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### Course: Physics library>Unit 16

Lesson 2: Minkowski spacetime

# Measuring time in meters in Minkowski spacetime

As counterintuitive as it might sound, it will become extremely convenient to use the same units for space and time. Happily, we can do that just by multiplying the time axis by c, the speed of light. Here's how.

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• This to me looks like a trick that works only because we made the speed of light absolute. Now, I don't know on what observations that is based... So ok... But if the speed of light was not absolute in vacuum, we couldn't do it... Right?
• Every measurement we have done for the speed of light gives the same value. The idea of a universal speed limit is part of the core that theories of Relativity were built off of and it is one of the most accurately verified theory we have.

Here is a post by Sean Carroll about the speed limit of the speed of light: http://www.preposterousuniverse.com/blog/2016/11/24/thanksgiving-11/
• I still can't understand how can we measure time in meters and what does measuring time in meters actually mean,does it mean that the unit for measuring time is meter (because it's going to seem strange if we tried to measure speed in meters per meters(m/m))or does it mean that the 1 second of time is equivalent to 300000000m on the space time diagram (the distance between (t=1 & t=2) on the graph is equal to the distance between (x=300000000 & x=600000000)on that same graph)?
• I'm lost as to how it follows from time differing between frames of reference that it's continuous/synonymous (?) with space?
• at which point is that said? can you identify it for me please? See if I can clarify it for you
(1 vote)
• I still don't understand the essence of using ct axis i.e. just blindly multiplying c with the time on y axis and calling it 'spacetime'? How is it useful and what if we didn't do that?
• If it is possible to measure time in meters then it is possuble to measure space in seconds either since space and time merge into spacetime?
• I understand that distance and location are not independent of time and vice-versa, but it seems that they are not equivalent; special relativity still indicates some restrictions on how one can move in time, in ways that do not similarly restrict left vs right or up-down,does it not? One can not travel into the past or to a time before an event that has taken place, correct?
• some studies are being done into this field. Before you go saying that, you might want to consider this: 200 years ago, man had never even dreamed of flying, and now, here we are, with an office room floating in space and a flag on the moon. So don't say it isn't possible. Just say it may not work.
(1 vote)
• Are we plotting DISTANCE against DISTANCE?
• No, we are plotting distance against time, but the UNIT for time here is meters.
(1 vote)
• is this the reason why when measure the distance between planets, astronomers mostly use light years instead of traditional distance unit?
(1 vote)
• A light-year is the distance light can travel in a year. I guess in a spacetime sense it does refer to time, but in a classical sense it only means distance. A parsec is about 3.26 light-years.
• If we can use the same units for both distance and time,
does that make speed, which is distance divided by time,
a dimensionless unit? What then, does speed even mean?
(1 vote)
• We don't use the same units for distance and time. In this video Sal has to multiply t by c to get things in the same units. ct is a unit of distance.
(1 vote)
• can i have a ruler that measures in seconds and minutes?
(1 vote)
• For one second you would need a ruler that was 299,792,458 meters long. Or about 3 nanoseconds would be a ruler a meter long.
(1 vote)