Relativity and Cosmology

Jorrie,

Interesting post. I wish I could say I understood string theory but I haven't learned the necessary math yet (someday I hope). You say that string theory assumes a "fixed space time". Can you clarify that.

What I mean to ask is, in what way is spacetime not fixed? Do you mean in the sense that it is curved by matter, or do you mean that it is expanding due to inflation
? or both or neither?

Thanks,

Roger

ps. My next entry (should be up by Thursday) is an overview of fundamental particles such as quarks, mesons, baryons, etc., just if your interested.

Hi Roger, you asked: "What I mean to ask is, in what way is spacetime not fixed? Do you mean in the sense that it is curved by matter, or do you mean that it is expanding due to inflation? or both or neither?"

Both and more...

Wikipedia's article describes it as: "Background independence is a condition in theoretical physics, especially in quantum gravity, that requires the defining equations of a theory to be independent of the actual shape of the spacetime and the value of various fields within the spacetime, and in particular to not refer to a specific coordinate system or metric. The different configurations (or backgrounds) should be obtained as different solutions of the underlying equations".

Looking forward to your Blog entry on the particles.

Jorrie

Hi Jorrie,

How many dimensions has a string of 'string theory' got? I undestood from 'Brief History of time' by Stephen Hawking that as per the theory it has 11 or 26 dimensions. What does this mean. why not in between the two? Does it mean that only with these alternatives a satisfactory mathematical theory is possible?

kash

Hi Kash.

The number of dimensions depends on the brand of string theory - there are many of them. Depending on which one, a certain number of dimensions are required in order for that theory to give a photon with zero mass. Any other number of dimensions violate this principle.

Wikipedia String Theory says amongst other things: "Therefore, the photon will be mass-less — and the theory consistent — only for a particular number of dimensions".

Jorrie

Hallo, Jorrie (and hallo to everybody)…

Elementary particles are very mysterious… 1) Fermions (that constitute the ordinary matter) have spin ½… This means that if such a particle makes a full rotation of 360^o we'll not see the same "side" (meaning the same quantic state) but actually the opposite "side"… It has to do two full rotations in order (for us) to "see" the same "side"… It's like the particle "sees" a greater space (or "exists" in a greater space if you like… maybe more dimensions…???…)… 2) An electron, for example, is neither a "dot of zero dimension" (because the electric field should be infinite on it) nor a "sphere" (because the electron should be resolved due to the mutual repulsion of its distributed charge)… 3) There is duality in their behaviour: They behave as a wave or as a particle, depending on the way that we observe them… Furthermore, it seems that they change their behaviour earlier than we make our decision in which way we'll observe them (it's like they, already, know the way that we are going to observe them)…

All these thoughts may seem irrelevant with this discussion (although we could discuss the physics of the particles in another forum… Jorrie…???…)…but I wanted to show that there is a lot of ignorance about the real "nature" of the particles (electrons, quarks, photons e.t.c.)… Maybe our human perception is not enough to fully understand the deeper reality (or not???)…

And now, the String Theory comes to make things even worse and more complicated… It claims that, actually, the elementary particles are tiny strings, much smaller than the particles themselves, which vibrate in a "larger" space of 11 (or 26???) dimensions… These extra dimensions are intense bended in a same tiny space (and that's why we cannot conceive them)… The strings vibrates in several modes… And each mode corresponds to a specific particle… Although, this theory is incomplete and seems to have problems… But, maybe, it's a good start…

I almost know nothing about the other theory that you have mentioned… Can you give us some more tips???

Hi George.

As I understand things, the quantum gravity part of string theory attempts to incorporate gravity into the theory of the basic particles and forces, i.e., it's the particle physicists way of looking at gravity. From the Livingreviews web pages that I quoted above: "Thus, for a physicist with a high energy background, the problem of quantum gravity is now reduced to an aspect of the problem of understanding: What is the mysterious nonperturbative theory that has perturbative string theory as its perturbation expansion? And how does one extract information on Planck scale physics from it?"

Loop Quantum Gravity (LQG) is an attempt from the relativistic side to quantize general relativity's gravitational field, while staying away from the 'background space' dependence of string theory. From Livingreviews: "Loop quantum gravity is thus a genuine attempt to grasp what quantum spacetime is at the fundamental level. Accordingly, the notion of spacetime that emerges from the theory is profoundly different from the one on which conventional quantum field theory or string theory is based."

Reading further in Livingreviews, one finds: "... strings and loop gravity may not necessarily be competing theories: There might be a sort of complementarity, at least methodological, between the two. This is due to the fact that the open problems of string theory are with respect to its background-independent formulation, and loop quantum gravity is precisely a set of techniques for dealing non-perturbatively with background independent theories. Perhaps the two approaches might even, to some extent, converge."

Maybe the main problem is getting the particle physicists and the relativists to talk to each other more...

Jorrie

Hi Jorrie,

I've read a few things about string theory, but hadn't heard of LQG before. At one time gravity was theorized to be an electromagnetic wave, and as such would have quantization. I think that thinking has gone out of favor. What can you tell us of 'proofs' that gravity is not an electromagnetic wave?

S

Hi S.

Gee, I do not know of a time when "gravity was theorized to be an electromagnetic wave, and as such would have quantization." There are similarities, but at the fundamental level they are quite different. Einstein spent the latter half of his life attempting to reconcile electromagnetic theory with general relativity, but he failed. LQG is more or less a continuation of Einstein's quest.

What I do know is that a gravitational field acts much like an electromagnetic field and that changes to the gravitational field is theorized to propagate at the speed of light - gravitational waves in general relativity, or virtual gravitons in quantum gravity.

Finally, electromagnetic waves do not 'pull' objects towards each other over huge distances, like gravity does. When photons are absorbed or reflected by an object, they cause it to recoil due to radiation pressure.

Jorrie

Hi Jorrie,

"I do not know of a time when gravity was theorized to be an electromagnetic wave"

It may have been my assumption based on the description of gravitons as "particles" in a famous Scientific magazine in America. Since photon "particles" are really waves in my opinion, and since the phenomena travels at the speed of light, it was a natural assumption.

"electromagnetic waves do not 'pull' objects towards each other"

I left open the possibility that at some high frequency they could do that.

The only "proof" that I know of for gravity not being an electromagnetic wave is that black holes emanate gravity beyond the event horizon. Have you heard of any other?

S

Hi S.

"The only "proof" that I know of for gravity not being an electromagnetic wave is that black holes emanate gravity beyond the event horizon. Have you heard of any other?"

Others that jump to my mind is: the fact that one can apparently not shield anything from a gravitational field or from gravitational waves. Gravitational waves must theoretically operate in the audio and sub-audio frequency ranges (0.1 - 10 KHz, I think).

Jorrie

Jorrie, you said:"Finally, electromagnetic waves do not 'pull' objects towards each other over huge distances, like gravity does. When photons are absorbed or reflected by an object, they cause it to recoil due to radiation pressure."

We could say that gravitational waves when they meet an object, also, cause it to recoil... Thus there is a similarity with electromagnetic waves (or photons) that meet an object... Steady gravitational field "pull" objects but a change (I mean instant increase-decrease, like a pulse) of the gravitational field - which produces a gravitational wave - makes the recoil of an object (when the wave meets it)...

Both (electric and gravitational field) have infinite range... Their difference is in their intensity as the gravitational field is much less weaker than the electric field... (The gravitational field "pull" big objects (like planets) over huge distances, although it is weaker, because it always produces attraction ("adding" behaviour)... Electric field "pull" or "push" electrically charged objects but not objects like planets, whose positive and negative charges are equal)... They both interchange virtual particles: virtual photons cause "attraction or repulsion", while virtual gravitons cause "attraction"... [So, their differences are the intensity and that gravity is always attractive]... The absorption or reflection of virtual photons causes the "attraction or repulsion"... So, I didn't get what you were saying...

Hi again George.

I understand that gravitational waves do not work in the radial direction from the source, but rather in the transverse directions. It stretches and squeezes objects perpendicular to the direction of wave travel, like this.^[1][2]

'Hyper-space' is a non-existing dimension just used to show space curvature. the space-space diagram on the right shows how space is alternatively stretched and squeezed as a gravitational waves passes by.

Jorrie

[1] http://www.einsteins-theory-of-relativity-4engineers.com/gravitational-waves.html

[2] A nice animation at: http://en.wikipedia.org/wiki/Gravitational_waves

Very interesting links, Jorrie...

Thus, (as I said) we could assume that a fluctuation of the gravitational field can be transmitted as fluctuations of space-time, forming gravitational waves... These waves travel with the speed of light, making every object that they meet to fluctuate in accordance with them... There is a similarity to the electromagnetic waves: They are produced by fluctuations (oscillations) of an electromagnetic field, travel with the speed of light and make any particle with electric charge that they meet to fluctuate... For example, when they arrive at an antenna (i.e. of a radio receiver) the electrons of the antenna oscilates (in accordance with the waves)... What do you thing???...

At the other hand, I tend to believe that gravitons is an essentially different thing... They are emited from a massive object in order to attract another massive object... (They are not emited if the massive object is perfectly isolated... imagine an object in an empty universe... am I right???...)... But there is no need for a fluctuation of the gravity field in order be emited (as the gravitational waves need)... They are emitted anyway... They are "virtual particles" that "carrie" the gravitatinal force... Just like "virtual photons" that carrie the electromagnetic force... (But, as I said, the virtual gravitons always "pull" while the virtual photons "pull" or "push"...)

What is your opinion???... Is it right to consider "gravitons" and "gravitational waves" as completely different things???... or I have misunderstood something???...

We had a similar discussion in another blog entry in the past... But I would like to clarify things better... I would like to have your comments...

Hi George.

"Is it right to consider "gravitons" and "gravitational waves" as completely different things???... "

As far as I can make out, general relativity sees them as the same thing. Here the only effect that propagates at the speed of light is gravitational waves and if it has to be quantized, it must have gravitons (real gravitons, not virtual ones).

String theory sees the virtual graviton as the carrier of the gravitational force and does not have gravitational waves in their picture. (They do not really have gravity!)

Loop Quantum Gravity tries to reconcile these views and haven't succeeded so far. So we can perhaps ask: are real gravitons and virtual gravitons the same things things? With present knowledge, I think they are different things, but I guess many other will differ.

Perhaps Roger can help a little...

Jorrie

I view the Gravitational Field as the representation of the distortions of space which is inherent in whatever structural patterns give rise to "mass". Furthermore, I think any "mass" is due to a bundling effect from the other forces traveling in /along resonant pathways. (it is these "resonances" that give entities location and form within space-time.)

From any particular location on the ever changing space-time grid, the summation of Fields is the total of ALL fields emanating from all points within reach (i.e. within the current "Light Cone" passing through this location). I think that somehow, the interaction of all these Fields at any point seeks to minimize the pathways. In other words, these Fields travel the path of least resistance, which is both defining and appearing to define "distance". Perhaps this tendency to shorten the path is what is manifest in the PULL of Gravity. The more regions of matter that are involved, the proportionally stronger are the "pulls".

I believe in Gravitons. I interpret them as the manifestation of what Nature can recognize as the smallest interaction. This is essentially the same at the "Plank Size" (as it must be if it is due to the interaction of all other forces which are governed by this "size"). Therefore the same minimum discernable chunk of space-time that quantisizes the other Fields, acts to quantisize Gravity. Any oscillation smaller than that is simply indiscernible, and hence unable to communicate with (i.e. to affect ...interact ...resonate ...pattern along) other matter or energy forms.

I blurted this out without taking a lot of time to explain, or furnish background. It probably looks like a lot of babble. Hopefully, it will make sense to others, and help to create a mental vision of these phenomena. I do not claim to completely understand this ...not by a long shot! Still, it seems to fit together, and to work along the lines of describing a "reality" that is uniquely local and generally global ...and must ultimately be entirely self-generating.