objects are interacting, their behaviors are correlated – a change in one object will effect a change in the other because of their mutual interaction. This can be observed in a wide variety of circumstances at many different scales of size, it is a “classical” behavior. However, when this interrelationship is investigated in subatomic particles the observed behavior can be quite different – the particles can be *strongly *correlated. This has been thought to be a non-classical, or quantum phenomenon, and is the result of maximum entanglement. Two particles can become so strongly correlated, that no matter the distance of separation between the two, they appear to still be interacting – instantaneously, even if they’re across the universe from each other!

Instantaneous interaction implies a faster-than-light, or superluminal, transmission of information, and it is this fact that led Einstein (who is famous for having set the cosmic “speed limit” at 300,000 kilometers per second, the velocity of light) along with physicists Rosen and Podolsky to propose the EPR paradox. Einstein, Rosen, and Podolsky pointed out that, as formulated, quantum mechanics suggested that there could be instantaneous interaction between strongly correlated particle pairs. Without a physical mechanism to explain such superluminal behavior, EPR suggested that this was a highly unlikely scenario. However, far from being unlikely, it has been confirmed and is routinely observed in quantum experimentations. Eventually Einstein called this quantum entanglement “spooky action at a distance”.

A conundrum arises – because if it is true that nothing can travel faster than the speed of light (which is a supposition that is becoming more and more tenuous) how can two particles that are separated by a sizable distance, kilometers in some instances, be interacting with each other faster than any light signal can travel between them? Fascinatingly, the answer may lie in a structure normally reserved for science fiction movies, but which many top physicists consider very seriously – wormholes. Wormholes are tunnels that are made out of the very “fabric” of space itself (yes, space has structure and substance to it!) and as such they can connect separate regions of space.

So how did the concept of wormholes become associated with quantum entanglement? The development has come in the wake of many publications such as *Building up spacetime with quantum entanglement*, by Mark Van Raamsdonk, and most recently *Cool Horizons for Black Holes*. The latter paper, written by Juan Maldacena, who is renowned for having derived a correspondence principle between quantum field theory and general relativity, and Leonard Susskind, a string theorist who has pioneered work on black holes and information theory – suggests how wormholes can entangle not only black holes, but subatomic particles as well. Their theory is offered as a possible solution to paradoxes that have arisen from the investigation of the quantum properties of black holes. Paradoxes that many physicists feel are threatening the very foundation of quantum mechanics.

Traditionally, black holes have been thought of as very simple objects – they can be completely described with just a few physical parameters, such as their mass and size. This made the situation very simple, because no matter the complexity of the matter that existed before forming a black hole, after collapsing down into a volume so dense that not even light can escape, it’s only distinguishable characteristics would be its mass, rate of spin, and possibly electric charge (this is called the no-hair theorem, apparently some physicists thought as a stars ages it would suffer from male pattern baldness).

Yet the situation became more complex when a consideration was given to the gravitational effects on the quantum properties of a black hole. This is a remarkable area of investigation, because generally at the quantum scale (think infinitesimally small sizes) gravity is considered to play no significant role – it is regarded as negligible. But black holes contain a very unique gravitational field – they form an event horizon – a virtual boundary demarcated as the volume (like a sphere) from which light cannot escape the gravitational “pull” of the condensed mass within (the singularity). Because of this gravitational horizon – a literal light-like boundary – quantum fluctuations in space, which would normally cancel out as annihilating particle-antiparticle pairs, actually emit real particles from the surface of the boundary as one of the partners in the pair crosses the event horizon while the other escapes.

This situation confounded investigators, as evident by the terminology used in this emerging field of quantum gravity – such as the *information loss paradox*, and most recently, the AMPS paradox. The AMPS paradox, which is an acronym derived from the names of the physicists who proposed it, chief of which is Joseph Polchinski (the ‘P’ in AMPS, another string theorist) describes a seeming paradox that would arise from the strong correlation – that is entanglement – associated with the emitted particle and its partner that crossed into the event horizon of the black hole. The problem, as they describe it, is that if a newly emitted particle becomes entangled with another outside of the horizon, it would break the entangling bond between the emitted particles and those in the interior. Just as when chemical bonds are broken in chemistry – like igniting gasoline – there is a release of energy. Therefore, they proposed that the black hole must have extremely hot particles at the horizon – a literal firewall.

This would mean that not only do black holes radiate – with emission of particles from the quantum vacuum of space – they would shine from the high energy quanta at the surface horizon! This is further confounding for many physicists because the “disentanglement” of the trans-horizon particle pair represents a loss of information, which violates quantum mechanical principles of the conservation of information. Juan Maldacena and Leonard Susskind have proposed a way to preserve a cool horizon for black holes (hence the name of their paper), and the solution is that the black hole is not just a point of infinite density – a singularity – it is a wormhole connected to another black hole.

In this case, even though the interior of a black hole is entangled with another system – namely another black hole, which is analogous to the entanglement of the radiated particles with the interior of the black hole in the AMPS paradox, there is still a smooth horizon (meaning it is undetectable – you would never know it is there, under this regime). That is to say that production of a wormhole does not necessarily produce a firewall. Therefore, if the radiated particles (known as Hawking radiation), are considered to be connected with the interior of the event horizon through wormholes as well – then there is no need to conjecture for a firewall. This is remarkable because it demonstrates how modeling entanglement at the subatomic scale with wormhole connections is resolving seeming problems that are arising from considering entanglement as merely “spooky action at a distance”.

It should be noted that this is not the first time that quantum entanglement has been proposed to be the result of spacetime connections through wormholes. Sergio Santini, a Brazillian physicist who has done pioneering work in causal quantum cosmology, suggested how quantum entanglement could be a result of wormhole connections in a 2006 paper. Interestingly, none of the physicist who have later done work in this area have cited Santini for his groundbreaking contribution. As well, Nassim Haramein has done work elucidating a wormhole network connecting all of spacetime. In his latest work, Haramein utilized a geometric and holographic approach to describe the fundamental characteristics of quantum particles, such as the origin of their mass, size, and nuclear binding forces. Remarkably, he demonstrated how protons have a holographic mass that is equal to the mass-energy of the entire observable universe.

Essentially, this means that the interior of protons are one single volume sharing all information. How is this possible? Haramein’s calculations show how a proton is connected to every other proton through Planck-sized wormholes (a Planck volume is several billions of times smaller than a single proton). The strength of the entanglement between the particles is dependent on their relative distance, and the degree to which they have interacted. Therefore, while they are all fundamentally connected, the strength of the entanglement varies in degree to which they have interacted physically – just like what is actually observed empirically.

The potential of wormhole formation is an integral element of black holes, with the possibility to connect extremely distant regions of spacetime with a nonlocal – or superluminal – shortcut (as Susskind has explained, the black holes connected by the wormhole are in a state of maximum entanglement). If wormholes are such an essential feature of black holes, does this perhaps suggest that quantum particles entangled by Planckian wormhole tunnels are more like black holes themselves? When the quantum gravitation and holographic mass description of black hole like particles are considered in addition to the wormhole entanglement, it becomes a very appealing theory indeed.

The AMPS paradox is an extremely subtle one whose resolution, we believe, will have much to teach us about the connection between geometry and entanglement. AMPS pointed out a deep and genuine paradox about the interior of black holes. Their resolution was firewalls, but in our view the solution more likely involves Einstein-Rosen bridges [wormholes]. The interior of an old black hole would be an Einstein-Rosen bridge constructed from the micro-degrees of freedom [entropy – or information] of the black hole as well the radiation.”– Juan Maldacena and Leonard Susskind

By: William Brown

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