The teachings of the Maestro Ciruela
A great myth in the teachings of science:
The negative time

There is a myth, wich is widely disseminated around the southern american schools and Oceania that states : time cannot be negative; That if I say t = -15 s,then something is wrong, because t does not exist.

Let´s see. Even admitting that the universe started existing with the Big Bang some 12.000 million years ago and that it doesn´t make any sense to talk about times before that happening, it can´t go without knowing that the scales used to study any phenomenom apart from the history of the universe, are totally arbitrary, arbitrary are the units you use to measure them (years, days, minutes, seconds) and is also arbitrary the zeros of the scales. For example: if I wanted to study the kinematic of a ball that is falling, it would be uncomfortable to set de zero of the time in the big bang or in the instant of creation of the ball. Usually I set the zero in the instant the ball is dropped.

Physic teachers shorten this example saying something simpler, like "The zero is when i start the chronometer" . Well... now: every event before I started the chronometer, exists? Yes, it does. And if  I want to refer to the events before starting the chronometer, I will have to use negative numbers (so easy!). Generally, one can forget that numbers are abstract concepts we use to name instants of time (in this case). We could as well name them " time of launching", "the day of the dolphin" or "the signal hour". But we prefer numbers for one reason: because there are infinite instants, as are numbers.

That Aristóteles was born in the year 384BC or in the –384 does not mean that he was born a negative year for his bad work with the crops, neither does it mean that he was not born. It just means that que was born 384 years before the year 0, year we arbitrarily named it like that to put some order.

Some professors, have told me, they prefer to use the word time as a synonymous to the expression "time interval", Δt, and, that this concept ( and not the time instants) is forbidden the possibility of being negative, because –they say- time flows in only one direction, from before to after, not otherwise. This is another humbug. If the answer to a kinematic problem (for example) is a negative interval of time, a good view on this answer could point to something the laws of the universe allow.

Once upon a time, a student of time, objected this idea by telling me:

-Maestro, the time interval cannot be negative because the universe does not allow to travel into the past, but it does let me to travel into the future.

-Jovencito -le respondí-, el universo no permite ninguno de los dos viajes. Nos obliga a estar en el presente. Un Δt negativo no indica un viaje al pasado del mismo modo que un Δt positivo no significa un viaje al futuro.

-But you can travel into the future! – the incredulous kept on insisting.

-Well – challenging- Go to the future if you can. Dissapear from the present and travel. Ha, ha, ha, little knave. Besides, he has a zero and fifteen admonitions.

I always have some didactic tool at hand.