Gravity and the Infinity Problem 2 Questions

[mikejp56]

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 space-time? 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

 

[Naty1]

Hey Mike, good questions

1) If I understand properly, isn't gravity the effect of a massive object warping the fabric of space-time? 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,

 

[mikejp56]

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 pig-headed, but I'm just trying to understand.
Thanks again Naty1

 

[soarce]

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.

 

[mikejp56]

Hi soarce,
Thanks for the reply.
Mike

 

[stevendaryl]

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 pig-headed, but I'm just trying to understand.
Thanks again Naty1

Whether or not we "unite" gravity with the other forces, we certainly need to understand how gravity interacts with the other forces. Even more basic: we need to understand how gravity interacts with quantum-mechanical objects. If you have a massive object that is in a superposition of two states, and the location of the object is different in those two states, then what happens to the gravitational field due to that object? (For the purposes of this question, it doesn't matter whether we are talking about GR or Newtonian gravity.) I think most people would say that the gravitational field (or the metric tensor) would have to be in a superposition of two different states, as well. So you need a quantum theory of the gravitational field in order to understand how gravity interacts with quantum mechanical massive objects.

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 one-way: 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 Weinberg-Salaam-Glashow 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?

 

[Naty1]

"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]

I never completely understood why physicists cared about forces being unified. It's a beautiful thing that the Weinberg-Salaam-Glashow 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?

It seems the motivation goes back to the earliest moments of the universe...no space, no time, no particles, just, apparently, very high energy??...and a very high and unstable vacuum expectation energy...and then 'poof' spontaneous symmetry breaking and out popped particles, forces, different forms of energy...so the supposition they were all 'unified'...all one entity when all was perfectly symmetric...leads us to think there should be a unification theory, a 'theory of everything'

Here are a few perspectives from my notes: [with source where I have them]

Wikipedia LQC: [Loop Quantum Cosmology]

The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of LQG. In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low space-time curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction and thereby resolving singularities of general relativity. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues —e.g., the ‘horizon problem’— from a new perspective... In LQC the big bang is replaced by a quantum bounce...

[The prior quote links gravity to cosmology and may reveal a new force.]

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

...the FLRW space-times of interest are invariably incomplete in the past
due to the big bang singularity where matter fields and space-time curvature diverge...
It is widely believed that general relativity is simply not applicable once curvature reaches the Planck scale... to encompass the Planck regime, one needs a quantum gravity extension of the standard cosmological perturbation theory.

[Neither GR nor QM can describe what appear in current models as 'singularities]

Quantum Nature of the Big Bang: Improved dynamics
Abhay Ashtekar,Tomasz Pawlowski,and Parampreet Singh

http://arxiv.org/pdf/gr-qc/0607039v2.pdf

...The scalar field continues to serve as ‘emergent time’, the big bang is again replaced by a quantum bounce, and quantum evolution remains deterministic across the deep Planck regime.

[The above quote hints at the possibility of further understanding time.]

Marcus:

...In a BH collapse, there is some MATTER that collapses, but the surrounding space does not. In a LQC cosmological collapse the whole of space collapses and rebounds...

[So what are the similarities and differences between these? Still a mystery.]

from a Roger Penrose lecture..in response to an audience question:

... A true theory of quantum gravity should replace our present concept of spacetime at a singularity. It should give a clear-cut way of talking about what we call a singularity in classical theory. It shouldn't be simply a nonsingular spacetime, but something drastically different...

Quote by Haelfix
No one has solved the reason for the low entropy initial conditions of cosmology. The problem exists for almost every single proposal. Loop or other. Taken at face value, it rules out almost all of cosmology.

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 space-time associated with horizons...But there is much to learn...
for those interested, try reading about Unruh effect, Hawking radiation, and black hole horizons...

 

[Naty1]

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.

 

[Naty1]

Mike:

but not really answering the questions.

well, yeah I did! [LOL] ,,,
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.

 

[mikejp56]

Hi Guys,
Thanks for all of your responses.
Mike