Information Extraction: Relations, Events, and Time

21.4 Representing time

1. What is Allen’s interval algebra?
   A widely used model in which all events and times are intervals (no points), related by 13 primitive relations such as before/after, meets, overlaps, starts, during, finishes and equals (with inverses).

   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 21, §21.4, p. 486
2. Why is the relation between verb tense and time not straightforward?
   The present can refer to the future (“we fly from San Francisco to Boston at 10”), and past tenses differ in ways not captured by “before now” (“Flight 1902 arrived late” vs “had arrived late”).
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 21, §21.4, p. 486
3. What is Reichenbach’s reference point?
   A time R separate from the event time E and utterance time U, used to explain tenses: in “When Mary’s flight departed, I had eaten lunch”, the departure is the reference point and the eating precedes it (E < R < U); with “I ate lunch”, eating starts at R.

   tense	order
   simple past (I ate)	E = R < U
   past perfect (I had eaten)	E < R < U
   present perfect (I have eaten)	E < R = U
   simple future (I will eat)	U < R = E

   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 21, §21.4, p. 487
4. How are temporal expressions in English often organized metaphorically?
   In spatial terms: at, in, somewhere, near (“somewhere around noon”, “at 6:45 in the morning”), one domain systematically expressed through another.
   • Jurafsky & Martin, Speech and Language Processing, 3rd ed. draft, ch. 21, §21.4, p. 487