What are the fundamental quantities / as far as we can tell / that really tell us something about the structure of nature? // One would be the speed of light. / Everything that is massless travels at the speed of light, / at this speed, whatever it is. // If you have any mass at all, / you cannot travel, / you cannot accelerate to this speed. // Another one would be the strength of the gravitational force. / What is the force between two objects of a particular mass? / Or in Einstein's theory, / how does a particular amount of matter or energy distort the fabric of the universe? // The number that tells you about that is Newton's gravitational constant, / which was first measured back in the 1780's, 1790's. // And then there's Planck's constant itself. // In 1900, Max Planck made a revolutionary proposal. // You could say, for example, / that there's a fundamental limit on how accurately we can know the position of a particle / and the momentum of a particle. / You can't know them both with absolute precision. // There's a fundamental limit, / and it's around about Planck's constant. // Planck first introduced it in the context of the frequency / or the wavelength of light emitted from hot objects. // Photons- what's the energy of a photon? / A packet of light: / It's Planck's constant multiplied by the frequency. // So those three things- speed of light, / strength of gravity, / and Planck's constant- allow you to define some distances, / a particular distance called the Planck length. //