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Gravity and the Infinity Problem 2 Questions 
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#1
Jan2413, 04:39 PM

P: 6

I am an avid reader of physics, cosmology, quantum mechanics; the entire genre. I have 2 physics questions:
1) If I understand properly, isn't gravity the effect of a massive object warping the fabric of spacetime? If this is correct, then is gravity not really a 'force', but a manifestation of that warping? If these are true, then isn't trying to equate gravity with the other 3 forces of nature; the strong, weak, and electromagnetic, like trying to merge apples and oranges? 2) I have seen Professor Michio Kaku explain that the infinity problem crops up when a point particle is represented as having no radius; again if I understand properly. Would this problem be cleared up if instead of using 0 for the radius of a particle, the planck length was used instead? I only have math up to mid college level, so tensors and the higher level stuff have eluded me. Sorry if this is in the wrong forum. Thanks, Mike 


#2
Jan2413, 04:56 PM

P: 5,632

Hey Mike, good questions
1) If I understand properly, isn't gravity the effect of a massive object warping the fabric of spacetime? If this is correct, then is gravity not really a 'force', but a manifestation of that warping? If these are true, then isn't trying to equate gravity with the other 3 forces of nature; the strong, weak, and electromagnetic, like trying to merge apples and oranges? in a sense yes.....that's why nobody has been able to do it yet...that's the objective of quantum gravity....so far the Standard Model of particle physics is incomplete for a number of reasons...it has three forces and relativistic quantum mechanics, but not gravity, which so far is best described by general relativity [GR}. Neither quantum mechanics nor GR works at extreme curvatures of space and time, singularities, like inside a black hole and at the big bang. 2) I have seen Professor Michio Kaku explain that the infinity problem crops up when a point particle is represented as having no radius; again if I understand properly. Would this problem be cleared up if instead of using 0 for the radius of a particle, the planck length was used instead? Yes, stuff like Coulombs electrostatic potential F = qq/4pi rr goes to infinity as r approaches zero....so there is a realm where it's 'not so good', but still very useful. That's an aspect of the electromagnetic force in the Standard Model. That's a big issue for sure. String Theory gets around that by positing particles as extended vibrating bits of energy...one dimensional extensions instead of points. But string theory has its own set of major issues.... String theory is an aspect of quantum mechanics and loop quantum gravity an aspect of GR, so people are working with the complex mathematics trying to figure out how to make different pieces fit. You may have heard of "M" theory discovered by Ed Witten: There were six different string theories, each seemed different, but Witten figured out how they were related via a more general approach..M theory....and ignited lots of new research... . Thanks, 


#3
Jan2413, 05:23 PM

P: 6

Hi Naty1,
Thanks for the quick response, but not really answering the questions. If gravity is not a force, then why try to unite it with the other 3? And Coulomb's electrostatic potential would go very high, but not infinity. Sorry for being pigheaded, but I'm just trying to understand. Thanks again Naty1 


#4
Jan2413, 05:40 PM

P: 18

Gravity and the Infinity Problem 2 Questions
1) In quantum mechanics there are no interacting forces. The interactions take place by "exchanging particle mechanism", so if one comes up with a quantum version of gravitational interaction it must be term of exchanging particles.



#5
Jan2413, 06:40 PM

P: 6

Hi soarce,
Thanks for the reply. Mike 


#6
Jan2513, 06:58 AM

Sci Advisor
P: 2,211

In a practical sense, this problem can be avoided under most circumstances, because quantum effects are only significant for tiny objects, and gravitational effects are only significant for large objects, so it's difficult to come up with a situation in which both effects are significant at the same time. (Hawking radiation is a situation where both gravity and quantum mechanics come into play, but the influence only goes oneway: spacetime curvature affects quantum processes, but the effect of quantum processes on spacetime curvature is negligible, unless you consider vast intervals of time.) Getting back to the actual unification of gravity with the other forces, I never completely understood why physicists cared about forces being unified. It's a beautiful thing that the WeinbergSalaamGlashow model of weak interactions shows them to be an aspect of the same force that gives rise to electromagnetic interactions, but I don't see why we need all forces to be unified in this sense. Is that just aesthetic preference, or is there some technical reason? 


#7
Jan2513, 12:05 PM

P: 5,632

"If gravity is not a force, then why try to unite it with the other 3?"
[short answer: because we think they ARE related...see below] Here are a few perspectives from my notes: [with source where I have them] An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era Ivan Agullo, Abhay Ashtekar, William Nelson (Submitted on 6 Nov 2012) http://arxiv.org/abs/1211.1354 Quantum Nature of the Big Bang: Improved dynamics Abhay Ashtekar,Tomasz Pawlowski,and Parampreet Singh http://arxiv.org/pdf/grqc/0607039v2.pdf Marcus: from a Roger Penrose lecture..in response to an audience question: And none of this even mentions one of the most profound questions affecting the Standard Model of particle physics: How are all the masses and strength of the forces determined in our universe? Where do particles come from?? Does the graviton exist? So far, the Standard Model is a hodge podge of different theories coupled with experimental observations, like the mass of the electron. Nobody knows how to calculate ANY particle mass from fundamental principles. PS: Particles have been associated with inflationary cosmology {LQC} and the expansion of spacetime associated with horizons...But there is much to learn... for those interested, try reading about Unruh effect, Hawking radiation, and black hole horizons.... 


#8
Jan2513, 12:17 PM

P: 5,632

Regarding 'point particles'....and infinities.....I was just reminded of Leonard Susskind in his book THE BLACK HOLE WAR [a book for the general public] where he provides truly fascinating explanations of particles. [A great book, cheap if already used online.]
In one view, he explains via the holographic principle [ which despite Wikipedia claims to the contrary, IS a widely accepted] understanding how particles can become 'smeared' over the horizon of a black hole...Although he does not say this, it sounded to me like one could resort to the wave description of a particle to get an intuitive insight. In any case, whatever goes into the black hole remains as an appearance on the horizon. In another description, he likens particles to the spinning of an airplane propeller: you can see the hub, less so the blades [with typical experimental resolution], and maybe not the blade tips at all. He says this is what particles may really be like.... So if you slow down the spin rate, or equivalently slow down time by hovering outside a black hole, more of the extended object comes into view...In the string theory perspective, you see larger and larger portions of extended strings....and you are exposed to more and more radiation. Hence the description via general relativity than a free falling observer passes the horizon without effect while a hovering observer is fried by high energy radiation. 


#9
Jan2513, 12:22 PM

P: 5,632

Mike:
suggest you go back and read really carefully because my first post IS the answer...all the rest is detail..... another simple way to answer your question: "because questions remain which we cannot answer." anyway, now you have examples above as explained by experts. 


#10
Jan2613, 09:58 PM

P: 6

Hi Guys,
Thanks for all of your responses. Mike 


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