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Francesco Biscani

The End Of Gravity As a Fundamental Force - 6 views

science.slashdot.org/...Gravity-As-a-Fundamental-Force

phy

shared by Francesco Biscani on 11 Jan 10 - Cached
LeopoldS liked it
  • Francesco Biscani on 11 Jan 10
     
    "At a symposium at the Dutch Spinoza-instituut on 8 December, 2009, string theorist Erik Verlinde introduced a theory that derives Newton's classical mechanics. In his theory, gravity exists because of a difference in concentration of information in the empty space between two masses and its surroundings. He does not consider gravity as fundamental, but as an emergent phenomenon that arises from a deeper microscropic reality. A relativistic extension of his argument leads directly to Einstein's equations."
  • ...8 more comments...
  • Dario Izzo on 12 Jan 10
     
    Diffcult for me to fully understand / believe in the holographic principle at macroscopical scales ... potentially it looks though as a revolutionary idea.....
  • pacome delva on 13 Jan 10
     
    never heard about it... seems interesting. At first sight it seems that it is based on fundamental principle that could lead to a new phenomenology, so that could be tested. Perhaps Luzi knows more about this ? Did we ever work on this concept ?
  • Luzi Bergamin on 13 Jan 10
     
    The paper is quite long and I don't have the time right now to read it in detail. Just a few comments: * We (ACT) definitely never did anything in this direction? But: is there a new phenomenology? I'm not sure, if the aim is just to get Einstein's theory as emergent theory, then GR should not change (or only change in extreme conditions.) * Emergent gravity is not new, also Erik admits that. The claim to have found a solution appears quite frequently, but most proposals actually are not emergent at all. At least, I have the impression that Erik is aware of the relevant steps to be performed. * It's very difficult to judge from a short glance at the paper, up to which point the claims are serious and where it just starts to be advertisments. Section 6 is pretty much a collection of self-praise. * Most importantly: I don't understand how exactly space and time should be emergent. I think it's not new to observe that space is related to special canonical variables in thermodynamics. If anybody can see anything "emergent" in the first paragraphs of section 3, then please explain me. For me, this is not emergent space, but space introduced with a "sledge hammer." Time anyway seems to be a precondition, else there is nothing like energy and nothing like dynamics. * Finally, holography appears to be a precondition, to my knowledge no proof exists that normal (non-supersymmetric, non-stringy, non-whatever) GR has a holographic dual.
  • Luzi Bergamin on 13 Jan 10
     
    Update: meanwhile I understood roughly what this should be about. It's well known that BH physics follow the laws of theormodynamics, suggesting the existence of underlying microstates. But if this is true, shouldn't the gravitational force then be emergent from these microstates in the same way as any theromdynamical effect is emergent from the behavior of its constituents (e.g. a gas)? If this can be prooven, then indeed gravity is emergent. Problem: one has to proof that *any* configuration in GR may be interpreted as thermodynamical, not just BHs. That's probably where holography comes into the play. To me this smells pretty much like N=4 SYM vs. QCD. The former is not QCD, but can be solved, so all stringy people study just that one and claim to learn something about QCD. Here, we look at holographic models, GR is not holographic, but who cares... Engineering problems...
  • LeopoldS on 13 Jan 10
     
    is there any experimental or observational evidence that points to this "solution"?
  • Luzi Bergamin on 13 Jan 10
     
    Are you joking??? :D
  • pacome delva on 14 Jan 10
     
    I was a bit fast to say it could be tested... apparently we don't even know a theory that is holographic, perhaps a string theory (see http://arxiv.org/abs/hep-th/9409089v2). So very far from any test...
  • LeopoldS on 14 Jan 10
     
    Luzi, I miss you!!!
  • Luzi Bergamin on 14 Jan 10
     
    Leo, do you mean you liked my comment on your question more than Pacome's? Well, the ACT has to evolve and fledge, so no bullshitting anymore, but serious and calculating answers... :-) Sorry Pacome, nothing against you!! I just LOVE this Diigo because it gives me the opportunity for a happy revival of my ACT mood.
  • pacome delva on 14 Jan 10
     
    haha, today would have been great to show your mood... we had a talk on the connection between mind and matter !!
jcunha

Gravity hidden aspects of electrodynamics - 0 views

phys.org/...n-aspects-electrodynamics.html

gravity theory electrodynamics

shared by jcunha on 13 Apr 17 - No Cached
  • jcunha on 13 Apr 17
     
    "Gravity spoils the symmetry regardless of whether magnetic monopoles exist or not. This is shocking. The bottom line is that the symmetry cannot exist in our universe at the fundamental level because gravity is everywhere"
Marcus Maertens

NASA's GRAIL Mission Solves Mystery of Moon's Surface Gravity - 1 views

www.nasa.gov/...grail20130530.html

space

shared by Marcus Maertens on 07 Jun 13 - No Cached
  • Marcus Maertens on 07 Jun 13
     
    Uneven gravity patterns on moon probably caused by asteroid impacts.
  • Ma Ru on 07 Jun 13
     
    Nooo, it's TMA-1!
Paul N

Quantum gas goes below absolute zero - 4 views

www.nature.com/...es-below-absolute-zero-1.12146

science

shared by Paul N on 03 Dec 13 - No Cached
  • johannessimon81 on 04 Dec 13
     
    Quite intriguing!
  • LeopoldS on 09 Dec 13
     
    this is fantastic! If built, such systems would behave in strange ways, says Achim Rosch, a theoretical physicist at the University of Cologne in Germany, who proposed the technique used by Schneider and his team3. For instance, Rosch and his colleagues have calculated that whereas clouds of atoms would normally be pulled downwards by gravity, if part of the cloud is at a negative absolute temperature, some atoms will move upwards, apparently defying gravity4. Another peculiarity of the sub-absolute-zero gas is that it mimics 'dark energy', the mysterious force that pushes the Universe to expand at an ever-faster rate against the inward pull of gravity. Schneider notes that the attractive atoms in the gas produced by the team also want to collapse inwards, but do not because the negative absolute temperature stabilises them. "It's interesting that this weird feature pops up in the Universe and also in the lab," he says. "This may be something that cosmologists should look at more closely."
Luzi Bergamin

IOPscience::.. Highlights of 2009-2010 - 5 views

iopscience.iop.org/...Highlights%20of%202009-2010

PHY gravity

shared by Luzi Bergamin on 21 Aug 11 - No Cached
  • Luzi Bergamin on 21 Aug 11
     
    Highlights of the year 2009 and 2010 of "Classical and Quantum Gravity". There's an ACT paper among them!
  • Ma Ru on 23 Aug 11
     
    Congrats!
LeopoldS

Phys. Rev. D 93, 024014 (2016) - How current loops and solenoids curve spacetime - 1 views

journals.aps.org/...PhysRevD.93.024014

gravity fundamental physics

shared by LeopoldS on 18 Jan 16 - No Cached
jcunha liked it
  • LeopoldS on 18 Jan 16
     
    modifying gravity? Jai?
jaihobah

Tabletop Searches For Extra Dimensions And Dark Matter | Quanta Magazine - 1 views

www.quantamagazine.org/tra-dimensions-and-dark-matter

PHY

shared by jaihobah on 03 May 16 - No Cached
  • jaihobah on 03 May 16
     
    Another story about the search for extra dimensions through modifications to gravity on small scales.
jaihobah

The Network Behind the Cosmic Web - 1 views

cosmicweb.barabasilab.com

PHY

shared by jaihobah on 18 Apr 16 - No Cached
  • jaihobah on 18 Apr 16
     
    "The concept of the cosmic web-viewing the universe as a set of discrete galaxies held together by gravity-is deeply ingrained in cosmology. Yet, little is known about architecture of this network or its characteristics. Our research used data from 24,000 galaxies to construct multiple models of the cosmic web, offering complex blueprints for how galaxies fit together. These three interactive visualizations help us imagine the cosmic web, show us differences between the models, and give us insight into the fundamental structure of the universe."
santecarloni

ScienceShot: Garden Gnome Tests Earth's Gravity - ScienceNOW - 2 views

news.sciencemag.org/...shot-garden-gnome-tests-e.html

garden gnome earth gravity science

shared by santecarloni on 22 Mar 12 - No Cached
  • santecarloni on 22 Mar 12
     
    This look like a perfect ACT project :)
Isabelle Dicaire

Measuring height by connecting clocks - 2 views

www.sciencedaily.com/...131002103036.htm

shared by Isabelle Dicaire on 06 Oct 13 - No Cached
annaheffernan liked it
  • Isabelle Dicaire on 06 Oct 13
     
    They were able to compare the ticking rates of two optical clocks separated by 2000 km, with the objective of computing sea level based on the effect gravity has on the clock ticking rate. They did the experiment using glass optical fibers, but I wonder if we could one day do the same from orbit, to measure the gravitational field around Earth.
  • Nicholas Lan on 09 Oct 13
     
    isn't this is effectively what pacome has been doing with his time for the last few years? e.g. http://arxiv.org/pdf/1308.6766v1.pdf also mentioning the ACES experiment
duncan barker

Zero Gravity water sphere experiment Video - 0 views

www.pakistan.tv/periment-%5BT4JrhW2aO6I%5D.cfm

shared by duncan barker on 28 Oct 10 - No Cached
  • duncan barker on 28 Oct 10
     
    ssssoooooooooo  coooooooooool
pacome delva

Special relativity passes key test - 2 views

physicsworld.com/...40834

phy quantum_gravity special_relativity gamma_ray_burst

shared by pacome delva on 15 Dec 09 - Cached
  • Granot and colleagues studied the radiation from a gamma-ray burst – associated with a highly energetic explosion in a distant galaxy – that was spotted by NASA's Fermi Gamma-ray Space Telescope on 10 May this year. They analysed the radiation at different wavelengths to see whether there were any signs that photons with different energies arrived at Fermi's detectors at different times.
  • According to Granot, these results "strongly disfavour" quantum-gravity theories in which the speed of light varies linearly with photon energy, which might include some variations of string theory or loop quantum gravity. "I would not use the term 'rule out'," he says, "as most models do not have exact predictions for the energy scale associated with this violation of Lorentz invariance. However, our observational requirement that such an energy scale would be well above the Planck energy makes such models unnatural."
  • Dario Izzo on 16 Dec 09
     
    essentially they made an experiment that does not prove or disprove anything -big deal-... what is the scientific value of "strongly disfavour"??? I also like the sentence "most models do not have exact predictions for the energy scale associated with this violation of Lorentz invariance" ... but if this is true WHAT IS THE POINT OF THE EXPERIMENT!!!! God, physics is in trouble ....
  • pacome delva on 17 Dec 09
     
    hum, null result experiments are not useless !!! there is always the hope of finding "something wrong", which would lead to a great discovery. For the state of theoretical physics (the "no exact predictions" quote), i totally agree that physics is in trouble... That's what happen when physicists don't care anymore about experiments...! All you can do now is drawing "nice"graph with upper bounds on some parameters of an all tunable weird theory !
pacome delva

"Quantum trampoline" measures gravity - 2 views

physicsworld.com/...41208

phy atom_interferometry atom_gravimeter

shared by pacome delva on 15 Dec 09 - Cached
  • Physicists in France have come up with a new way of using bouncing ultracold atoms to measure the acceleration due to gravity. The technique involves firing vertical laser pulses at a collection of free-falling atoms, which bounces some atoms higher than others. When the atoms recombine at the centre of the experiment, they create an interference pattern that reveals that g is 9.809 m/s2 – just as expected for their Paris lab.
  • pacome delva on 15 Dec 09
     
    That's the lab I worked...
  • Dario Izzo on 16 Dec 09
     
    just being cinical ... but did not we know that g = 9.809 in Paris? I can also create a complex measurement procedure that will held pi = 3.1415, just as expected!!!
  • pacome delva on 17 Dec 09
     
    well, sure... the interest of such gravimeter is to be absolute, and for now slightly more accurate than the other type of absolute gravimeter which uses retroreflector and interferometry ( http://en.wikipedia.org/wiki/Gravimeter ). While the latter ones reached their limit in term of sensitivity, the atomic ones can be enhanced in many ways (using cooler atoms, better optics, etc...)
Christos Ampatzis

Butterflies in the ISS - 5 views

anon.nasa-global.edgesuite.net/...exp22_butterfliesc.asx

shared by Christos Ampatzis on 10 Dec 09 - No Cached
  • Christos Ampatzis on 10 Dec 09
     
    learning to flap wings without gravity
  • pacome delva on 10 Dec 09
     
    is there something more than a video where nothing happens...?
Giusi Schiavone

cost-utility analysis of abolishing the law of gravity - 3 views

www.ncbi.nlm.nih.gov/...20071204s00038p1536.pdf

anti-gravity

shared by Giusi Schiavone on 19 Jan 11 - No Cached
  • Giusi Schiavone on 19 Jan 11
     
    Crazy and funny
Dario Izzo

paper shoving the advantages of morphological changes during artificial evolution - 2 views

m.pnas.org/...1015390108

ai alife evolution

shared by Dario Izzo on 20 Jan 11 - No Cached
  • Dario Izzo on 20 Jan 11
     
    Might be worth looking at for our project on evolution of gaits at different gravity level
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