Wormholes: real physics, no known route to building one

A wormhole is a topological shortcut — a connection between two separated regions of spacetime, shorter through the connection than through ordinary space.

It is not science fiction borrowed by physics. It is physics that fiction borrowed, and the distinction is worth setting out because it cuts both ways.

Where the idea comes from

Einstein and Nathan Rosen described a bridge structure in 1935 as a feature of the field equations. The Einstein-Rosen bridge is a genuine solution and it is not traversable — it pinches off faster than anything could cross it.

The modern treatment dates from 1988, when Michael Morris and Kip Thorne published a paper on traversable wormholes.

The prompt was unusual. Carl Sagan, writing the novel Contact, asked Thorne whether there was a physically defensible way to move a character across the galaxy. Thorne and his student worked the problem, found the conditions a traversable wormhole would require, and published them.

The exotic matter problem

Negative energy density is not entirely hypothetical. The Casimir effect — two uncharged conducting plates placed very close together experience an attraction arising from the quantum vacuum between them — involves a region of negative energy density relative to the surrounding vacuum. It has been measured.

The quantities are microscopic. The amounts required to hold open a wormhole large enough to pass through are, on current estimates, comparable to the mass-energy of astronomical objects, in negative form, arranged stably.

There is no known physical process that produces this. Not a difficult engineering problem — an absence of any mechanism.

Where the negative energy limits come from

Quantum field theory does not forbid negative energy density outright. It constrains it, through results known as quantum energy inequalities.

These say, roughly, that a region of negative energy must be compensated by positive energy nearby, and that the more negative energy you want, the more briefly and the more locally you can have it.

The constraint bites hard on wormholes. Holding a throat open requires the negative energy to persist in a fixed region, which is precisely the configuration the inequalities restrict.

The causality problem

This one is arguably worse.

If a traversable wormhole exists and one mouth is moved relative to the other, relativistic time dilation makes the two ends disagree about when it is. Passing through then places you in your own past, which permits closed timelike curves.

That is not a paradox to be waved at; it is a structural problem that any working wormhole appears to create. Hawking's chronology protection conjecture proposes that the laws of physics prevent this, possibly by vacuum fluctuations destroying the wormhole as it forms. The conjecture is unproven.