Time Stops at the Speed of Light. What Does that Mean?

if you're interested in space travel and who's not looking for a way to get off the planet at this point then you've probably heard that time doesn't pass for light or that time stops when you reach the speed of light or something like that what sense does this make I think I can help this video comes with a quiz that lets you check how much you remember the idea that time doesn't pass for light is an interpretation of Einstein's theories of special and general relativity yes that guy again both of these theories are based on the inside that time is a coordinate and must be joined with the three dimensions of space to a four dimensional SpaceTime the other key ingredient is that the speed of light in vacuum is always the same the key to understanding Einstein's theories is that they have two different Notions of time the one is the coordinate time the other one is the personal time which is different for each Observer the easiest way to see the difference is to draw a diagram unfortunately last time I checked YouTube didn't support four-dimensional Graphics so this is why we'll make do with just one dimension of space that we draw on the horizontal axis and time on the vertical axis this is called A Spacetime diagram in this SpaceTime time diagram the motion of an observer who just sits still is described by a vertical line that of an observer moving a constant velocity is a straight line at some angle to the vertical by convention the speed of light is at a 45° angle the curve on which an observer Moves In A Spacetime diagram is called their world line now about these two Notions of time the time AIS in this diagram measures what we call the coordinate time that could for example be the time in your bedroom with zero marking midnight on Tuesday it's a convention like its convention to measure location on Earth by latitude and longitude and longitude zero is in a place called Granite but this coordinate time is physically meaningless because anyone could pick a different one like there's nothing preventing you from picking a Time coordinate that looks like this it's just that it would be very cumbersome the physically meaningful time is the time that passes for an observer and that time is measured by the length of the world line of the Observer which I remind you is just a fancy word for the curve that he or she makes in the SpaceTime diagram this physically meaningful time is called the proper time of the Observer so the proper time is the length of the world line which is the curve of the observer in SpaceTime to see the the difference between the coordinate time and the proper time consider I give you the coordinates of Cape Town in longitude and latitude you can then say that well that's 18.4 de east of granite all right but that doesn't tell you what it physically takes to get there does it for this you'd have to measure the length of the path from granich to Cape Town and it's the same in SpaceTime if you want to know the time that passes you must measure the length of the path and this is where things get a bit complicated because the length of a path in space time is not calculated the same way that we calculate the length of a path in space I mean look somehow it must matter that it's space time and not space and more space right if it was just space space we'd use what's known as the ukian distance suppose the two coordinates are X and Y if you have a straight line you take the distance between the initial and end points in each Direction Square them and then you take the square root of the sum in space time now you take the square of the distance and time but subtract the distance in space this is known as the lorenzian distance and that finally brings us to the question how much time passes for light let's ask how much proper time passes for an observer between two instances of Co coordinate time two instances of coordinate time are those two parallel lines there are many ways to get from one to the other depending on how fast you move remember that the faster you move the closer the angle of Your World line is to 45° which is the speed of light if you calculate the length of this curve with the correct lorenzian distance you find that it gets shorter the closer you get to the speed of light and indeed if you calculate the proper time for anything that moves exactly with the speed of light then the result is always zero this is how SpaceTime works and this is where the idea comes from that time doesn't pass for light because light doesn't have an internal time the proper time for light is always zero the question is now what this means light isn't known for being very communicative so we can't ask it how it's doing though well of course we can ask if you're light please let us know in the comments how you're doing while we wait for light to reply best we can do is try to interpret the maths and my interpretation of the maths is that if you move with the speed of light then everything along your path happens in the same instant if you were to cross the entire universe the entire trip would take a duration of 0er seconds it all happens at once maybe that's why why light doesn't talk all that much unfortunately we can't move at the speed of light to our best current knowledge the only thing that can move with the speed of light is well light light is made of the Quantum of light the photons and those are Elementary particles Elementary particles don't experience time because they don't experience anything so the question of what the math means for experience is rather philosophical I believe the reason that people get confused about what it means that time stops for light is that they don't distinguish between the proper time of light itself which in some sense could indeed be set to stop and the coordinate time that describes the universe we inhabit of course coordinate time doesn't stop and I hope that you won't stop coming up with interesting questions please let me know in the comments what you want me to talk about next Science and Mathematics are important show but they're also just super interesting and a constant source of surprise if you want to take a deeper dive into the science behind solar panels or neuron networks or Quantum Computing I recommend you check out brilliant.org who've been sponsoring this video on brilliant you find courses on a large variety of topics in science Computer Science and Mathematics all their courses come with interactive visualizations and followup questions some also have videos for demonstration experiments or executable Python scripts this really gives you a feeling for what's going on it's both fun and easy to fit into a busy schedule I've learned a lot on brilliant and now even have my own course there that's an introduction to Quantum Mechanics it's a beginner's course and covers topics such as interference superpositions and entanglement the uncertainty principle and B serum sounds good I 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Channel: Sabine Hossenfelder

Views: 437,287

Keywords: science without the gobbledygoook, hossenfelder, science, science humor, physics, special relativity, general relativity, Einstein, space travel, space, can time stop, does time pass for light, does time stop for light, does time stop in space, does time stop at the speed of light

Id: vqwLKLc4gMg

Length: 8min 20sec (500 seconds)

Published: Sun Apr 21 2024