how to get info from topic ~/physics/contacts from the world file

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Hey

I created a world file that includes three blocks. I want to know the contact normals between them so I subscribed to ~/physics/contacts topic. On the start when I start the gazebo and echo the topic the topic doesn't show any contacts (publish empty messages) although I can see the blocks. the topic start publishing contact normals after I move the blocks or add another one.

Does somebody know why is that happening and how I can fix that?

Thanks in advance