Imagine a hypothetical situation in which you had the ability to place, and the time to observe, a delicately placed household LED flashlight in the middle of outer space. Would, given enough time, the flashlight propel itself forward?
Yes, but very slowly.
Even though it is massless, light has momentum; and if a beam of light is projected in one direction, the principles of conservation of momentum indicates that a small, yet discernible force, will push the object in the opposite direction.
A reasonably powerful LED flashlight will use about 3 Watts of energy, and the efficiency of an LED is in the region of 33%. This as a result, provides a net amount of light output of 1 W.
The ratio between the momentum and energy of light is 299,792,458 (which is also the speed of light). So in 1 second, the flashlight produces 1 Joule worth of light, which is equal to 0.33×10-8 kg m/s. If the flashlight is not too heavy, say 100 gram or 0.1 kg, it means that 1 second of light would propel the flashlight to a velocity of 10-7 m/s (or in other words, 0.000001 m/s). This assumes that all light is directed in straight line, whereas perhaps a more conventional flashlight would produce a more cone-shaped the bundle of light. And, by extension, the more cone-shaped the bundle of light, the lower the momentum transfer would be.
Leaving the light on for one day, and presuming that the light left the flashlight in a straight line, would propel the flashlight to about 0.009 m/s or almost to a velocity of 1 cm per second!
Unfortunately, operating a 3W LED for a day uses about 260 kJ of energy. Regular AA batteries have somewhere around 10 kJ of energy (depending on the type). And at a weight of 20-30 grams per battery, you can’t carry or put more than 2-3 in the device without violating the original assumption of a 100 gram device.
Though it’s not just visible light that will produce a tangible thrust, any wavelength of light will. This becomes a problem for space probes, because electronics and power supplies turning on and off create heat in the infrared spectrum, and these infrared photons cause a small thrust which over long periods of time will cause the probe to veer off course. As a result, NASA spends significant amounts of time developing computer simulation and modelling tools to account for this infrared-thrust effect when setting probe trajectories and course corrections.
Though even contemplating this sort situation is highly absurd – it may just be able to provide further insights into improved futuristic space propulsion methods. One such example is a photon rocket – a hypothetical rocket that uses thrust from emitted photons (radiation pressure by emission) for its propulsion.
Although we may be very far away from flying around space in photon rockets, this particular example highlights how something as little and insignificant as a flashlight holds a lot more power than you initially think.
Read more on the same topic: