pic 1. Quantum entanglement is sharing a deep connection – in a quantum scale of things.
When the physical and statistical properties of a particle are fundamentally dependent on the properties of one or several other particles, these particles are said to be entangled. Without any physical interaction these particles can remain deeply connected to each other, even when they are vast distances apart.
Entanglement is a theoretical prediction that comes from the equations of quantum mechanics. Two particles can become entangled if they share the same state in a way that makes it possible to consider them not individually, but as a system. A laser beam fired through a certain type of crystal can cause individual photons to be split into pairs of entangled photons. Remarkably, quantum mechanics says that even if you separate those particles and send them to opposite directions they can remain entangled and inextricably connected. At least in theory. Conservation of the entanglement is very much susceptible to noise and “quantumness” disrupting decoherence (read more in Superposition text) if we are dealing with anything else but a perfectly isolated laboratory environment.
To understand the profound meaning of entanglement you can consider the quality that an electron possesses called the “spin”. Generally, just as it is common for other quantum qualities, the spin of an electron remains uncertain and fuzzy until it is measured, as explained by Superposition. With two entangled particles whenever one of them is measured with spin up the other one must, no matter how far away it is from its entangled pair, be spin down. In other words whenever we inflict a measurement on one entangled particle, we automatically change its counterpart to correlate no matter the distance and with nothing, no physical force, attaching these two particles to one another.
Fig 1. Changing one of the entangled particles spin will immediately do so with the other one, seemingly faster than light. This is what Einstein called “The spooky action at a distance”.
The physicists Niels Bohr and Werner Heisenberg argued in 1934 among other quantum theory questions that an object’s state only truly existed once it became associated with a measurement, which meant somebody needed to observe it experimentally. Until then, its nature was merely a possibility. Upon measurement the system’s spin is fixed either up or down.
To other physicists, such as Albert Einstein and Erwin Schrödinger, this was as preposterous as saying a cat inside a box is neither alive nor dead until you look. A paradox in other words. No action taken on the first particle could instantaneously affect the other, since this would involve information being transmitted faster than light, which is forbidden by the theory of relativity. Theory of relativity states for example that if anything were to travel faster than light it would violate the laws of causality and is a theory many times tested to not fall short. From Einstein’s work with Podolsky and Rosen an idea to solve this “spooky action at a distance” was argued to be solved with the thought of a more deterministic theory still unknown to science and hidden local variables that were coded in the particles and that could not be later influenced.
In 1964 John Stewart Bell made a theoretical article that argued quantum physics to be incompatible with the local hidden variables theories, and that was later proven correct. Decades later, Bohr’s ideas still stand strong, and the strange nature of quantum entanglement is a solid part of modern physics. An interesting theory tested in the laboratory (using entanglement) is one that aims to quantize general relativity and unify the foundations of modern physics by leaving out time altogether. The results of testing this in 2013 with a toy Universe model suggested that time itself is an emergent phenomenon that comes about because of the nature of entanglement and that. While not working as the unifying theory for modern physics it paves a way for more research regarding entanglement. Entanglement continues to boggle the minds and remains as a part of the strange world of subatomic physics that we call quantum.
More to read and links to text:
Malin, Shimon, World Scientific 2012: Nature loves to hide: Quantum Physics and reality, a western perspective
Science alert website, What is Quantum Entanglement? https://www.sciencealert.com/entanglement
Fobes 11 August 2015, Chad Orzel: How Quantum Randomness Saves Relativity https://www.forbes.com/sites/chadorzel/2015/08/11/how-quantum-randomness-saves-relativity/
Space.com website 31 July 2019, Yasemin Saplakoglu: ‘Spooky’ Quantum Entanglement Finally Captured in Stunning Photo https://www.space.com/quantum-entanglement-photo.html
Medium webpage 23 October 2013, The Physics ArXiv Blog: Quantum Experiment Shows How Time ‘Emerges’ from Entanglement https://medium.com/the-physics-arxiv-blog/quantum-experiment-shows-how-time-emerges-from-entanglement-d5d3dc850933
Ekaterina Moreva, Giorgio Brida, Marco Gramegna et al. 17 October 2013, Time from quantum entanglement: an experimental illustration, https://arxiv.org/pdf/1310.4691
Noora Heiskanen with thanks to Silvia Cotroneo and Jani-Petri Martikainen