Exploring the Relationship between QFT, GR, and Backreaction in Curved Spacetime

  • B
  • Thread starter star apple
  • Start date
  • Tags
    Gr Qft
In summary, the conversation discusses the concept of spacetime as a fluid, which could potentially solve the problem of quantizing gravity. The question then arises if this would still be compatible with the back reaction approximation used in QFT in curved spacetime. The answer is not clear, but the idea of spacetime being made up of smaller things is one way to approach the problem. The conversation also touches on the idea of scientific evidence and the limitations of mathematical evidence.
  • #1
star apple
I know we do not have a version of QFT (?) in which we can dynamically solve for the QFT and the background spacetime at once. What we can do is where if we come up with a QFT whose expectation value of the stress-energy tensor doesn't match the fixed background spacetime geometry via the Einstein Field Equation, we go back and try again taking or taking *back reaction* into account.

The question I’d like to ask is. Is this normal due to simply incomplete QFT tool.. or is it really necessary to quantize spacetime--or at least to build a quantum theory of something whose classical limit looks like spacetime?

In our daily life. We think Schroedinger Equation is enough in our biochemistry and we walk visualizing Galilean space. Sometimes we may think of QFT a bit as we navigate in minkowski spacetime. But I want to get used to think in terms of QFT in curved spacetime as this is the more complete mode. Therefore I’d like to know if the backreaction thing described above can exist by its own (meaning can be complete itself) without quantizing spacetime or create something where the classical limit looks like spacetime.
 
Physics news on Phys.org
  • #2
In other words, if something is approximate.. does it always mean there is another more complete theory lurking around.. like in our QFT using perturbation approximation.. does it mean it's a sign of incomplete theory? Isn't there anything in physics in which approximation is the final thing and nothing more to it? In regards QFT and General Relativity.. I guess the incompatibility is more than having to take backreaction into account.. or more about the infinities.. how is this related to the problem of backreaction.. are they separate problems?
 
  • #3
Let me rephrase my question so it makes more sense.

Bee said that http://backreaction.blogspot.com/2017/10/space-may-not-be-as-immaterial-as-we.html

"Physicists have gathered evidence that space-time can behave like a fluid. Mathematical evidence, that is, but still evidence. If this relation isn’t a coincidence, then space-time – like a fluid – may have substructure...
<bypassing several paragraphs later>
"If space-time is made of smaller things, this could solve a major problem: How to describe the quantum behavior of space time. Unlike all the other interactions we know of, gravity is a non-quantum theory. This means it doesn’t fit together with the quantum theories that physicists use for elementary particles. All attempts to quantize gravity so-far have either failed or remained unconfirmed speculations. That space itself isn’t fundamental but made of other things is one way to approach the problem."

Of course spacetime being not fundamental makes more sense that its being fundamental. It's like a stone age man asking if a pixel in a computer screen is infinitely small.

But my question is. If spacetime was not fundamental and made up of smaller things. Would the back reaction approximation used by QFT in curved spacetime still valid?

(A Yes or No answer is enough.. lol.. thanks)
 
  • #4
Small nit, but this statement makes no sense: "Mathematical evidence, that is, but still evidence." I can come up with mathematical "evidence" for anything. Scientific evidence is a whole other matter.
 
  • #5
bapowell said:
Small nit, but this statement makes no sense: "Mathematical evidence, that is, but still evidence." I can come up with mathematical "evidence" for anything. Scientific evidence is a whole other matter.

Thanks for pointing that out. Maybe she meant mathematicaly consistent.
 
Last edited by a moderator:

FAQ: Exploring the Relationship between QFT, GR, and Backreaction in Curved Spacetime

What is the relationship between QFT, GR, and backreaction in curved spacetime?

The relationship between quantum field theory (QFT), general relativity (GR), and backreaction in curved spacetime is a complex and ongoing area of research in theoretical physics. QFT is a framework for describing the behavior of subatomic particles and their interactions, while GR is a theory of gravity that describes the curvature of spacetime in the presence of massive objects. Backreaction refers to the effect of the quantum fluctuations of matter and energy on the curvature of spacetime, and how this feedback loop may impact the behavior of both QFT and GR.

Why is it important to explore the relationship between QFT, GR, and backreaction in curved spacetime?

The study of this relationship is important because it can help us better understand the fundamental laws of nature and potentially reconcile the two currently incompatible theories of QFT and GR. It can also provide insights into the behavior of the universe at the smallest scales and during the early stages of its evolution, which may have implications for our understanding of the origins of the universe.

What are some current theories or models that attempt to explain the relationship between QFT, GR, and backreaction in curved spacetime?

There are various theories and models that attempt to explain this relationship, such as semiclassical gravity, where quantum effects are taken into account in a classical spacetime, and quantum gravity, which seeks to unify QFT and GR into a single framework. Other approaches include loop quantum gravity, string theory, and black hole thermodynamics.

What are some challenges or obstacles in exploring the relationship between QFT, GR, and backreaction in curved spacetime?

One of the main challenges is the difficulty in combining the two vastly different theories of QFT and GR into a coherent framework. Another challenge is the lack of experimental evidence to test these theories, as the effects of backreaction are extremely small and difficult to measure. Additionally, the extreme conditions of curved spacetime, such as those near black holes, make it challenging to apply these theories and make accurate predictions.

How might a better understanding of the relationship between QFT, GR, and backreaction in curved spacetime impact our daily lives?

While the potential applications of this research are not immediately apparent in our daily lives, a deeper understanding of the laws of nature can have far-reaching impacts on technology and society. For example, a unified theory of QFT and GR may lead to breakthroughs in fields such as quantum computing, space exploration, and energy generation. Additionally, insights into the behavior of the early universe could have implications for our understanding of the origins of life and the universe itself.

Similar threads

A Approaches towards quantum gravity
Replies
3
Views
2K
I Polarization mode symmetries of massless particles
Replies
1
Views
1K
A "Monstrous Higgs and Leech Spacetime"
Replies
0
Views
2K
CC Problem Beyond the Standard Model
Replies
1
Views
1K
A Will QG leave GR unchanged or QT?
2
Replies
60
Views
6K
Studying How to learn QFT on curved spacetimes by myself?
Replies
1
Views
1K
A How to construct a spin-3/2 theory from the ground up
Replies
2
Views
5K
A A Proposed Black Hole Entropy Calculation
Replies
21
Views
4K
B Solving Ambiguity in QFT in Curved Spacetime
Replies
6
Views
1K
Exploring String Theory on Curved Spacetime: How Does it Explain QFT?
Replies
12
Views
3K

Hot Threads

Recent Insights

Back
Top