theory and general relativity into a few paragraphs! Those familiar with quantum theory and general relativity may proceed to the first road in section III. The first aspect of quantum theory that we recall is the quantization of light into photons. According to Planck and Einstein light of frequency ν and wavelength λ = c/ν can only be emitted and absorbed in multiples of the energy where ω is the frequency in rad/s. Thus we may think of light as a rain of photons, each with E = hν . 15,16 According to Einsteins relation for mass-energy equivalence, E = mc $^{2}$, a photon should interact gravitationally as if it has an effective mass Thus, for example, a single photon captured in a mirrored cavity increases the effective mass of the cavity according to Eq.(4). Next we recall a standard heuristic derivation of the Heisenberg uncertainty principle. 15 In Fig.1 we show the Heisenberg microscope thought experiment; using the microscope we view a particle with light entering from the bottom of the page. It is well-known from wave optics (and also rather clear intuitively) that the position of the particle can be determined to an accuracy of about the wavelength λ of the light used; a bit more precisely the uncertainty in position is given by According to classical physics we could determine the position as accurately as desired by using very short wavelength light, of arbitrarily low intensity so as not to disturb the particle by the electric field of the light wave. However the quantization of light as photons with energy hν prevents this since the intensity cannot be made arbitrarily low. A single photon scattering from the particle and into the microscope (at angle less than ϑ ) will impart momentum of order Δ p ∼ = psinϑ = ( h/λ ) sinϑ to the particle, so that Eq.(5) implies which is the Heisenberg uncertainty principle (UP). (In our rough estimates we do not distinguish between h and $_{ℏ}$, thus taking 2 π ≈ 1; this has been referred to as “using E = hν = $_{ℏ}$ω (3) M$_{ef}$ = hν/c 2 (4) Δ x ∼ = λ/sinϑ (5) Δ x Δ p ∼ = h ≈ ℏ (6) 4