People felt that to know what light is, they had to be able to visualize it. In other words, they had to be able to point to something in the macroscopic world, and say, "It is like that." In their attempt to do this, they ended up pointing to two things, waves and particles. In reality, light isn't like anything in the macroscopic world. People say light is both a particle and a wave. Light is neither a particle nor a wave. It's a subatomic entity in no way similar to anything in the macroscopic environment. Many people say, "light is a subatomic entity that under some conditions manifests itself mathematically as a particle and under other conditions manifests itself mathematically as a wave." This is true in a limited sense. When you say "manifests itself mathematically" you're referring to our model of light. Within our model, light is both a particle and a wave. When you say "light is a subatomic entity" you're referring to light as it really exists. You have to differentiate between our model, which was invented by humans, and actual reality. Whenever anyone talks about light being two things at once, they're talking about the model. In reality, light is not two things at once. It's one thing unlike anything in our daily lives or the macroscopic world. It's ironic that people who mistake our model for reality say that you can't visualize light as either a wave or a particle, and yet cling tenaciously to the right to visualize light as a particle and wave superimposed on top of each other like a photographic double exposure, or something similar. When people do this, they are trying to force the subatomic world to be similar to the macroscopic world. Even though I've used photons as an example here, this is of course true for all particles, quarks, leptons, bosons, etc. Richard Feynman said, "If you get rid of all these old-fashioned ideas, there is no need for an uncertainty principle!"
Does this insight shed any light on the electron going through the double slit experiment? No it doesn't. The electron is a subatomic entity that goes through the barrier with two slits and hits the screen behind. We put a transparent electron detector in front of each slit. One detector registers something and the other does not. We still have no idea why the one that registered something registered something, and the one that registered nothing registered nothing, instead of the other way around. Our only attempts to explain it are the models I have described. Who knows what's really true, or what models will exist in the future, but currently these models are all we have. We try to think up explanations for what we observe, so all we have are the observations, and our attempts at explanations.
In the early 20th century, quantum mechanics emerged with strange predictions: particles could be in superpositions, entangled with distant particles, and lack definite properties until measured. Many physicists hoped these oddities were just signs of an incomplete theory, that beneath quantum mechanics, a more "realistic" and "local" theory existed.
Locality means that objects are only influenced by their immediate surroundings, and that no information or effect can travel faster than the speed of light.
Realism holds that physical properties exist with well-defined values prior to and independent of measurement. In 1964, John Bell demonstrated that these assumptions, local realism, impose strict constraints on the outcomes of certain quantum experiments. These constraints are expressed as Bell Inequalities.
According to any local hidden variable theory (i.e., a theory that retains locality and realism), measurements on entangled particles must obey these inequalities.
But quantum mechanics predicts correlations that violate Bell inequalities. These correlations arise from entangled quantum states, where the measurement outcome of one particle is statistically linked to the other, even across vast distances.
Alain Aspect (1981 - 1982): Performed a series of experiments using entangled photon pairs. By switching measurement settings while photons were in flight, Aspect addressed the concern that information about the measurement setting might influence the source. His results clearly violated Bell inequalities.
Anton Zeilinger (1998): Pushed the boundaries by increasing the distance between entangled particles and improving detector efficiencies. This reduced the possibility of experimental loopholes, reinforcing that quantum mechanics predictions hold true.
Delft University (Netherlands): Used electron spins in diamonds and ensured space-like separation between measurement events.
NIST (USA): Used entangled photons with very high detector efficiency.
Vienna Group: Added randomness and fast switching to prevent signaling.
All these confirmed the violation of the Bell Inequalities while rigorously closing major loopholes:
1. the locality loophole (ensuring spatial separation) and
2. the detection loophole (ensuring all relevant outcomes are measured).
These results rule out any theory that preserves both locality and realism. At least one, or both, must be abandoned. If you keep locality, then you must accept nonrealism: the properties of particles are not determined until they are measured. If you keep realism, then you must accept non-locality: distant events can influence each other instantaneously. Some interpretations (like Bohmian mechanics) accept non-locality to preserve realism. Others (like Copenhagen) reject realism altogether. No consensus exists but the mathematical predictions of quantum theory are correct. Importantly, these violations do not imply that information travels faster than light. The correlations observed are strong, but do not allow controllable signaling, preserving consistency with special relativity. Bell's Theorem is not just a test of quantum mechanics. It's a test of how the world fundamentally works. It tells us that the Universe does not obey classical principles both locality and realism together.
There is a popular misunderstanding that consciousness is required for wavefunction collapse.
1. DOUBLE-SLIT EXPERIMENT BASICS :
When particles (like electrons or photons) pass through two slits without any measurement, they form an interference pattern—a wave-like behavior.
When which-path information is measured, the interference pattern disappears, and the particles behave like particles—suggesting "collapse" of the wave function.
2. QUANTUM ERASER VARIANTS :
In some experiments (like the quantum eraser), information about which-path the particle took is recorded but not accessed, or is scrambled.
In these setups, the interference pattern can reappear if the which-path info is truly unavailable, suggesting the availability of information affects the outcome. Some people misunderstand and think that consciousness causes collapse.
1. INFORMATION AVAILABILITY VS. CONSCIOUSNESS :
The experiments show that when which-path info is in principle unknowable, the system behaves like a wave. But this doesn’t mean a conscious mind must be involved. It suggests the physical availability of information, not an observer, is what matters.
2. NO-SIGNALING PRINCIPLE :
In quantum mechanics, information cannot be transmitted faster than light, and the outcome of a measurement does not depend on whether someone consciously observes it later. The idea that "consciousness collapses the wavefunction" has been proposed (e.g., by Wigner), but it is not experimentally proven.
3. ALTERNATIVE INTERPRETATIONS :
Decoherence explains wavefunction collapse as the system interacting with the environment, not any consciousness. Many-Worlds Interpretation says no collapse occurs, just branching realities. Objective Collapse Models like GRW add a spontaneous collapse mechanism.
It is not consciousness per se that causes wavefunction collapse, based on current evidence. Availability of information, not necessarily conscious observation, appears more relevant.