Weightless refers to the state of an object or substance that is not being acted upon by any external force, such as gravity. Although something may be weightless, it can still be seen and experienced through senses such as sight, sound, touch and smell. Examples of things that are weightless but visible include air molecules, infrared light waves and electromagnetic radiation. These particles cannot be weighed because they do not possess mass in the form of matter; however their presence can still be detected with special instruments or even the naked eye.