There Is Something Faster Than Light

In 1935, Einstein presented a thought experiment showing that quantum mechanics breaks the principle that nothing can travel faster than light. Physicists initially dismissed him as an old man past his prime unable to accept radical new physics.

30 years later, one physicist stumbled upon Einstein's forgotten paper and realized the prediction could actually be tested experimentally. When scientists ran the experiment, they were shocked to find quantum physics really does violate the universal speed limit.

The experiment reveals that something like actions going faster than light from one place to another must occur in quantum mechanics, contradicting relativity's fundamental postulate.

This is about one of the spookiest and most misunderstood experiments in physics, and may be the strongest evidence that we live in many worlds.

Newton's theory predicted that gravity acts instantly across any distance, so if the sun disappeared, we would feel the gravitational change immediately. Newton himself was deeply disturbed by this concept, calling it 'a great absurdity.'

In 1905, Einstein realized that instantaneous action at a distance doesn't just seem absurd—it leads to outright paradoxes when combined with relativity.

If observers moving at different speeds can disagree about when events happen, then one observer sees the sun disappearing and Earth flying off simultaneously, while another observer sees Earth flying off before the sun disappears—reversing cause and effect.

Einstein spent 10 years fixing this issue, completely overhauling our understanding of gravity by showing that gravity causes spacetime to bend, with effects spreading at the speed of light rather than being instant.

Gravity is caused by the bending of spacetime. When there's a change in gravity, it only affects local spacetime, and that ripple spreads to nearby regions at the speed of light.

This theory of gravity is local because effects spread from place to place at the speed of light instead of being instant. All observers agree on the order of events because of the delay between cause and effect.

The delay between cause and effect ensures all observers agree on the order, which is why nothing can go faster than light. This universal rule protected all of classical physics.

After fixing gravity, Einstein studied the new theory of quantum mechanics and made a terrible discovery—it violated this crucial locality principle.

At the 1927 Solvay Conference, around 60% of the attendees would win Nobel prizes. Einstein thought the architects of quantum theory had gotten something fundamentally wrong and presented a thought experiment to prove it.

Einstein imagined firing a single electron through a narrow slit toward a detection screen. Quantum mechanics says the electron has a wave function that spreads through space, and when detected at one point, the wave function collapses to zero everywhere else instantly.

The measurement at one location must instantly affect the wave function at other locations, no matter how far apart. This violates locality and contradicts relativity's postulate that nothing travels faster than light.

Einstein's argument was so simple and his talk so short that people didn't know what to make of it. Even Niels Bohr, the most influential figure in quantum physics, admitted he didn't understand what point Einstein wanted to make.

Niels Bohr's Institute in Copenhagen became the hub for the new field of quantum physics. Bohr didn't write the mathematical rules but instead told everyone what they meant through his Copenhagen interpretation.

• The wave function describes everything you can know about a particle or system • It evolves according to the Schrödinger equation • At measurement, the wave function collapses • Physics only needs to predict measurements in the lab, not explain what's really happening

Bohr argued it's wrong to think physics should find out how nature really is. The job of physics is just to predict measurements, which quantum mechanics does incredibly well. What the electron is doing when not observed is not a valid question.

Einstein couldn't stand the Copenhagen interpretation, calling it a tranquilizing philosophy or religion. He believed the wave function collapse mechanism revealed a critical weakness and that a local hidden variable theory must eventually replace quantum mechanics.

Einstein, with colleagues Boris Podolsky and Nathan Rosen, imagined a high-energy photon becoming two particles: an electron and positron. Both particles have spin properties that must conserve total spin, but the electron can spin in multiple directions until measured.

When the electron is measured and determined to have one spin direction, the positron must instantly have the opposite spin to conserve spin. This interdependence of particle states is entanglement, and measurement of one particle instantly determines the other's state.

The authors realized that when the electron is measured, the positron's wave function must instantly collapse to the opposite state, even if far away. This requires a message traveling faster than light to tell the distant particle how to collapse—violating relativity.

The EPR paper proved that the only local alternative explanation is that particles carry hidden variables assigned when still together. These hidden variables contain information about how particles will respond to any measurement, eliminating the need for faster-than-light communication.

The physics community's response to the EPR paper was mixed. Some older allies of Einstein supported it, while others like Bohr rushed to defend quantum mechanics. Bohr's response was famously obscure and difficult to understand, with some believing it either nonsensical or containing actual mistakes.

Both the Copenhagen interpretation and Einstein's local hidden variable theory made exactly the same predictions about experimental outcomes. Since they were empirically identical, physicists considered debating different interpretations to be armchair philosophy rather than real physics.

The general attitude became that the debate was settled in Bohr's favor and didn't matter. The Copenhagen interpretation made good predictions, so why debate interpretations? The field moved forward with 'shut up and calculate' as the dominant approach.

Einstein died still questioning quantum mechanics, but the majority of the physics community had moved on without him. Bohr later lamented that Einstein wasted decades on fruitless thought experiments because he could not accept quantum mechanics.

John Bell was an undergraduate student after World War II, taught the Copenhagen Interpretation. From his first quantum mechanics class, he was deeply skeptical, getting upset with instructors and asking what measurement really means.

Bell was discouraged from studying quantum foundations and instead pursued nuclear physics, having an accomplished career at CERN. After eight years in particle physics, in 1963 he took an academic sabbatical to finally focus on his doubts about quantum mechanics.

By re-examining old debates and dismissed papers like EPR, Bell realized Einstein's logic was sound: either quantum mechanics is non-local or there must be local hidden variables. The crucial question was whether an experiment could prove which was true.

By Bell's time, creating entanglement in the lab had become feasible, unlike in 1935. Scientists like Madam Wu had reproduced the EPR experiment as a real experiment, though simply doing EPR in reality isn't enough since both theories predict the same results.

Bell wondered if there was a version of the EPR experiment where non-local and local theories would give different results. The key innovation: experimenters can choose independently how to orient their Stern-Gerlach machines among three different angles: zero, 120, and 240 degrees.

When experimenters choose different measurement axes, quantum mechanics predicts a disagreement rate of 25%. The geometry ensures that any two different axes produce the same 25% disagreement rate.

Local hidden variable theory requires particles to pre-assign responses to all three possible questions before separation. The best strategy particles can use results in a disagreement rate of at least 33% when different axes are chosen—higher than quantum mechanics' 25%.

Non-local quantum mechanics predicts 25% disagreement when different axes are measured, while local hidden variables predict at least 33%. This measurable difference makes the theories empirically distinguishable for the first time.

Alain Aspect performed experiments at the Institut d'Optique forty years ago to measure Bell's inequalities. The experiment used entangled photons created by a pair of crystals, with measurements performed on separate arms by rotating polarizers.

• Create entangled particle pairs • Start with both measurements in the same direction to establish baseline disagreement rate • Rotate one axis and measure new disagreement rate • Calculate disagreement percentage to test against theoretical predictions

The experiments showed disagreement rates matching quantum mechanics' 25% prediction, not the 33% or higher predicted by local hidden variable theory. This ruled out local hidden variables and confirmed that quantum mechanics is fundamentally non-local.

Bell expected quantum mechanics to be correct because who would bet against the most successful theory in physics? He said it would be very hard for him to doubt the outcome of such experiments, recognizing the strength of quantum mechanics despite its non-locality.

Physics textbooks and papers widely claim that Bell's theorem rules out local hidden variables or local realism. This is presented as the definitive conclusion of Bell's work.

Bell himself said this interpretation was an error. He remarked it's quite remarkable how many people make this mistake about what his theorem actually proves.

• Assume locality for entangled particles • EPR showed the only way for local coordination is through hidden variables • Bell proved local hidden variables predict incorrect experimental results • Therefore, the assumption of locality must be wrong

Bell's theorem proves that any theory correctly describing the experiment must be non-local. It doesn't rule out hidden variables—it proves that if hidden variables exist, they must operate non-locally, like quantum mechanics itself.

Einstein's main concern from the beginning was about locality. He discovered two of the most important aspects of quantum mechanics: entanglement and non-locality.

Bell showed that all the concerns Einstein raised about locality were completely justified. Einstein was right to be worried about non-locality; it's a real problem in quantum mechanics.

People often claim Einstein's problem was that he couldn't accept quantum mechanics. But it was only because he refused to 'shut up and calculate' that he discovered these fundamental aspects of quantum mechanics that others had overlooked.

The debate between Einstein and Bohr was about whether there was a genuine problem to be concerned about. Bell proved that there genuinely is a problem—non-locality is unavoidable in any quantum mechanical theory.

If particles are really acting non-locally, this should cause paradoxes. Different observers disagree about which measurement caused the collapse of the entangled state, since relativity says observers in different frames see events in different orders.

One observer sees you measure first, then your friend, thinking your measurement caused the collapse. Another observer sees it reversed. But both viewpoints are equally valid in relativity.

Quantum mechanics sidesteps these paradoxes through a fundamental constraint: outcomes are random. You can't send messages faster than light because measurement results are completely random, even though they're correlated with entangled particles.

Quantum mechanics is non-local but doesn't lead to catastrophic paradoxes like relativity would. However, it violates the spirit of relativity even though it doesn't break the letter of the law—it's an uneasy truce between theories.

Despite Bell's theorem, Bell himself championed alternative interpretations like pilot wave theory or Bohmian mechanics. Bell's theorem doesn't rule out this interpretation because it's also non-local, making it compatible with experimental results.

In many worlds, measurement doesn't collapse the wave function. Instead, both outcomes happen with you becoming entangled with the particle, existing in two parallel versions of yourself seeing each outcome.

Many worlds avoids the faster-than-light communication problem because there's no collapse to communicate. The positron doesn't need to know what the electron measured since both versions of both particles already exist with the correct correlations.

In many worlds, Bell's proof doesn't technically apply because it assumes all measurements have one outcome. Many worlds has multiple outcomes, so the logical chain of Bell's theorem breaks down in this interpretation.

Many worlds is not local in one sense because entangled particles separated by huge distances still share state. However, it is local in the crucial sense that far-away entangled particles don't influence each other faster than light.

Many worlds obeys Einstein's universal speed limit. Unlike Copenhagen quantum mechanics where measurement collapse seems to travel faster than light, many worlds avoids this problem entirely.

• Copenhagen's definition of measurement always feels arbitrary—when does a quantum system become observed? • In many worlds, entanglement between quantum particles and larger systems happens consistently • The framework feels more logically consistent without drawing an artificial line at measurement • No need to invoke collapse as a special mechanism

The biggest problem with many worlds is dealing with the infinity it brings forth—accepting parallel universes for every quantum event seems counterintuitive, though difficulty imagining something doesn't prove it's not happening.

If many worlds is correct, everything changes. The conflict between quantum mechanics and relativity vanishes, as non-locality is only a feature of Copenhagen, not of many worlds.

Physicists have struggled for decades to unite quantum mechanics with general relativity to build a theory of quantum gravity. Perhaps this failure stems from trying to marry relativity to a non-local theory.

If quantum mechanics ultimately turns out to be local through many worlds, then Einstein's dream of a local description of reality might not be dead after all. A local quantum theory could finally marry smoothly with relativistic principles.

Bell showed that mere thought experiments and armchair philosophy can have real consequences in physics. His work made studying the meaning of quantum mechanics respectable again and revealed that how we interpret quantum mechanics deeply matters.