What is the space time
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Understanding Space-Time: Concepts and Theories
The Block Universe and Evolving Block Universe
The Block Universe concept suggests that spacetime is a fixed, unchanging entity where the flow of time is an illusion. This view is based on time-reversible microphysical laws, which do not account for the time-irreversible macro-physical behaviors and the development of complex systems like life. To address these limitations, the Evolving Block Universe model proposes that spacetime itself evolves, with the future continually becoming the past. This evolution is not tied to any preferred surfaces in spacetime but is associated with the evolution of proper time along families of world lines1.
Causal Set Theory
Causal Set Theory posits that at the smallest scales, spacetime is a causal set—a locally finite set of elements with a partial order that defines past and future. This theory suggests that a Lorentzian manifold can approximate a causal set, and the effective dimensionality of a causal set can vary with length scale. The quantum dynamics of causal sets may reproduce general relativity in the classical limit, providing a bridge between quantum mechanics and general relativity2 5.
Space-Time Singularities
Space-time singularities are points where the curvature of spacetime becomes infinite. Under certain conditions, such as strong causality and genericness, future timelike or null geodesically incomplete spacetimes contain either curvature or intermediate singularities, or primordial singularities. These singularities mark the boundaries of spacetime where the known laws of physics break down3.
Quantum States Across Space-Time
In quantum mechanics, time is often seen as a marker of evolution, while in general relativity, spacetime is curved around mass. A unified approach in quantum theory constructs quantum states across spacetime rather than on spatial slices. This approach treats different instances of time as different quantum modes, leading to new insights into the similarities and differences between spatial and temporal correlations4.
Quantum Gravity and Non-Spatiotemporal Structures
Theories of quantum gravity often suggest that the underlying reality of relativistic spacetimes is non-spatiotemporal. To maintain empirical coherence, these theories must show how relativistic spacetime emerges from non-spatiotemporal structures. This involves recovering the functional roles of spacetime features that are relevant for producing empirical evidence. Causal Set Theory and Loop Quantum Gravity are two prominent approaches that illustrate this idea5 7.
Spacetime in String Theory
String theory offers a radically different view of spacetime compared to Einstein's relativity. At a perturbative level, the spacetime metric appears as coupling constants in a two-dimensional quantum field theory. Nonperturbatively, spacetime is not fundamental but must be reconstructed from a holographic, dual theory. This perspective challenges the traditional notion of spacetime as a continuous entity6.
Philosophical Perspectives on Space and Time
Since the 1950s, the assumption of a spacetime continuum has been questioned, leading to the development of alternative models. These models are supported by a confluence of philosophical ideology, mathematical innovation, and experimental possibilities. The distinction between space and time, often taken for granted, is rooted in the standard presentation of general relativity, where the metric of spacetime is locally Lorentzian. However, some argue that the existence of time should be deduced from gravitational theory rather than enforced upon it8 9.
Conclusion
The concept of spacetime is central to our understanding of the universe, yet it remains one of the most mysterious aspects of physics. From the Block Universe and Evolving Block Universe models to Causal Set Theory and the implications of quantum gravity, various theories offer different perspectives on the nature of spacetime. These insights not only challenge our traditional views but also pave the way for new understandings of the universe's fundamental structure.
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Most relevant research papers on this topic
Physics in the real universe: time and spacetime
The Block Universe view of spacetime is best replaced by an evolving block universe, encompassing time-irreversible macro-physical behavior and the development of complex systems, including life.
Highly CitedSpace-time as a causal set.
Space-time at smallest scales is a causal set, with a partial order defining past and future, and a Lorentzian manifold can approximate it, with effective dimensionality varying with length scale.
Highly CitedSpace-time singularities
Future timelike or null geodesically incomplete space-times can contain either curvature or intermediate singularities, or primordial singularities, based on strong causality and genericness assumptions.
Different instances of time as different quantum modes: quantum states across space-time for continuous variables
This paper proposes a unified approach to space and time in quantum theory, treating different instances of time as different quantum modes, leading to desirable properties and highlighting the differences and similarities between spatial and temporal correlations.
Spacetime is as spacetime does
Quantum gravity theories must establish how relativistic spacetime emerges from non-spatiotemporal structures, focusing on functionally relevant features for empirical evidence.
Highly Cited