Experimental Evidence of Quantum Gravity?

In summary: Quantum gravity effects would appear under extreme conditions (where ordinary classical GR breaks down and develops "singularities") such as in connection with cosmological big bounce or black holes.Quantum gravity effects would also be detectable in the laboratory under extreme conditions such as high energy collisions or in the presence of strongly interacting particles.In summary, most of the research papers studying ways to test are about BB or BH, and most have to do with either the CMB (ancient light from early universe, showing fluctuations in density) or with radiation that might be detected coming from the evaporation of black holes. Some experimental proposals for detecting QG effects involve launching a spacecraft with the right instruments to look for a "foot
  • #1
ryan albery
67
1
The proportionality constant of big G (like Planck's constant) seems to suggest that gravity is a quantized 'thing'... does anyone know of any experiments (perhaps with a Watt or torsion balance) that show evidence of this?
 
Physics news on Phys.org
  • #2
Quantum gravity effects would appear under extreme conditions (where ordinary classical GR breaks down and develops "singularities") such as in connection with cosmological big bounce or black holes.

So most of the research papers studying ways to test are about BB or BH, and most have to do with either the CMB (ancient light from early universe, showing fluctuations in density) or with radiation that might be detected coming from the evaporation of black holes.

I can only toss out some links to give an idea of the kind of things being studied. I don't know of anybody who has the immediate direct ANSWER of how to test today's QG theories with today's technical means.
This is just to get the flavor. It is not something to try to understand in depth, all still preliminary and nowhere near ready for mass audience.

http://physics.republika.pl/PRD7.pdf (Physical Review D 2010)
Observing the big bounce with tensor modes in the cosmic microwave background:
Phenomenology and fundamental loop quantum cosmology parameters


Some slides for a conference talk by the same people--more visual and less texty:
http://indico.in2p3.fr/getFile.py/a...onId=13&resId=0&materialId=slides&confId=2779

According to one version of QG theory clouds of microscopic BH would exist and show characteristic radiation. The particular theory could be tested (and perhaps falsified) if one looks for that characteristic radiation (and perhaps does not find it.) Strange rather bold idea...FWIW here is the paper:

http://arxiv.org/abs/1202.0412
Emission spectra of self-dual black holes
Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 2 Feb 2012)
We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. ... In this limit, we then derive the number-density of the primary emission particles,... We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.
15 pages, 6 figures

Here's a more popularized account of some different proposals for testing Loop QG:

Physicists propose test for loop quantum gravity
www.physorg.com/pdf244805864.pdf
 
Last edited:
  • #3
G

Here's an earlier paper from Physical Review Letters 2009.

http://arXiv.org/abs/0902.0145
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Accepted by Physical Review Letters, 7 pages, 2 figures
(Submitted on 2 Feb 2009)
"The primordial spectrum of cosmological tensor perturbations is considered as a possible probe of quantum gravity effects. Together with string theory, loop quantum gravity is one of the most promising frameworks to study quantum effects in the early universe. We show that the associated holonomy correction should modify the potential seen by gravitational waves during the inflationary amplification. The resulting power spectrum should exhibit a characteristic tilt. This opens a new window for cosmological tests of quantum gravity."

This by Grain and Barrau is one of the most highly cited papers of this type. Since 2008 there have appeared are some 40 or 50 papers exploring various ways to test QG theory. I'll get a search link that can dig up some of them.
http://inspirehep.net/search?ln=en&...Search&sf=&so=d&rm=citation&rg=100&sc=0&of=hb

Many of the ideas here involve sending up a spacecraft with the right instruments to detect whatever "footprint" of QG they want to look for.

The technical specialty of figuring out how to test scientific theories is called phenomenology. What we are talking about are "QG Phenomenology" research papers. Designing tests that would show this or that theory is wrong or not wrong---observational tests that would favor or disfavor some QG theory compared with some other.

It's a small challenging research area that is getting started. Sabine Hossenfelder a QG phenomenologist in Stockholm, has been active in organizing a couple of conferences/workshops on it so far--getting the people who do this kind of work together to compare notes and share ideas.

Here is a write-up she did on a workshop on the Experimental Search for Quantum Gravity
http://arxiv.org/abs/1010.3420
It covers a whole range of ideas for testing various QG theories.
Maybe that link is the best way of answering your question.
 
Last edited:
  • #4
I can't say I fully understand the words and equations that these papers present, but thanks for the references and I will read them again, and probably again. It makes sense that we'd look for something so hardly measurable as the quantum of gravity in the CMB.

I'm guessing that the present LIGO configuration in Louisiana has too many seismic (and especially temperature) unknowns for us to be detecting a gravity wave... does the theory of a gravity wave also imply that gravity is a quantized 'thing'?
 
  • #5
ryan albery said:
I can't say I fully understand the words and equations that these papers present, but thanks for the references and I will read them again, and probably again. It makes sense that we'd look for something so hardly measurable as the quantum of gravity in the CMB.

I'm guessing that the present LIGO configuration in Louisiana has too many seismic (and especially temperature) unknowns for us to be detecting a gravity wave... does the theory of a gravity wave also imply that gravity is a quantized 'thing'?

There was just recently a video seminar talk by Raphael Porto about LIGO and what it might be able to detect. Years ago Raphael used to post here and I conversed with him. He's very bright and gives a really good slide presentation, fast talking and lots of visual information.
Google "pirsa porto" and get http://pirsa.org/12030088/

I can't speculate as to whether or not LIGO type instruments will be able to identify some kind of quantum effect. But you can listen to Rafael, or just glance at his slides PDF and see if anything grabs your interest.

A lot of people have the basic view that "gravity = geometry" so a gravity wave is a ripple in geometry itself. And quantum gravity effects should appear in the geometry under extreme conditions.
Fortunately early universe expansion was so extreme that the CMB is like looking at things under a huge microscope. Have to go. Supper.
 
Last edited:
  • #6
marcus said:
There was just recently a video seminar talk by Raphael Porto about LIGO and what it might be able to detect. Years ago Raphael used to post here and I conversed with him. He's very bright and gives a really good slide presentation, fast talking and lots of visual information.
Google "pirsa porto" and get http://pirsa.org/12030088/

I can't speculate as to whether or not LIGO type instruments will be able to identify some kind of quantum effect. But you can listen to Rafael, or just glance at his slides PDF and see if anything grabs your interest.

Terrific talk. Nothing about quantum gravity, but lots of stuff I'm interested in. I somehow thought LIGO got killed off in funding? Seems not, which is nice.
 
  • #7
ryan albery said:
... does the theory of a gravity wave also imply that gravity is a quantized 'thing'?

No. Gravity waves exist in a purely classical (i.e. non-quantum) theory of gravity.
 
  • #8
atyy said:
Terrific talk. Nothing about quantum gravity, but lots of stuff I'm interested in. I somehow thought LIGO got killed off in funding? Seems not, which is nice.

As regards funding trouble, we've been hearing more about LISA
http://lisa.nasa.gov/mission/ [Broken]
Laser interferometer SPACE antenna. The idea was an equilateral triangle orbiting the sun (side = 5 million km)

"Due to NASA budget outlook and to the astro2010 decadal rankings, in April 2011 NASA and ESA ended their ten-year-long LISA partnership. As a consequence:

ESA is now developing a redesigned European-only gravitational-wave mission, NGO. A new European science team already completed a science performance study, and an industrial study is currently in progress. ESA will decide next year if NGO will go forward as a Cosmic Vision L-class mission, in which case NASA may participate as a minor partner."
 
Last edited by a moderator:

1. What is quantum gravity?

Quantum gravity is a theory that attempts to reconcile the principles of quantum mechanics, which explain the behavior of subatomic particles, with the principles of general relativity, which explain the behavior of objects on a larger scale. It is a potential solution to the problem of how gravity can be explained in the context of quantum mechanics.

2. What is the experimental evidence for quantum gravity?

Currently, there is no direct experimental evidence for quantum gravity. However, there are several experiments and observations that support its existence, such as the behavior of black holes and the cosmic microwave background radiation. Additionally, ongoing research in areas such as quantum entanglement and gravitational waves may provide further evidence in the future.

3. How do scientists test for quantum gravity?

One of the main ways scientists test for quantum gravity is through mathematical models and simulations. These models allow them to predict the behavior of particles and objects under the influence of both quantum mechanics and general relativity. Other methods include studying the effects of gravity on subatomic particles and measuring the effects of gravitational waves.

4. What are the challenges in finding experimental evidence for quantum gravity?

One of the main challenges in finding experimental evidence for quantum gravity is the fact that it occurs on a very small scale, making it extremely difficult to observe and measure. Additionally, the principles of quantum gravity may contradict our current understanding of physics, making it difficult to design experiments that can accurately test for its existence.

5. How important is finding experimental evidence for quantum gravity?

Finding experimental evidence for quantum gravity is crucial for our understanding of the universe and the fundamental laws that govern it. It would also have significant implications for fields such as astrophysics and cosmology. Additionally, it could lead to new technologies and advancements in science and technology.

Similar threads

  • Beyond the Standard Models
Replies
13
Views
1K
Replies
13
Views
2K
  • Beyond the Standard Models
Replies
7
Views
1K
  • Beyond the Standard Models
Replies
6
Views
588
  • Beyond the Standard Models
Replies
5
Views
2K
  • Beyond the Standard Models
4
Replies
105
Views
10K
  • Beyond the Standard Models
Replies
24
Views
3K
Replies
2
Views
2K
  • Beyond the Standard Models
Replies
5
Views
739
Back
Top