LAB Physics - file 04

To overtake Light – Relativity to Quantum

In my previous post, we saw that the actual speed of light is slowed down by medium, while a Photon travels constantly the 100% of the speed of light (“c” = about 300,000 km/s) as its property. This discrepancy, as well as the interference with molecules of water induce the Sonic Boom of light, called the Cherenkov Radiation, as the visible fantastic blue glow in a nuclear reactor pool.

Consequently, photons can relatively overtake actual speeds of light in various media, while the speed of photon is still kept up to the speed of light in vacuum (c) constantly.

The speed of light in a vacuum (c) is commonly called the universal speed limit. Then, is there anything else exceeding the speed of light? Is the mythical speed of light in a vacuum (c) really the fastest and the upper limit in the Universe? Let's examine together!

The Absolute Speed Limit

Although you would be disappointed, the answer is that Nothing travels through space faster than light. But it's too early to sigh… Note that it's the case through space. Okay, let's seek a ray of light!

According to Albert Einstein’s Theory of Special Relativity, as an object with mass (a Matter, like a spaceship, a ball, an atom, or your own body) moves faster and faster, it effectively becomes heavier and heavier. That is, its Relativistic Mass, which corresponds to the total energy (E), is increasing along with the speed, getting faster.

If you try to reach the speed of light, you would need an infinite amount of energy. Because your mass would become infinite, as the formula of Special Relativity below shows.

This is why Nothing with mass can ever reach or overtake the 299,792 km/s (“c”) limit.

In this formula, as velocity (v) approaches the speed of light (c), the energy (E) required is increasing exponentially and approaches infinity. Because, as you can see, the denominator of the formula above is approaching the minimum infinity.

In Einstein’s Theory of Relativity, energy is included in the mass (E = mc2). Rest mass (= invariant mass; “m” or m0) is, in Special Relativity, an invariant quantity of mass, which is common for all observers in all various reference frames, regardless of the kinetic energy or momentum.

For example, if you are on a train with a ball, the mass (rest mass) of the ball is invariant regardless of the speed of the train, even if the train stops or runs with nearly the speed of light. Because you, the observer, are on the train together with the ball (= 0 km/h)!

The Sonic Boom of Light

While nothing is faster than light in a vacuum (“c”), in my previous post, we could see that light slows down in various media.

And in specific cases, high-energy particles like electrons in a nuclear reactor can travel through water faster than the actual speed of light through the same water. When this happens, it creates a Sonic Boom of light, called Cherenkov Radiation, emitting the fantastic blue glow of photons.

So in this case, some high-energy electrons and photons can relatively overtake the actual speed of light in the medium, although they can NEVER exceed the speed of light in vacuum (“c”).

Expanding Space

This is big news! While we have seen that Nothing can move through space faster than the speed of light (“c”), space itself can expand at any speed. Actually, the Universe is growing faster than the speed of light!

For example, two galaxies very far apart are Not necessarily swimming away from each other. Rather, the ocean of space between them is growing!

The cumulative expansion of the Universe across billions of light-years means that distant galaxies are receding from us faster than the speed of light! This is why there is the horizon of the observable Universe. We human beings are too infinitesimal, too frivolous in the Universe, let alone our brains or histories... The nature of the Universe is beyond any scientific laws.

Quantum Entanglement

Quantum Physics goes further than Einstein's Theory of Relativity. According to quantum physics, two particles can become entangled, regardless of the space between them. If the state of one has been changed, the other changes instantly, even if it’s on the other side of the Universe.

This weird phenomenon is called Quantum Entanglement. Pairs or groups of particles are generated and interact with each other in ways such that the state of one particle cannot be described independently of the others, even if they are separated by vast distances.

“God does not play dice with the universe.” (Albert Einstein. In a letter to Max Born in 1926)

In his another letter in 1947 to physicist Max Born, as well, Albert Einstein derided Quantum Entanglement as a “Spooky action at a distance.” Because this theory of quantum physics suggests that particles instantaneously, faster than the speed of light, influence each other over vast distances, which plainly violates the speed-of-light limit in Einstein's Theory of Relativity.

Einstein believed in the Local Realism of classical physics. The local realism has 2 principles;

● Locality

Objects are only influenced by their immediate surroundings, with NO information transfer faster than the speed of light.

● Realism

Objects possess definite, invariable inherent properties independent of the reference frames of observations.

This implies that the Universe follows deterministic, local laws, intuitively aligning with classical physics. So Einstein objected to quantum mechanics as an incomplete, probabilistic description of reality, since he believed in a deterministic, orderly universe where physical outcomes could be predictable and are governed by the law of causality.

However, despite Albert Einstein’s skepticism, experimental evidence has surprisingly supported this ‘Spooky’ quantum phenomenon.

Local Hidden-Variable Theory vs. Bell's Inequality

Local hidden-variable (LHV) theory was explicitly designed to support local realism, that is, Realism, which suggests that particles have underlying, unobserved, ‘hidden’ pre-existing properties, and Locality, which advocates that measurements are only affected by their immediate surroundings, aligning with the view of classical physics.

On the other hand, Bell's inequality, which was suggested by John Stewart Bell in 1964, provided a mathematical framework to be tested, and violated to disprove local realism. And he heavily supported quantum entanglement by demonstrating that the statistical predictions of quantum mechanics (QM) were incompatible with classical local hidden-variable theories.

Bell's prime intention was to investigate whether local hidden-variable theory could explain quantum mechanics. So Bell’s inequality was designed to be violated. By his inequality, Bell showed that if quantum mechanics was correct, the correlations between entangled particles would violate his mathematical inequality defined by local realism. So the violation means that local realism can NOT account for quantum correlations.

Bell's experiments, so called Bell tests, consistently proved the violations of Bell's inequality, disproving the local hidden-variable theory. He showed that local hidden-variables were constrained by his inequality, and couldn't reproduce the predictions of quantum mechanics through the tests, so that nature was NOT subject to the principles of local realism, which rejects faster-than-light influence. Bell’s tests consistently confirmed quantum entanglement and ruled out local realism.

Experimental Proofs of Quantum Entanglement

A superconducting nanowire single photon detector (SNSPD) by Optics Research Group and Delft University of Technology
(source: Optics Research Group)

Since the 1970s, experiments in physics have consistently demonstrated that this ‘spooky’ entanglement is real, and confirmed the theory of quantum mechanics as the NEW paradigm of science. For example;

● Clauser-Freedman Experiment (1972)

American physicists John Clauser and Stuart Freedman conducted this groundbreaking experiment in 1972 at UC Berkeley. This was the first experimental test of Bell’s theorem, supporting quantum mechanics over local hidden-variable theory. They used entangled photons from calcium atoms, and successfully demonstrated that measuring one particle instantly determined the state of the other, regardless of distance. So this test confirmed ‘spooky’ entanglement for the first time in history.

● Aspect Experiments (1981–1982)

French physicist Alain Aspect, who was awarded the 2022 Nobel Prize in Physics together with the above-mentioned John Clauser, conducted pioneering tests of quantum mechanics in 1982. His experiments provided definitive evidence for the violation of Bell's inequality, which means to confirm the existence of quantum entanglement and reject the local hidden variable theory.

Aspect excited Calcium atoms by lasers to emit pairs of entangled photons with correlated polarizations. And he improved to incorporate extreme high-speed acousto-optical switches to alter the orientation of polarization filters while the photons were in flight. These switches operated at extremely high frequencies around 50 MHz (= 20 nanoseconds period), switching the measurement settings in a time up to 10 nanoseconds (10-8 second = 0.00000001 seconds = 100 MHz, in which it takes light to travel roughly 3 meters), shorter than the time it would take for light to travel between the detectors. These settings disturbing the speed of light, closing a major locality loophole, which allows for communication between photon detectors.

Through these rows, we could say that the paradigm of physics in the Nobel Prize had already shifted to support quantum physics as the NEW standard from Einstein's Relativity.

● Loophole-Free Bell Tests (2015)

Nitrogen-vacancy (NV) center in diamond
(source: NIST)

In 2015, 3 landmark experiments achieved loophole-free Bell tests for the first time. In order to close the detection loophole, all pairs of entangled electrons were measured without omission, while in order to close the locality loophole, the researchers ensured to keep a significant distance, simultaneously.

Delft University of Technology (TU Delft, the Netherlands) team led by Ronald Hanson

National Institute of Standards and Technology (NIST, the United States) team led by Saulius Valiulis

University of Vienna (Austria) team led by Anton Zeilinger et al.

By entangling nitrogen-vacancy (NV) centers in diamonds separated by 1.3 km, the physicists provided definitive evidence to confirm quantum non-locality and reject local realism.

● Long-Distance and Satellite Experiments (since the 2000s)

Long-distance and satellite-based experiments are of far bigger scales. The researchers utilize vast distances to distribute entangled photon pairs not only on the ground basis, but also expanding to space-to-ground, or free-space links, far exceeding fiber-optic limitations on the Earth.

Atmospheric Link Studies by the European Space Agency (ESA)

Canary Islands

In September 2005, the European Space Agency (ESA) conducted a landmark study involving a free-space atmospheric link across 144 km for quantum entanglement to aim at testing the viability of space-based quantum communications.

The experiment used ESA's Optical Ground Station (OGS) 1-meter telescope on Tenerife Island in the Canary Islands (Spain) as the receiver, and a specially built quantum optical terminal at the Roque de los Muchachos Observatory on the neighboring island of La Palma. The distance between Tenerife and La Palma, 144 km, created an atmospheric link, which simulated the conditions of a satellite-to-ground link.

The ESA team generated pairs of entangled photons using the Spontaneous Parametric Down-Conversion (SPDC) method on La Palma. This nonlinear instant optical process converts one high-energy pump photon into an entangled pair of low-energy signal and idler photons. And then they sent one of the entangled photons across 144 km to Tenerife Island, and kept the other in La Palma for comparison.

Their experiment proved that the entanglement remains intact over such a significant long distance, 144 km away. This had built a crucial step toward creating secure free-space, satellite-based Quantum Key Distribution (QKD).

The world of Quantum Mechanics is very deep, and the concepts, mathematical formulae, and terminology are unique and distinct from classical physics, so we need to dive deeper in other posts.

In my future posts, let's take a look further at how quantum entanglement is currently applied for our future technologies! Coming soon and stick around!

LAB

Tuesday, April 7, 2026

LAB Physics - file 04: To overtake Light – Relativity to Quantum