Unblockable Communication With Gravity

Abstract

All modern wireless communication technologies are based on electromagnetism. However, electromagnetic signals are susceptible to screening and blocking, so their availability cannot be guaranteed in adverse environments. This raises a fundamental question: Can information be transmitted through a truly unblockable channel? Here we show that gravity, unlike electromagnetism, offers such a path. We propose and implement a wireless communication protocol in which a broadcaster encodes a binary message by moving a mass, while a receiver detects the resulting gravitational signal with a gravimeter. We validate this scheme experimentally, successfully transmitting a gravitational message a distance of $\approx$ 0.7 m through a brick wall at a rate of 1 bit min$^{-1}$. These results establish gravity as a viable platform for unblockable communication.

Figures (4)

• Figure 1: Schematic of gravitational communication. Alice (inside a perfectly shielded room) can communicate with Bob (outside the room) by moving a mass between two positions '0' and '1' to create a binary change in the local gravitational field. The inset shows a schematic of the gravitational acceleration $g$ measured with Bob's gravimeter as Alice moves the mass between the two positions.
• Figure 2: (a) Schematic of a point mass moving between vertical positions $+\Delta z$ and $-\Delta z$, forming a gravitational dipole antenna. The gravimeter, at position $(r,\alpha)$ in the $xz$-plane, measures a change in gravitational field as the mass moves between the two positions. (b) Polar plots of the vertical signal $\Delta g_z$ from the gravitational antenna, as a function of the polar angle $\alpha$ from the axis of the antenna. Line colours denote the radial distance $r/\Delta z$ from the centre of the antenna, indicated by the legend. The strength of the signal at each radius is normalised to the largest value at that radius. For comparison, an electromagnetic half-wave dipole antenna radiation pattern is shown as a black dotted line. Note that all patterns are symmetric with respect to rotation around the $z$-axis.
• Figure 3: Schematic of experimental setup (not to scale). (a) The counterweight in the lower (0) and upper (1; dotted boundary) position. The force $\mathbf{F}_\mathrm{g}$ on the gravimeter's test mass due to the gravitational attraction to the counterweight changes in both magnitude and direction between the two positions, as indicated. (b) Schematic of the expected vertical gravitational signal $g_z$, as a function of the counterweight's vertical position. The lower and upper counterweight positions are indicated as $z_0$ and $z_1$, respectively. The maximal vertical signal occurs at an intermediate height $z_+^*$, resulting from the tradeoff between the directionality of the sensor and $\sim z^{-2}$ fall-off of gravity.
• Figure 4: Gravimetry signal $g_z$ over the transmission sequence (corrected for tide and drift supplement). The start and end points of each 7-bit ASCII character are denoted with vertical dashed lines. The 60s temporal window for each bit is indicated by the alternating blue and white shading, and horizontal solid lines show the corresponding mean $g_z$ for that interval, where the average is taken over the final 25s. All measurements above (below) $g_z=25nms^{-2}$ (horizontal black dotted line) after correction are recorded as a $1$ ($0$). Note that this is a relative gravitational measurement, and hence the offsets of the data are arbitrary. The sign convention of the data is such that a positive change in $g_z$ corresponds to an increase in downward acceleration.