Warp Drive News
backreaction.blogspot.com
backreaction.blogspot.com
Also, any article that ends with a phrase like "while it may look now like you can’t do superluminal warp drives, this is only correct if General Relativity is correct" does not bode well for practical applications of the subject matter. There is also the logical fallacy that it might be the case that GR is incorrect and you still can't do superluminal speeds.
Debunking with a critical eye is good and necessary but sometimes one gets the feeling that certain internet debunkers feel a need to debunk at all costs because that's their label, even if their own "rational" interpretations make leaps of logic much worse than simply admitting that something inexplicable was observed.
https://patents.google.com/patent/US10144532B2/en
It looks like the secret US navy TR-3B. The language here is oddly specific. Is this real?
> https://patents.google.com/patent/US10144532B2/en
> It looks like the secret US navy TR-3B. The language here is oddly specific. Is this real?
This is an EMDrive variant that AFAICT hasn't yet been tested or validated (contrast with the Shawyer frustum design, which has been tested, but not validated).
A warp drive that lets your local frame feel like .1c while you are actually traveling at .3c would still be a civilization-altering development.
Special Relativity is about velocity, not bending spactime.
It's not clear that this method of manipulating space-time would create a shockwave (I would be surprised if it didn't on start/stop) or would interact with matter differently from subliminal travel in transit.
The "problem" with General Relativity is that it is so general it can describe arbitrary universes exactly, but doesn't in itself provide a way to tell if an exact description is a good match to our universe. So we have to use some heuristics to reject candidates.
For example, general curved spacetimes have any number of space and time dimensions. General Relativity will let you exactly solve the geometry of a universe with one dimension of space and one of time, or ten dimensions of space and two of time, and so on. However, we have excellent (and so far never contradicted) evidence that our entire observed universe must be in one of a very particular family of general curved spacetimes, those that are Lorentzian. Lorentzian spacetimes have three dimensions of space and one of time. When we examine a solution to the Einstein Field Equations and see only two dimensions of space, or twenty-five dimensions of space, we can reject it as unphysical.
Next, there are an infinite number of possible Lorentzian solutions to the Einstein Field Equations, and we can make some very silly ones. We can reject any that are spatially smaller than a breadbox, for example, or which have a total existence of about ten seconds, or in which literally nothing ever happens for eternity. Indeed, we can reject a Lorentzian spacetime as unphysical if its properties could never produce galaxies (since we see those, and live in one) and the conditions for creating galaxies includes chemistry, nuclear physics, and so on. There are lots of possible Lorentzian universes that can produce "us", but lots more that cannot, and we can reject those as unphysical (because we exist; this is a flavour of the Anthropic Principle).
Relativists, however, like to work with simplified models to make calculations tractable, even if those simplified Lorentzian universes are obviously unphysical. Most of them are vacuum solutions (i.e., there is zero matter content at all, so therefore no galaxies), or use some very simple splash of matter (the Standard Cosmology models the matter content of our Lorentzian universe as a handful of smooth fluids, ignoring the relatively small-scale lumpiness of stars and planets). These models are manifestly unphysical, but [a] still usefully approximate observed physical systems and [b] typically can be made to more and more closely approximate our physical universe (or at least a region of it) perturbatively, meaning that we can add things to the simple model making it much less simple and a little more in accord with observation and experiment.
But how do we connect a manifestly unphysical model with an apparently physical one? Heuristically, again. Among the things we expect to decide whether a Lorentzian spacetime is physical or not are various energy conditions [1]. When a region of some arbitrary Lorentzian spacetime is determined to violate some energy condition, its physicality is in doubt. It may be possible to rescue such a region of spacetime in some cases, but most of the time one would not bother. "Seemed promising until I found it violated some energy condition at galactic scales (kiloparsecs)" can still be good for developing intuitions or numerical methods or whatnot, and might even work as an effective theory -- reasonably modelling our physical universe -- at a much smaller scale (like in a star system, at the scale of tens of astronomical units or microparsecs) or a much larger scale (like at the scale of tens of billions of light-years or gigaparsecs).
"Warp drive" solutions similar to Alcubierre's are vacuum solutions which are almost always a bump function on an otherwise perfectly flat Lorentzian spacetime. For short trips from near some outer planet of our solar system to near some outer planet of a nearby star system (scale of parsecs), the presence of gravitating matter nearby the path of travel can be treated as small perturbations on this model. So heuristically, we look OK. However, the "warp bubble" in these solutions are eternal features of the spacetime, and that's both a giant warning sign of unphysicality (is any complex structure eternally old in our universe, given that everything we know of is consistent with a hot big bang in the finite past?) and also completely useless for the purposes of on-demand travel. So, we ask the question: how does one go from a vacuum flat spacetime to a spacetime with a suitable bump function?
Here one takes a "sources-first" approach, where sources are matter and anything else that generates nonzeroes in the stress-energy tensor that sits on the right hand side of the Einstein Field Equations. As a slogan one would put this as "matter tells spacetime how to curve". So what type of matter can create a bubble?
As one investigates this question, one generically runs into a violation of the positive energy condition. I would describe the postive energy condition as "the joint ground state of all the spacetime-filling fields serves as a floor, and no local perturbation of this state leads to a globally-lower energy", or "empty space is really really cold, and adding things to it can only make it hotter rather than colder". A solution which upsets this -- negative energy -- is something never seen in any experiment since the discovery of the second law of thermodynamics, and negative energy is so torturously hard to sweep under carpets like non-isolation or vacuum energy that after a while it must begin to feel self-deceitful to keep pursuing a region of spacetime that contains it as anything other than an unphysical model.
Apparently people who try also run into comparable problems when trying to (for want of a better word) steer a warp bubble that exists (ignoring how to make it exist in the first place, or how to make it cease existing at some later time). I'd wager the model runs into trouble for trips between galaxy clusters, by virtue of interacting with the metric expansion of space: having to make big changes to simple models to accommodate the cosmological constant is a common pattern in the history of General Relativity.
There is something of a literature on problems like this, and while it's interesting in the sense of hunting for heuristics to shoot down other seemingly physical solutions to the Einstein Field Equations, I don't think it's very interesting to keep looking for loopholes to try to save the family of Alcubierre-like spacetimes. Beating a dead horse into some sleek-looking and seemingly aerodynamic shape won't make a stagecoach be more likely to reach Mach 6.
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[1] https://en.wikipedia.org/wiki/Energy_condition#Motivation
The lack of interest hangs on the fact that we have highly-tested descriptions of our universe which have relativity baked right in. Even Alcubierre's "drive" paper [1] is written in the language of relativity. And thus we have to take seriously the problem that we have in relativity no obvious way to induce a warp bubble onto a patch of flat spacetime by scattering any configuration of known matter through that region.
We can create stars, including relativistic stars (neutron stars are an example) and black holes by depositing practically an infinite variety of matter into a region of flat spacetime. (In fact, if we use a solar mass of hydrogen gas then Raychaudhuri's focusing theorem makes it hard to avoid it coming together as a star eventually, and then probably a white dwarf).
Likewise, "large scale structure formation" is nearly inevitable with a handful of parameters which in the standard cosmology we capture and try to measure. There's lots of ways in an expanding universe to wind up with galaxies.
We can even get silly and contrive extremely unlikely initial configurations of known matter and simulate what happens. We can even simplify known matter by removing physical features that complicate the picture. This can even be useful. Historically one started with perfectly spherical arrangements of gas that does not self-interact (doesn't clump together, doesn't repel, doesn't induce twisting or spiralling) to try to understand the formation of spheroidal objects like stars, planets, and black holes. Then as tools to simulate such collapses became better and faster, adding back in the stripped-out physical aspects of real interacting matter (which clumps into molecules, and those in turn break apart as they get hot, etc., and probably never found in perfect spheres rather than discoids or blobs) for a better approximation of such systems.
No known matter has the properties necessary to create negative energy.
If we start with an extremely simple form of a purely classical non-interacting negative-energy-density non-relativistic dust that we can drop into the stress-energy tensor of the Einstein Field Equations, we can do some simulations and see what happens. [3] But we are complicating something wholly fictional. Nothing known has the requisite property (negative energy), and so there is no path to adding more complex behaviours to such a basic test set up to try to approach a physically plausible system.
Such an unknown substance is politely called "exotic matter" for the simple reason that it foreign to our solar system. "Unobtanium" is just as fair, if a little less polite.
And that, for me, leads to it being an uninteresting family of solutions.
In the very unlikely event we find some mechanism to generate any negative energy, then great, general interest will revive (and we'll start asking questions about what suppresses that negative energy such that it is undetected in our solar system and in various-sized systems (galaxy clusters, etc) we see in our sky).
Until then, I will tend to see such solutions to the Einstein Field Equations as starting with highly contrived metrics ab initio. I don't object to anyone investigating such things on strictly that basis: Schwarzschild's solution was highly contrived too, and was a matter-free eternal setup. But there were already pretty spherical gravitating systems around to examine (Schwarzschild lived on one!) and Newtonian methods to study those were useful for validating the Schwarzschild solution for objects wholly outside their own Schwarschild radius. But we should also think about the work of Kerr on axisymmetric rotating systems : it too was a contrived vacuum solution, but almost nobody (and certainly not Kerr himself [1]) thinks that we should think of the inside-the-horizons region as physical (and probably most relativists don't, for various reasons).
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[1] Alcubierre has left a copy at https://arxiv.org/abs/gr-qc/0009013v1 -- note the very first sentence of the abstract ("within the framework of relativity").
[2] "... negative mass instead of positive: that's NOTHING to do with physics." Around 28:05 at https://youtu.be/jji2pgfq7oE?t=1685 -- this lecture also shows that a very simple complication of an exact solution, just adding angular momentum to Schwarzschild -- is both very hard work and will often produce obvious garbage. Even Kerr's solution, as he himself says, is part garbage. Fortunately the garbage in Kerr's solution is hidden behind an event horizon, so we can decide not to care at all about that for systems that aren't dense enough to have event horizons, and can say that the inner parts of black holes are for all practical purposes purely a theoretical problem, since we know no way of investigating what goes on in there. Relatedly, Kerr at around that point in the video points out that we know how to arrange matter to form a rotating black hole with the features of the Kerr solution, and frankly, to me, since that is a true statement, that overrides any theoretical objections about the interior garbage. (We don't know how to arrange matter to form a warp bubble, and warp bubbles have nice quiet interiors and distant exteriors, but have tremendous garbage in the thin shell).
[3] Results tend to follow a pattern that we can think about using the idea of a gravitational charge. Unlike the electromagnetic charge, where like charges to repel, the gravitational charge causes like charges to attract. All known matter -- including dark matter -- has the same gravitational charge. So protons, neutrinos, photons, whatever the hell dark matter is, and so on all tend to collapse into structures like galaxies. "Negative" stuff has the opposite gravitational charge. Things with the opposite elecromagnetic charge tend to attract one another, but things with the opposite gravitational charge tend to repel one another. So if we brought a small amount of "negative" matter, with its opposite gravitational charge, into our solar system, it would be thrown out by all the normally-charged stuff (the sun, the earth, etc). Things get really absurd when one couples equally-massed equally-but-oppositely-gravitationally-charged matter together: you can create infinite power in some other charge (like the electromagnetic one) for instance. Again, this oppositely-gravitationally-charged negative stuff is called exotic because if there wasn't much of it in the early universe, it's all been thrown out of it since, and if there was much of it in the early universe, where the hell are all the side-effects ? (if we're talking about a tiny percent of negative stuff, a fraction of baryonic matter, then wormholes and warp bubbles should fill our sky almost as much as stars do, making all sorts of weird observables from gravitational lensing far from galaxies to odd emissions and absorption lines on light from distant quasars).
As the Casimir effect pulls two surfaces toward each other, the net motion is zero. Not very useful.
(One might also try to use a Casimir setup to lift some heavy object off the floor, or do other gravimetry experiments).
The usual explanation for the Casimir results involves modes in the fields of the Standard Model, all of which couple in the same way to the curvature of spacetime.
In the language of particles and charges (which we conventionally get to by considering the spin statistics of gravitational waves in General Relativity), all the Standard Model particles have the same gravitational charge. That includes everything involved in a Casimir experiment, including "vacuum energy" if any.
However, so far most of the thought that has gone into "how do we scatter matter around flat spacetime to induce an Alcubierre metric on it" requires stuff with the opposite gravitational charge.
Back to the language of metric theories of gravitation: if there is only one gravitational charge that everything possesses, then everything couples to a single metric tensor. If we allow for a gravitational charge that has both positive and negative signs, so far as we know (from among other things analysis under the PPNF[1]) we are forced (by the Universality of Free Fall) into having matter couple to a metric tensor according to the sign of its charge. This is what we would call a bimetric theory of gravity, and such theories have been studied for decades motivated by understanding the very early universe https://en.wikipedia.org/wiki/Bimetric_gravity
Generically, one asks "where's all the stuff that falls differently?" and almost inevitably has to say "it fell out of our universe much earlier than the formation of the cosmic microwave background, or decayed into ordinary stuff, or dilute away much faster during the metric expansion than ordinary stuff and became undetectably sparse before the first stars shone", or in other words that either the second metric tensor has decayed to all zeroes everywhere in the observable universe, or that nothing is in the observable universe that couples to it. Otherwise there are lots and lots of bullets to bite about why we don't see any observables that would support the second metric.
Alcubierre's "drive" paper (he has left a copy at https://arxiv.org/abs/gr-qc/0009013v1 ) uses one metric, not two. The warp bubble is also eternal. Importing a whole extra metric tensor into our fundamental theory of gravity is a really high price to pay for having a mechanism which can produce a non-eternal warp bubble (one that forms in the finite past and un-forms in the finite future). And then one has to really spread around a lot of matter that couples with (i.e., it sources as well as follows) the second metric if one hopes to steer the warp bubble. We probably end up in some ratholes of inventing entire chemistries and nuclear physics of "negatively gravitationally charged" matter, and then figure out how it interacts with normal matter when they're confined together in some system.
So although a terse "well maybe it's hiding in the Casimir effect" massively underplays what is needed, or what the consequences would be of reproducible Cavendish (or other gravimeter) experiments on Casimir apparatuses that show that when the plates are being drawn together they are also lighter (in a weight-is-the-quantity-that-bathroom-scales-measure sense). Show it happens at all first, even if it's a very very weak effect, then take seriously thoughts about scaling it up.
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[1] PPNF, or PPN formalism: https://en.wikipedia.org/wiki/Parameterized_post-Newtonian_f...
Regarding practicality, you may have missed the thesis, which is that warp drives are more practical than previously thought, because the theory allows subliminal warp drives at finite energy, not only superluminal warp drives at unphysical negative energy.
The part about GR is just the stretch part. It's obvious what she means about GR, that our current arguments against superluminal drives are based on General Relativity.
I don’t buy into the UFO phenomenon hype, but does this mean that a disk shaped “saucer” requires less energy than a craft with an elongated geometry? Wow
For example; instead of saying "faster than the speed of light", would the problem not be better described using "faster than the speed of causality" ?
After all, isn't c really just the speed of Causality in a vacuum? (i.e. the time it takes for a beam of light from a source to be detected at, say, a distance of 500,000 kilometres, is basically the rate at which Causality allows that light to get from the emitter to the detector.)
And isn't the speed of Causality, basically like some kind of inertia-like quality of spacetime? (Apologies in advance - I'm trying to describe what's in my head as best I can using... language ;) ) For example - the speed of "light"/Causality through water is slower than the speed of "light"/Causality through a vacuum.
(And mea culpa - I am a layman, trying to make sense of all this in as best a way I can. I'm not sure if what I wrote makes any kind of sense (it does in my head))
I'm probably misunderstanding what you're saying.
And it's more to do with the use of language to describe the problem (how to break the "light barrier") than forgetting something. And the use of language in abstracting/describing solutions to it.
[0]https://en.wikipedia.org/wiki/Special_relativity
[1]https://en.wikipedia.org/wiki/Speed_of_light
[2]https://web.pa.msu.edu/courses/2000fall/phy232/lectures/emwa...
It merely gets translated into math after that - math being a tool used to attempt to solve that problem.
No one thinks and speaks purely in math. It must be language, then math, then language again to help describe and explain the solution which was found using math.
I disagree for any non-trivial use of "language".
By that I mean that things like "wonder what will happen if I use a different boundary condition" is a trivial use of language, as it does not affect the understanding of the problem or how to solve it.
Well, the speed that the photons travel between the water atoms is still c.
An underwater explosion travels slower in water than it does in a vacuum. That's not just to do with the coefficient of friction of the water. The light from the explosion will take longer to reach you than it would if you and the explosion were in a vacuum.
Which leads me to another thought - does that mean that the matter of water is basically just another form of spacetime, in which the speed of Causality is different from the speed of Causality in spacetime in a vacuum?
The speed of light is slower in water, yes, which is why Cherenkov radiation exists -- that's basically the photon equivalent of a supersonic shockwave. The speed of causality doesn't change, and it's easy to demonstrate that by throwing particles through it at superluminal speeds; nuclear reactors do it all day long.
Cherenkov radiation is still light/electromagnetic energy, and it still must obey the speed of light (Causality) through the water, which means it will still be slower to arrive from source to observer at a slower speed than it would have had it been travelling through a vacuum.
You're not supposed to measure the speed of the Cherenkov radiation. You're supposed to measure the particles that are causing Cherenkov radiation.
If you measure those fast particles, you'll see that they will cross a tank of water just as fast as they will cross a tank of vacuum. They can cross faster than a beam of light will.
I am confused, what particles could reach superluminal speeds?
A.k.a. hydrogen oxide.
It might be useful to watch this earlier episode, "Are Space and Time an Illusion?" [2], first.
There is a later episode (or maybe a couple of them) that go into how interactions with the Higgs field make it so things with mass travel slowed than the speed of causality.
To tie back to the warp drive news, they have an episode that would be a good place to start on warp drives, "Is The Alcubierre Warp Drive Possible?" [3]. There's also "Superluminal Time Travel + Time Warp Challenge Answer" [4], and "Will Wormholes Allow Fast Interstellar Travel?" [5]. A non-warp unknown physics drive that comes up a lot also got an episode, "The EM Drive: Fact or Fantasy?" [6].
There is at least one episode covering interstellar travel without using any unknown physics, "5 REAL Possibilities for Interstellar Travel" [7].
The scope of the series is basically anything vaguely connected to astronomy, cosmology, and quantum mechanics, and the episodes are all pretty short (5-12 minutes), so it is a great thing to watch when you've got a short wait for something.
Here's the series home page [8], and their YouTube channel [9]. Also on the PBS streaming app, for those who like to get their physics on their big screen TVs.
[1] https://www.youtube.com/watch?v=msVuCEs8Ydo
[2] https://www.youtube.com/watch?v=YycAzdtUIko
[3] https://www.youtube.com/watch?v=94ed4v_T6YM
[4] https://www.youtube.com/watch?v=HUMGc8hEkpc
[5] https://www.youtube.com/watch?v=ldVDM-v5uz0
[6] https://www.youtube.com/watch?v=hqoo_4wSkdg
[7] https://www.youtube.com/watch?v=EzZGPCyrpSU
[8] https://www.pbs.org/show/pbs-space-time/
[9] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g
It’s a natural extension of knowing that humans can’t fly. We need only look up to see that constraint didn’t hold up to the rest of time nearly as well as contemporary humans assumed it would.
How do we know that some immutable property of space/time causes light to travel at the speed it does rather than the other way around where some immutable property of light causes it to be incapable of traveling any faster (the old strap wings to a human and look now it can fly; so strap X to light and look now it can go faster to!)
Again, the extent of my knowledge in this space is from Star Trek not science so please be gentle with my clear level of ignorance.
See, I'd try and frame it as "just because Causality constrains light to only go that fast..." and try to think what Causality is. And if Causality is a property of spacetime then how would I go about building a "Causality Engine" in order to alter spacetime in such a way that the speed of Causality can be broken ;)
My mind is warped - pun intended.
Our mechanics allow only one such speed, and if you try to use one that doesn't have this property for causality, you'll get all kinds of paradoxes.
But it's probably something that can be solved. All said, the reason we assume causality moves at that speed is because it agrees with the experiments.
The same strength shove hardly moves an artic truck, it wobbles a Ford F-150, it pushes a sedan, it rolls a small sports car, without air resistance and axel friction, it’s only the mass which resists your shoving and determines the speed. and you can imagine waving a magic wand over the car to lower its mass then the same shove pushes it faster and faster. At the mass of a ball bearing the car pings away. Like a tennis racket swing can’t move a car but can move a tennis ball at 100mph.
Until there is no mass left, at that point any shove and it skitters away enormously fast. As mass drops the shove pushes it faster. As mass goes to zero, it goes “infinitely” fast.
There isn’t any mass left to remove, so the only way you can speed it up from here is to push harder instead... but now as soon as you touch it at all, at any pressure, it skitters away and you aren’t touching it anymore so you can’t push - damnit. The first inkling whisker of a skin cell touching the car and it’s off, the time it takes to “wind up” your shove power is too slow, the massless car is 300,000m away in a thousandth of a second. You can’t keep your hand on it, let alone go faster than it to provide more pushing force!
In fact the only way you can move your hand fast enough to keep up with it, so you can keep touching it long enough to give it more push, is if your hand also has no mass. And now your massless hand can’t be pushed by your real muscles for the same reason, once stuff has no mass you can’t get a grip on it. So pushing harder is out.
Maybe you can push it faster in a non-mechanical way like electromagnetically? Ok, you send some magnetic waves after it. Photons. They have no mass, so they travel at the same speed as well. They can never catch the car to push it. Like a game of tag where you run behind someone waving your arm at them but never quite able to touch them, you sure can’t push them harder if you can’t ever quite touch them. Nothing can catch up to provide more push.
So it can’t go faster than this “infinite” speed.
> How do we know that some immutable property of space/time causes light to travel at the speed it does rather than the other way around where some immutable property of light causes it to be incapable of traveling any faster
Because the speed isn’t how fast light goes, it’s how fast spacetime lets massless things go through it and light happens to be a massless thing. If mass is what resists movement, and you take all of it away, there isn’t any way to go faster because you can’t take more mass away than all of it.
(Wish I’d written this with whacking tennis racket on a car, bowling ball, baseball, ball bearing...) seems easier to imagine.
You cannot accelerate masses to the speed of light, because if General Relativity holds, you'd require infinite energy to do so.
In addition, every bit of radiation arriving from the direction of travel will be turned into high energy gamma radiation and cook you something fierce real quick; not to mention dust or tiny rocks that would release the energy equivalent of a thermonuclear explosion upon impact.
Since these effects alone will create a practical upper limit to your interstellar travel speed, there's still a huge difference between FTL and reactionless drives. An FTL drive might get you to Proxima Centauri in a month, whereas even the most optimistic subluminal option will take at least a decade.
This is a common misconception. There is exactly 1 mass you can accelerate to any speed you want. YOU. Time dilation canceling out acceleration from the external perspective is why it looks like externally that it takes infinite energy to reach C. Light travels at C and thus experiences zero travel time. Things going slower than C will experience "some" travel time. Not 1 year per lightyear.
On the ship it is locally Newtonian the entire way. Sure your destination and origin are doing weird gymnastics out the view screen, and the radiation is getting a bit energetic, but if you have the fuel and willingness to take Gs, you can travel any distance in any subjective amount of time. There exists a fuel budget (a lot) (at overwhelming Gs) to reach another star in 5 minutes.
> In addition, every bit of radiation arriving from the direction of travel will be turned into high energy gamma radiation and cook you something fierce real quick; not to mention dust or tiny rocks that would release the energy equivalent of a thermonuclear explosion upon impact.
This is a very real problem I offer no solution to other than adding shielding budget to the tyranny of the rocket equation. "warp drives" might offer a solution to this problem but all flavors of reactionless drives are currently SciFi. "If you have enough fuel" here likely means "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency"
Still not true. YOU have rest mass ant therefore the faster you go, the more of the energy used for momentum transfer will be converted to relativistic mass, thus preventing you from ever approaching the speed of light in any frame of reference.
Local time has nothing do with that. Sure, you once you exceed ~70.7% c time in your local frame of reference will tick so slow that you perceive less time than an observer on Earth would see light take to get to your destination.
But in any frame of reference that's more or less "at rest" relative to you, time will tick faster so any message you send once you get to say Alpha Cen, will only be received about ~8.74 years after you left Earth.
An FTL vessel, however, would allow you to go to Alpha Cen, take a picture, send it to Earth, return and arrive back on Earth before the message containing the picture arrives. THAT's the difference.
> you can travel any distance in any subjective amount of time
Even that doesn't seem to be true [1] as the CMB seems to slow relativistic spacecraft down even in intergalactic space.
> "a couple Jupiters worth of hydrogen that you fuse at perfect e=mc^2 efficiency"
You don't fuse hydrogen to get that kind of efficiency. Only matter-antimatter reactions are capable of coming close to that. While antimatter is hard to come by, it's the ultimate rocket fuel.
Ultimately, even a reactionless drive would be limited by the human physique - it takes about 3½ months to accelerate to 30% light speed at 1g constant acceleration. So unless inertia negation devices are invented, that sets the practical lower limit for any interstellar travel time.
> Still not true. YOU have rest mass ant therefore the faster you go, the more of the energy used for momentum transfer will be converted to relativistic mass, thus preventing you from ever approaching the speed of light in any frame of reference
Rest vs relativistic mass only exists from the outside observers perspective. The traveler doesn't see any change in it's own mass. On a ship traveling near the speed of light, people walking around won't have to spend extra energy to move in the direction of ships motion. Relativistic mass just accounts for the energy being spent to accelerate that doesn't result in an increase in velocity (and thus momentum) from an external perspective.
> Ultimately, even a reactionless drive would be limited by the human physique - it takes about 3½ months to accelerate to 30% light speed at 1g constant acceleration. So unless inertia negation devices are invented, that sets the practical lower limit for any interstellar travel time.
This is a really good point! This means the Andromeda galaxy could be only 30 years of travel away. The primary point I am trying to make is that "FTL" isn't important for interstellar travel if a reactionless drive gives us most of the benefits we need (getting to other stars within human lifespans).
Honestly I don't think either reactionless or FTL drives are actually possible, so generation ships or ships full of human popsicles coasting most of the way are most likely how we will get to other stars.
You are right that the reactionless drive will be limited by the speed of light, but thanks to time dilation that 10 years might only feel like a couple of months to the passengers. It's cold comfort for their aging friends and relatives back home, but for the passengers it's not that much difference.
It was just an arbitrary example. If we're in the realm of the purely fictional we can just as well forego FTL entirely and use teleportation instead - instant travel to anywhere, why not?
Let's say we want to travel in 1 hour to the nearest star 4 light years away. That would require compressing roughly 6.5 billion miles/second of spacetime into this bubble!
All matter in those 6.5 billion miles will be sucked into your passenger area of the warp bubble and decompressed back to Earth spacetime. This would be a weapon of galactic destruction.
But that's not the real issue, how exactly would we be able to warp spacetime faster than light at such large distances? The main engine would have to already be breaking the speed limit as part of its technical specification, which becomes a paradox, trying to use the solution as the foundation.
Find an alternative solution to breaking the speed limit, and we can overcome the paradox.
This presumes that we're doing the moving all in one big operation, doesn't it?
What if each nanosecond, the warp bubble is moving forward by a small fraction, slipping through without imparting any momentum on the matter it passes by.
It would make interplanetary and orbital settlement not just possible but easy.
I don’t know about “easy”, but it removes a logistical wall.
I wonder if this sidesteps the time dilation effects of high speed travel.
On yet another note, I read an unsettling sci-fi/theory about how the expansion of space is perhaps being caused by whatever tech allows interstellar travel in lightspeed-like time. So whatever intelligent species that evolved first may have a monopoly on such tech, while inadvertently altering the fabric of physics to make interstellar travel gradually impossible for any younger species.
I thought the "alien magic" behind the gates technology was the same old K. Thorne wormholes.
> On yet another note, I read an unsettling sci-fi/theory about how the expansion of space is perhaps being caused by whatever tech allows interstellar travel in lightspeed-like time. So whatever intelligent species that evolved first may have a monopoly on such tech, while inadvertently altering the fabric of physics to make interstellar travel gradually impossible for any younger species.
Do you have a link to read more about that? Reminds me of the FTL technology in the books "The Dark Forest" and "Death's End" by Liu Cixin.
Spoiler:
Bar glcr bs nqinaprq beqanapr orvat hfrq va vagrefgryyne jnef jnf n qvzrafvbanyvgl erqhpgvba jrncba. Bapr npgvingrq, vg cebprrqrq gb erzbir bar qvzrafvba sebz gur fheebhaqvat fcnpr. Gur rssrpg rkcnaqrq ng gur fcrrq bs yvtug naq arire fgbccrq, fb vg hygvzngryl - va ovyyvbaf bs lrnef - jbhyq qrfgebl gur ragver bofreinoyr havirefr.
Gur obbx erirnyf gung gur irel ernfba bhe havirefr vf n guerr-qvzrafvbany bar, vafgrnq bs sbhe be gjryir be jungabg, vf orpnhfr bs guvf jrncba qrcyblrq va cnfg vagrefgryyne jnef.
Uh... what?
If objects underwater can supercavitate, can objects in the atmosphere also supercavitate and can we have objects orbiting the earth inside the atmosphere?
In the atmosphere, you're already in a gas, so there's not an obvious other state of matter that would be advantageous. Furthermore, reducing pressue would certainly reduce drag, but not by orders of magnitude like a phase transition.
But in the spirit of 'yes, and': if you reduced pressure near the skin by A LOT (near vacuum) you might see a dramatically lower drag. Maybe that could be accomplished by ionising the incoming air and then electrostatically repelling it? Who knows. It's fun to speculate though.
without the LSD Wave.
Or are we conjecturing what would happen if this was even slightly viable?
We've only very recently managed to experimentally demonstrate that unimaginably huge cataclismic events can make this happen, but, has anyone ever built some device that can make a crease of any significance at all in reality, and do so at will?
Bends in spacetime creating measurable effects is how LIGO’s interferometers see gravity waves.
You’re asking for a light bulb when we’ve barely begun grinding lenses for a telescope.
That distinction matters, since I personally find it easier to derive enthusiasm from concrete, tangible tools that may prevent the entire species as a thing from dying choked in a toxic miasma of our own creation than from the purely numeric advances in our grasp of the rules of a universe we're basically guests in.
We have witnessed spacetime ripples bending the length of an inferometer chamber. These ripples are far weaker than those we can, with known physics, create. (Most of LIGO’s work involves removing the effects we unintentionally create.)
> tangible tools that may prevent the entire species as a thing from dying choked in a toxic miasma of our own creation
Bending materials with spacetime ripples is known physics. You are asking for (or about or whatever) something we have.
If you are unsatisfied with anything short of a functioning interstellar drive, see the earlier point about demanding light bulbs from those just grinding lenses. There are those doing the work. There are those oblivious of it. And there are those shouting at it.
(She mentions she explains it in another video but I couldn’t find it.)
In the presence of negative energy you can form closed time-like curves in general relativity, which would violate causality, but these are not valid solutions because negative energy is not a thing.
It's just like saying you can travel faster than the speed of light in special relativity, you just need imaginary mass (ie. m^2<0), but mass is unfortunately a real quantity rather than a complex quantity so you can't.
If it's NOW in the Sol system, it's NOW in Alpha Centauri as well. (I guess it's difficult to think in these terms because we always only see a 4.367 years old "NOW" from Alpha Centauri.)
There would be no violation of Causality if you had a ship which could instantly Jump between Sol and Alpha Centauri - you'd simply arrive there at NOW+<however long this Jump took> - whether it's seconds or picoseconds.
If you sent a radio signal "Hi there!" from Sol to Alpha Centauri before this Jump, you'd be waiting for 4.367 years for it to arrive - which still isn't a violation of Causality, because it'd be NOW+4.367 years in Alpha Centauri and the same would be true in the Sol system.
If your faster-than-light (faster-than-Causality) Drive wasn't an instantaneous Jump like above, Causality is still not violated, because you'd still be arriving at NOW + <however long it took> to travel between Sol & Alpha Centauri. Maybe you could only travel at twice the speed of light (twice the speed of Causality), in which case you'd arrive there in NOW + 2.1835 years
Maybe it’s the same as how I can move a beam of light across the moon faster than light but that’s ok since it can’t send information.
Am I misunderstanding it?
(a lazy writing device that was a step too far IMHO)
> Why it's almost as if Star Trek is fiction and the "canon" of a franchise spanning over fifty years of television, film and books has always been fluid and arbitrary.
Despite it being fiction, you cannot arbitrarily mess with the basic mythology and still keep what makes the thing magical.
e: I'm curious what the opinion of professional physicists would be.
1 - Your comment isn't about content of the article. I don't use apple hardware or software. There are tons of apple posts, especially about the M1. How would it work out if all the people who aren't part of the apple ecosystem hit every apple story with a "Sheesh, not another apple article! Can't we get something more interesting?"
2 - You are free to submit things you think are interesting. Be the change you want to see, etc.
3 - People who whine about being downvoted tend to get more downvotes
It is a commentary on the article. Having read the article, I think regardless of Sabine's credentials, she has gone the wrong path in terms of science communication. Apple is a topic, Sabine is an author. It's the physics equivalent of asking for a source other than the Daily Mail.
> People who whine about being downvoted
I'm legitimately curious about the opinion of professional physics. I've worked with quite a few professional AMO physicists and the opinions of Sabine's content there are very low.
Dr. Hossenfelder seems to have a lengthy, strong background in physics. On the other hand, every time I read one of her transcripts, I see something along the lines of the "superluminal travel doesn't necessarily result in the possibility of causality violations" claim in this one. Every modern text I've read about physics has indicated that superluminal travel (or signaling) inexorably means that causality can be violated, unless Einstein was missing something.
In the last one I read, there were claims about Hawking radiation and virtual particles that also seemed to contradict everything I've read. Now, it may be the case that those texts really are simplified down and one learns what's really going on in some advanced physics course, but twice in as many transcripts is a red flag for me. It's not at the Dr. Salvatore Cezar Pais level yet, but usually people who consistently claim that all of the experts are wrong are themselves wrong, and so I'd like to see some non-self-referential confirmation.
> Neither does this faster-than light travel necessarily lead to causality paradoxes.
I take this to mean that if you could travel FTL to a very distant place that you could never get to otherwise, like say whatever universe there is that is farther away than light can travel in age of the universe, then there isn't a causality problem. Which isn't controversial. I'm pretty sure there are other FTL scenarios in which there is no paradox.
Yes, precisely. She made an extraordinary claim, as if it were a minor footnote, instead of "every mainstream description of relativity is wrong in this very fundamental way."
If it were the only time I'd seen her do that in her writings, it wouldn't trouble me as much. It seems to be common, and that makes me concerned that e.g. she may be off in some branch of physics that's not well accepted at this time.
"In his book, Hawking painted a neat picture for black hole evaporation that is now widely used. [...]. It’s simple, it’s intuitive, and it’s wrong."[1]
When I read that quote, I originally took it at face value, because she seems like an expert in her field, and it certainly sounds plausible. But now, in the next writeup of hers that I've come across, I find a claim that the description of FTL and causality portrayed by every legit source I've ever encountered is wrong, and that FTL without violating causality is somehow possible within Einstein's framework. That's two incredible claims in as many writeups. Why haven't I seen similar claims from e.g. Brian Greene or David Deutsch? Is it something specific to her own theories? Is it from some branch of theoretical physics that's not accepted by mainstream physicists? Am I misunderstanding the claims? Etc.
[1] http://backreaction.blogspot.com/2015/12/hawking-radiation-i...
edit: I can appreciate the snark value, however.
I stand by what I said: you weren't commenting on the article. You were commenting about the author of the article, and that you were tired of seeing news about her.
I doubt anyone is aware of all of their biases. I'm assuming you're making reference to gender bias - totally possible that I am susceptible to that, although I think:
a. Sabine is a more unique name than Brian.
b. In almost all discussions I see of her, she is referred to as Sabine - I was just following along with that precedence.
She also goes by "Bee" and dislikes her last name [0] [1]
[0]: https://blogs.scientificamerican.com/cross-check/physicist-s...
Think for yourself. Learn the math or physics you need and try to form your own conclusions. All books are free. This is the great responsibility for anyone alive in our time and it's hard, thankless work: to think from first principles and to do our own homework. There is no way around it. Needing other people's opinions before rigorously forming our own quickly devolves into talking about people and other people's opinions instead of talking about fundamental ideas.
Unless of course one is more interested in talking about opinions and personalities than understanding ideas. If her "content" is poor in someone's eyes, the question that remains relevant is: what are the ideas and their principles?
I studied physics at Harvard, I'm hardly unstudied in the area, but yes, I'll be the first to admit that I don't know QFT or really anything beyond basic QM and general relativity.
> This is the great responsibility for anyone alive in our time and it's hard, thankless work: to think from first principles and to do our own homework
Absurd. I do not need to learn advanced GR/QFT from first principles to know enough to comment on Sabine, nor do I need to verify every principle from first axioms in order to come to conclusions.
Humans have to learn to rely on trusting the judgements of others in order to make scientific advancement. Physicists can't spend their whole life validating that basic mathematical axioms are true, they have to trust the body of academic mathematicians who have done that work. I reject the entire premise of your comment.
Trust but verify. Better yet, trust your effort first, then trust others later.
> I studied physics at Harvard,
Wow. Harvard!
> Absurd. I do not need to learn advanced GR/QFT from first principles to know enough to comment on Sabine, nor do I need to verify every principle from first axioms in order to come to conclusions
Consider if all the people whose opinions you want studied at Harvard like you and also believe the statement you made. Will the blind lead the blind?
As my cello teacher said: the effort is the thing. You probably won't become a master but you have to make the effort. The effort is all.
Importantly, everything valuable that is known was once unknown by everybody, including masters. If we don't make the effort, there's no chance of going beyond frontiers. We are left discussing what so-and-so said they think about what so-and-so thinks about what so-and-so wrote about a paper and the author's personality.
Stick with playing the cello then.
> Importantly, everything valuable that is known was once unknown by everybody, including masters.
We as a species are way beyond the point that laymen - no matter their intellectual capacity - can acquire detailed knowledge about every special topic in even just a single scientific area.
This isn't as simple as music theory or perfecting motor functions by means of practising. Just to give you an idea of the scale we're talking here:
The journal "General Relativity and Gravity" [1] alone published 167 papers in 2019.
Yesterday(!) alone, 20 papers were announced on arxiv.org [2]
So assuming you already know GR by heart, you'd need to spend every waking minute of your existence just reading up on what's currently being discussed within the community. This leaves absolutely no time for reflection on and understanding the material, let alone working on original ideas.
You are delusional if you think that 200 years of modern mathematics, physics and astronomy can be mastered by a single person, let alone an average person, who rarely has the mental capacity to even understand most of the material en detail.
[1] https://link.springer.com/journal/10714/volumes-and-issues
You are delusional if you imagine I said you should master everything. In fact, I said you should make the effort. Recall, John Nash said that in graduate school he had to reinvent 300 years of mathematics for himself? And just how many books did Witten say he studied in a year in graduate school on his own?
Try to calm down and question your assumptions. I'm not your enemy.
Trying to understand stuff before asking for an opinion is a habit. If you don't practice it, you can't do it, and you shouldn't be surprised if you instead believe that the default should instead be relying on others who practice the habit.
Importantly, the habit of doing your own homework is actually a muscle: if you don't strengthen it, it will atrophy or never develop. The stronger it is, the faster working through physics and mathematics and chemistry papers becomes. Perhaps we can do better than our masters from the Renaissance if we try?
> Stick with playing the cello then
Thanks! That's kind of you to say. It's a struggle but I'm getting there.
> This isn't as simple as music theory or perfecting motor functions by means of practising.
You think music is simple? You are totally trolling me. I like your sense of humour.
> When disagreeing, please reply to the argument instead of calling names.
> Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.
> Please don't comment about the voting on comments. It never does any good, and it makes boring reading.
I'm a good audience for her writeup - someone who hasn't deep-dived into FTL for decades.
Maybe it helps I'm not at all familiar with Sabine.