It's never wrong to confirm by experiment, but there was never really any doubt among professional physicists that antimatter falls down. Similar doubts are expressed about the feasibility of the warp drive by Alcubierre himself: https://twitter.com/malcubierre/status/362011821277839360
Even among people who hold out hope that a warp drive is physically possible (as opposed to merely mathematically possible), I don't think anyone ever suggested that antimatter was a candidate for the negative mass required by the equations. It's always been some kind of unobtainium.
I can give an example. If something moves back in time, it interacts as if it has negative mass relative to us, who move forward in time. Of course nothing moves back in time but tachyons which exist only on paper. The worlds of things moving forward in time and backward in time never meet because they are just two different solutions to the same equation and we have absolutely no evidence that the second world exists. Inverted matter from Tenet should have received negative mass but we most likely will never see it because time may be an illusion likewise to rotation.
Well, i think some trouble lies in that things with 0 mass are already defined here. Light has 0 mass, and anything with 0 mass must move at the speed of light since it has no inertia to stop it. What other point of reference can we use when 0 has been so solidly defined?
No, the gravitational analogue of voltage is the gravitational potential energy. We can freely shift them by constants because absolute potentials are unobservable: only potential differences have physical consequences. Mass, like charge, is directly observable.
As I understand it, dilithium crystals control the matter/antimatter reaction somehow. Zephram Cochrane's first flight used a ship powered by a nuclear reactor, I think.
The warp field is made with an array of "warp coils" which apparently require the large amount of energy from the matter/antimatter reaction to create a subspace distortion field that allows warp travel. What is subspace? It's never adequately explained, but sometimes monsters come out of it and kidnap you to a pocket dimension where they rearrange your organs.
Antimatter is only used because, when combined with matter, it has a higher energy density than anything else known, including fusion. That part is completely within known physics. The "dilithium" is used somehow to control the matter-antimatter reaction; they have nothing to do with the warp field. Why the writers came up with this I don't know, but it does make for a convenient plot device sometimes when they have some problem with dilithium mines on some distant planet. Like various hard-to-acquire materials today, there are political problems caused by the scarcity of dilithium, and this comes up in many episodes. And, conveniently, dilithium somehow can't be replicated by the regular matter replicators.
The warp field is created by the warp nacelles. How? Who knows.
Dilithium is a mediator for a deuterium/antimatter reaction that generates the fantastic amounts of power required. The warp field is generated by some sort of coils in the nacelles.
It always seemed like a reasonable, but still big, assumption that antimatter behaved the same way under gravity. Anti-particles have opposite charge, so maybe it could have made sense that they have opposite "gravitational charge"? But also gravity doesn't have "charge".
So yeah, agreed. A good thing to confirm, even if (especially if) they expected the result to be unexciting.
Not a big assumption at all. Gravity is believed to be a property of space as opposed to a property of matter. That is, a satellite orbiting around the Earth is really going in a straight line but space is curved.
Seen that way it’s just insane that any kind of matter could fall up. See
> a satellite orbiting around the Earth is really going in a straight line but space is curved.
This is not quite how it works, I think. The gravity of the Earth isn't high enough to cause space to curve around in a circle, and the path of the satellite depends on its orbital velocity. If it fired a rocket forwards to reduce speed, it would fall towards the Earth, but that's not due to a change in space curvature.
The paths taken by light will follow the curvature, however. That would give a very small deflection for light passing near the Earth.
The gravity of Terra is exactly enough to cause the curved orbit. You are just missing how shallowly curved it really is, by squashing out the time part of spacetime. In 4 dimensional spacetime, the circle is (to simplify a little) really a helix, because it does not return to its starting point, and it's very stretched out along the time axis. (Or very narrow along the space axes, depending from how one choses one's units.) The curvature only looks greater because you are discounting how much greater length of time it extends across in order to do all of that curving that gets it all of the way around Terra.
The curvature is quite small but is responsible for orbital motion in the steady state condition. I don't think the parent comment was suggesting that space would "curve into a circle" unless you are implying something else specifically.
All ideal orbits, with no other forces involved, are equivalent to geodesics. However that only applies in true free fall. Actual orbits decay due to tiny non-ideal characteristics which have a cumulative effect or stochastic "butterfly effect"-like characteristic over long periods of time. For example in the ideal case, as soon as a satellite fires maneuvering thrusters, it is not following a geodesic for the time that the rockets are active. After the maneuver is completed, a new steady state condition can be calculated piecewise and then it's following a geodesic again. Another example is the ideal state of a satellite being in a low orbit that decays due to friction with the atmosphere, for which there are good estimates but not exact predictions. Due to the friction, there is a tiny acceleration (which happens to become larger over time) and this makes it follow something that starts close to a geodesic but deviates from it.
The quantity coupled to the curvature is the stress-energy tensor. In local coordinates, this can be represented as a (1+n)x(1+n) matrix where n is the number of spatial dimensions, with components given by
[mass density, mass flux = momentum density]
[energy flux, stress]
Photons are massless, but still carry momentum and energy, so they do gravitate.
A negative mass particle would repel a positive mass particle but be attracted by a positive mass particle so the two would accelerate to ever increasing speeds but the positive and negative kinetic energy would cancel out so energy would still be conserved.
If photons had mass they would attract each other as well.
Gravitational lensing is pretty strong proof that photons follow the curvature of space affected by a gravitional field. If photons had mass themselves they would not follow straight lines in free space but they'd clump together over astronomical distances and long periods of travel. Though there is a way to get them to act in unison which gives some pretty odd effects:
Photons do interact with each other gravitationally. It's not as simple as just attracting each other - the Newtonian limit doesn't work here - but parallel photon beams going in opposite directions, for instance, will be deflected towards each other.
But photons traveling in (almost...) the same direction (such as the photons that arrive at some destination after gravitational lensing do not gravitationally attract.
I thought photons also contributed to the curvature of spacetime because they have momentum (and thus energy). I would expect it to be a pretty miniscule contribution, though.
Hm... just an interested layperson here but would that not require some kind of mechanism by which the photon sheds some of that momentum? Which would seem to be pretty hard if it really is an 'elementary' particle. Unless 'photon === graviton' and you find photons shedding other photons!
I'm a layman as well, but I think photons can effectively lose energy in various ways, one of them being the well-known redshift effect. And one of the main causes of redshifting is photons climbing out of a gravitational well (this is called a gravitational redshift).
However, whether that translates to a loss in momentum is a bit more fuzzy and I really can't tell whether it does or doesn't. Although I'm far from being a physicist, so hopefully someone more knowledgeable chimes in to enlighten us...
But I'm curious: what is your reasoning for asking whether photons have a mechanism to lose momentum as a consequence of them affecting the curvature of spacetime? It's not at all obvious to me the relationship between these two concepts.
> However, whether that translates to a loss in momentum is a bit more fuzzy and I really can't tell whether it does or doesn't.
Yes, redshifted photons lose momentum. Momentum (really the stress-energy tensor) is conserved locally, and along trajectories that preserve the metric, but global momentum conservation in GR isn't even well-defined.
If they don't lose momentum then there is no interaction (in order for any kind of interaction you need to lose some energy). Momentum is pretty much all a photon has and it could conceivably toss off much lower energy photons to shed that momentum.
If you treat GR as a gauge theory, and plug local Poincare symmetry into Noether's theorem, the corresponding conserved current and charge you get out are the stress-energy tensor and 4-momentum. It's not exactly the same situation as typical examples of gauge theories, since you need to impose some extra geometric conditions to pick out the Einstein field equations, but I think it's close enough that 4-momentum could reasonably be called gravitational charge.
It does not. Time-reversal is antiunitary, so you when you apply it to p_0 = i d/dt, you pick up one -1 from i and another from d/dt, and they cancel out.
If antimater particles were time reversed then I could see how they could have negative energy and thus antigravity. That's not at all how any observed particles behave though.
I don't think anybody expects a wrap driver to be possible. It's just one of those odd things where the math says something, and it's well work looking at just in case. (Just like negative mass matter is something that nobody expects to exist, but likes to appear on equations here and there.)
That said, I don't follow it closely, but didn't somebody recently worked out one that works without negative mass matter?
>That said, I don't follow it closely, but didn't somebody recently worked out one that works without negative mass matter?
Yes. There was recently a few papers published showing that a static warp field is mathematically possible without negative energy. However, the field doesn't move or impart acceleration on its own. The best you can do is drag the warp field behind your ship with normal thrusters. Such a field seems to be pretty useless right now, but maybe that research will lead to something else in a few years
Perhaps not useless, but definitely not as convenient as we hope warp technology will be someday. Could such a device be used as a way to improve thruster efficiency? If you can drag a large enough warp bubble with conventional thrusters, and it "slopes" space in front of you, could it make moving in the direction of the slope easier so you use less fuel?
I don't remember the details, but they explored some different field geometries which were more useful, but required an impossible amount of energy. Like, more energy than the entire universe.
The static warp field doesn't allow you to go faster than light because it doesn't move relative to the generator. You can drag the field around at sublight speeds but that's about it.
Though IIRC they played around with some different geometries that did move, but the energy required was many times more than the entire universe contains.
Without reading any of the mathematics, I propose the warplev equivalent of maglev trains. We build a long train of these devices that we then turn on and off at the right times in order to propel something. It's genius and I will be making a wikipedia page, .io domain, and putting up an Angelist RUV.
There are generally speaking, two interpretations in scifi. I don't really understand the physics, so apologies if I get it wrong.
1. Better known as the "Alcubierre Drive", Compress space infront of you and expand the space behind. Essentially inching forward like the inch worm. (If we presume that visually, the inch worm stays in the hunched position and slides forward because our eyes cannot see it's movement)
Doing this smoothly in theory lets you accelerate and decelerate infinitely.
2. Warp space infront of you to create gravity, and pull you forward. Essentially, you make a pot hole in space time, and "fall" into it. This is basically the same Alcubierre Drive, but without the expansion at the rear.
2.1: One version holds that pothole at a specified distance infront of the ship.
2.2: The other version creates the pothole, then removes it before you reach it.
I've often wondered about option 2, but wouldn't that be very risky if you were near something/someone and you created a large gravitational field?
Also, if this was common and you had lots of ships warping the gravity field then would that not be some kind of space pollution?
I know space is big and all that, but this seems like an irresponsible option.
Yes actually, it would be. It would also depend on whether you have fine control over the size of the spacetime pothole. It is also possible that the larger nearby gravity well would pull on the pothole.
I like to think that concepts such as "you must be at the edge of the solar system to initiate faster than light travel" or "cannot initiate faster than light within the gravity well of planet" originate here.
I have nothing to backup that theory of course.
As for space pollution, I never really thought of that. But considering some of our observation tools observe 'gravitational waves'? I could imagine another "starlink and light pollution is getting in the way of space observation" situation.
It's pretty much what it says on the tin. A field which warps nearby space into a different shape.
What that means is complicated to explain, but the gist of a warp drive is that you expand space behind you and compress space in front of you. The normal space in between gets pushed forward, ideally at speeds faster than light. There's lots of good explainers on YouTube if you want to learn more.
I always liked the neat dodge of hyperspace proposed by Larry Niven. Idea was that humans never noticed it in their experiments, because they always ran their tests on Earth, too close to a gravity well, where it resolved to a singularity. At this point I believe the hands on the other foot. It's not the speed of light - but causality itself - that's not allowed to be broken by the universe.
The author directly addresses this sentiment in the concluding paragraph:
> Although there are physicists who wonder “Why did we even need to do this experiment; we all knew that antimatter has positive mass,” that sentiment is absolutely foolish. We must remember — and I say this as a theoretical physicist myself — that physics is 100% an experimental science. We can be confident in our theory’s predictions only insofar as we can test and measure what it predicts; as soon as we step outside of the realm of what’s been validated by experiment, we run the risk of stepping outside the realm of where our theory is valid. We just learned that Einstein’s general relativity passed another test, the antimatter test, and with it, our greatest science-fiction hope for achieving warp drive has completely evaporated.
"Why do we need to measure the speed of light coming from a moving source. We all know that the velocity of all objects compounds with the velocity of their emitter"
Agreed, though also I really appreciate this sort of "obvious" experiment, because the incentives really aren't set up to encourage it and we've been surprised often enough over the centuries that Actually Double Checking is probably something we should do more of.
(there's a parallel here to 'more negative results' and 'more replications')
