When Carter and Mitchell are walking down the hall just after being transported to an alternate dimension, one of the people they pass can be seen moving out of the way.
Mitchell and Carter are sent into a parallel dimension where they cannot be seen or heard, and supposedly cannot interact with physical objects in their original dimension (as when Carter's hand passes through a coffee cup). Yet they are able to walk around on the same floor that everyone else uses, can hear words spoken by those in the original dimension (which would require the air to carry sound vibrations to their ears), and can see things in that dimension (requiring light to reflect off their retinas). They also are still able to breathe the air, and Mitchell travels through the Stargate as if he were still composed of matter that can interact with the gate.