Note: this feature only available in 3.1.1 or later builds. Also at the moment tekkit 3.1 uses Immibis's Modular Forcefields v1.97rev 7.
Force Fields are blue-ish transparent blocks generated by force field machines at a cost of force power. they have the following traits:
1) Indestructable (instantly reforms if destroyed)
2) Displaces water, oil, lava, redstone torches, redstone wires, levers (tested) and torches when generated.
3) Any block in a position where a field should be generated not listed in the above list will prevent the field from forming in it's space until the block itself is destroyed.
4) Every block of force field generated takes 1 force power per second to maintain. There is more information on this on the Forcefield Core page.
5) Force field blocks are considered solid to players and mobs, who can only pass through them if they were standing in a block where the forcefield block spawned. Mobs cannot see through the forcefield blocks. Light can, however, pass through the forcefield blocks
There was a concern raised that large force fields may take a few moments to spawn. This is not the case, and has been proven false by using a Wireless Transmitter. All forcefields, no matter the size, are instantly spawned when activated.
Note: the images below were taken using the MISA texture pack so prepare for 16 and 64bit blocks side by side.
The Equivalent Exchange mod adds a number of items that can destory blocks with in a Forcefield with out destorying the field itself, as the Forcefield is ment to be un-breachable. This is considered a bug. Confirmed items are The Destruction Catalyst, The Catalytic Lens, The Red Morningstar and The Hyperkinetic Lens (others may exist). Players should be warned not to trust a forcefield for complete protection from players, it is recommend that players use personal safes in combination with a Forcefield for complete protection of their items if the server doesn't not have any grief/chest protection.
A player wearing gem armor standing on a forcefield block will generate lava particles.