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

Apr 24, 2024If you're interested in space travel and aren't looking for a way to get off the planet right now, you've probably heard that

## time

## does

n't pass for### light

or that## time

### stops

when you reach the## speed

of### light

or something like that. What sense## does

this have? I think I can help. This video comes with a quiz that allows you to test how much you remember. The idea that time does not pass for light is an interpretation of Einstein's theories of special and general relativity. Yeah, that guy. Again, both theories are based on the fact that time is a coordinate and must be joined with the three dimensions of space to a four-dimensional space-time.The other key ingredient is that the

## speed

of light in a vacuum is always the same. The key to understanding Einstein's theories is that he has two different notions of time, one is coordinated time and the other is 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 it didn't. It does not support four-dimensional graphics, so we settle for a single dimension of space that we draw on the horizontal axis and time on the vertical axis. This is called a space-time diagram.In this space-time time diagram, the motion of an observer. who simply stands still is described by a vertical line that of an observer moving at constant speed is a straight line at some angle to the vertical by convention the speed of light is at an angle of 45° the curve in which an observer moves in a space-time The diagram is called his world line now about these two notions of time. The AIS time in this diagram measures

#### what

we call coordinated time which could be, for example, the time in his bedroom with zero marking midnight on Tuesday.It is a convention like his convention for measuring. The location on Earth by latitude and longitude and zero longitude is in a place called Granite, but this time coordinate is physically

### mean

ingless because anyone could choose a different one as if there was nothing stopping them from choosing a time coordinate that looks well, it's just that it would be very cumbersome physically### mean

ingful time is the time that passes for an observer and that time is measured by the length of the Observer's world line which I remind you is just a fancy word for the curve that he or she ago in the Space-Time diagram.This physically significant time is called the observer's proper time, so the proper time is the length of the world line, which is the observer's curve in spacetime. To see the difference between coordinated time and proper time, consider that I give you the coordinates of Cape Town in longitude and latitude, then you can say well, that's 18.4 of this granite, okay, but that's not It tells you

#### what

it takes physically to get there, for this you would have to measure the length of the path from Granich to Cape Town and it is the same in space-time if you want to know the time that passes you must measure the length of the path and here is where things get a little complicated because the length of a path in space-time is not calculated in the same way that we calculate the length of a path in space, I mean, look somehow, it must matter that it is space -time and not space and more space.If it were just space we would use what is 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 start and end points in each direction square them and then 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 Lorenzian distance and that finally brings us to the question of how much time passes for light. Let us ask how much proper time passes for an observer between two instances of coordinated time Co. two instances of coordinated 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 the line to Your World will be at 45°, which is the speed of light if If you calculate the length of this curve with the correct Lorenzian distance you will find that it gets shorter the closer you get to the speed of light, and in fact if you calculate the proper time for anything moving exactly with the speed of light, then the result is always zero, that's how SpaceTime works.

It works and from here arises the idea that time does not pass through light because light does not have an internal time. The appropriate time for light is always zero. The question now is what does this mean? Light is not characterized by being very communicative, so We can't ask him how he's doing, although hey, of course, we can ask him if he's Light. Let us know in the comments how it goes while we wait for Light to respond. The best we can do is try to interpret the mathematics and my interpretation of the mathematics is that if you move at the speed of light, then everything along your path happens at the same instant.

If you crossed the entire universe, the entire trip would take a duration of 0 seconds, everything happens at once. maybe that's why light doesn't talk much, unfortunately to our best current knowledge we can't move at the speed of light, the only thing that can move with the speed of light is light, light is made of the Quantum of light. photons and those are elementary particles Elementary particles don't experience time because they don't experience anything, so the question of what mathematics means for experience is quite philosophical. I think the reason people get confused about what it means for time to stop for light is that they don't distinguish between the proper time of light itself, which in some sense could stop, and the coordinated time that it describes. the universe we inhabit.

Of course, coordinated time doesn't stop and I hope you do. Don't stop asking interesting questions, let me know in the comments what you want me to talk about in the next show. Science and math are an important program, but they are also super interesting and a constant source of surprises if you want to delve deeper. about the science behind solar panels or neural networks or Quantum Computing. I recommend you visit shiny.org, which has been sponsoring this video on shiny. You will find courses on a wide variety of science topics, Computer Science and Mathematics. All their courses come with interactive programs. visualizations and follow-up questions, some also have videos for demonstration experiments or executable Python scripts.

This really gives you an idea of what's going on. It's fun and easy to fit into a busy schedule. I have learned a lot brilliantly and now I even have my own. This course is an introduction to Quantum Mechanics, it is a beginner's course and covers topics such as interference and entanglement superpositions, the uncertainty principle and serum B sounds good. I hope so. You can try it yourself free for 30 days, but be sure to use our link. bright.org zabina because that will give you a 20% discount on an annual premium subscription thanks for seeing us see you tomorrow

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