The Real Science of Forensics

The Real Science of Forensics


[Intro/Outro Music] Let’s talk about crime shows. In a nonstop media stream filled with reality shows, cooking competitions and
whatever is happening in Westeros, police procedurals are probably as close as most of us are going to get to seeing science
portrayed in prime time. And the techniques that crime fighters use to catch
“bad guys” vary from show to show, but a lot of the time it involves forensics, which is basically the use of science in the field of law, in this case, criminal law. Different kinds of forensic investigators have different roles, like analyzing crime scenes or running tests in the lab, and they can specialize beyond that, focusing on analyzing DNA or bullets for example. Generally, they all have an undergraduate degree in a scientific field, like chemistry or biology, or a more targeted degree in forensic science itself. Some have a graduate degree, too, and medical examiners, or ME’s, usually have a degree in medicine. But they all have one thing in common: using science to find, gather and analyze
evidence that can be used in court. However, Hollywood seems to think that real science doesn’t always make for entertaining TV, so writers tend to take some liberties
with how forensics really work. Most of the time they aren’t completely off the mark, for example, the tests they use on the show
might actually exist. But they wouldn’t be nearly as fast
or accurate in real life. And the technology they use is just … ridiculous. We’re here to clear that up, and talk about
what forensics can actually do, which turns out to be pretty interesting all by itself. And to do that we’re going to solve
a hypothetical crime. So here’s our case: someone finds a dead guy
in an alley in Chicago. The cops secure the scene and the forensic investigators show up around 11 PM to gather clues. When they go through the victim’s pockets, they find a receipt for a bottle of soda from a nearby convenience store time-stamped at 5PM, 6 hours earlier. And, according to the ID in his wallet, his name is Bob. The medical examiners wanna know
how long Bob has been dead, which could be key to finding and catching his killer. So there are a few things they can check,
and they all happen to end in the word “mortis,” which makes sense,
’cause that just means “death” in Latin. First there’s livor mortis, or how the blood pools. Now that Bob’s heart isn’t distributing his blood anymore, it just goes where gravity takes it, and that makes the skin look
purple-ish from the outside. But if a body’s been dead for more than 12 hours,
the blood will have coagulated or dried. It stays in place, and if you shift the body,
the blood won’t pool in a new spot. Now, Bob’s blood seems to still be very liquid,
so he’s been dead less than 12 hours. Though of course, the examiners already knew that, since he was alive and well in a convenience store
only 6 hours ago. Next they check if rigor mortis, the stiffening of the muscles after death, has set in. Rigor mortis is proof that your muscles work in kind of the opposite way than you might expect. Since running and lifting weights, and doing things that require your muscles is hard, you might think making your muscles contract
requires a lot of energy, but, that’s not true. Your body actually uses energy
to make your muscles relax, not contract. So after somebody dies, and their muscles
stop getting chemical energy, their muscles can’t un-contract, so their bodies stiffen. The effect starts about 2 hours after death
and lasts until about 36 hours in, when the muscles decompose enough
that they can’t hold their position anymore. In Bob’s case, rigor mortis does seem to have set in,
he’s frozen in place, so the body is probably more than 2 hours old.
They would like to get a more accurate number though. If the body is close to 6 hours old, that means he was probably murdered right after he left the store. So, they take the body’s temperature … rectally … a detail they don’t normally show in crime dramas,
and it’s 29 degrees Celsius. Now, normally, a body loses heat at a rate of
about 1.5 degrees Celsius per hour, a process known as algor mortis. When Bob was alive, his body temperature
would have been 37 degrees, so it has lost 8 degrees so far, you’d think Bob’s been dead very close to 6 hours, and in a TV show, the ME would probably say that. But there’s a problem, this is a cold winter evening in Chicago, and it’s about 5 Degrees outside. The body is going to lose heat a lot faster to the colder air, but it is hard to tell exactly how fast. Given all the information they’ve gathered, our MEs
put the time of death between 5 PM and 7 PM, there’s no way to tell if Bob was murdered right after
he left the store, or two hours later. So, the detectives head to the store and ask to review the security camera footage, hoping they’ll be able to figure out
if anyone was with Bob when he bought his drink. Turns out that as Bob left the store, the camera picked up someone quickly emerging behind a nearby tree, to follow him down the street. But it was so far away that the stalker’s face
is all pixilated and blurry, you can hardly even tell it’s a face, let alone whose it is. Now if this were a TV show, usually the detectives would zoom in on the face, and enhance the image … somehow … and then run the magically clear photo through a facial recognition database. And then maybe the next part of the story is they get
a match, which leads them to another clue. But, in real life, there’s no way they could
enhance the picture like that. When a camera captures a digital image, it’s recorded as data that forms a map of the colors in each point, or pixel, in the picture, and those pixels cover a bigger or smaller space depending on the resolution, or how many pixels are in that image. The color of each pixel is recorded as the average
of all the colors within that space. But once the color is stored as the average, that’s it,
you can’t enhance the resolution of a photo. Because there’s no way to tell which amount of which colors went into that average in each pixel. Let’s say that your camera has 8 megapixels, which is pretty typical for a smartphone. That means it takes a picture with 8 million pixels in it. That sounds like a lot, but let’s just say you wanna
take a picture of something really small, or really far away, like a person
at the other end of a field. You can zoom in a lot, but you still probably
won’t be able to make out much detail. Whatever’s written on the T-shirt, for example,
might just look like a few blocky, dark green squares, and there’s no way you can enhance those squares to see that the dark green pixels are just averaging together the bright green letters on a black background that spell out “SciShow”, which, of course, are available at DFTBA.com/SciShow. 😉 If you wanted to be able to make out what’s on the shirt, you need a lot more pixels that would each depict
a smaller area of that mysterious figure. But let’s say our real life detectives look through
some more of the footage and realize the person who was following our victim was actually back in the store about three hours later. They can tell, because he was wearing
the same clothes. The camera captures him as he puts something down on a shelf, then leaves. They get a close up picture of his face, and run it through the database. Facial recognition actually has a long history, because it’s one of those things humans tend to be very good at. But it’s hard to get computers to do well. Humans are excellent at finding patterns, but the computers have to be taught what to look for. Human features, as it turns out, are arranged
in very specific ways, but the specifics are unique to each person. For example, everyone has a certain curvature to their eye sockets, or distance between the nose and mouth. Those dimensions are different for everyone, but computers can be programmed to measure them, then use the data to identify faces. Together, these metrics make up a faceprint, and there are actually databases of faceprints
compiled from things like mugshots. A computer can take an image of a face,
like the one of a man following Bob, and compare his faceprint to ones
already in the database. On TV shows, that database will usually shown as a sophisticated system, with all the data in one place. All the detectives have to do
is type in some commands on a keyboard and the computer starts cross-referencing
with every picture ever taken in the country. But that kind of law enforcement database
doesn’t really exist, at least, not yet. The FBI is working on what they say will be
the world’s biggest database of biometrics, with tens of millions of records. But for now, if cities have searchable
faceprint databases at all, they’re usually local. Chicago, for example, has one called NeoFace
that looks for matches in the police photo database. In our case, the investigators catch a lucky break. When they run the suspect’s faceprint against the Chicago police department’s
database of mugshots, they find a match. His name is Charlie, and he owns a hardware store a few blocks away. When the investigators inspect the shelf they saw him putting something on, they find a wrench with a dark red stain. Thinking that it might be their murder weapon,
they take a swab of whatever’s on the wrench then do something called a Kastle-Meyer test
to see if it’s blood. On TV, you might just see them spraying
some liquid onto the swab to see if it changes color, but in real life, they’ll need to use
two different substances. First they add the chemical phenolphthalein to the swab, then a couple of drops of hydrogen peroxide. If there’s blood in the sample, the two compounds
will react with each other, turning the phenolphthalein a vivid shade of pink. Blood contains hemoglobin, which acts as a catalyst, basically, the substance that makes a reaction happen. With hemoglobin’s help, the peroxide reacts with
the hydrogen in phenolphthalein and becomes water. The new hydrogen-less form of phenolphthalein
then turns pink. If there were no blood on the wrench, the reaction wouldn’t happen because it wouldn’t have a catalyst to help it along. But in this case, the swab from the wrench does
turn pink, meaning that the stain is probably blood, so the investigators take the wrench for further testing. Back at the lab they run a DNA analysis on the blood from the wrench and compare it with Bob’s. If it’s a match, they’ve probably found the murder weapon, and their murderer. Now this test actually works
pretty much like it does on TV. DNA is the molecule that makes you who you are, long strings of four different compounds or base pairs, in a particular order, and everybody has their own unique set,
except for identical twins. So, if you have a DNA sample, that’s a really good way
to identify someone. But forensic teams don’t just sequence everyone’s DNA, instead, they usually use a technique known as
STR analysis to match DNA samples. It’s based on the idea that everyone’s DNA has
certain sections with repeating patterns of base pairs, but the number of times the pattern repeats itself
varies from person to person. The STR looks at 13 of those repeating sections, and the odds of two people having the exact same base pairs in all 13 are about one in a billion, meaning there are probably only about six other people
in the entire world who have the same STR profile as you and forensic experts figure that’s accurate enough. Plus, it takes less than an hour and a half to run. So, in our case, investigators find that the blood
on the wrench did come from Bob, which certainly makes Charlie a suspect.
But there still are open questions. Was Bob’s encounter with the wrench what killed him? What someone else involved besides Charlie? Unfortunately I can’t answer those questions for you because we’re out of time
and Game of Thrones is about to come on. But thanks for watching, and thanks especially to our
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