Physical Shaders work very similar to the standard shader system, with a few key differences. Rather than using a specular channel Physical Shaders use a Glossiness and Reflectivity channel in conjunction to create realistic materials.
If the material is not already set to Physical Shaders, you can change from Standard to Physical with the drop down at the top of the material settings.
This controls the specular and blurriness of the reflection. A higher value closer to 100 will create a tight specular with a sharp reflection, and a lower value will create a softer more spread out specular with blurred reflections.
Specular and Eviroment Multiplier
Since Specular and Reflection values are based on physical settings to create realistic renders based off the Environment, in some cases you may want the look to be more embellished or intense. You can use the Specular Multiplier and Environment Multiplier to increase those values past the physical settings.
Reflectivity is closely related to the Glossiness. In many cases, if you have a map loaded into the Reflectivity you can set the Reflectivity to 100 and just adjust the Glossiness value above to adjust the specular and blur. In other cases to dampen the reflection you can set it to a lower value to decrease the intensity.
LOADING MAPS TO CREATE REALISTIC SHADERS
Load texture maps into the Glossiness and Reflectivity channels to create variation in the way those channels affect the materials. You can drag texture files straight to the None Set slot.
Tip: Click the None Set slot to bring up the Texture Channel options. You can adjust the Gamma, Contrast, and Brightness to adjust the intensity of the grayscale of the map. Closer to black will block out light and closer to will let light through.