Short answer: light moving backwards in time. Long answer: Take a light detector (such as a CCD camera) and place it in front of a light source. The light sour

Short answer: light moving backwards in time. Long answer: Take a light detector (such as a CCD camera) and place it in front of a light source. The light source emits a "pulse" of light. What you detect on the detector is that initially there is no light falling on it, then some light, then maybe a bit more, a bit less, then eventually no light anymore once you switch off the source. A pulse of light in the time-domain is just some varying level of light that can be measured by a detector. the pulse is of finite temporal length in the real world i.e. it has a definite start and a definite end. You don't have to do the math in the time-domain. You can take the function that describes the pulse in the time domain and take the Fourier transform of it to take it to the frequency domain. In the frequency domain, the pulse is described by a function that can extend from negative to positive infinity. Each point on this function can be interpreted to be a sinusoidal wave of that frequency, which when take to the time-domain extends from negative to positive infinity time. Obviously, its not possible for light from a source to start from negative infinity time, so this is just an nonphysical interpretation of the math. For positive frequencies, the corresponding time-domain sinusoidal wave is moving forward in time, and for negative frequencies, the sinusoidal wave is moving backwards in time. In the real world, you can't create light pulses that are exclusively negative frequencies, but you can create pulses that have both positive and negative frequency components and then exploit those separately in clever ways.