Universe or Multiverse?

[Wuwei]

It seems to me that you are attempting to explain the fundamentals of universe/multiverse based on laws that are only valid for the bubble we live in. Vacuum State is entirely possible in a certain bubble. As a matter of fact, string theory versions paint very depressing pictures of most states/bubbles in our universe/multiverse. Life is impossible in most of them. Gravity is a luxury that only seem to be found in some bubble/states. And so on. This makes Mach's principle irrelevant to our discussion.

That's right. I am thinking about our universe with an adjunct of a dark matter part. If Mach's principle is invalid then a few of the conservation laws that we know are not valid. Since symmetries imply conservation laws, I would presume other multiverses have their own conservation laws that are quite unrelated to ours. If we are not talking about this universe, any possible singularities in other universes would be different.

Warped space is real but it is not responsible for mediating gravitational force. Graviton is the force carrier for gravity in string theory and it makes sense because gravity just like EM is a field. Neither special nor general relativity is adequate in explaining everything we observe in the bubble we live in. I am not even talking about particles that are too small for us to see. Let us take an example of black holes. Black holes are massively dense objects that can arrest light. Yet, gravity has compacted them into a point called singularity. This pretty much is as small as an object can get. So then the question arises: do you use theory of relativity or quantum mechanics to explain black holes? As it turns out, there are aspects of black holes that neither quantum mechanics nor general/special relativity can explain. In light of this, you cannot say that either of those two grand theories are complete. We need to fill in the gaps left by general/special relativity and quantum mechanics. This task is enormous but string theory so far seems to be rising to the occasion.

I did not suggest that there are two different properties for gravity in our bubble. What I suggested was that gravity may not even exist or it may have fundamentally different or similar property in a different bubble. There is no requirement that gravity has to exist in a given bubble or that it has to have same properties as the gravity we observe in our bubble.

Yes, I read about Hawking radiation.

I still don't understand how the graviton and warped space can meaningfully coexist. If matter under the influence of warped space travels along geodesics that dictate orbits and trajectories what do gravitons add? Or conversely, if gravitons mediate the force why should space be curved.

 

[Wuwei]

Update on college. I'll be transferring to University of Missouri S&T next year. I start Columbia College on August 29th.

Algebra, Finite Mathematics, Calculus and Analytic Geometry, and of course, Computer Science, are the agenda for the next 12 months.

I've never felt like a bigger nerd in my life, nor have I ever felt so happy about it.

Good luck. I am a nerd too and proud of it.
Nerds of the world, arise!

 

[Vikrant]

SixFoot,

Look at the Light-Cone model; it is fairly simple to conclude from the model that when the speed of a matter starts to approach the speed of light, time starts to dilate. Time stops when the speed reaches that of light. If a matter could travel faster than light, it would enter what we call past event (going back in time).

Now ask yourself, what can you do to go in future without having to wait for the future to happen? What is your thought on that?

Seems to me that if something were to travel through physical space faster than light, something else other than sheer speed must come in to play in order for said matter to "change directions" and flow backward in time.

If we accelerate fast enough, we will leave Earth's orbit. If matter/mass-less particles accelerated fast enough, it would leave spacetime's "orbit" (for lack of a better word)?

If matter were to travel faster than the fields themselves of spacetime can handle, would it "rip" through, causing the change in direction/time?

My mind always... LOOPS back to these differing "directions". lol

In addition to speed, gravity too affects time. As gravity decreases, time speeds up and you age faster.

 

[Vikrant]

That's right. I am thinking about our universe with an adjunct of a dark matter part. If Mach's principle is invalid then a few of the conservation laws that we know are not valid. Since symmetries imply conservation laws, I would presume other multiverses have their own conservation laws that are quite unrelated to ours. If we are not talking about this universe, any possible singularities in other universes would be different.

Mach's principle is very vague but that is not the point. Point is, if there is no gravity then what are you applying the Mach's principle to? Similarly, if there is no mass then what are you going to apply the law of conservation of mass to?

Yes, I read about Hawking radiation.
I still don't understand how the graviton and warped space can meaningfully coexist. If matter under the influence of warped space travels along geodesics that dictate orbits and trajectories what do gravitons add? Or conversely, if gravitons mediate the force why should space be curved.

Gravitational field behaves similar to electromagnetic field. We can explain mechanics of electromagnetism in one of the two ways:

a. Particles exchange photons and that is how they influence each other (similar to graviton)
or
b. The electromagnetic field influences the particles by acting on them (similar to curved space)

Of course, I am over simplifying the analogy but those two explanations for electromagnetism have been around for quite sometime now. So I do not see why we cannot accept curved space and graviton side by side when explaining gravitational field.

 

[Vikrant]

^ I just wanted to add one more tidbit.

Gravity is not caused by curved space. It is the other way around. Gravity causes curved space. So existence of curved space is secondary when it comes to explaining gravitational force.

 

[Wuwei]

Mach's principle is very vague but that is not the point. Point is, if there is no gravity then what are you applying the Mach's principle to? Similarly, if there is no mass then what are you going to apply the law of conservation of mass to?

I certainly agree. I was just commenting that the conservation principles would be totally different in a different universe. It wasn't a critique.

Gravitational field behaves similar to electromagnetic field. We can explain mechanics of electromagnetism in one of the two ways:

a. Particles exchange photons and that is how they influence each other (similar to graviton)
or
b. The electromagnetic field influences the particles by acting on them (similar to curved space)

Of course, I am over simplifying the analogy but those two explanations for electromagnetism have been around for quite sometime now. So I do not see why we cannot accept curved space and graviton side by side when explaining gravitational field.

Yes, I understand all that. My point is that all the gravitational dynamics of objects can be derived from a curved space including a closed universe, and black hole singularities. Is it expectaed that modeling gravity using gravitons gives those same results? That is certainly different than the EM force. Coulomb's law is not known to break down in the same way as the inverse square law of gravity. But of course Coulomb's law can't be measured at galactic distances.

Gravity is not caused by curved space. It is the other way around. Gravity causes curved space. So existence of curved space is secondary when it comes to explaining gravitational force.

Yes, I understand the model that gravity somehow causes curved space. But to me the idea of force is obviated by the existence of curved space. The standard picture of a bowling ball sun on a rubber sheet with the earth as a marble following a geodesic implies that the idea of gravitational "force" is replaced by geometry and geodesics. The earth is just following it's inertial path. So if gravity is not a force, what does the graviton do if the graviton is supposed to mediate a force?

If gravity actually is a force mediated by a graviton then why would string theory even consider curved space being a process that would influence anything.

I'm not trying to critique theories that I don't understand, I simply want to more clearly understand the gravitational nature of string theoy.

 

[Vikrant]

^

I work as a software engineer not a physicist. Quite often, I participate in discussions on this forum after long hours of grueling work which is not glamorous by any stretch of imagination. I think miscommunication on my part perhaps was the byproduct of mental fatigue. Your point on Mach's principle and conservation is correct. That was indeed not a critique of what I said earlier.

Also, I agree with you that we cannot simply plug in gravitons in the Standard Model just like we do with gluons, photons, W+, W- and Z. It is because unlike other mediator particles, graviton's probability amplitude yields infinity whereas we were expecting a very small number because gravitational force between particles is very small.

String theory solves this problem very elegantly. However, I would like to differ that for some other night. Tonight before I crash, I just want to talk about the original problem which string theory sets out to solve. The original problem stems from applying Feynman rule to graviton. In Feynman diagram, you have four interaction vertices. These vertices represent elementary events in space and time. Together, these four elementary events form what is called a complete process. In order to calculate the probability amplitude of the process, we have to use integration technique from calculus and calculate the integrand of these four elementary events. The result (integrand) starts to diverge instead of integrating when two of the events share same space and time. This is where we are left off as far as Standard Model is concerned. Beyond this is the territory of String Theory.

For reference, here is the Feynman diagram without the graviton:

 

[Vikrant]

^

Also one more parting tidbit

This is purely my thought so take it with a bucket of salt

Gravity mediated by graviton influences the space and time; this results in curved space. From this point on, we have no major disagreement and I will quote you:

The standard picture of a bowling ball sun on a rubber sheet with the earth as a marble following a geodesic implies that the idea of gravitational "force" is replaced by geometry and geodesics.

 

[Wuwei]

My understanding is that the infinities do not come from first order Feynman diagrams. They come using perturbation theory which can be pictured as an infinite number of higher order graphs such as these.

To consider all possibilities would be formidable. Each term in the perturbation theory (or complex Feynman picture) is finite, but the sum of all of them would diverge to infinity. However a renormalization process was discovered that neutralized the infinities and allowed the integrals to converge.

Gravitons would be more difficult because they would interact with all particles with mass. Photons are the one exception. As I understand it a renormalization method for gravitons has not yet been discovered.

Gravity mediated by graviton influences the space and time; this results in curved space. From this point on, we have no major disagreement and I will quote you:

Yes, that would make me happy.

 

[Vikrant]

^

I think we have a slight misunderstanding here. I said if you were to apply gravitons to Feynman diagram, the integrand will result in infinity. I did not say Feynman diagram itself was an issue. I was sleepy so I did not want to go through the trouble of extracting the image from the PDF of Ashoke Sen, creating a photo sharing account and uploading the image. So I just pointed you to an image of Feynman diagram from WikiPedia hoping that my intended meaning was clear. I think I even noted in the caption that it did not apply gravitons to the Feynman rule.

Today however, I went through some trouble of extracting the diagram from Ahsoke Sen's PDF, creating a new photo sharing account and uploading the image which depicts infinite contribution to gravitational scattering.

The image is from this article:
http://www.hri.res.in/~sen/current2.pdf

 

[IsaacNewton]

Whether there is or is not has not been determined, but thankfully there are many smart people who don't just say 'this is how it is everywhere forever there is no need to study it' and get on with studying it.

Leave dogma to the dogs.

 

[Wuwei]

Thank you for going through the trouble. I was going to read a couple pages of the article before going to bed, but unfortunately it was a "page turner" so I had to finish it. It is now 12:40 AM. The paper is very readable and seems to be at the level of a Scientific American article, or it was possibly a chapter in a text book. It had no date but I gleaned that it was written before the Hadron Collider was running.

I would like to go one step further and learn a little more about the Calabi-Yau manifold. I have had some experience with group algebras of euclidean spaces, but not the topologically complex manifold of Calabi-Yau.

My unfounded gut feeling is that when a phase of M-theory is found that is consistent with the Standard Model, it will become clear why that phase is special, and worthy of forming a universe based on it.

Thanks again

 

[Vikrant]

^

That article is a very good spot to step into string theory. It is a text format of Ashoke Sen's lecture on introduction to string theory. Also, it contains some good material on Standard Model which is a prerequisite to understanding string theory. It has some original stuff related to string theory as well. The article it seems was written before Higgs particle was confirmed but it is definitely at the edge of our current knowledge. I think some scientists do not believe that the experiment that confirmed the existence of Higgs particle was conclusive. So in a sense, we still have some ground work to do as far as confirmation of Higgs particle is concerned.

Happy learning on Calabi-Yau! That definitely is one of the useful tools for understanding string theory.

 

[waltky]

A dozen black holes may lie at the centre of our galaxy, the Milky Way...

Dozen black holes found at galactic centre
4 Apr.`18 - A dozen black holes may lie at the centre of our galaxy, the Milky Way, researchers have said.

A new analysis provides support for a decades-old prediction that "supermassive" black holes at the centres of galaxies are surrounded by many smaller ones. However, previous searches of the Milky Way's centre, where the nearest supermassive black hole is located, have found little evidence for this. Details appear in the journal Nature. Charles Hailey from Columbia University in New York and colleagues used archival data from Nasa's Chandra X-ray telescope to come to their conclusions. They report the discovery of a dozen inactive and low-mass "binary systems", in which a star orbits an unseen companion - the black hole.

Black hole​

The supermassive black hole at the centre of the Milky Way, known as Sagittarius A* (Sgr A*), is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars. These stars live, die and could turn into black holes there. In addition, black holes from outside the halo are believed to fall under the influence of Sgr A* as they lose their energy, causing them to be pulled into its vicinity, where they are held captive by its force. Some of these bind - or "mate" - to passing stars, forming binary systems. Previous attempts to detect this population of black holes have looked for the bright bursts of X-rays that are sometimes emitted by black hole binaries.

Faint and steady

"The galactic centre is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years," said Prof Hailey. Instead, the Columbia University astrophysicist and his colleagues decided to look for the fainter but steadier X-rays emitted when these binaries are in an inactive state. "Isolated, unmated black holes are just black - they don't do anything," said Prof Hailey. "But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable." A search for the X-ray signatures of low-mass black hole binaries in the Chandra data turned up 12 within three light-years of Sgr A*.

By extrapolating from the properties and distribution of these binaries, the team estimates that there may be 300-500 low-mass binaries and 10,000 isolated low-mass black holes surrounding Sgr A*. Prof Hailey said the finding "confirms a major theory", adding: "It is going to significantly advance gravitational wave research because knowing the number of black holes in the centre of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them." Gravitational waves are ripples in the fabric of space-time. They were predicted by Albert Einstein's general theory of relativity and detected by the Ligo experiment in 2015. One way these ripples arise is through the collision of separate black holes.

Dozen black holes at galactic centre

 

[IsaacNewton]

A dozen black holes may lie at the centre of our galaxy, the Milky Way...

Dozen black holes found at galactic centre
4 Apr.`18 - A dozen black holes may lie at the centre of our galaxy, the Milky Way, researchers have said.

A new analysis provides support for a decades-old prediction that "supermassive" black holes at the centres of galaxies are surrounded by many smaller ones. However, previous searches of the Milky Way's centre, where the nearest supermassive black hole is located, have found little evidence for this. Details appear in the journal Nature. Charles Hailey from Columbia University in New York and colleagues used archival data from Nasa's Chandra X-ray telescope to come to their conclusions. They report the discovery of a dozen inactive and low-mass "binary systems", in which a star orbits an unseen companion - the black hole.

Black hole​

The supermassive black hole at the centre of the Milky Way, known as Sagittarius A* (Sgr A*), is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars. These stars live, die and could turn into black holes there. In addition, black holes from outside the halo are believed to fall under the influence of Sgr A* as they lose their energy, causing them to be pulled into its vicinity, where they are held captive by its force. Some of these bind - or "mate" - to passing stars, forming binary systems. Previous attempts to detect this population of black holes have looked for the bright bursts of X-rays that are sometimes emitted by black hole binaries.

Faint and steady

"The galactic centre is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years," said Prof Hailey. Instead, the Columbia University astrophysicist and his colleagues decided to look for the fainter but steadier X-rays emitted when these binaries are in an inactive state. "Isolated, unmated black holes are just black - they don't do anything," said Prof Hailey. "But when black holes mate with a low mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable." A search for the X-ray signatures of low-mass black hole binaries in the Chandra data turned up 12 within three light-years of Sgr A*.

By extrapolating from the properties and distribution of these binaries, the team estimates that there may be 300-500 low-mass binaries and 10,000 isolated low-mass black holes surrounding Sgr A*. Prof Hailey said the finding "confirms a major theory", adding: "It is going to significantly advance gravitational wave research because knowing the number of black holes in the centre of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them." Gravitational waves are ripples in the fabric of space-time. They were predicted by Albert Einstein's general theory of relativity and detected by the Ligo experiment in 2015. One way these ripples arise is through the collision of separate black holes.

Dozen black holes at galactic centre

I actually read today that there may be tens of thousands which seems unlikely. They'd gravitate towards each other to form a supermassive blackhole, which is the theory I've heard most often regarding what is at the center of galaxies.

They've tracked stars near the center of our galaxy over a couple decades and found a number that will orbitting something at the center at millions of miles per hour. Thus the theory of the supermassives at the center of galaxies.

Fascinating in any case.