Time and space in Quantum gravity

In summary, the question of what spacetime is remains an open problem in theoretical physics. While loop quantum gravity and string theory attack the problem from different viewpoints, it is still unclear which approach will be the winner.
  • #1
Timeismatter
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0
In Einstein equation of general relativity, one side is stress-energy tensor and the other side is the Einstein tensor (functions of metric tensor). But the problem is in order to describe the matter field/stress-energy tensor, we have to use the space and time, which are determined by the metric tensor. So there is some circular reasoning here. Is the spacetime just some geometry or a real field carrying energy?
In the string theory, a string exists in a 26 or 11 or 10 dimensional spacetime. But could the spacetime exists as some kind of container for the string?
Is spacetime some kind of material or a priori notion of physics?
 
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  • #2
One should distinguish between spacetime topology and spacetime metric. The former may be defined even without the latter. Only the latter is explicitly dynamical in general relativity (GR), including the most of quantum versions of GR.
 
  • #3
Timeismatter said:
Is spacetime some kind of material or a priori notion of physics?

Time is ticks on the observer's clock.
 
  • #4
You may be interested to know that one of the endpoints (I hesitate to say conclusions) of the mini-program on singularity resolution at the Kavli Institute of Theoretical Physics (Jan. 2007) was that a new view of space-time is needed by both the Loop Quantum Gravity approach and the String Theory approach to understand what happens physically at or near black holes and/or the Big Bang.

In Thomas Thielmann's first seminar at the mini-program, he considers the problem of time to be 1. the lack of a canonical Hamiltonian in generally covarient theories, and 2. there is no time evolution of observables, resulting in a frozen picture.

More simply, I should say your question is an open problem of some importance in the theoretical physics community.

R
 
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  • #5
I don't think it is too unexpected/freak to discard the present concept of space and time. The question is what spacetime is. Could we think/experiment withouth the concept of spacetime? Loop quantum gravity attacks the problem mainly from GR viewpoint, while string theory attacks the problem from QM viewpoint. But I am not sure whether the last winner will be either of the two. Without attacking at the spacetime problem directly there will be no real results of the physics concerning the world.
Many results from the string theory are beautiful and have a clear brand of Ed Witten's influence. But I guess to distinguish physics and mathematics is a necessary job we need to do. Compared with the era of Newton, Newton needed to develop the necessary mathematics calculus to describe his mechanics. Einstein didn't need to develop Riemann geometry to write the equation of GR. Heisenberg didn't need to invent the matrix and spectral theory to develop the Matrix mechanics. But today we even don't know what kinds of basic mathematics tools we need. String theoreticians are just speculating on every kind of physics/mathematics.
 

1. What is the concept of time and space in quantum gravity?

In quantum gravity, time and space are considered to be emergent properties that arise from the underlying quantum structure of the universe. This means that they are not seen as fundamental concepts, but rather as approximate descriptions of the behavior of matter and energy at a microscopic level.

2. How does quantum gravity explain the relationship between time and space?

According to quantum gravity, time and space are intertwined and cannot be separated. This means that events that occur in space also have an effect on the flow of time, and vice versa. This is in contrast to classical physics, where time and space are treated as separate and independent entities.

3. What is the role of the uncertainty principle in quantum gravity?

The uncertainty principle, which states that the more precisely we know the position of a particle, the less precisely we can know its momentum, plays a crucial role in quantum gravity. This is because it limits our ability to measure both the position and momentum of particles at the quantum level, making it difficult to fully understand the behavior of matter and energy in the fabric of space-time.

4. How does quantum gravity affect our understanding of black holes?

In classical physics, black holes are described as objects with an infinite gravitational pull that suck in everything, including light. However, in quantum gravity, black holes are seen as complex systems that have a finite temperature and emit radiation, known as Hawking radiation. This challenges our traditional understanding of black holes and raises new questions about their behavior and properties.

5. Is it possible to reconcile quantum mechanics and general relativity in the context of quantum gravity?

This is one of the main goals of quantum gravity research. While quantum mechanics and general relativity are both successful theories in their respective domains, they have not yet been successfully unified to create a theory of quantum gravity. Scientists are continuously working to find a way to reconcile these two theories and create a unified framework that can explain the behavior of matter and energy at all scales.

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