Issues with String Theory | PF Answers

[Baraccus]

Hey there PF, as you can see I am newly registered here, but I have been floating around for a little while now reading the forums.

That being said, I come to you today in hopes that I can get some answers and resolve these problems I have regarding String Theory.

I just recently finished reading "The Elegant Universe" by Brian Greene. It was my first true exposure to the particulars of String Theory. I had heard about it, but did not know what it consisted of. While reading, there were 3 points that did not seem reasonable, so without further ado, here they are.

1. Strings are said to be Planck's Length. Planck's Length has importance because at distances smaller than it General Relativity and QFT are not compatible. Unless I am mistaken, this is due to the Uncertainty Principle causing fluctuations, even in "empty" sections of space-time. However, Greene states that because you cannot accurately observe phenomena smaller than your measuring device. In this case, a string is the device and the phenomena would be fluctuations. Since you cannot observe the fluctuations, Greene states you can say they do not exist. This seems wrong to me, not only because it seems mistaken to disregard an occurrence because you cannot directly observe it, but also because it seems to be disregarding the Uncertainty Principle itself, which is at odds with my second issue.

2.It is found that strings must be under incredible tension. Therefore, it requires enormous energy to cause a string to vibrate. As Einstein said, energy is mass, so therefore vibrating strings should create huge masses. However, this is not the case, Greene states, because quantum fluctuations cause the strings to lose energy, just enough to create small mass particles. First he states that the fluctuations do not exist because they can not be directly observed, but now they are used to cause strings to lose energy? You can not simply disregard one section of the Uncertainty Principle and then use another part, it just is not consistent.

3. The third issue is one about string theory in general. From Greene's book, it seems that String Theory gained a lot of popularity because it stated that a particular string vibration explains the graviton, but I do not see why the graviton needs to exist. General Relativity states that gravity is caused by space-time curvature. Why must the gravitational field be quantized by the graviton? As far as I know, the infinity problems caused by the combination of General Relativity and QFT is when one attempts to apply GR to the graviton. Both energy and mass cause space-time curvature, so why can the same concept not be applied to particles? It seems that QED, QCD, and General Relativity should be combined to be able to explain the forces (assuming the weak force can be connected to QED). This issue may only be born out of my ignorance on the particulars of the GR and QFT incompatibility, but it seems just plain silly to have two different theories on the same force, and then expect them to work together.

Thank you for your assistance and time.

 

[torquil]

Regarding points 1 and 2, I would need page number references in order to comment on that.

As for point no.3, nobody knows if gravity is correctly described by some quantum theory or not. But theoretical physicists are trying to find such a theory because of their urge to unify the different forces into the same framework, as before with the electromagnetic and electroweak forces. There is a feeling that everything ought to be explained by an incredibly simple law.

It might not be correct for gravity, but who knows. Most likely, any quantum consequences would be inaccessible to experiment anyway... (but not necessarily).

It was known that within a quantum field theoretical framework, the graviton would have to be a spin two particle, and when such an entity was predicted by string theory, people were (understandably) very exited about that.

 

[Baraccus]

Regarding points 1 and 2, I would need page number references in order to comment on that.

As for point no.3, nobody knows if gravity is correctly described by some quantum theory or not. But theoretical physicists are trying to find such a theory because of their urge to unify the different forces into the same framework, as before with the electromagnetic and electroweak forces. There is a feeling that everything ought to be explained by an incredibly simple law.

It might not be correct for gravity, but who knows. Most likely, any quantum consequences would be inaccessible to experiment anyway... (but not necessarily).

It was known that within a quantum field theoretical framework, the graviton would have to be a spin two particle, and when such an entity was predicted by string theory, people were (understandably) very exited about that.

Thanks for your comment. See, I figured that was much the case, but I just feel that an urge to put every force under one umbrella, while having two differing explanations, is not getting anyone anywhere.

However, from what I understand, it seems that String Theory tailored a string to suit the graviton. I am not saying that it undermines the credibility of String Theory's concept of the particle, just that the Theory said a particular string can exist that fits the criteria of the graviton.

As for page numbers, you can find point one being developed in his book around pages 152-157. Point 2 is 148-150.

 

[torquil]

Thanks for your comment. See, I figured that was much the case, but I just feel that an urge to put every force under one umbrella, while having two differing explanations, is not getting anyone anywhere.

However, from what I understand, it seems that String Theory tailored a string to suit the graviton. I am not saying that it undermines the credibility of String Theory's concept of the particle, just that the Theory said a particular string can exist that fits the criteria of the graviton.

As for page numbers, you can find point one being developed in his book around pages 152-157. Point 2 is 148-150.

If I got my string theory history right, I believe the string graviton state was sort of "discovered" within all of the five string theories, so I wouldn't say that string theory was tailored to produce a suitable graviton.

When the forces are unified under this umbrella, then the classical geometric gravity would only be an effective theory valid under non-quantum conditions, so there should be no problem of inconsistency between the two formulations of gravity. The string theory would just describe the same at a greater level of detail and be valid also under quantum conditions.

Re. inconsistency between the arguments summarized in your points 1 and 2 (I reread those sections in the book):

Firstly, on pages 148-150 he is talking about the negative mass corrections due to higher order quantum field effects (quantum fluctionations) resulting in the graviton mass being zero. This is related to what is called mass renormalization. That's something well established in ordinary quantum field theory, although I don't know the details of such calculations within string theory.

Although an expert should probably comment on this, because I though that the bare mass of the graviton was already zero both in bosonic and fermion/boson string theories (one step up from the negative mass states that are removed by the GSO projection)? Perhaps his statement concerns something a bit more subtle that I'm not picking up on? So his statements about the necessity of negative contributions to cancel the mass of a Planck mass graviton confuses me. But I'm no expert in string theory...

After that, on pages 152-157 he discusses the inability for us to detect any structural properties of spacetime smaller than the Planck length, and his argument is pretty sound. At the end (p156-157) he mentions a philosophilal viewpoint that these fluctuations therefore doesn't exist. This is of course highly questionable and not really a question of physics at all so its best to just ignore such statements. If something is not measurable then it is not determinable and therefore it makes no sense to say anything about it.

If the theory predicts quantum fluctuations "below" the observable level, then one cannot check the existence of these if the theory works correctly. One cannot say that the fluctuations exists for sure either, because at that level the theory is untestable.

 

[Baraccus]

Alright, I think I see what you are saying. Yet, it still irks me about the fluctuations not existing. Isn't the very basis of Hawking Radiation the fact that these fluctuations can be considered virtual particles, and so black holes cause a separation of the particle pair? It is impossible to directly observe the virtual particles, you can still indirectly observe them by way of their results, in this case black hole radiation. If fluctuations cause black holes to radiate, I would not consider them immaterial.

 

[torquil]

Alright, I think I see what you are saying. Yet, it still irks me about the fluctuations not existing. Isn't the very basis of Hawking Radiation the fact that these fluctuations can be considered virtual particles, and so black holes cause a separation of the particle pair? It is impossible to directly observe the virtual particles, you can still indirectly observe them by way of their results, in this case black hole radiation. If fluctuations cause black holes to radiate, I would not consider them immaterial.

I think he is in essence saying that in general quantum fluctuations are important and measureable (of course).

But the existence of just those tiny "unmeasurable" quantum fluctuations that are smaller than the smallest possible size of a string are put into question by that philosophical idea where one says that they don't exist because one cannot measure them.

But I thought of a different viewpoint in this matter: Since the fluctuations themselves are spontaneous quantum exitations of string states, those states must be larger than a string. So there should be no fluctuations on smaller scales than a string, right (if it is true that string effects are always larger than some nonzero length (Planck length))? This must be determined mathematically of course.

Of course, it would still not make sense to claim that there were no fluctuations smaller than the string, since the theory might be completely incorrect at smaller length scales without us knowing about it...

I agree that the proposed philosophical standpoint in the book is a somewhat inconsistent viewpoint, although one can say anything one wants about unmeasureable quantities (if they indeed are unmeasureable, who knows). It makes no sense no matter which viewpoint one takes since one can never determine the truth about it if the theory is correct.

 

[Baraccus]

I think he is in essence saying that in general quantum fluctuations are important and measureable (of course).

But the existence of just those tiny "unmeasurable" quantum fluctuations that are smaller than the smallest possible size of a string are put into question by that philosophical idea where one says that they don't exist because one cannot measure them.

But I thought of a different viewpoint in this matter: Since the fluctuations themselves are spontaneous quantum exitations of string states, those states must be larger than a string. So there should be no fluctuations on smaller scales than a string, right (if it is true that string effects are always larger than some nonzero length (Planck length))? This must be determined mathematically of course.

Of course, it would still not make sense to claim that there were no fluctuations smaller than the string, since the theory might be completely incorrect at smaller length scales without us knowing about it...

I agree that the proposed philosophical standpoint in the book is a somewhat inconsistent viewpoint, although one can say anything one wants about unmeasureable quantities (if they indeed are unmeasureable, who knows). It makes no sense no matter which viewpoint one takes since one can never determine the truth about it if the theory is correct.

I like your way of thinking about it, it seems much more consistent than what Greene stated in his book.

So, it seems that there is no way, yet at least, to prove that gravitons should/do exist, but at least the first two matters are cleared up, thanks a bunch.

However, it does make one think. Feynman, among many other Physicists, believed that the point-particle model is wrong, because particles have to have some size, no matter how small. Using the same logic as String Theory, it could be said that particles have a length equal to that of a String, and the GR-QFT issues should be solved. Sadly, I doubt that framework has the same potential as String Theory to describe phenomena.