The right to privacy cannot be absolute. Sometimes, people do things that endanger others. We need rules that define when people's actions justify searching their possessions and perhaps arresting them based on the evidence so gathered. The Fourth Amendment says that search or arrest is reasonable if the officials doing it have gotten a warrant from a judge. The Supreme Court has ruled that some searches are reasonable even without a warrant, such as when circumstances make it very difficult for the officials to stop what they are doing to get a warrant before continuing.
Officials have to show the judge that there is probable cause before a warrant can be given.
That means they have to convince the judge that a reasonable person would believe a crime has taken place and that the warrant is needed. To prove probable cause, the officials need to gather evidence, which can't be obtained through unreasonable invasions of privacy. In less technologically advanced times, it was easier to draw the line between acceptable means of gathering evidence and unreasonable invasions of privacy. If police officers sneaked onto a suspect's property without a warrant to listen to a conversation at an open window, they were trespassing and the search was unreasonable. All evidence gathered was illegal and inadmissible in court.