The Real Science of Forensics
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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 😉 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
patrons on Patreon who make this show possible. If you want to help us make episodes like this,
just go to And don’t forget to go to
and subscribe. [Intro/Outro Music]


  • Nia Bride

    4:50 What if, just what if, you can find out the pixel value using reverse math? Like, this specific value holds not only the colour, but another infos too. such an pixel value disclose the color values and position of neighbouring pixels too. not in an expended way, of course, then the compression would have no use. I am almost sure, that there is some math who can figure this out. Is it?

  • macsnafu

    2015? How did I miss this episode? This is great stuff! While modern crime dramas certainly seem more realistic than say, shows from the 1970s, they are still very much works of fiction. But it's time for a follow-up to this episode, as it is now 2018. Has forensics technology had any recent breakthroughs?

  • Nicole Jenanian

    Why do females nest when pregnant or sometimes when moving? Do men do this, or something else that is similar?

  • Kailen Patel

    Somethings not right. Charlie jumped out of a bush and they found out the 'killer was him' by looking at his shirt. If he was waiting outside and ,as explained in the beginning of the video, as it was a 'cold winters evening in Chicago' why would he even be wearing a shirt because he would have been really cold as he was waiting for the victim before he entered the shop and when he came out. Also, why did the camera not catch charlie going into the bush if it had seen him leaving it.

  • feebleh!

    Wow, great video! I was really curious as to how forensics and the people involved work and it's pretty interesting to me now that I understood how it is ?

  • Pete Rhein

    That's it? "We're out of time" ? Fine, I'm gonna throw a temper tantrum right now.
    Why couldn't Sci-show have a part 2 of this? A very rare Thumbs-down.

  • Meta Somma

    Not one mention of the cause of death? I feel like blunt force trauma would be easy to spot. Or did I miss that part?

  • AgainstOdds

    Okay, but why the chalk outline from 1:30 to 1:40?

    Did you feel like, as part of a YT video, you are part of the entertainment industry and have to add at least some obligatory inaccuracies in there?


    Im finishing on my Forensic Science Bachelor degree and this video is a really nice compilation of knowledge i had to go 4 years to school to learn.New subscriber ???

  • Lilith Alucard

    I thought the temperature drop rate was 1.4, But I suppose .1 doesn't really make that big of a difference in the end.

    Love these kinds of videos. Thanks for that and keep up the good work.

  • Shannon Kelly

    Fun fact: Insects can also be used on forensic investigations to more accurately ID the time of death. They can sometimes even indicate the cause of death e.g. drug overdose.

  • marinus18

    One thing I saw once that I'm curious about is that one was beaten and then hit on the head just after death. Could you discern an injury that was sustained less than a minute after death from one sustained before death?

  • Angry-Moth-Noises

    I wish you would have talked about finger prints. Everyone thinks finger prints is a definite way to catch someone. But what they don't tell you
    1. dusting for finger prints is REALLY messy. Because of well, dusting for them. A fine powder is dusted over a finger print and it sticks to the oils that the skin as left over. Then they use scotch tape (or any other clear tape) to stick onto the dust and lift the imprint and then put the peice of tape on paper (normally in the opposite color of the dust). Lifting them can be rather tricky to get a readable one, as its very easy to mess them up or when you lift one only part of it is lifted.

    2. Reading finger prints is done by humans and is open for human error. When you are checking finger prints, your looking for similarities between the one at the crime scene and at a database. When you match a characteristic between the two its called a point. There is no stranderd for how many points someone needs to find. This means they could look for at little as 5 or 50 points. A study was also done that the more emotionally charged a person was about a crime (Like the murder of children come paired to robbery) they more likely people will try to find points on finger prints to force a match.

    3. Not everyone's finger print is in a database.

  • Forensic Marts
    World's first educational profiler in the field of Forensic science
    A platform to share information regarding events, books, research papers, current news in the field.

  • Gsuave

    Wait, WHERE had rigor mortis set in? Depending on the extent of it, the body could be 2-12 hours old, maybe longer (though that end doesn't matter in this case due to the receipt. Rigor mortis starts at the head, then over time works its way down the body until its at its peak at 12 hours. Then it wears off, taking about another 10 hours to do so. If someone was exercising, running for example, then the body part they were using would show signs of rigor earlier as well.
    In case anyone is wondering why i know this, i took a forensic science class.

  • John-Luke Lindsay

    I’m going to be completely honest, I live in Florida and my school was competing in a competition on forensic science, we were the underdogs as we have never competed before and I didn’t expect much since we were just testing the waters, so for three months me and my partner did not study as we should have due to the fact that we didnt think we would win anyway and just prepare for next year afterwards. After the three months had passed we watched this video on the bus ride there, this 10 minute video allowed me and my partner to place second place in my division against schools that have been studying all year, my team and two others are now going to nationals. So I just want to give a huge thanks to this channel for literally being the only thing that helped me make second place and move on, and spark my interest for forensic science. Now that I’m going to nationals though, I will have to study lmao. Thanks again guys!!!!

  • Cherry Blossom

    We had to tell our teachers what we wanted to be when “we grew up” and we all went up on by one. And I say “a forensic scientist.” And my teacher says “well that’s a fist.” And now everyone tells me I’m gonna be “a murder inspector.” It’s kinda funny.

  • Jackie

    wouldn't the police have checked for any signs of possible injuries- so like a wounding the body; and fingerprints on the possible murder weapon

  • Lilian N

    My opinion on the crime scene:
    Lawyer: Igiari
    Everyone: What!??!!?
    Lawyer: If Charlie is a murderer, why would he leave all the clue behind and hide the weapon in a place where someone could find it? Why would he go back to the store and hide the weapon there even though he knew that there is a camera? Why did he commit it? And if that Charlie the real Charlie?
    Also, the police are terrible at investigating crime scene. I think they should have checked behind the tree for hair, fingerprint, anything that contain DNA. They should also do a DNA check on the weapon, it could be someone using it to kill Tom and then force Charlie to take it to the store. They should also investigate further to see if Charlie has been blackmail.
    They should also check if there is any other DNA on the victim body because I don't think that Charlie couldn't kill a person by simply hitting the head with a wrench. Surely, Tom would scream and try to get away or fight back. Charlie would have to stop Tom and that means leaving fingerprint on Tom.
    There is also a possibility that the Charlie we see in the camera is not the real Charlie. In the video, they didn't check if there is Charlie DNA nor question him and other witnesses. That Charlie could be a fake and is trying to frame the real Charlie. How could this be a fake Charlie, you may ask. Is the Charlie really there.

    What happens:
    At 5:00, when Tom leaves the store, the fake Charlie was following him (with a wig, some makeup and a different shirt). He kills Tom at the alley and leaves all the evidence there (It could be a piece of evidence if Charlie is really clumsy and it's his first kill). Then he took off the costume and head to the store at a different time, before the real Charlie come. He pretends to buy something at that section, but he was really just hiding the murder weapon there (The wrench could use one by Charlie for something else and fake Charlie could have stolen it or he could be a friend or relative to Real Charlie. The wrench could also be hidden so that no one else see it). He leaves the store and the real Charlie come and look around, mistakenly into the section with the murder weapon (Charlie could have seen something interesting that he might want to buy). Charlie leaves the store.
    And that is how the murder work.

    Why am I analyzing a fake crime scene this much?
    Thanks for reading. I always like to watch detective show and movie and this crime scene just seem odd to me.
    This is all just my opinion.

  • Meme Meme

    Am I the only one planning to take up Political Science and Forensic Science? Just really want to be a CSI and a lawyer

  • Forensic Marts

    Please watch the video and join us on the website and also subscribe the channel.

  • Manj J

    Can we have a part 2 to this pretty please? It's so interesting and as a writer this is really helpful especially when you want to include accurate stuff without too much detail or having to do too much research because of time and other restrictions

  • Tári Faelivrin

    Yes! Just yes! I'm a university student and I study forensic science. So just yes. You are right. This dear friends is basically how it works. This video made me so happy.

  • Axel CHEMIN

    Superresolution (use in astrophotography) can be used to zoom in a pixelated image

  • Mr Jerkface

    great. except for the almost total lack of science in 'forensic science':

  • Willow Smith

    A catalyst doesn't magically make a reaction happen, it just speeds it up. So, it might seem like a reaction doesn't happen without the catalyst, it's just very slow

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