Your question, Why will there a a half hour of silence in heaven (Revelation 8:1)? Actually it states about a half an hour which indicates it will be less. The event is fulfilling scripture. We see Paul explaining that in order for the man of sin can be revealed the Holy Spirit must be taken out of the way for a time. That starts here when the seventh seal is opened. This is also fulfilling the scripture of a famine to take place. That God speaks of in Amos. Amos 8:11 “Behold, the days come, saith the Lord GOD, that I will send a famine in the land, not a famine of bread, nor a thirst for water, but of hearing the words of the LORD: And they shall wander from sea to sea, and from the north even to the east, they shall run to and fro to seek the word of the LORD, and shall not find it. This event also begins with the seventh seal. As to the amount of time that the Holy Spirit is taken out is debatable. We see it stars with the seventh seal and we see John is told the word must be preached again starting in the timeframe of seventh trumpet. As to how much time is between the two only God can tell.