Calculating at what time electromagnetic wave reaches highest and lowest value? I thought electromagnetic waves travel forever and that the peak (both positive and negative) only decreases when the magnitude decreases cus of attenuation but it would never reach 0?
How is it possible to calculate at which time the field strength reaches its lowest and when its highest value?
Especially if you only know the frequency (which is 5GHz).
So, the question I'm asking is "At what time do the field strengths reach their highest and lowest value". I don't know how do you measure at what point the strength peaks at highest and when it 'peaks' at its lowest point.
 A: lets start here :


Electromagnetic waves carry energy as they travel through empty space. There is an energy density associated with both the electric field E and the magnetic field B.

As wavelength $λ=c/ν$ , so if you know the frequency you know the wavelength.
Highest to lowest is 1/2 wavelength as seen in the figure, talingthe y axis..
Now if you mean how to measure the time it takes, at the velocity of light and the angstrom distances it is not possible with a ruler and a clock.There are various experimental methods that use lasers and mathematics to do so. Thomas Young in the beginning of last century did so. (Original experiment) 
When you know the wavelength, and the velocity of light you can answer your:

"At what time do the field strengths reach their highest and lowest value". 

by assuming that t1 is known  and is in the positive peak and t2 is in the negative peak, half a wavelength away..
A: What you are asking for is the period of the wave.
Electromagnetic waves have the same fundamental wave characteristics as all other harmonic waves.   
At every point through which the wave passes the electric field oscillates with a fixed frequency - in this case $f=5GHz$. The time between the wave reaching consecutive peak values is the period $T$. As usual this is related to the frequency of the wave by $f=1/T$.
The 'lowest values' of the electric field (ie the maximum in the opposite direction) occur at the half time points between peaks.
