Heisenberg Uncertainty Principle The HUP is important in quantum mechanics. It states that you cannot determine the position and the momentum (or velocity) of a particle at the same time. If you determine the momentum, the position is uncertain and vice-versa. With macroscopic objects this effect is also present, but it is negligible. I can easily measure a car’s speed and position at the same time. Speed cameras do this all the time.. we know how fast the car was driving and were it was at that time. However, in the quantum world this is not so easy.. Why? What do we do when we make a photo of something? We shine a light on it. We know that at the quantum level particles are extremely tiny.. and if you want to look at a subatomic particle like an electron, you have to shine a light on it to detect it.. Now here is the catch.. The energy of a light wave comes in distinct packages, called photons, so you will be bombarding the electron with photons to be able to see it. And.. If you want to take a photo of something, you have to use a wavelength that is shorter than the particle you want to see, otherwise it will be blurry. The problem is that the wavelength of a photon is inversely proportional to its momentum so, the shorter the wavelength, the higher the momentum of the photon. what does that mean? If a photon has a high momentum, it has more energy, so it will hit the electron hard, transferring energy to it.. and because the electron is very small, this will make a huge difference to its momentum.. You will get a great indication of the electron’s position at that time, but the momentum of the electron will be changed because it has been knocked around by the bombarding photons. The momentum is uncertain. So now, you could turn down the energy of the bombarding photons, the wavelength will decrease and you may get a good indication of the electrons momentum. Only you won’t get a clear picture of its position, the position becomes uncertain because the wavelength is larger than the image you want to make. For our car it would make no difference, you could not change a car’s momentum measurably by shining a light on it. This is why the HUP is negligible at macroscopic scales. Image: Werner Heisenberg Return to the Homepage