Warp Drives

“Warp drives” originated in the science fiction shows “Star Trek” in the 1960s, but in 1994, the Mexican Miguel Alcubierre published a paper arguing that under certain conditions exceeding light speed was not forbidden by Einstein’s General Relativity. Alcubierre reached his solution by assuming it was possible, then working backwards to see what was required while rejecting those awkward points that arose. The concept is that the ship sits in a bubble, and spacetime in front of the ship is contracted, while that behind the ship is expanded. In terms of geometry, that means the distance to your destination has got smaller, while the distance from where you started gets longer, i.e. you moved relative to the starting point and the destination. One of the oddities of being in such a bubble is you would not sense you are moving. There would be no accelerating forces because technically you are not moving; it is the space around you that is moving. Captain Kirk on the enterprise is not squashed to a film by the acceleration! Since then there have been a number of proposals. General relativity is a gold mine for academics wanting to publish papers because it is so difficult mathematically.

There is one small drawback to these proposals: you need negative energy. Now we run into definitions, and before you point out the gravitational field has negative energy it is generated by positive mass, and it contracts the distance between you and target, i.e. you fall towards it. If you like, that can be at the front of your drive. The real problem is at the other end – you need the repulsive field that sends you further from where you started, and if you think gravitationally, the opposite field, presumably generated from negative mass.

One objection often heard to negative energy is if quantum field theory were correct, the vacuum would collapse to negative energy, which would lead to the Universe collapsing on itself. My view is, not necessarily. The negative potential energy of the gravitational field causes mass to collapse onto itself, and while we do get black holes in accord with this, the Universe is actually expanding. Since quantum field theory assumes a vacuum energy density, calculations of the relativistic gravitational field arising from this are in error by ten multiplied by itself 120 times, so just maybe it is not a good guideline here. It predicts the Universe has long since collapsed, but here we are.

The only repulsive stuff we think might be there is dark energy, but we have no idea how to lay hands on it, let alone package it, or even if it exists. However, all may not be lost. I recently saw an article in Physics World that stated that a physicist, Erik Lentz, had claimed there was no need for negative energy. The concept is that energy could be capable of arranging the structure of space-time as a soliton. (A soliton is a wave packet that travels more like a bubble, it does not disperse or spread out, but otherwise behaves like a wave.) There is a minor problem. You may have heard that the biggest problem with rockets is the mass of fuel they have to carry before you get started. Well, don’t book a space flight yet. As Lentz has calculated it, a 100 m radius spacecraft would require the energy equivalent to hundreds of times the mass of Jupiter.

There will be other problems. It is one thing to have opposite energy densities on different sides of your bubble. You still have to convert those to motion and go exactly in the direction you wish. If you cannot steer as you go, or worse, you don’t even know for sure exactly where you are and the target is, is there a point? Finally, in my science fiction novels I have steered away from warp drives. The only times my characters went interstellar distances I limited myself to a little under light speed. Some say that lacks imagination, but stop and think. You set out to do something, but suppose where you are going will have aged 300 years before you get there. Come back, and your then associates have been dead for 600 years. That raises some very awkward problems that make a story different from the usual “space westerns”.

Advertisement

Seaweed and Climate Change

A happy and prosperous New Year to you all. The Great New Zealand Summer Vacation is coming to an end, so I have made an attempt at returning to normality. I hope all is well with you all.

Last year a paper in Nature Communications (https://doi.org/10.1038/s41467-021-22837-2) caught my eye for two reasons. First, it was so littered with similar abbreviations I found it difficult to follow. The second was that they seemed to conclude the idea of growing seaweed to absorb carbon dioxide would not work, but  they seemed to refuse to consider any option by which it might work. We know that much of seaweed biomass arises from photo-fixing CO2, as does biomass from all other plants. So there are problems. There were also problems ten thousand years ago for our ancestors in Anatolia or in the so-called fertile crescent wanting to grow some of those slightly bulky grass seeds for food. They addressed those problems and got to work. It might have been slow, but soon they had the start of a wheat industry.

So, what was the problem? The paper considered the Sargasso Sea as an example of massive seaweed growth. One of the first objections the paper presented was that the old seaweed fronds get coated with life forms such as bryozoans that have calcium carbonate coatings. They then state that by making this solid lime (Ca⁺⁺ + CO3^— -> CaCO3, a solid) it releases CO2 by reducing seawater alkalinity. The assertion was from a reference, and no evidence was supplied that it is true in the Sargasso. What this does is to deflect the obvious: for each molecule of lime formed, a molecule of CO2 was removed from the environment, not added to it as seemingly claimed. Associated with this is the statement that the lime shields the fronds from sunlight and hence reduces photosynthesis. Can we do anything about this? We could try harvesting the old fronds and keep growing new ones. Further, just as our ancestors found that by careful management they could improve the grain size (wild wheat is not very impressive) we could “weed” to improve the quality of the stock.

I don’t get the next criticism. While calcification on seaweed was bad because it liberated CO2 (so they say) they then go on to say that growing seaweed reduces the phytoplankton, and then the calcification of that gets reduced, which liberates more CO2. Here we have increased calcification and decreased calcification both increase CO2. Really?

Another criticism is that the seaweeds let out other dissolved carbon, which is not particulate carbon. That is true, but so what? The dissolved sugars are not acidic. Microalgae will gobble them up, but again, so what?

The next criticism is if we manage to reduce the CO2 levels in the ocean, we cannot calculate what is going on, and the atmosphere may not be able to replenish the levels for a up to a hundred years. Given the turbulence during storms I find this hard to believe, but if it is true, again, so what? We are busy saving the ocean food chains. Ocean acidification is on the verge of wiping out all shellfish that rely on forming aragonite for their shells. Reducing that acidity should be a good thing.

They then criticise the proposal because growing forests on land reduces the albedo, and by making the land darker, makes the locality warmer. They then say the Sargasso floating seaweed increases the albedo of that part of the ocean, and hence reflects more light back to space, which reduces heat generation. Surely this is good? But wait. They then point out that other proposals have seaweed growing in deep water and this won’t happen. In other words, some aspect of some completely different proposal is a reason not to proceed with this one. Then they conclude by saying they need more money to get more detailed information. I agree more detailed information would be helpful, but they should acknowledge possible solutions to their problems. Thus ocean fertilization and harvesting mature seaweed could change their conclusions completely. I suspect the problem is they want to measure things, possibly remotely, but they do not want to actually do things, which involves a lot more effort, specifically on location. But for me, the real annoyance is that everyone by now knows that global warming is a problem. Growing seaweed might help solve that problem. We need to know whether it will contribute to a solution or merely transfer the problem. They may not have the answers, but they at least should identify the questions that need answers.