Physical Limits to Communication

The limits posed by physics to the quantity of information that can be transmitted with a certain amount of power are investigated. The same ultimate limits are found for transmission of information encoded using matter and massless fields.

Figure 1

Transmission line for massive objects. Alice continuously sends Bob information encoded into slabs of material of cross section $A$ (with largest transverse dimension $a$) and rest length $L$ at a speed $v$. The upper graph depicts the situation in Alice and Bob’s reference frame where the slab is Lorentz contracted by a factor $\gamma$; the lower graph depicts the situation in the slab frame where Alice and Bob are Lorentz contracted by the same amount.

Figure 2

In order to apply the Bekenstein bound (4) we enclose the channel with overlapping spheres of radius $r$ which are separated by a distance $l$ (as measured in the rest frame of the information carriers).