My plan to attend the Tennessee Valley Interstellar Workshop this week fell through when I became ill two days before departure date. Talk about bad timing. TVIW is a wonderful conference, and not only was my son Miles flying in for the event, but I had planned many good conversations with friends in the interstellar community. I was also looking forward to the Homo Stellaris working track led by Robert Hampson. I’ve been at all previous TVIWs and deeply regret having missed this one.
I’m hoping for a less foggy mind by tomorrow, at which point I’ll resume the schedule here, which is four or five posts per week unless interrupted by travel. Complicating this past few days has been a server migration which apparently went well (this, thankfully, was out of my hands), and the need for a PHP upgrade, which should be occurring by the end of the week. Fingers crossed, I am hoping for no disruption. I’ll hope to get some reports from TVIW and pass along links to presentations from the Chattanooga conference as they become available.
Why didn’t you name your son Parsecs? Get well soon…
Wish I had thought of that!
Eh, I’m kind of glad you hadn’t… :)
Another neat conference: http://space-access.org/updates/sa16info.html Living in Arizona I get to attend it sometimes. At earlier Space Access conferences I met Henry Spender and George Herbert, two very knowledgeable folks on Space exploration.
I’m going to try to attend this year’s conference. In my opinion propellent depots are one of the technologies with the potential to enable economical spaceflight. Jon Goff and Frank Zegler will be attending. Both those guys have been doing work to make depots a reality.
Jeff Greason and John Schilling will be attending. Would love to see of the Centauri Dreams crowd there.
Get well soon, Paul. And good luck with the server migration. My web site went through a similar migration a couple of months ago and I was originally told it would take as long as several days… I lucked out and it was all done in less than a day. Hopefully you’ll get lucky as well :-)
I hope you have a speedy recovery Paul, I am sure they will have plenty to share with you.
I kept reading “server migration” as ‘severe migraine’ and nothing after that made sense. During the third reading and approaching my own migraine, I found my error.
Hope you feel better soon and glad you don’t have that migraine.
Well, no migraine here yet, but that PHP upgrade seems a bit ominous…
Get well soon Paul. The starflight community is important, but your health more so.
I’m very sorry to read of your missing out on the workshop, connecting with friends and son; and being ill, of all things, instead.
I hope that you’ll more than make up for that at the next workshop.
Very sorry to hear about your illness and disrupted plans, Paul. I hope you feel better soon.
Off-topic, but possibly of interest: I stumbled across a mention of this paper somewhere on Twitter:
“Creating spacetime shortcuts with gravitational waveforms” by Charles J. Quarra.
http://arxiv.org/abs/1602.01439
I don’t really understand it, partly because I lack both the math and the vocabulary, but apparently the author speculates that it may be possible to use gravitational waves to set up a limited region within which, in a specific direction, it is possible to travel or communicate between points faster than the speed of light as perceived by an observer outside the region.
It is apparently similar to using an Alcubierre drive to travel FTL as perceived by an outside observer, without some of the Alcubierre drive’s difficulties (one of which is that the Alcubierre drive requires exotic matter that may not exist, whereas at least we appear to have some observational evidence that gravitational waves exist).
The biggest difficulty with this new idea is setting up the sources of gravitational waves in order to create the “FTL” region. The author speculates that a highly advanced civilization might be able to do this, though.
The Next Big Future website has a summary of the paper minus the math. I still don’t really understand it. Unfortunately, as of this writing, the NBF site appears to be down (I viewed the summary via Google’s web cache).
I spent 5 minutes skimming the paper. Offhand this looks to me to be nothing special. However my understanding of precisely what’s going on is incomplete.
My take is that this is a gravitational wave equivalent to group velocity in electromagnetics whereby a feature of a wave front can be seen to propagate FTL. But nothing is actually moving FTL, nothing physical and no information transfer.
I recall Charles Quarra having appeared here before so perhaps he’ll weigh in.
@RonS:
You obviously know more about these things than I do — I can tell by the terms you use — but bear with me for a few minutes.
OK, conceptually: I have previously read about some phenomena that naively _seem_ to be moving FTL but really aren’t. For example, you can stand outside under the night sky with a laser pointer and sweep your laser beam from one star to another in a couple of seconds. In a naive sense, it would appear that the “end” of your beam of light has moved from star A to star B faster than a beam of light could move from A to B. But nothing has _really_ traversed the actual distance of A-B faster than light — no particle, no object, no information. To my limited understanding, this seems to be at least roughly analogous to your “nothing special” interpretation of the paper. (Which, as you said, was only based on a quick skim of it.)
However, although I don’t really understand the paper, I think there’s a bit more going on than that.
The paper seems to be saying that, within a region R, “geodesics” (which I understand to mean the spacetime path within which a particle can travel) can actually move faster (from the viewpoint of an observer outside R) in a particular direction than could a beam of light without R ” because spacetime has shrinked [sic] in a preferential direction for their [that is, the geodesic’s] whole transit inside R.”
So, as I understand it, the spacetime path that a particle is traveling along is actually shrinking in a particular direction — and that allows the particle to travel from point A to point B faster than light could without this effect. But as with an Alcubierre drive, this isn’t violating relativity, because locally, spacetime itself moves, or warps, in some way that makes the movement within that spacetime region slower than light. (Later the paper says the spacetime dilates as well as shrinks.) It is FTL only from the viewpoint of someone outside that region.
Also, in the “Discussion” section that comes right before “Appendix A,” the paper says things like:
“Assuming the daunting problem of astronomical scale gravitational wave generation is somehow solved, this method could in principle enable FTL travel without appealing to exotic physics. However a detailed
analysis of tidal forces is required before assessing the feasibility of this scheme for transit of payloads.”
“The nature of the shortcut generation involves the creation of waveforms that compress and dilate spacetime in the direction of flight. In order for signals (or ships) to be able to take advantage of the metric-contracting fields, they must carefully control their timing synchronization, in order to cross the field regions as close as possible to the compression valley, where the distance is minimal between opposing sides of the field region. The region must be crossed in substantially less than T /2, with T being the period of the gravitational wave. Even if the compression of
each field region is small, large distance reduction could be accomplished by bridging many pre-configured field regions in a timely manner. It is conceivable that other field configurations exist that achieve better distance compression patterns. Even without exploiting the FTL aspects of the field, time-like geodesics can still be substantially accelerated
or decelerated with special field configurations of this type, while remaining in free fall during the transit.”
From this wording, I think it is clear that the author believes that physical objects, as well as communication (signals), could actually be moved from A to B faster than otherwise — fast enough, from the viewpoint of an observer outside the special region of spacetime (including observers at the origin and destination), to be perceived as FTL.
I don’t know if this is accurate, but the image I am getting from all this is something like the following:
You have your special region of spacetime, going from point A to point B. Within this region moves a long caterpillar that humps itself along. I’m not clear on whether I should says the caterpillar is analogous to the spacetime stretching from A to B, or is analogous to a geodesic that is moving within the spacetime region. But at any rate, the caterpillar’s inching/humping along is such that the crest of each moving “hump” in the caterpillar’s back is actually touching its neighbor hump before the caterpillar’s motion stretches things out again.
And there is an ant walking along the caterpillar’s back. Let’s say that the maximum speed that an ant can walk is the equivalent of “C.” As the ant walks along the caterpillar’s back, it times its movements so that it travels primarily from the crest of one hump to another while those humps are adjacent, with no “valley” to cross in-between. This has the effect of shortening the path that the ant actually has to travel. (Plus I guess the caterpillar itself is moving? Is that also part of this analogy? Maybe not.)
As a result, the ant can get from A to B much faster than could an ant that was just walking along the ground and did not have the benefit of walking along the cresting bumps of the caterpillar. Both ants could be traveling at “C” (maximum ant walking speed) and neither would perceive itself as walking faster than “C,” but the ant using the caterpillar effect would cross from A to B faster than “C” from the viewpoint of the unaided ant.
Or I could have badly misunderstood something. But this is the impression I am getting from the portion of the paper that I think I understand.
Stevo, let’s forget about the paper for a moment. The more basic question is whether GW can cause a gravitational gradient that can be exploited for propulsion. The answer is effectively no. Interference (as in the paper) doesn’t change that.
https://en.wikipedia.org/wiki/Gravitational_wave
This is a useful starting point. Notice that the plane wave applies an oscillating strain. This deforms objects by stretching and compressing space. The strain has an amplitude and polarity. In a binary merger, for example, the polarity rotates in sync with the system while the amplitude is in proportion to the intensity of the binary interaction.
But there is no gradient as in the Alcubierre warp. So there is no net movement.
However, I do know that GW can carry linear momentum. It is this recoil that can “kick” a BH to high velocity. But I don’t know that the radiation itself has any utility, and if it did you’d have to be awfully close to the merger. For more I think an expert would have to speak up.
My best wishes for your quick recovery.
Best wishes and hope you have a speedy recovery Paul.
Thanks to all who have wished me well re my incredibly poorly timed illness! Your thoughts are much appreciated. Am gradually emerging back to the world, though with laryngitis and a hacking cough.
Dear Paul
We all missed you at TVIW. Feel better!
Regards, Greg
Greg, I was especially sorry to have missed the event when I realized you would be there. Was C with you? Would have been great to see you both. Let’s hope we can do that some time soon.
Hope you are feeling better Paul.