Hidden behind Bohr and Einstein:
Time Separation
They could have talked the Lorentzboosted Bohr radius, but they did not. The time separation was a hidden variable to them. 

Niels Bohr (18851962) was born six years after Einstein. Whenever Bohr
mentions the word "space" in his philosophical writings, he adds "time."
This is how much Bohr respected Einstein. Einstein also had a
great respect for Bohr. At the same time, Einstein had his objections to
the Copenhagen interpretation of quantum mechanics. This aspect is
well known and is still actively debated by the present generation of
physicists.
 Niels Bohr is also well known for his Bohr radius. To him, the proton
was sitting at the center of the universe, even though he knew the existence
of the time variable associated with the space. Einstein perhaps thought
about the hydrogen atom in a moving frame, but he never said anything
about it. His Lorentz transformations are strictly for point particles.
Before saying there was something wrong with them, let us look at Thomas Jefferson. He said "All men are created equal." But he forgot to mention "women" there, and he was not able to practice his concept of equality with some of the the people who served him at his Virginia mansion called Monticello. Jefferson's freedom remained as a hidden variable to at least one half the world population for many years. It took many years of evolutionary processes to see the full extent of this variable. It is quite possible that this variable is still not completely discovered.
Let us go back to Bohr and Einstein. Physics is an experimental science. It is still unthinkable for the hydrogen atom to move with a relativistic speed, but the proton can be accelerated to a speed very close to that of light. The question then is whether the proton has it own quantum mechanics like the hydrogen atom. Yes, the quark model formulated in 1964 could tell the story. We can therefore illustrate the evolution of the hydrogen atom has gone through with this figure.
Needless to say, this evolution was made possible thanks to the advances in the accelerator physics. At the proton proton speed of 0.9999999999c. At magnitude of time separation is essentially the same as that of the spacial separation.
In order to study this hidden variable, I spent many years with Marilyn Noz. We published many papers, but we also had many problems with the referees, because the timeseparation variable is thoroughly hidden to them.
 The first person who recognized the existence of this variable was
Richard Feynman. Based on his talk given at the 1970 APS spring meeting,
he published his paper with his students in
Phys. Rev. D 3 , 2706 (1971)]. In this paper, Feynman
et al. state the existence of the timeseparation variable.
They said they could not do anything about this variable because they do
not anything about it. I like very much their honest statement.
 As for the quantum mechanics of the time variable, Paul A. M. Dirac
worried about the timeenergy uncertainty relation in his paper
 P. A. M. Dirac, Proc. Roy. Soc. (London) A114, 243 (1927).
He noted in this paper that the timeenergy uncertainty relation is a cnumber relation, and that this lack of timelike excitations makes it difficult to combine quantum mechanics with special relativity. In his 1945 paper:
 P. A. M. Dirac, Proc. Roy. Soc. (London) A183 , 284 (1945).
Dirac uses the fourdimensional harmonic oscillator to understand the Lorentzcovariant world. The fourth variable is the time variable. I assume he had in mind the timeseparation variable. If we combine Dirac's 1927 paper with his 1954 paper, we end up with this figure with a circle.
In 1949, Einstein became 70 years old, and the Reviews of Modern physics published a special issue. This issue contains Dirac's paper:
 P. A. M. Dirac, Rev. Mod. Phys. 21 , 392 (1949).
It is then not difficult to combine the circle and the rectangle to construct the ellipse. Does this sound like a science fiction to you? We observe this effect whenever we turn on a particle accelerator. This is a picture of the proton released from the accelerator. Click here for a detailed story.
Paul A. M. Dirac was of course a great physicist. I seem to work strictly within his framework. I could do my own research because he left some soft spots in his papers.
 Dirac's papers are poems, but have you seen figures there. I seem to
enjoy translating his poems into cartoons.
 Dirac wrote many history making papers. However, Dirac seldom takes
advantage of his own papers published earlier. It is therefore profitable
to link up the papers Dirac published at different times.
 Dirac had a "famousbrotherinlaw" named Eugene Wigner who wrote
a paper on representations of the Poincare group in 1939. In his 1949
paper, Dirac said the task of constructing relativistic quantum mechanics
is the same as that of constructing a suitable representation of the
Poincare group.
When I talked with Dirac in 1978, he was still uncomfortable about the lack of timelike excitations in the Lorentzcovariant world. He used to call this spacetime asymmetry. He could have learned about Wigner's O(3)like little group to clarify the question of spacetime asymmetry.
 Time Separation as a Hidden Variable: In spite of all I said above, the
time separation remains as a hidden variable to the absolute majority of
physicists even these days, especially to the orthodox Copenhageners. How do you
define the word "hidden" in physics? This concept came from the uncertainty
principle which does not allow a point in the phase space of position and
momentum.
The best way to approach this problem is to use the Wigner phase space, where position and momentum is confined in an area not smaller than Planck's constant. The area of uncertainty can increase if we fail to measure other variables. In that case, the region of uncertainty expands. In order to clarify this, we can use Feynman's rest of the universe.
 What is Feynman's rest of the universe? In
his book on statistical
mechanics (1972), Feynman says
The best way to understand what Feynman says here is to use two coupled oscillators. One representing the system in which we are interested, and the other representing the rest of the universe.
 Click here for a webpage.
 Click here for an article published in the American Journal of Physics(1999).
The mathematics of the coupled oscillator system is exactly the same as the covariant harmonic oscillator which leads to Dirac's form of relativistic quantum mechanics, where one of the variable is the timeseparation variable. It is not necessaary to write down lengthy formulas. The best way is to use a cartoon. It is even better if the cartoon is the one you have seen before. The net result is
The spacetime portion of this paper is from the paper I published with Wigner in 1990. The title of the paper is Entropy and Lorentz Transformations. If we do not measure the timeseparation variable assuming it is hidden in Feynman's rest of the Universe, the net result is an increase in entropy and an expansion of the uncertainty region in the Wigner phase space. Click here for the phase space expansion.
This figure can be used for the twophoton coherent system or twomode squeezed state. The labels "first photon" and "second photon" were added in my paper with Noz entitled "Time separation as a hidden variable in the Copenhagen school of quantum mechanics," in Advances in Quantum Theory, American Institute of Physics Conference Series No.1327, pp 138147 (2001). ArXiv. As far as I can see, this story can be traced to Dirac's 1963 paper.
 P. A. M. Dirac was quite fond of harmonic oscillators and the
Lorentz group. He attempted to construct a representation of using two
oscillators, and ended up with the deSitter group applicable to five dimensional
space with three space and two time coordinates. Here is his paper.
 P. A. M. Dirac, J. Math. Phys. 4 , 901 (1963).
The result is that twooscillator representation later became the language for twomode squeezed state. I spent many years on this subject. Many people are saying I am crazy about the squeezed state. If so, it is because I am crazy about Dirac's papers. The original circleellipse figure for the covariant harmonic oscillator is again applicable to the twophoton states. In this case, both photons are observable, but we can choose not to observe the second photon. In this way, we can study the hidden timeseparation variable using observable photons.
copyright@2009 by Y. S. Kim, unless otherwise specified.
The photos of Bohr and Einstein and of Dirac and Wigner are from the E. Segre Visual Archives of the American Physical Society.