At a basic level, the main contradiction between general relativity (GR) and quantum physics arises from their differing mathematical frameworks and conceptual foundations. Here are a few key points of tension:
Scale and Gravity: General relativity describes gravity as the curvature of spacetime caused by massive objects. It provides a classical, continuous description of gravity. On the other hand, quantum physics deals with the behavior of particles and fields on very small scales, where discrete units or quantization becomes important. Combining gravity with quantum physics requires a theory of quantum gravity, which is currently an active area of research.
Singularities: General relativity predicts the existence of singularities, such as black holes or the Big Bang, where the curvature of spacetime becomes infinitely large. These singularities are regions where our current understanding of physics breaks down. Understanding the behavior of matter and spacetime at these singularities requires a theory that incorporates both quantum physics and gravity.
The Measurement Problem: Quantum mechanics introduces the measurement problem, which raises questions about how observations and measurements affect the behavior of quantum systems. Quantum mechanics provides probabilistic predictions for the outcomes of measurements, but it does not specify how the measurement process itself occurs. This is known as the measurement problem, and there is ongoing debate and exploration regarding its resolution.
Renormalization and Infinities: In quantum field theory, calculations involving interactions between particles often lead to infinite results. These infinities arise in the context of processes involving quantum fluctuations and need to be removed through a technique called renormalization. However, when attempting to combine quantum field theory with general relativity, these renormalization techniques break down, leading to challenges in reconciling the two theories.
It's important to note that resolving these contradictions is an active area of research, and several approaches, such as string theory, loop quantum gravity, and other quantum gravity models, aim to provide a consistent framework that unifies general relativity and quantum physics. However, a definitive resolution is still a subject of ongoing scientific exploration.