Science fiction becomes science: Study investigates traces of warp drives

Traveling through space at faster-than-light speeds is a dream that has fascinated more than just Star Trek fans for decades. What was considered pure science fiction has been the subject of scientific research for 30 years. In 1994, physicist Miguel Alcubierre discovered a theoretical solution to Einstein's field equations (Alcubierre metric) that describes a "warp drive".

The concept: a spaceship creates a "warp bubble" that contracts the space in front of it and expands behind it. This would allow it to effectively move faster than light without locally exceeding the speed of light or violating the fundamental laws of physics, Lorentz invariance. Although the drive would require exotic matter with negative energy density - a concept that so far only exists in equations – the topic is nevertheless interesting for physical thought experiments.

A paper published on arXiv in early June raises an intriguing question: even if humanity is still a long way from constructing such propulsion systems, could there be more advanced extraterrestrial civilizations that have already achieved this? And if so, would we be able to detect the traces of such a warp drive? A team from the Universities of Oxford, Potsdam and Cardiff has investigated these questions using computer simulations to determine what would happen if a hypothetical warp drive collapsed. The results provide surprising insights into the physics of such exotic spacetime distortions and their possible signatures. One of the researchers involved is astrophysicist Prof. Dr. Tim Dietrich from the Max Planck Institute for Gravitational Physics. We spoke to him about the publication and asked how much "Trekkie" there is in someone who deals scientifically with warp drives.

Details on the results of the study can be found below the interview.

heise online: Does it help to be a Trekkie as a gravitational physicist?

Tim Dietrich: I can't say for sure whether it really helps, but for some of us, series like Star Trek have been a motivation to study topics like gravitational waves, black holes or neutron stars.

But I guess only Trekkies get the idea to deal with warp drives?

Although all those who worked on the article are Trekkies, many colleagues (including some "non-Trekkies") are of course familiar with the Alcubierre metric and are therefore well acquainted with the theoretical principles. However, it can be assumed that you have to have a certain affinity for the topic in order to invest the necessary time to deal with it intensively.

Do the gravitational wave signatures depend strongly on the specific model and the equation of state chosen for the matter?

It seems to us that the qualitative form of the gravitational wave signal is relatively independent of this. However, the exact strength of the signal and also the frequency of the gravitational waves would of course depend on which equation of state is used and also which metric is assumed. We have used the Alcubierre metric here, but there are also similar metrics that would certainly lead to quantitatively (but not qualitatively) different results.

Can the simulations be extended to higher speeds - and what particularly interests me as a Trekkie - faster-than-light speeds? What would one expect in these cases?

Higher speeds up to the speed of light (or just below) should be possible without major changes in the code, but will need higher resolution and thus more computing time to give accurate results. Above the speed of light there may be some problems with the warp bubble. The problem with faster-than-light travel is that a kind of horizon forms, making it impossible to leave the bubble. This means that spacetime inside the bubble and outside the bubble are no longer causally interchangeable. We suspect that this is not so easy to simulate; definitely not with the methods we have used.

How would the gravitational wave signal change if longer simulations with higher resolution were performed?

For the configurations we have shown, we have done tests with different resolutions and verified that everything is correct. Therefore, no changes are actually to be expected in this area. Regarding the runtime of the simulation: here too, we made sure to run the simulations until the space-time change is small. But of course it can be assumed that - as described above - longer simulations with higher resolution are necessary for other parameters. Of course, we can only speculate what the simulation results will look like, but I suspect that the amplitude of the signals in particular will become stronger if the warp bubble has a higher energy.

What role does the gravitational wave memory effect play in particular?

To investigate this, we would need to run more simulations and focus precisely on the memory modes. These are normally more difficult to calculate, so we have only made an initial estimate here. Regardless of this, however, it is very interesting that we were able to observe that the remaining spacetime has a higher mass than our initial conditions, i.e. negative energy (as described below) is also radiated. I had not suspected this in advance, but it is very interesting.

How realistic is it that future detectors could actually measure such signals?

Of course, the answer to this question is very speculative, but despite all my love for Star Trek, I don't think we will see such signals with future gravitational wave detectors. Ultimately, the amount of negative energy required to power a warp drive makes it very unlikely that this technology will actually be used.

What would be the effects of the interaction of the warp drive's "exotic" matter with normal matter, if such an interaction exists beyond gravity?

In fact, this is not known. We have discussed this within our group and suspect that this interaction could lead to a possible multi-messenger event, i.e. that neutrinos or electromagnetic radiation can also be emitted through the interaction, but we cannot verify this.

Can the results be generalized to other "exotic" spacetimes and phenomena, i.e. wormholes or other models for warp drives?

This is a very good question and something we would like to look at in the future. We suspect that the methods used make a more general application possible, but this also needs to be tested thoroughly.

What no human has ever seen before - gravitational waves from collapsing warp bubbles

The researchers simulated for the first time what would happen if the warp bubble of a spacecraft became unstable and collapsed. To do this, they developed a numerical model that solves Einstein's complex field equations for this case.

The results are surprising: when the warp bubble collapses, characteristic gravitational waves are emitted. Unlike known astrophysical sources such as merging black holes, the simulated gravitational wave signal of a collapsing warp bubble shows a unique pattern, according to the researchers. It begins with a sudden burst, followed by an oscillating phase with a characteristic frequency of the order of the reciprocal of the bubble radius.

For a warp bubble of 1 km in diameter(editor's note: the spaceship must be completely enveloped: the largest Enterprise NCC-1701-E will have a length of 685 m), this would correspond to a frequency of around 300 kHz - significantly higher than the frequencies that earthbound detectors currently in use can record. But the amplitude would be considerable: at a distance of one megaparsec (about 3.26 million light years), the signal would still cause a stretching of space-time ("strain") of 10^-21- an order of magnitude that could in principle be measured if gravitational wave detectors were available for the corresponding frequency.

"Strain" (elongation, stretching, load) of a gravitational wave

The strain of a gravitational wave is the amount by which distances are stretched or compressed by a passing gravitational wave in relation to the original length. It is a dimensionless number and in the case of events such as the merging of two neutron stars at a distance of 130 million light years is of the order of 10^-21. For comparison: the diameter of a proton is 1.7 × 10^-15 m, so the elongation is less than one ten-thousandth of the size of a proton.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) - with which the first gravitational wave measurements were made - can register strains of the order of 10^-22; work is currently underway to achieve even greater sensitivity.

The work also investigated the dependence of the results on the speed of the warp bubble. They were able to carry out stable simulations for speeds of up to 50 percent of the speed of light, with higher speeds requiring a much higher numerical resolution. The team suspects that new phenomena could occur when approaching the speed of light.

Total energy of the system increases

According to the researchers, one particularly interesting aspect of the simulations is the flow of energy during the collapse: While the gravitational waves carry positive energy out of the system as expected, the exotic matter of the warp bubble shows a more complex behavior. It alternately emits waves of positive and negative energy. Overall, this even increases the total energy of the system - which would not be possible with "normal" matter.

This behavior is reminiscent of early theoretical work by Hermann Bondi (developer of the steady-state theory) on the interaction of positive and negative matter. The behavior raises interesting questions about the dynamics of spacetimes that violate the zero-energy condition. The authors emphasize that further investigations are needed to understand the influence of different equations of state on the gravitational wave signal and the matter fluxes.

Even if questions about how to create or even control a warp bubble(and what would be a counterpart for the "dilithium crystals" needed to control the flow of matter-antimatter reactors according to Star Trek canon, editor's note) sound like science fiction at first, such thought experiments and simulations sometimes reveal exciting scientific findings.

(vza)