The physical quantity that has the same value for waves belonging to different parts of the electromagnetic spectrum is the speed of light in a vacuum (c).

Here's why:

* Electromagnetic Spectrum: This spectrum encompasses all types of electromagnetic radiation, from low-energy radio waves to high-energy gamma rays.

* Speed of Light: All electromagnetic waves travel at the same speed in a vacuum, which is approximately 299,792,458 meters per second (m/s).

* Variations: While the speed of light remains constant in a vacuum, it can change when it travels through different mediums like water, glass, or air.

Therefore, regardless of the wavelength, frequency, or energy of an electromagnetic wave, its speed in a vacuum will always be the speed of light (c).