Einstein was working within classical field theory-but, more importantly, the fundamental properties of matter that led to the realization of a unified theoeretical framework  for  the non-gravitational interactions, were discovered, experimentally, after his death. He was ``wrong'' in the sense of not identifying what were the problems that *could* be addressed at that time in this direction-after he *had* been so successful in identifying and solving other problems (special relativity, Brownian motion, the photoelectric effect, general relativity, spontaneous and stimulated emission, quantum chaos, particle statistics, entangled states).   His approach towards a unified field theory doesn't provide insights towards the correct solution-that *was* developed. The fact that his approach was not successful doesn't mean that unification wasn't possible; it wasn't possible with the tools he was using.

I strongly believe God will only write one set of equations.  Einstein was too early and he did not have much experimental results to guide which direction he should have gone through.  The standard model of SU(3) is already quite successful.  Now, how to put gravitation into a Grand Unification Theory is one of the new tasks for physicists.  Either modify Einstein's general relativity, or find a new mathematics to include  both SU(3) and gravitation without the renormalization troubles. Maybe, a new theory is already somewhere to be published.

• In the 1920s, when Einstein began his work on a unified field theory, electromagnetism and gravity were the only known forces, and the electron and the proton were the only known subatomic particles. Most physicists at the time were excited about the newly discovered quantum theory, and spent their time absorbed with exploring its bizarre and interesting consequences. They didn’t see any great need for a theory uniting electromagnetism and gravity.
• But Einstein, and several other scientists, did work on the problem of unification. In 1918, Hermann Weyl proposed a unification scheme based on a generalization of Riemannian geometry. Inspired by Weyl’s work, Theodor Kaluza showed that by extending space-time to five dimensions, one could produce the Einstein equations in four dimensions, plus an extra set of equations that is equivalent to Maxwell’s equations for electromagnetism. The fifth dimension would be compact, curled up so small that we can’t detect it. Oskar Klein later refined this idea.

Einstein liked the five-dimensional approach. In 1919, he wrote to Kaluza, "The idea of achieving unification by means of a five-dimensional cylinder world would never have dawned on me...At first glance I like your idea enormously." Kaluza published his paper in 1921, and Einstein published his first paper on unified field theory, with Jacob Grommer, following a similar approach, in 1922. Another approach Einstein tried involved extending general relativity to include the equations of electromagnetism by generalizing the metric tensor while keeping the 4-dimensional geometry.

One reason for Einstein’s failure to discover a unified theory may be his rejection of quantum mechanics, which caused him to ignore new developments in physics and distance himself from the rest of the physics community. Einstein was aware of his position, and commented in 1954 that "I must seem like an ostrich who forever buries its head in the relativistic sand in order not to face the evil quanta." But the more he worked on unification, the farther away Einstein drifted from the rest of the physics community. He also became more and more absorbed in formal mathematical arguments, rather than following the physical intuition that had guided him in his youth to his great discoveries. Many people say that Einstein failed because he was simply ahead of his time. The knowledge and tools needed to complete a unified theory simply hadn’t been developed before Einstein died in 1955.

Today, many physicists are taking up his quest. The most promising approach appears to be string theory, which requires 10 or more dimensions and describes all elementary particles as vibrating strings, with different modes of vibration producing different particles. String theory has not yet made any testable predictions, and some scientists worry that string theorists have, like Einstein in his later years, strayed too far from physical reality in their obsession with beautiful mathematics. But many others believe string theory does indeed hold the key to completing Einstein’s quest, and researchers are hoping to find ways to test some of the predictions of string theory. Though his own work never produced a useful physical theory, Einstein established unification as an important goal of physics. Indeed a theory of everything is commonly called the “holy grail” of modern physics. Einstein would probably be pleased that so many physicists are enthusiastically devoting their careers to pursuing his dream.

Einstein looked after a *geometrical* unification. He was trying to extend the idea of general relativity to electromagnetism.  In some sense, the Yang-Mills theory, and electroweak theory, achieves that.

As to string theory, it isn't the "most promising" approach, but the only one considered but the current community.  The number of dimensions is not dictated by the need of unification, but is necessary because it is the only possibility to quantize the theory.  With 26 dimensions, they are already too many to take account of the known interactions à la Kaluza-Klein.  With 11, they aren't enough.

Dear Manuel

I disagree with you. Einstein was unsuccessful, but he was right. Gravity and electromagnetic are unified.

Dears Stam

I agree with you about;"The fact that his approach was not successful doesn't mean that unification wasn't possible; it wasn't possible with the tools he was using."

First of all, let's assume it is possible to unify the four fundamental forces. In my opinion answer is yes.

Question is that which of these forces (gravity; in fact graviton, electromagnetic, strong or weak) is fundamental?

I my opinion Einstein was right. Gravity and electromagnetic are unified.

If we unify the gravity and electromagnetic forces, then we are able explain the mechanism of other forces production.

Dear Qiubao

I agree with on two following items;

1- Einstein was too early and he did not have much experimental results to guide which direction he should have gone through.

2- Maybe, a new theory is already somewhere to be published.

But I am disagree with about the standard model.

There are some unanswered questions or complex concepts in standard model. The questions that modern physics does not have answers for, and the physicists believe that it is due to the inability of theories. Please see

Kane, G, “Tthe dawn of physics beyond the standard model”, Scientific American, vol 288(6), p.68-75, 2003

Dear Mohsen

I appreciate your interesting comment, thank you.

I am thinking that questions are;

1- How we can explain the relationship between gravity and electromagnetic?

2- How we can explain the mechanism of electric field production by charged particles?

3- How spacetime is producing the electromagnetic energy?

The answers can help us to unify the gravity and electromagnetic.

Dear Claude

In my opinion, Einstein found there is a strongly relationship between gravity and electromagnetic (same as Planck), and he was right. Also I agree with you about; "...because it is the only possibility to quantize the theory."

In addition; maybe we need a sub quantum theory.

There is a very interesting development in the quest for unification of gravity with the electromagnetic force. A wave-based model of the universe has been developed which predicts that there should be a first order force between fundamental particles which scales with wave amplitude squared (shown to be the electrostatic force) and a second order force which scales as wave amplitude to the 4th power (shown to be the gravitational force). For example, an electron has wave strain amplitude of As = 10-22. Therefore, the electrostatic equation has a term (10-22)2 which is vastly larger than the (10-22)4 term in the gravitational force equation. This new way of modeling forces generates several previously unknown equations which relate gravity to the electrostatic force. For example, if two of the same mass particles had Planck charge and if these particles are assumed to be separated by a distance equal to their reduced Compton wavelength, then the following relationship exists: Fg / Fe = Fe / Fp where Fg is gravitational force between two of the same mass particles, Fe is electrostatic force between two particles with Planck charge and Fp is Planck force. Therefore, at this distance there is a symmetry between Planck force, the electrostatic force and the gravitational force. Other equations show that at arbitrary separation distance the electrostatic force is related to the gravitational force by a simple difference in the exponents of the wave amplitude term. A preprint of the paper is attached below:

Dear John

I have studied you interesting theory. There are some new and considerable concepts and analyzes. I will not be disagree with you. But in generally my work is beyond of modern physics. I have started with reconsidering the relativistic Newton's second law.

To date, there is no way to explain the process that describes how particles produce exchange particles in modern physics. According to the results of reconsidering relativistic Newton's second law, we can definitely say that the best way for unifying the interactions is generalizing interaction between charged particles to photon structure and vice versa. This new view on photon means that we can redefine the graviton and electromagnetic energy. Electromagnetic energy converts to matter and anti-matter such as charged particles. Charged particles use gravitons and generate electromagnetic field. This way of looking at the problem shows how two same charged particles repel each other in far distance and absorb each other at a very small distance.

For more see

Reconsidering relativistic Newton's second law and its results

http://gsjournal.net/Science-Journals/Essays/View/5518

Dear Manuel

As I told before, I have found that the problems in modern physics is based on a fundamental complex concept. I have started with reconsidering the relativistic Newton's second law. Link below;

http://gsjournal.net/Science-Journals/Essays/View/5518

Hossein:   As the paper referenced in my last post shows, it is possible to write gravitational and electrostatic force equations between fundamental particles in a way that both equations are identical except that the gravitational equation has one term squared and that term is not squared in the electrostatic equation. Furthermore, the paper shows that both the electrostatic and gravitational force equations undergo a dramatic simplifications when the separation distance is expressed in the natural unit of length which is the number of reduced Compton wavelengths rather than expressing separation using an arbitrary unit of length such as meters.

Even if you choose not to agree with the model that predicted these results, it is necessary to acknowledge that a new set of equations have been introduced into the discussion of the relationship between gravity and the electrostatic force. Any force model must be able to accommodate these new constraints on the model. I claim that these new equations appear to be incompatible with the gravitons and virtual photons but this should become a topic for discussion. The equations do fit perfectly with a proposed wave-based model of the universe that combines the general relativity description of spacetime on the macroscopic scale with the quantum mechanical requirement that vacuum has a vast energy density (zero point energy, vacuum fluctuations, etc.) on the microscopic scale.

@Manuel

Please see 

Reconsidering relativistic Newton's second law and its results ; pages 29-35, link below

http://gsjournal.net/Science-Journals/Essays/View/5518

Dear John

I've already read your article..

You and I have almost the same problems.

There are deeper problems in modern physics and theories are not able answered to.

To answer these questions unanswered, we need a new look and thought.

Your solution is interesting to me.

In my opinion, without a review of the relativistic Newton's second law, we can not be the way we address the problems of modern physics.

Dear Manuel

I explained relationship between gravity and electromagnetic energy in reconsidering the relativistic Newton's second law article.

Also I explained zero point energy, then I have analyzed production of virtual photon by charged particle.

If you think this approach is weak or wrong, I do not now what to add.

Dear Manuel

What means E = G2?

Dear Manuel

I read your paper. What are E and G?

Is E electromagnetic energy? and is G , gravitational constant?

Dear Manuel

You should give a mathematical analyze that shows how you have obtained the G2 

Dear Manuel

For understand, I need a relationship between you G and physical fundamental laws. 

Dear Amrit Sorli

Thank you for your interesting comment and article. I read your article.

I have review the quantum vacuum energy, it is like yours in different look.

Maybe my attitude would be interesting for you.

Article Graviton and Newton's second law