Daniel and Kelly’s Extraordinary Universe - What is a light cone?

Episode Date: August 19, 2021

Daniel and Jorge explain how the speed of light limits how events play out in the Universe. Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

This is an I-Heart podcast. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System

On the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone. Hold up. Isn't that against school policy? That seems inappropriate.

Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Every case that is a cold case that has DNA. Right now in a backlog will be identified in our lifetime. On the new podcast, America's Crime Lab, every case has a story to tell. And the DNA holds the truth. He never thought he was going to get caught. And I just looked at my computer screen. I was just like, ah, got you.

This technology's already solving so many cases. Listen to America's Crime Lab on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Hi, it's Honey German, and I'm back with season two of my podcast. Grasias, come again. We got you when it comes to the latest in music and entertainment with interviews with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't auditioned in like over 25 years.

Oh, wow. That's a real G-talk right there. Oh, yeah. We'll talk about all that's viral and trending with a little bit of cheesement and a whole lot of laughs. And of course, the great bevras you've come to expect. Listen to the new season of Dacus Come Again on the IHeartRadio app, Apple Podcast,

or wherever you get your podcast. Hey, Jorge, do you ever wish that the speed of light was faster? I guess I'm not dissatisfied with the speed of light. I'm just glad it's not slower, because, you know, I don't want to wait 10 minutes after I flip on a light switch. But, you know, if it was faster, and then we could see, like, further out into the universe. That's true. And then we might be able to travel further to explore the stars. Also true, but there's another side to it, as always.

What? How could it be bad to have a faster speed of light? light. Well, if light speed was a million times faster than technically any alien out there in the universe could kill us instantly with a death ray. That's true, but you know, witnessing an alien death ray would be pretty cool. It would be pretty cool for about a nanosecond and then you'd be dead. Also, I'm pretty sure a death ray would be hot and not cool. Might still be worth it. Just for that nanosecond, you would know that aliens existed. As my brain is getting fried. Hi, I'm Jorge. I'm a cartoonist and the creator of PhD comics.

Hi, I'm Daniel. I'm a particle physicist and I don't spend a lot of time worrying about alien death rays. You don't? Do you know something I don't? I know that the speed of light protects us from most of the universe shooting us with alien death rays. Really? Like it forms a force field around us? Protecting us? Like, Like a blanket, helping us sleep at night. It just makes it kind of ineffective. If you're going to shoot an alien death ray millions of light years, then you're going to kill your target in millions of years.

And like whose geopolitical strategy really has that long an arc to it? But what if there are patient aliens? Then we're toast. Then we might literally be toast. Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of iHeartRadio. And we beam our joke rays into your brain. Hope it to transmit some of the incredible wonder and knowledge about

the universe. All the things that science has learned about this bonkers, wondrous, beautiful glittering cosmos and all the things that science is still working on. All the big questions about how the universe got here, where it's going, and who lives in it. Yeah, and how to actually build a joke ray. I mean, that's probably the next Nobel Prize. The question is, does it make you grown or does it make you laugh? Is that where you're going to ask when the aliens come? I'm going to be like, tell us the secrets of space time. And you're like, have you developed the joke ray yet? Well, think about what would mean for the mental health of humanity. You know, we could just have a laugh about everything.

Laughter is the best medicine. So anytime you go to the doctor, they're just like, yep, we're prescribing you five shots with the joke ray. And then as the aliens, as they, you know, enslave us and possibly eat us, we could be laughing all the way. But it is a bizarre and jokey universe because it seems like a lot of it is beyond our control or even beyond our ability to ever interact with it. That's true.

And it's also sometimes beyond our ability to. to really understand. We have one way of thinking about the world, the way things move and slide, the way time works. But about 100 years ago, we discovered that that wasn't really the way it works, that there were different rules.

The things change when you move fast or when space is bent. And that we needed new rules and new ideas to understand what actually goes on in this universe. Yeah, because I guess we realized a few hundred years ago that the universe, the world, doesn't really behave the way it behaves like on an everyday basis.

Like when you're juggling baseball, or, you know, bouncing at basketball. Like, that's one way that we think of the world is working. But really, like, if you get down to it in some situations, it's totally different. Yeah. In fact, there are really fundamentally different rules that govern how things move and how time flows. It just happens to be that in certain situations, like when you're moving really slow and when you're far from a black hole, things look a lot simpler.

So we're living in sort of a simplified corner of a very weird, very complicated universe. right it's almost like we're in a special case like our perception of the universe it's like a small example of all the ways that the universe could be it's like you're playing the warm up levels on some video game before all the bad guys come in just so you can sort of like figure out how the controls work right that's what we've been doing for physics for like thousands of years and now we finally understand the full scope of the game and wouldn't it be great if we could respawn in this universe we can just keep trying well we sort of do i mean like physicists start from where previous generations generations of physicists ended, right? We don't have to start from scratch on physics every single

generation. So physics sort of responds in our minds. Right. Are we going to run out of lives at some point? Do we need to put in more quarters into the arcade of the universe? That depends on the alien death rate. Or the joke rate. Either one might help us or not help us. But yeah, the universe is kind of tricky. And it's tricky not just in like extreme situations, like in a black hole or moving close to the speed of light. Because who knows? Like we could be moving right now. really fast compared to other aliens and other galaxies, right? We are. We are moving really fast compared to the sun

and the sun is moving really fast compared to the center of the galaxy. But all those things are kind of abstract. What really matters is your speed relative to the people who are watching you and relative to your clock. And so that's something that special relativity has taught us is that

the world looks different from different perspectives. Based on who's doing the measuring, you can get kind of a different account of what's going on and what's happened in the world. Yeah, you might not agree with your relatives about relativity. But yeah, things are kind of tricky in this universe. And so even physicists sort of struggle with it, right? I mean, it's sort of hard to wrap anyone's head around these topics and how it actually works. And so physicists have some ways to kind of think about how these things work that might help other people. Yeah, because in

science we have our intuition, like you have a feeling about how things might work and what might happen and how things are put together. And then there's the mathematics. And the mathematics is what actually happens, what really is going on, the rules that really do control the universe. But sometimes you're looking at like a page of equations and you're like, hmm, I don't see it. I don't really get it. And for me personally, it's easier to think about these things like geometrically to understand like the relationship in space and time of these things. So sometimes you can get more of an intuition for weird things. If you think about their structure and space and time, geometrically like the shapes that are created by all these ideas right and then you draw cartoons

about it which is where i come in right exactly it helps to draw cartoons no joke cartoons are a great way to visualize this stuff that's why you and i work together on a lot of these books and folks out there should check out our book we have no idea which has all sorts of really clever cartoons visualizing really tricky concepts yeah in our upcoming book which is coming out in a few months, frequently asked questions about the universe, which you can pre-order right now. Man, we totally snuck in those book plugs, Daniel. Totally unplanned. I keep getting emails from listeners saying, you guys should write a book.

I'm like, what? Do we not talk about the book enough on the podcast? I guess we don't. We should talk about it every time. Yes, I think we should, honestly. Anyway, back to the podcast. Yeah, back to the podcast. Let's talk about one of these tools that physicists use to kind of wrap your head around

some of these tricky concepts about relativity and the speed of light. So today on the podcast, we'll be asking. the question. What is a light cone? Now, Daniel, this is a light cone. I guess you mean light, not like not heavy cone. Yes, exactly. Light as in photons.

Is in that thing that speeds along at the maximum speed of the universe. Now, this sounds sort of like maybe a dessert, but maybe it would not a dessert you want to eat or that would taste very good. Sounds like the kind of dessert your mom would order for you when what you really want is a scoop of ice cream.

What you really want is the heavy cone, the heavy cream cone. You don't want the like fat-free light cone. But yeah, so this is a concept that comes up in special relativity and it has some pretty interesting consequences about causality and like what's actually real or like what's actually happening or happened in the universe. Yeah, and it helps you think about what's going on inside a black hole. And even possibly maybe allows you to figure out a way to do

real life time travel. Sounds like a very powerful dessert here. All right. Well, before we go on, we just want to give a quick shout out to Bjorn, right? He sort of helped inspire this episode, Daniel. Yeah, thanks, Bjorn from Sweden, who sent me a link to his solar system inspired music, which I listened to while preparing for this podcast. You can find him on Spotify at Orch Cloud Services.

Check it out if you like the sort of space ambiance music. Thanks a lot, Bjorn, for sharing your music. Yeah, and again, that's Orch Cloud. O-O-R-T cloud services on Spotify, which is a pun, because it's like the org cloud and also cloud services. And what is the org cloud done for us recently? Well, I guess it delivers spacing music. And you can upload your pictures up to it.

You might never get them back, but yeah, you can upload them. Well, it probably takes like 20 years just to get there. And then the download speed is probably even worse. All right, well, we were curious as usual how many people out there are familiar with the concept we're talking about today. So Daniel went out there into the internet to ask people, what is a light cone? And if you are out there on the internet and always wanted to participate in a podcast, please write to us to questions at danielanhorpe.com and you can be unprepared to answer future

questions for the podcast. Right. And if you're not on the internet, please tell us your secret. I mean, I would like to sometimes not be on the internet. I guess we're on the radio too, right, Daniel? I think some stations broadcasts, so they might be a little. listening on the radio. Yeah. So thanks everybody who's out there listening to us on the radio as well. We appreciate all of your support and all of your listening. Did you know that we recently passed

15 million episode downloads? Wow. Really? That's amazing. That's like 50 million hours that people have been listening to us. That's like 15 million banana jokes at least. At least because we're a multi-banana joke death ray, a joke ray here. We try to in our best episodes. It's like a shotgun rate joke rate, really. We just splatter, banana peels all over the place. Now I'm imagining 15 million tiny bananas coming out of the nozzle of a gun at very high speed. Coming out of our mouth. That's a little disturbing.

But anyways, think about it for a second. If someone asks you what you think a light cone is, what would you say? Here's what people had to say. I would say that's what comes out of the shiny end of a flashlight. I believe this is a space time diagram where it shows a three-dimensional. object and as you go up that's time passing so it makes a cone like if you look at the diagram of the big bang and we show a tiny point then it slowly goes up into a cone as time progresses um i don't know what a light cone is i'm thinking about there was some propulsion idea that was

i think it was largely theoretical but it was to do with kind of using light to push spacecraft and it would capture the light using light sails. I don't know if that would be to do with it. I remember seeing a model in the Science Museum in London, but it was a cone, but that was probably just because it was

made of plastic. A light cone is a representation of how light travels from a point. For instance, if you have if I have a flashlight in my hand and I turn it on and the light will travel in space and time

like a cone you can represent it as a cone that is something I've heard of before it's the realm of like what you can see so it's like 93 billion light years across in our observable universe because that's like how far we could observe or something into space or like the photons that have reached us from that distance although i don't understand how that's bigger than 13.8 billion light years since that's the

age of the universe but i'm still quite unclear on that but i know that the light cone is basically all that we can see within our realm of light uh anything that can reach us in any form i think a light cone is an ice cream cone that's been left out in the sun too long and all the ice cream melted. A light cone is an ice cream cone that has no chocolate ice cream in it whatsoever, which is fundamentally flawed, and I think it violates one of the laws of thermodynamics, because why would you want vanilla ice cream when chocolate is an option? So light cones make no sense. All right. A lot of people went with my interpretation, which is that it sounds like a dessert, possibly, and that it's light. Yeah, and we also.

heard some people throwing shade on vanilla ice cream. Wow, that was rough. I know. How can you be down on vanilla ice cream? It's like the nicest flavor. Gets along with everybody. That's true. I mean, I would always pick chocolate over vanilla, but you know, if all you got is vanilla, I'm happy to eat it. What if you take vanilla and sprinkle chocolate on it? See, that's the versatility of vanilla. Or take chocolate and sprinkle chocolate on it. Still better. But you can't take chocolate and sprinkle vanilla on it. That's just wouldn't like. No, no, what do that. See, vanilla doesn't add to anything. It's just sort of like the substrate. What do you mean?

It makes most cookies taste good. It's just sort of vanilla. There's a reason why they call it vanilla vanilla, you know? Because it's so dependable and solid and just always there for you. Right, yeah. If somebody reads your writing and they describe it as vanilla, would you take that as a compliment? Depends.

Am I writing the back of an ice cream carton? Then it would meet its goal. But yeah, so a light cone, Daniel. Tell me, this is something I was surprised to see. in our topic because it is like a graphical interpretation of physics, right? And so it's a little tricky to describe on a podcast. It's a little tricky to describe on a podcast. We'll see how we do. But I thought it was useful because I find that every time we talk about special relativity, I have light cones in my mind. I got these things in my brain. They really help me think about sort of the structure of

space and time. And they lead to a lot of interesting consequences. And so I hope if we can somehow download the concept of a light cone into the minds of our listeners. It'll help them think about special relativity. We need like a light cone ray to shoot this into people. Yeah, I think it's a podcast starring me and you talking about physics. Throwing out banana jokes. All right, well, step us through it. What is technically a light cone? So a light cone defines the part of the universe that you can influence, the part of the future that you can visit. Geometrically, you can think about it like if you turn on a flashlight for a moment, so you shine photons in every direction, they leave you in sort of a sphere. After one second, they are one light second away from you.

After one year, they are one light year away from you. So imagine a sphere, and then that sphere sort of growing as time goes on, right? So that set of spheres versus time, as those spheres grow, this is your future light cone. This is the part of the universe that you can influence where your choices, the things that you do as a person, could have sort of an effect downstream. It's very easy to understand why. The reason is that everything that's outside of that growing set of spears, you can't influence because information would have to travel faster than the speed of light to get there.

So it's a direct outcome of the fact that we have a maximum speed limit to the universe. It divides the universe into parts you can influence and parts you cannot. So the light cone is the part of this universe, including the future, right? We're talking space and time here, the part of the future universe that you can have any sort of influence over. So I turn on a light, like a light bulb,

and that light is growing, comes out of the light bulb, grows in a sphere, and you're saying that's the light cone. So really it should be called like a light sphere, maybe. Wouldn't that be a better name? Like the light sphere is the sphere

around you, which you can reach with light. The reason it's often called the light cone is that it's hard to draw four dimensions on a sheet of paper. The most we can really draw on a sheet of paper is three dimensions, you know, by having like some perspective. And so you need to have time on there.

So you can really only have two other dimensions of space on your sheet of paper. So when you're drawing these diagrams, people typically treat space as if it was a 2D surface instead of like our 3D world, just to sort of simplify it. So in two dimensions, the sphere we were just talking about becomes a circle, right? Imagine, for example, taking a smooth sheet of water and a huge sheet of water and a huge. hitting it somewhere. What happens? You get a wave that leaves where you hit it and it travels outwards. So now you get a set of circles. Now, if you take those set of circles and you extend them like vertically above the surface in time, make time that vertical direction, then what you get

is a cone, right? It starts from a point when you hit the surface of the water and then as time goes on, that circle gets larger and larger and that defines a cone. So it's a cone only in a sort of universe that has two spatial dimensions and one time dimension. Right. It's kind of like if you take a slice of those light spheres we were talking about, like if you take a slice of it and you see those, that circle on that slice growing and then you imagine that circle growing but growing and also moving up maybe at the same time,

then it kind of makes like a cone. Yeah, exactly. And it moves up because now we're imagining time to be one of our axes, right? We have two dimensions of space and we have another direction, which is time. And obviously things just flow forward in time at one second per second. And so this circle just sort of grows in time.

If you instead had something which wasn't moving, right? If you just like drew a circle around you, then that circle would slide through time and make a cylinder, right? Because it wouldn't be growing. But if it is growing, then it makes a cone. It starts from the point where you've made it and it makes a cone. And I guess if you're having true visualizing,

you can always just hop on Google images or something and look for light cone. I imagine you'll get a picture of this, right? How do people do podcasts before Google image search anyway? I don't know. They probably avoided topics that involve geometry and multiple dimensions. But it's kind of an interesting concept.

And I think you, as a physicist, you like it because it defines kind of like a volume, right? It defines kind of like a set of space time that is either inside the cone or outside the cone. That's kind of the power of this kind of visualization. Yeah. And it's not just arbitrary. Like you can make whatever shape you want in space time. But this one is special. This one is really important because everything that's inside your future light cone are things that you can influence. You can't send a message to anything outside your future light cone. Like anything outside your future light cone, by definition, is too far away for any message to reach because you would have to travel faster than the speed of light. And if you can send a message, it means you can't send any information. It means you can't. influence it at all. And one of the really tricky, complicated things about special relativity is that it plays sometimes with like the order of events and it seems to have consequences for causality.

So this helps you sort of keep track of that. The part of the universe that you can influence and also looking backwards, the part of the universe that can influence you. Right. All right. Well, let's get into some of these weird consequences for causality and simultity and alien death race. But first, let's take a quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal.

Apparently, the explosion actually impelled metal, glass. The injured were being loaded into ambulances, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and Order Criminal Justice System is back. In Season 2, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio

app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam, maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now hold up, isn't that against school policy?

That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor, and they're the same age. It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast.

I had this overwhelming sensation that I had to call it right then. And I just hit call. Said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation. And I just wanted to call on and let her know. There's a lot of people battling some of the very same things you're battling. And there is help out there. The Good Stuff Podcast, season two, takes a deep look into One Tribe Foundation,

a nonprofit fighting suicide in the veteran community. September is National Suicide Prevention Month. So join host Jacob and Ashley Schick as they bring you to the front lines of of One Tribe's mission. I was married to a combat Army veteran, and he actually took his own life to suicide. One Tribe saved my life twice. There's a lot of love that flows through this place, and it's sincere. Now it's a personal mission.

Don't have to go to any more funerals, you know. I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. Hello, it's Honey German. And my podcast,

Grasas Come Again, is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't audition in, like, over 25 years.

Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters, sharing their real stories of failure and success. You were destined to be a start.

We talk all about what's viral and trending with a little bit of chisement, a lot of laughs, and those amazing Vibras you've come to expect. And of course, we'll explore deeper topics dealing with identity, struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash because you have to do the code switching?

I won't say whitewash, because at the end of the day, you know, I'm me. But the whole pretending and code, you know, it takes a toll on you. Listen to the new. The new season of Grasas Has Come Again as part of My Cultura Podcast Network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. effect because I guess if something is far enough away, you can't affect it right now, right? Because you have to wait for the speed of light to get from here or there or there to here. It doesn't even have to be far away.

Something that's one foot away from you, like literally one foot away from you right now, you can have no influence on what's happening right there right now. Because even if you shot your death ray, it would only affect what's happening a foot away from you in the time it takes for light to travel from the tip of your death ray one foot. there's nothing you can do about what's happening one foot away from you right now oh wow that's kind of weird right to know that there's stuff around me that is inaccessible to me yeah i mean even if you're seeing something that happens a foot away from you that means it actually happened in the past so it's already done anyway right so all you can do is affect the future right that seems obvious but you also can't affect

the future everywhere like if you want to change the future of andromeda you know intuitively you can't change the future of Indromeda tomorrow or the next day or even the next year, it would take millions of years for any decision you make to affect anything in Andromeda because nothing you do will get there. No choices you make can propagate that far before a million years because it's a million light years away. Right. I guess that's the big consequence of having a speed limit in the universe. It's this idea that, you know, there are things in the present, but you can't really touch them or affect them or even know. know what they are right now in the same present as you.

That's right. There's a limit on the information and how information travels. It's really fascinating. You can only ever see one slice of the universe. And your slice of the universe might tell a different story from other slices of the universe. And this is really hard to get into your head because you imagine that what we're doing with physics is trying to build up like an absolute picture of the universe. Like what's really out there? What is really going on?

What happened in the universe and what's going to happen? And it's hard to accept that one, you can never really know everything about what's going on in the universe because you're limited to the slice of information you're going to get. But also that what you do learn about the universe might not be like the absolute truth. It might just be like what things look like from your perspective. Right. I think we talked about this once in the podcast. It's like imagine like how the human society was before the internet or before telecommunications, you know, when we had to rely on writing letters and, you know, waiting for news. paper to be delivered to your door.

It's like there could be things happening right now in other parts of the earth, but you know, it'll be a long time before you find out about them or you know that they actually happen at the same time that they happen. That's right. And speeding up our communications has sort of expanded our cone of

understanding, right? It's shrunk the time it gets for information to get here, but not to zero. Even with cell phones and speed of light electronics, you still can't know exactly what's happening across the world right now. There's still a time delay. And that's why it's cool to see in some of these TV shows that they really incorporate that, you know, that if you're trying to coordinate your army on Mars from your base on Earth, it takes time for your messages to get there. If there's a battle going on around, you know, the atmosphere of Jupiter, that by the time you've heard about a beginning, it's probably over already and there's nothing you can do to change the outcome.

I guess like before when you had a battle and you had to like send a messenger or send a scout or something, there was just huge delay. And so now things are faster, but they're still limited. Like, there's still no similarity. Like, there's still no ability to know what is going on right now somewhere else. Yeah, exactly. And the same concept, this light cone, which tells you, like, what you can influence. It also works in the other direction. You have, like, a backwards light cone.

If you think in the other direction, like, how close does somebody have to be to me to shoot a death ray and hit me right now? You know, if somebody is in Andromeda and they shoot a death ray at you, you're safe. for a few million years, right? Nobody in Indromeda can kill you right now. Even if they shot their death rate 100,000 years ago and it's been hurtling towards you for 100,000 years, you're still safe. It's not going to get here for a while

because they're not in your backwards light cone yet. But if they shot it a million years ago, it would be hitting us right now, potentially. And if they shoot it right now, we're still going to get hit, but it's going to be a million years from now. That's right. If they shoot it right now, it's not in our past light cone.

But our past light cone moves up this time axis and eventually they will be inside our past light cone and yes, it will get here, right? But you're safe right now. Right now there's this past light cone that dictates all the things that could be influencing what's happening at your location. And somebody really far away or even a foot away can't influence what's happening to you right now. They can only influence your future. Right. Well, they can influence you right now. But I mean, it doesn't mean you're safe from a death array because they could have shot it away. while ago and you would still die from the death ray arriving now. You're right. And the weirdest thing, the worst thing about death race is that they travel at the fastest speed possible. So there's literally no way to get any warning, right? The moment you see it is the moment it arrives, the moment it impacts you. But hopefully, you know, a million years ago, you know, we were just cavemen and women and, you know, we didn't do anything to offend these agents. Hopefully. Or maybe it's a natural process. Maybe it's gamma rays from a

supernova that blew up, you know, a while ago. And those sterilized. death rays are just now arriving here at Earth. Man, this light cone makes me feel not at all safer, Daniel. See how you can draw comfort from this light cone. It's like a pretty flimsy protection against giant gamma ray birds. It just means that somebody's got to plan ahead. If they really want to fry us now, they're going to either be closed by or they have to plan ahead.

So that exploding star, are you saying it's not very good at planning, so we're safe? Is that what it's a procrastinator? Yes, it saves us from procrastinators, exactly. Okay. But what about type A supernovas? Oh, the kind of plants ahead. There's nothing we can do. Yeah, we're toast.

All right, so it sort of protects us, and I think it also kind of affects what we might agree on is kind of a consistent history of the universe, right? Yeah, it's really interesting. The whole universe can be divided into three regions. You have your future light cone, your past like cone, And then there's everything else, right? And physicists call this the absolute elsewhere.

It sounds like an awesome name for a Netflix show or something. So the absolute elsewhere is the spot that you can't affect. You can't influence, right? And that's really interesting because things that happen in the absolute elsewhere, you might disagree with people about the order in which they happen. Like you might look at a series of events and say, A happens before B. And somebody else might look at it and say, no, no, no, I think B happens before A. And you could both be right if these two things,

are not inside each other's light cones. So these events don't have overlapping light cones, then they can be differently ordered and not affect anything from a causality point of view. Whoa. So I guess I can have a light cone and you can have a light cone and if those two don't interact?

Is that what you mean? Like if they don't touch, then we're looking at literally like different universes. Well, they will eventually touch, right? Because their light cones will eventually impact. And so there is some overlap in their light cone far in the future where both of them can influence.

But it doesn't matter which one happened first. It doesn't matter if A happened in the B or B happened and then A. And because these two events are the events themselves are not inside each other's light cones, right? B is not in A's light cone and A is not in B's light cone, even though the two light cones do overlap. If they're not inside each other's light cones, then you can swap their order just by changing your speed. If you decide to zip by them in a spaceship that's close to the speed of light, you will see one happen before the the other. And if you go in the other direction at nearly the speed of light, you will see the opposite order of events. And it all works out downstream because these two things aren't

causally linked. They're not inside each other's light cones. So you're free to change their order of events. But then, you know, the two people who are involved would have a different interpretation, right? Like one of them will see one thing happening before the other. Yeah, exactly. But there is no absolute definition of what happened first. All the observers have their own interpretation and they're all correct because the answer depends on your position. That's not true for two events where one is inside the other one's light cone. If I shoot a joke ray and I hit you in the head, then you laugh, right? Put it in joke mode, please.

There's death, stun, and then there's joke. All right, we'll put it on banana mode. So I hit the ray and the banana hits you in the head, right? Now you are in my light cone because the two events, me pressing the button on the gun and you having a banana arrive in your mouth, they're not at the same time. What happens before the other, and there's a causal structure for them. I have to press the button before the banana ray arrives at your head. And that's okay because the banana ray arriving at your head is an event in my future.

It's inside my light cone. But if, for example, you and I both have these guns, and we point them in random directions or whatever, and we try to fire them at the same time. Some people might say we fired them at the same time. other people flying by in a spaceship might disagree about who pressed their trigger first. Right, because they would be moving and because of the way that light, how long light takes to get to them and the speed limit of the universe, they might reconstruct the events in a different way. Yeah, but it's not just because how long light takes to get to them. Even if you factor that in, right, even if you account for how long it takes the signals and the images to appear, there's still a difference in the order of events.

And that's because time flows differently based on where you are and how fast you are going. So your speed affects how you see time flowing. And so you can see one event happening before the other. You can see Jorge pressed his button first or somebody else going the other direction can see Daniel press their button first. Even if they account for the transmission delay from light. But they would be wrong. I guess what I mean is they would have their own interpretation, but other people would have their different interpretation. Yeah, but nobody's right or everybody's right.

You know, if these two events are not inside each other's light cones, right, their light cones can overlap in the future. But if the two events themselves are not inside each other's light cones, then their order is not well defined. And this is all because the universe has a speed limit, right? Like if it didn't have a speed limit, then you could talk about like an absolute history of the universe. That's right, exactly. And that speed limit defines the size of this cone, right? Imagine, for example, if the speed of light was much, much slower, then your light cone would be narrower. the part of the universe, the part of the future that you could influence would be a much smaller

part of the universe. If you shot death rays in every direction and the speed of light was much smaller, then you would kill a smaller fraction of the universe. Or if you shot joke rays, you would make a smaller fraction in the universe laugh than if the speed of light was 10 times faster than it is. Then you could influence more of the universe and aliens out there with their joke rays could make us laugh more effectively. So this number, the speed of light, really changes the whole structure of causality of

space. Right, but what if you make like a light joke? Like this, vanilla is the best. Ha, ha, ha, ha, that's a joke. Like a light vanilla joke. Yeah. That wouldn't harm anyone and people would just let it go by. But it really is sort of like a cone. I'm thinking like a dog cone. You know, those cones they put on dogs so that they don't harm themselves or just to protect them. It really is like a cone that you put on our, we put on our heads that kind of limits what we can see and what we can interact with, right? It sort of does. Now I'm imagining you with one of these on your head trying to eat a banana, even though you can't get it around the cousin. I'd be like, Daniel, shoot me with the banana, shoot me. Exactly. Then you need me to shoot you with the banana ray.

Or I could shoot one up in the air and then catch it. And the cone would help me catch this. There you go. There you go. Pretty soon you'd be surrounded with bananas. I'd have a cone around my head filled with bananas. And I'll come along with a scoop of vanilla on top. There you go. And then pour some chocolate. But the amazing thing to me is that we still don't know why the speed of light is this number. You know, it has such a dramatic effect on what part of the universe we can see and what part of the universe we can influence and how the universe is sort of like tied together, you know, what effect what.

It really changes everything. And it's just like a number. We measure it. We don't know why it is what it is. It could have been bigger. It could have been smaller for all we know. Our laws of physics just have like an empty spot there. If you're like right in the number you measure, there's no like reason it has to be this or that that we know of.

And that's like a real mystery, you know, makes you wonder why it's this and not something else. All right, cool. Well, let's get into now what consequences is this light cone has in the universe and possibly in our ability to travel through time. But first, let's take another quick break. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage,

kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass. The injured were being loaded into ambulances. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism.

Law. and order criminal justice system is back. In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly and now I'm seriously suspicious. Well, wait a minute, Sam.

Maybe her boyfriend's just looking. for extra credit. Well, Dakota, it's back to school week on the okay story time podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Now hold up, isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's former professor and they're the same age. It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well,

he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast.

Hola, it's Honey German, and my podcast, Grasias Come Again, is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition.

I haven't auditioned in, like, over 25 years. Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters sharing their real stories of failure and success. You were destined to be a start.

We talk all about what's viral and trending with a little bit of chisement, a lot of laughs, and those amazing vibras you've come to expect. And, of course, we'll explore deeper topics dealing with identity, struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash because you have to do the code switching?

I won't say whitewash because at the end of the day, you know, I'm me. But the whole pretending and cold, you know, it takes a toll on you. Listen to the new season of Grasasas Come Again as part of My Cultura Podcast Network on the IHartRadio app, Apple Podcast, or wherever you get your podcast. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depth.

of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy. Hi, I'm Danny Shapiro. And these are just a few of the profound and powerful stories I'll be mining on our 12th season of Family Secrets. With over 37 million downloads, we continue to be moved and inspired by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes. with you. Stories of tangled up identities, concealed truths, and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests for this new season of Family Secrets. Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or

wherever you get your podcasts. Right, we're talking about light cones and the speed of light and whether or not we can affect things in the universe that are happening right now, which is basically no, right? Like, we can't affect anything else in the universe happening at this moment. That's right. Nothing except for what's happening right here right now or stuff that's happening in the future. But at this very moment, you can't affect anything except for exactly the spot where you are. Well, technically nothing, right? because even things that are one millimeter away from my eye in the present moment, I can't even see or effect.

That's true. We have to go down to zero millimeters. And that's even only if you believe in free will and if your brain operates instantaneously. So technically you're right. Any decision you make right now can only influence the future. Except that maybe there's a possibility to use this concept to do time travel, Daniel. So explain that to me. Yeah. So what we've talked about so far, this idea of light cones, these circles of light expanding out into the universe and into your future. this is the way we think about space if it's not curved. So we call space flat if it's just sort of like has no shape to it, if there's no bending of it that affects like the path of photons. And we know that space can be bent.

We know that, for example, everything with mass and energy bends space, like the sun bends space. That's why the Earth is moving around it instead of just flying off into deep space. We know that you bend space because you have mass. And so everything out there changes. the shape of space, which means it changes how light goes through it. Remember that gravity is not just like a force that tugs on things, because light, for example, has no mass, but gravity can affect its path

because gravity changes the very shape of space and light travels through that space. And so we know that things are more complicated when space gets curved. So what we've talked about so far is only in the event that the universe is empty and there's no curvature, there's no mass or anything to change the shape. So in flat space. When there's a lot of mass around, things get curved and things get pretty different and weird. These light cones, which you can imagine sort of like going up and going down, now start to get tilted. Right.

This is something we can actually see, right? You can see light getting bent out there in space by heavy objects. Yeah. It's called gravitational lensing. It is like a really big heavy object between us and some source of light, like a background galaxy. Then as those photons come towards Earth, they get bent around. that massive object. And this is one way we can detect, for example,

the existence of dark matter and try to figure out roughly where it is based on how it distorts the light that's passing near it or through it. So it's definitely a real thing that's happening. It's something we've observed we're sure is true. So if a space can bend light or bent space can sort of affect the trajectory of light

that would make sense that it would bend these light cones that we have to kind of figure out what we can't see and cannot see in the universe. Exactly. because the shape of the light cone is defined by where light goes, right, where light can reach. And so if space is bent in a way that light can't get somewhere, then the light cones are also bent. So this idea of them being like simple cones is only in flat space near something really massive. They bend towards the mass, right? Because light moves towards mass, right? If you have,

for example, the sun and you shoot a photon near the sun, it will get curved sort of in the direction of the sun. And if you have a black hole, then it's going to get curved through towards the black hole. So as you get closer and closer to something massive, these light cones tilt more and more. Wow, they start to look like maybe horns and not so much cones or. Yeah, or like bow ties or something. Yeah, or like a bugle. Do you remember that snack? Yeah. The little like cones, curved cones. Yeah, exactly. Yeah, light bugles, we'll call it. Light bugles, exactly. And so inside a black hole, of course, you can think about what happens to a photon? Well, a photon inside a black hole always goes towards the center,

not because gravity is so strong, but because the shape of space requires it because there's only one direction inside a black hole. So inside a black hole, these things have tilted totally sideways. You shoot a photon and it goes just towards the singularity. All the light cones are now tilted towards the center. You hear people say sometimes this thing that inside a black hole, space and time switch rolls, right? This is like a geometric way to understand that. because now instead of time flowing up, it's sort of flowing sideways, right? Because the light cone has tilted over by 45 degrees. Whoa.

I guess I don't understand it at all. So the light cone tilts and somehow time and space switch positions? Is that what you just said? Yeah, because your future, all possible futures are the singularity. And so the only way to move forward in time is to move in this one direction of space. And so sort of like moving through space towards the singularity is the same thing as moving forwards in time. And you can graph that geometrically by thinking about the light zone being tilted over from being vertical to being horizontal. It's like there's only space and time become one way and everything goes in the same direction.

Yeah, exactly. All right. And so then how does that relate to time travel? You said it might be possible to do time travel? Yeah, it might be possible to do time travel if you use these cones. you can construct this really weird structure, right? So we talked about how near something really heavy, these cones can tilt like up to 45 degrees, right,

towards like the center of a black hole. Now, imagine if you could have a cone that tilted more than 45 degrees. And what that means is that you're tilting so that part of the light cone is now in the past, right? So it's possible if you could do this, it would mean that these particles are moving not just into the future, but like the future is now dipping into the past. What? You mean you bend the light cone so much,

it wraps around and touches kind of the past light cone. Mm-hmm, exactly. And you think, is this possible? Doesn't that contradict itself? Well, remember that like the definition of time and the definition of space depends a little bit on how fast you're going and the environment you're in.

And that gets even more complicated when space is bent. And so what you can have is like a particle, moving forward into its future like its local time still says I'm moving forward into future one second then the next second then the next second but it could be moving into the past

from the point of view of another observer so like from a distant observer who's watching this from far away whose space isn't curved they could see that particle moving into like their past even though if it's still moving into its own future whoa wait are you saying that space and time

actually curve around itself or it only looks curved around to a different observer. Now, the structure of it is actually occurred. And people think in some weird configurations, if there's a very special arrangement of mass, then you might be able to construct a structure of space that allows particles to have their local future,

their personal timeline extend into, like, the past from the point of view of other observers. Oh, from the point of view of other observers. But the particle itself wouldn't be traveling into the past. Or like me, I wouldn't be. be traveling into the past. Well, it's like into their own future, but that's sort of like what happens on a time travel show, right? If you are on a time travel show and you want to go into the past, you're still living your life forward on your timeline, right? You were 46 years old and seven

days and two minutes. And then when you go into the past, you're not like younger again, right? You're just like keep going forward. Now you're 47 years, two days and five minutes and seven minutes and 10 minutes. So what you want is your own timeline to continue, but you want the structure of space around you to change so that your timeline, your future now goes into like the rest of the universe is past. So this is called in physics, it's called a closed time-like curve. Because if you can tilts a bunch of these cones just the right way, then you can make this path that loops on around itself and comes back to the same point where it originated. So it's like shooting a light beam and having it hit you in the back of the head. Almost like a wormhole kind of in time.

Almost like a wormhole. Yeah. It's not actually a wormhole because a wormhole is like our shortcut between two distant locations. This is like a structure in space. It's like a loop in space time, right? It is a complicated reconnection and rearrangement of how space works, but it's technically a little bit different from a wormhole. Right. Wouldn't you be stuck in this loop?

Like, it'd be like groundhogs day for this particle. It'd just be reliving going around and around and around. It thinks it's going in the future, but really it's like revisiting the same time over and over and over. Exactly. It's a structure that reinforces itself and it's only stable if it does revisit it. itself. And so you're right. You wouldn't like get to go back and choose a different flavor of ice cream or set your death rate to banana instead of death. You have to make the same choices over and over

again. It's like this self-supporting structure in space and time. So not recommended. Well, unless you want to keep watching the same shows, maybe, or if you really like vanilla and you want to eat only vanilla flavor for the rest of your lives for eternity. Yeah, exactly. And this is something which is theoretically conceivable. Like people have worked this out on paper and it seems like it might work. But again, general relativity doesn't tell us like how to bend space to make this happen. People are like, if you could bend space in this particular way, then we think this could happen. That doesn't mean necessarily we know how to make space bend in that particular way. You know, people have ideas. They have this crazy structure that's like an infinite spinning cylinder

of dust and they do the calculations, they think maybe that would allow for these closed timeline curves to happen. But other people think it's impossible. There's no way it could work. And that we have to just sort of throw up our hands and say, let's figure it out when we get the unified theory of everything. Well, you're saying that according to the equations, it's possible

or like, you know, anything that can bend, if you bend it enough, this will happen. But we don't know if it's physically possible to do it. We don't know if it's physically possible. If you believe general relativity is, absolutely true and you can construct an infinite spinning cylinder of dust then you should believe this is physically possible and can actually happen personally i don't think general relativity is a true theory of nature because it ignores quantum mechanics and so it can't be

correct and we need some updated version and it might be that in that updated version and incorporates quantum mechanics this doesn't work anymore so this could be like sort of like a singularity of a black hole like a sign that the theory is wrong because it predicts this sort of nonsense result. Right. Because I guess if it's a closed loop, how would you even get into that loop, right? Yeah. You have to have somebody else build that infinite spinning cylinder for you in the infinite past because you'll always be in that loop. So it might exist and there might be a particle stuck in it or from the beginning of time, but you wouldn't be able to get in or out of it. That's right. If it exists, it would be stuck sort of forever. But, you know, now we're talking

about like construction of a time loop. So we're talking about the variation of the loop as a function of time. So it's like an extra level of meta time there, which gets tricky. You can't just use the same vanilla equations. You get to go extra spicy or sprinkle some bugles for some extra crunchiness. All right. So that's the light on, I guess. And I guess you're saying it's one way that it helped you think about all these weird situations of space and black holes and bending time and space time looping around itself. You use that in your everyday, you know, theoretical musings. I do. And also when people write to me and they ask me some like crazy question about special relativity involving, you know, spaceships and snowballs and whatever, I usually end

of drawing light cones so I can understand like, well, can this happen there? What will this person see or is it possible for them to know this? It's just a nice way to sort of like separate things. Because in physics, problems become much simpler when you can separate and say these things are independent from those things so I can just deal with it on its own. They're not like entangled. So light cone shows you how to like disentangle all the causal threads of the universe into a bunch of different packets. All right. Yeah, everything's better with the snow cone. That's our next episode.

What is a physics snow cone? It's a cone where physicists just say snow all the time. It's no. Can I travel back in time? Snow. C.S. C.S. All right.

Well, hopefully it helps all of you out there. think about causality and what we can and cannot affect about the present and maybe makes you appreciate a little bit more where you are and what we're able to see of this big, crazy universe. Thanks for joining us. See you next time. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio.

podcast from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. toys. Then everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend. just looking for extra credit.

Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. every case that is a cold case that has DNA right now in a backlog will be identified in our lifetime on the new podcast america's crime lab every case has a story to tell and the DNA holds the truth

he never thought he was going to get caught and i just looked at my computer screen i was just like gotcha this technology's already solving so many cases listen to america's crime lab on the iheart radio app apple podcasts or wherever you get your podcasts This is an IHeart podcast.