# Absolute Rest?

*Most recent answer: 08/27/2016*

Q:

I was doing a little poking around about just what is a massless particle and what that means when I came across this paragraph, "If we hop in a spaceship that can go 90% the speed of light, we are now going through spacetime with only 10% dedicated to the time dimension which means time slows down for us (relative to slower moving observers) by an equivalent amount. So, for light, using 100% of the speed of light, there is nothing being used in the time dimension so time is not a part of lightï¿½s eternal journey through spacetime." The full page can be found here: http://www.askamathematician.com/2010/09/q-how-can-photons-have-energy-and-momentum-but-no-mass/ Anyway, that got me thinking. If this is true, then is there a reverse? Light is a constant, a speed limit which nothing can exceed. Not only that, anything at that speed will always move that speed relative to an observer at a slower speed. My questions then are, is there anything that will always be stationary relative to an observer? If light is the cosmological constant of speed, is there a cosmological constant of rest? And if so, would that have infinite mass? And exist wholly in time and not in space? I'm not a mathematician or a physicist, but I do like thinking and would love to hear some answers to these.

- Michael Dominguez (age 32)

Philadelphia, PA USA

- Michael Dominguez (age 32)

Philadelphia, PA USA

A:

Nice question. Before getting to it, I should say that the site you link to has a huge error. The original post there is nice, but the comment you quoted from above is nonsense. It completely misses the central point of relativity, that nobody is "going 90% of the speed of light" more or less than anybody else is. Any of us can say we're at rest and use the same laws of physics.

So now to your question, is there anything that must be at rest according to any of its neighbors, just like light must be traveling at c according to any of its neighbors? (It gets more complicated when you consider remote observers.) The answer turns out to be no.

Mike W.

*(published on 08/27/2016)*