Does negative frequency actually exist or it is just theoretical?

To directly answer your question; no, there is no such physical thing as negative frequency. Frequency is the rate at which something happens, so by defintion it must be a positive real number. Likewise a quantity of physical objects can also only be a positive real number, e.g. I have 4 and a half apples.

1. Are negative frequencies real?
2. Can frequency be negative in statistics?
3. Why do we have negative frequencies?

Are negative frequencies real?

The meaning of negative frequencies is just mathematical(not physical) similarly to the imaginary part of a complex signal. In real world, the negative frequency does not exists and the spectral content on negative frequencies must be added to the spectral content at the positive frequencies, to save energy.

Can frequency be negative in statistics?

Therefore, negative frequencies are not considered, and attention is given to the zero and the M2 positive frequencies.

Why do we have negative frequencies?

The concept of negative frequency originates from the fact that spirals can rotate either clockwise or counterclockwise. This can also be conceived as the forward or backward phase angle in time. Real signals comprise of two equal but complex exponentials that revolve in opposite directions.