( we ( ( are ( ( ( reaching ( physical dimensions ) ) and ) ( ( switching times ) ( in ( ( ( the head ) and ) ( media ( ( at which ) ( ( electrical ( and ( magnetic properties ) ) ) ( are ( different ( than ( they ( were ( ( ( at ( lower speeds ) ) and ) ( at ( macroscopic sizes ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP we)) (VP (VBP are) (VP (VP (VBG reaching) (NP (JJ physical) (NNS dimensions))) (CC and) (VP (VBG switching) (NP (NNS times)) (PP (IN in) (NP (NP (DT the) (NN head)) (CC and) (NP (NP (NNS media)) (SBAR (WHPP (IN at) (WHNP (WDT which))) (S (NP (JJ electrical) (CC and) (JJ magnetic) (NNS properties)) (VP (VBP are) (ADJP (JJ different) (SBAR (IN than) (S (NP (PRP they)) (VP (VBD were) (PP (PP (IN at) (NP (JJR lower) (NNS speeds))) (CC and) (PP (IN at) (NP (JJ macroscopic) (NNS sizes))))))))))))))))) (. .)))
we are reaching physical dimensions and switching times in the head and media at which electrical and magnetic properties are different than they were at lower speeds and at macroscopic sizes.
(ROOT (S (NP (JJ Different) (NNS media)) (VP (VBP affect) (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (NN light))))) (. .)))