when you can’t wait for relativistic rockets
Here’s the recipe for an eye-catching post according to Popular Mechanics. Find a picture of a giant machine designed to be used for the LHC. Browse the arXiv blog for the oddest paper that mentions the collider, then combine the two into a brief write-up about putting relativistic rockets to the test while smashing atoms at nearly the speed of light. But as much as I like to read about ideas for hyperdrives and potential revolutions in space exploration, I’m afraid that I’ve got some bad news about this one. Franklin Felber, the physicist behind the paper in question, is not nearly as close to making relativistic rocketry a reality as the post makes it seem.
You see, Felber’s concept is based on an equation created by mathematician David Hilbert while working on some of the implications of Einstein’s general relativity formulas. This equation predicted that objects traveling at just over half the speed of light should appear to be deflected by a stationary mass to distant observers. So after flipping the idea on its head and adding in a flood of formulas from pretty much every famous theoretical physicist of the last century, Felber came up with a vague concept for what he calls “antigravity propulsion” that would rely on an object traveling at relativistic speeds to accelerate a stationary mass in front of it.
Why exactly could we call something like this an antigravity device? I’m not quite sure, but this is how a press release by his tech R&D shop advertised his work while providing this bizarre description of how a relativistic device built to Felber’s specifications would achieve about 90% of the speed of light…
Accelerating a 1-ton payload to 90 percent of the speed of light requires an energy of at least 30 billion tons of TNT. In the ‘antigravity beam’ of a speeding star, a payload would draw its energy from the antigravity force of the much more massive star. In effect, the payload would be hitching a ride on a star.
Wait, when does a speeding star come into the picture? What speeding star are we talking about? And where would we get a star traveling at relativistic speeds? The smallest possible size of a star is comparable to the diameter of Jupiter. Sending something that big careening at even a small percentage of the speed of light is going to require a black hole. In his latest paper, Felber doesn’t go into any detail as to how we could harness the phenomenon he predicts. Instead, he lays out a blizzard of sophisticated calculus and posits that the LHC could provide a proof of concept test. And if the LHC isn’t available, he’ll settle for Fermilab’s Tevatron.
All right, so his company’s public relations efforts may not be the best and the use of the term antigravity might not be a good choice. But since Felber is trying his best to put his theory to the test in controlled conditions, it might not be such a bad idea to give him a shot. Just because his concept might not work, doesn’t mean we’ll never learn anything from his work. And if it does, we’ll have a new quantum phenomena to study. However, to start calling such a test a stepping stone in hyperdrive propulsion is rather sloppy science journalism since a relativistic particle moving a stationary one doesn’t say much about what would happen when the masses are scaled up to the size of spacecraft and how the phenomena can be used in a practical way.
Spaceships designed to carry humans to alien worlds and solar systems would be very large and heavy. How big would the repulsive mass need to be? And how would we accelerate it to over 0.577c without expending a totally unrealistic amount of energy in the process? Not even full scale antimatter engines would be up to the task and hypothetical warp drive technologies would be more of a risky alternative than an enabler.
See: Felber, F. (2009). Test of relativistic gravity for propulsion at the Large Hadron Collider SPESIF 2010, the Space, Propulsion and Energy Sciences International Forum arXiv: 0910.1084v1
[ illustration by CG artist Luke Paxton ]
I can only hope that we’re not STILL playing Halo by the time we enter hyperspace.
And where would we get a star traveling at relativistic speeds?
The descending career paths of Jøm Hurum, the balloon boy, or Britney Spears?
Given all the questions raised by Einsteinian frame-of-reference issues, simply defining stellar velocities seems a bit problematic. F’rinstance, good ol’ Sol is orbiting the galactic center at a respectable clip; add in the galaxy’s motion within the galactic cluster, etc and hang on to your beret! Cherry pick your star, galaxy & cluster, perhaps you could get within an order of magnitude of c.
Leaving out expansion-of-the-cosmos factors for a moment, just what is the fastest-moving known interstellar object?
“…just what is the fastest-moving known interstellar object?”
On a micro scale, those would be the particles in cosmic rays which can travel at 0.999c because they have very little mass and are created in energetic events like stellar fusion, supernova explosions, or in the accretion disks of black holes.
On a macro scale, that would probably be RX J0822-4300, a neutron star that was slung around by Sgr A*, the supermassive black hole in the center of our galaxy, and is moving at about 4.8 million km per hour. That works out to around 0.004c.
… the particles in cosmic rays which can travel at 0.999c because they have very little mass…
I s’poze “very little mass” is relative, too. How much of their (human-measured) mass is due to their material substance (if that term even applies), and how much comes from their velocity? Does their aggregate mass have notable cosmological consequence?
… 0.004c …
Put that way, it sounds positively boring.
“How much of their mass is due to their material substance, and how much comes from their velocity?”
That’s kind of a tricky question. I think you may be wondering about their momentum more than their mass since they move so quickly because they have very little mass and are thus going close to the cosmic speed limit.
“Does their aggregate mass have notable cosmological consequence?”
And that’s one of the mysteries of modern science. So far, it doesn’t seem so, but who knows how that idea will be refined after the LHC smashes enough particles for us to make a definitive statement about the Higgs boson.
Apologies for the intermittent queries – this so-called real life stuff gets in the way of blog-browsing sometimes.
Let me step back & take another run at my previous question:
IIRC, an object zipping along at relativistic speeds experiences several counterintuitive effects according to Einstein’s theories. Most attention is paid to time dilation (cuz it’s fun in sf plotlines, and has also been measured), but there’s also an increase in effective mass (or gravitational pull, or spacetime curvature). A stray body passing by at a high fraction of c will alter another body’s course more than one of the same mass and path just ambling along.
Back to our cosmic ray particle, cruising at almost-lightspeed. Its hypothetical mass at rest is, poetically, as close to zero as its actual velocity is as close to c. But does that near-photonic rate of motion give it more influence on material bodies than it would otherwise have – and if so, how much, and what else might that imply?
The short answer is yes. Particles in cosmic rays rip through pretty much anything and can knock our molecules around because they have so much momentum. If they didn’t travel that quickly, they wouldn’t be a type of potentially deadly radiation.
One very important thing to remember is the size of those particles. They’re protons and electrons with some helium nuclei, so their small size and mass inherently limit the effect they can have on the surrounding matter, even when traveling at 0.999c.
Greg – I still suspect we’re not quite on the same page here.
If a cosmic “ray”/particle bangs into an atomic nucleus, that nucleus will be all messed up in ways that wouldn’t happen if it met a similar particle at a sedate 0.004c. Well and good, but similar in principle to the difference between being shot and having a little kid throw a bullet at you.
Now imagine, say, a loosely packed dirty snowball drifting free in interstellar space, and a kilotonne iron cannonball zipping by at .004c, passing within 1 mm of the snowball. Gravitational effect is close to zero; maybe a few water molecules shift position slightly.
The next cannonball, otherwise identical, skims the snowball at the same clearance, but 249.9 times faster. If (the weak point here) my understanding of relativity is anywhere near accurate, the spacetime curvature/gravitation pull of this object is significantly greater than that of its slowpoke twin, and its tidal effects on the snowball are detectable.
In reality, my thought experiment is for practical purposes impossible. Nonetheless, even though they don’t travel in unified bodies, the rays/particles emitted by supernovae (etc) amount to exatonnes of mass, multiplied (I think) by the relativistic effect of their momentum. Does that unevenly diffuse gravitational influence (like that of comparably diffuse interstellar hydrogen) have any measurable or theoretical cosmological consequences?
Ah, ok. I see what you’re asking now Pierce.
Yes, particles traveling at relativistic speeds produce a kind of space-time shockwave and this is a fact well known by particle collider designers who need to build special buffers and dampeners as they whiz by. I mentioned this in a post about the colliders of the future in which the energies they would create could knock out some of the insanely delicate and expensive machinery.
Relativistic wakefields don’t diffuse gravity because they’re usually generated by tiny particles and while they constantly interact with much larger chunks of matter, they don’t seem to have any serious cosmological consequences as far as we know.
Unlike in colliders where bunches of them are aimed to smash into each other at 0.997c, their wakefields are scattered over very wide areas and the mass of the objects they hit is so immense by comparison, it’s like driving through a field of flies. Sure, some will smack into your windshield and leave a little streak, but it won’t change the direction of your car. The biggest effect cosmic rays have on Earth is the production of short lived muons, close cousins of electrons, in the atmosphere.
… special buffers and dampeners …
Hard to imagine how engineers, already tasked to prepare for earthquakes, fires, storms, and perhaps collisions, cope with a requirement to deal with the bending of spacetime itself. (Guess I’ll have to go read the link.)
It still seems to me that, say, a supernova shockwave passing through a nebula might leave differently-shaped ripples than would a similar mass of non-relativistic particles, but the experiment would be rather difficult to set up properly.
… wakefields are scattered over very wide areas …
The terminology makes sense, but probably causes some confusion among readers of pre-Victorian English literature.
Thanks for your patience in explaining things to the physics-ly handicapped!