The constant that remains the same across the entire electromagnetic spectrum is the speed of light in a vacuum.

Here's why:

* All electromagnetic radiation travels at the same speed in a vacuum: This speed is approximately 299,792,458 meters per second (often rounded to 3 x 10^8 m/s). This applies to all types of electromagnetic radiation, from radio waves to gamma rays.

* The speed of light is a fundamental constant: It's one of the most important constants in physics, and it's not affected by the frequency or wavelength of the radiation.

While the frequency and wavelength of electromagnetic radiation vary greatly across the spectrum, their product is always equal to the speed of light.