By Daniel Hubbard | September 20, 2010
One of our physicist friends dropped in to visit last week when she was in Chicago for meetings. It’s always nice to catch up with someone you don’t see very often and also with what is happening in physics. As usual one thing reminds of another and eventually I find myself thinking about genealogy. Sometimes apparently very different endeavors require solving rather similar problems.
If we are lucky our ancestors left a paper trail behind them—a marriage record here, census entries there and a land record or two. The problem is that you don’t start with the trail, you start with the records, some of them from the right trail, some of them from other trails. That is very different from, say, talking to grandma. If you talk to her an hour later, you know that it is grandma. You don’t need to question if she is the same person that you talked to earlier. You probably don’t need to question if this story about grandpa is about the same person as the last grandpa story was. The connections are there from the start.
In particle physics the particles leave their own “paper trails” as they pass, not through life, but through detectors. One of the difficulties with studying minuscule particles is that you can’t see them or hear them or touch them individually any more than you can chat with a distant ancestor. In our daily lives we are used to things being on scales of space and time that match the workings of our senses and it is those kinds of things we find easy to think about. Our eyes can see just about anything that we might want to manipulate. When a marble rolls across a table we are capable of perceiving a continuous sound, not just occasional noises. Even those first long-ago scientific instruments, which allowed us to gather information beyond what our senses could detect, only enhanced the senses we already possessed. The microscopes in high school biology class allowed you to see tiny cells that you could not see otherwise. Yet you were still seeing. In short, our senses work very well for tasks like talking with grandma. Our brains have an easy time thinking about those tasks as well. Our daily experience of our surroundings is a lot like what our experience of genealogy would be if we could only talk to grandma and do nothing else to pursue our past.
Putting Together Pieces
You can’t have a conversation with the ’49er in your pedigree and get all the details. He is gone. It was too long ago. Instead you need to look at records. There are some objects that you just can’t examine and get all the details. Some objects are too small. At some point, they become too small to see directly no matter how good a microscope you might have. Instead, when you get down to high speed subatomic particles, you need very special detectors to examine them. The workings of our senses have been left far behind. We are used to being able to continuously sense those things that have importance for us and immediately perceive them as separate and whole things. With particles, that sort of continuous experience of clearly discernible things is virtually impossible.
A detector might be able to report that something passed through along this horizontal line, along that vertical line and at a specific distance from the source of the particles. Similar information may come in for different distances. None of this is like the virtually continuous information of our normal world. It comes in at different times from different places with nothing directly knowable about the gaps in between. It is more like a baptismal record here and a census enumeration there. Individual particles aren’t immediately apparent either. Each horizontal line that records the passage of a particle is totally independent of each vertical line. All those lines that were hit must be matched so that the point at which the particle crossed the detector can be reconstructed and the path of each particle can be known.
You can imagine putting a chessboard down in front of you and covering a row of squares with masking tape (please don’t do this for real with a cardboard chessboard). Now imagine putting down a strip of masking tape over a column of squares. Where the tape strips cross is where an imaginary particle went through your chessboard. If you had a real detector, electronics would report the hit strips to a computer that would tell you where the crossing point was. Next tape over a second row and a second column. Now there is a problem. There are four crossing points but probably only two particles. If more than a single row and a single column detect something you will need more criteria to decide what happened. You need more information. Perhaps data from other detectors or more information from your chessboard. Maybe each row and column that detected the passage of particles can measure an amount of energy that could help with matching horizontal to vertical.
Researching Records, Reconstructing Ancestors
There is much here that parallels genealogy. With ancestors, it isn’t size that is the problem but all the time that has passed. When we research long-dead ancestors, they don’t come ready-made like living people. There are only independent records that need to be matched together using some appropriate criteria. We like to talk about researching ancestors but at the beginning, it is records we are researching. We don’t know about the ancestor before we analyze the records any more than a particle physicist knows about an invisible particle before all the detectors’ data has been combined.
The physicist knows that the information from the detectors is the only reason to believe that a particle passed through and so can’t make any initial assumptions. The genealogist has the odd handicap of knowing that they have ancestors. Knowing that, assumptions can be made and the assumed ancestor, not the records, becomes the starting point. Records get matched to the one and only person that we have in mind whether they really match or are just close. Yet, there are often multiple people that leave records at the same time and same place in such a way that they can be confused for each other.
When a computer tries to figure out a complicated pattern in a set of particle detectors it may try several hypotheses for how the different measurements can be combined into sensible paths. One set of paths might work well until another bit of data is added that throws everything off. A genealogist may try to combine a set of records to reconstruct one person only to discover that adding a little bit of new data turns that person into two people. There is an “ah ha” moment. Maybe most of your records remain with the “John A. Doe” from your first hypothesis but some others become records concerning a different man, “John O. Doe.” Another discovery may lead to a solution with two people but not with the same records assigned to them as before. Perhaps some records for “John Doe” move from your John O. Doe pile back to your John A. Doe pile. Just like the physicist and computer program that needed to switch from one set of particle paths to another as they try to connect the dots through the detectors.
Separate bits of data need to be respected as being truly separate until you can begin matching them, one with another. There is both an art and a science to doing that matching, be it in genealogy, particle physics or some other endeavor. It might be a bit obscure to someone who hadn’t read this post but that final chessboard could have been entitled, “Which possible record matches were real individuals?”Twitter It!