# Why do we use seconds to measure time, and is this significant?

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#### thedoc

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##### Why do we use seconds to measure time, and is this significant?
« on: 13/01/2015 22:30:02 »
Peter Colenso  asked the Naked Scientists:

I think i have debunked Einstein once and for all. A second was first used in 900ad by al Biruni it is a man made concept to break up the time of the day and has no universal significance. Einstein uses the second in his theory, it should not be there it was made by man 1100 years ago. Print this up and get people talking, it has to be right.

What do you think?
« Last Edit: 13/01/2015 22:30:02 by _system »

#### PmbPhy

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##### Re: Why do we use seconds to measure time, and is this significant?
« Reply #1 on: 14/01/2015 02:03:23 »
Quote from: Peter Colenso
I think i have debunked Einstein once and for all. A second was first used in 900ad by al Biruni it is a man made concept to break up the time of the day and has no universal significance. Einstein uses the second in his theory, it should not be there it was made by man 1100 years ago. Print this up and get people talking, it has to be right.
I'm sorry, but can promise you that you haven't debunked Einstein in the least. No physicist, including Einstein, has ever created a theory which is dependent on the particular system of units chosen to express any equation in. When being educated, all physics students learn how to use the laws of physics in different systems of units. In fact there is a debate in electrodynamics on which set of units should be held as the standardized set of units. Some hold that it should be the SI system of units which is based on the meter-kilogram-second (mks) system while others hold that it should be the system which uses the centimetre–gram–second (CGS) system. See: http://en.wikipedia.org/wiki/International_System_of_Units

The text Classical Electrodynamics by J.D. Jackson used the cgs system from 1962 until 1999 when he finally felt the pressure to change the units he used in his text to SI units.

It's usually an exercise in many textbooks to transform physical quantities in one set of units to that in another set of units. For example; the change in units from Gaussian to SI of the electric field is given in Jackson 3rd Ed page 782 table 3 as

E($$\Phi$$, V)/$$\sqrt{4\pi\epsilon_0}$$ = E($$\Phi$$, V)

In the system of units used by particle physicists and relativists the speed of light has the value of 1.

There is nothing in Einstein's postulates in his theories that is dependent on a particular system of units which would constitute a flaw in his work. That's why no physicist has ever complained about it, i.e. we're all quite aware of this.
« Last Edit: 14/01/2015 02:38:58 by PmbPhy »

#### chiralSPO

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##### Re: Why do we use seconds to measure time, and is this significant?
« Reply #2 on: 14/01/2015 02:45:10 »
The second is a unit that we use to measure time. there is nothing universal about a second, but time is (essentially) of universal significance.

Similarly we made up the meter as a unit of distance, or the volt as a unit of charge etc.

There is nothing special about most of these units, but those properties that they pertain to are still quite important and "real"

There are some units that are more universal, for instance the charge of an electron or the speed of light in a vacuum etc. etc. but this is just because they are defined by observables that can be observed anywhere in the universe.