In a vacuum, all parts of the electromagnetic spectrum travel at the same speed. This speed is known as the speed of light, denoted by the letter "c", and is approximately 299,792,458 meters per second.

Here's why:

* Electromagnetic radiation is a wave: Light, radio waves, X-rays, and all other forms of electromagnetic radiation are fundamentally waves that consist of oscillating electric and magnetic fields.

* These waves are governed by Maxwell's equations: These fundamental equations describe the behavior of electric and magnetic fields and predict that electromagnetic waves travel at a specific speed in a vacuum, regardless of their frequency or wavelength.

Important Note: While all electromagnetic radiation travels at the same speed in a vacuum, their speeds can differ when they travel through different materials. This is because the interaction with the material's atoms can slow the waves down.