The measurement of an entangled particle can have a profound and instantaneous effect on its entangled partner, regardless of the spatial separation between them. This phenomenon is known as quantum entanglement, and it is a fundamental aspect of quantum mechanics.
When two particles become entangled, their quantum states become correlated in a way that the state of one particle is intimately connected to the state of the other, even if they are physically far apart. This means that measuring one particle's properties will instantaneously determine the properties of the other, regardless of the distance between them.
For example, let's consider a pair of entangled particles with their spin states entangled. Spin is an intrinsic property of particles, and it can be either "up" or "down" in a given direction. When the particles are entangled, their spins become correlated, so if one particle's spin is measured and found to be "up" in a particular direction, the other particle's spin, when measured, will be found to be "down" in the same direction. The measurement outcome of one particle is instantly correlated with the measurement outcome of the other particle, even if they are far apart.
This instantaneous correlation between entangled particles has been experimentally verified in a phenomenon called "Bell's theorem" experiments. These experiments involve measuring correlated properties of entangled particles in different locations and have consistently shown results that violate classical notions of local realism.
It is important to note that the entanglement itself does not allow for faster-than-light communication or information transfer. Although the measurement of one particle instantaneously determines the state of the other, this does not enable sending information or messages faster than the speed of light. The entanglement is a correlation in the measurement outcomes, but it does not violate the principles of causality or allow for faster-than-light communication.