I'm interested in exploring the following concept: people have discovered a certain kind of matter, let's call it "Unobtanium" (as it always goes).

Unobtanium actually has mass and is comprised of the known (to the day) elementary particles - albeit, perhaps, in some special conditions or combinations.

What is a special property I want to make "possible": if we add X kilograms of Unobtanium to Y kilograms of regular matter, the resulting substance mass is actually less than X+Y Also, I'm not referring to weight, I'm explicitly asking about mass.

My question: are there any known laws of physics today which will make it absolutely impossible, unless "magic"? To clarify: the answer "no, we do not know so far if mass manipulation is impossible" is a valid one.


I'm also not interested in destroying the matter, for which we already know about Antimatter. The impact on the matter to which Unobtanium was added should be as small as possible.

Bonus: If it's possible to explain that adding Unobtanium to the regular matter would (almost) completely nullify the resulting mass without (significantly) changing chemical properties of the substance.

Behind the scenes

The thing I'd like to achieve with this is the inertia reduction. Say, if we'd like to accelerate to 99.999% the speed of light very quickly provided that we have the tech to propel us that fast. Or to be able to do "very quick turns" during high velocity travels - normally it wouldn't be possible due to mass inertia which would not only prevent close to instant change of direction but also kill anything living due to insane G-Forces appearing in such circumstances.

This is also why mass and not weight, is what I'm aiming for.

EDIT: I'm not sure why it's "unclear what I'm asking" - especially given the fact that there are already 3 very good answers targeting exactly what I've asked :)

  • $\begingroup$ There's a very important difference between "mass" and "weight" of an object. If I ship 1kg steel and a scale to the moon and put the steel on the scale, it shows less that 1kg because of the lower gravity. The weight of the steel decreases, but the mass stays the same. Have a look at this related question. Does your Unobtanium decrease the weight of matter (defy gravity) or the mass (defy physics)? $\endgroup$
    – Elmy
    Jan 30, 2019 at 10:37
  • $\begingroup$ Yes, I'm aware about the difference between mass, weight and even apparent weight (such as the "lack of weight" for the astronauts on the ISS whereas in fact they are just "falling really really fast"). The question involves mass, so it should be not dependent to gravity $\endgroup$
    – Alma Do
    Jan 30, 2019 at 10:39
  • $\begingroup$ Anti-gravity matter is familiar from HG Wells' The First Men in the Moon. Though it's generaly considered science fiction rather than magic: en.wikipedia.org/wiki/The_First_Men_in_the_Moon $\endgroup$ Jan 30, 2019 at 10:39
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    $\begingroup$ @FaySuggers Wells' Cavorite was a gravity insulator. This might hypothetically change gravitational mass, but not inertial mass. Mass manipulation isn't the same as antigravity. $\endgroup$
    – a4android
    Jan 30, 2019 at 11:52
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    $\begingroup$ @Mazura Just because the majority doesn't know what the question is about, doesn't make it unclear. That is more a measure of their lack of knowledge. $\endgroup$
    – a4android
    Jan 31, 2019 at 6:39

4 Answers 4


Where mass comes from

The mass of an object is composed of the masses of its particles. When observed those form the so called inertial mass. The particles are quantum field excitations and include everything: the usual idea of particles, nuclear forces energy, electromagnetical energy between protons and electrons, etc.

The inertial mass in turn comes in 2 parts: a constant inherent rest mass and an inertial addition from movement, the latter you can't change as it comes from the very basic principles of movement in the space-time, but it's proportional to the rest mass. In everyday life with everyday things, it's very small anyway, so let's not care about it.

The rest mass of each particle is believed to be generated by its interaction with the Higgs bosons. There are no current known mechanism to break it simply by rearranging particles, but it's complex topic not exactly researched in the fullest, so outside of hard science we could think it's possible.

Break Higgs and you're done

The rest masses of many particles don't exactly add up, but in a large chaotic system of many particles of few types that usually can be ignored. So if you succeed in weaking the effect of the Higgs mechanism on individual particles, there you go, the overall mass of the body should reduce.

But beware!

It's extremely unlikely that a massive thing will ever turn to be massless and remain anything but a burst of gamma radiation. The zero and non-zero rest mass particles are of completely different sorts, and that difference is fundamental. For example, the massless can travel at the speed of light, but the massive can't. Turning one into another typically breaks any structure they've formed before.

UPD: Even without going massless, there is trouble. All the chemical bonds and interactions in anything is essentially electromagnetic. If you lower the mass but leave electricity the same, it starts pull everything stronger. Things get denser. The chemical bonds get stronger, the substances interact less. The complex biochemistry of the living things may break down, as it depends on parts of proteins turning in specific manners and staying that way for some time.

And that's assuming that all particles lose mass proportionally. But if, for example, you make atom cores 200 times lighter but leave electrons the same, they start to behave like real-life muons. And by replacing electrons with those, you jumpstart a thermonuclear reaction!

  • 1
    $\begingroup$ If the OP's unobtainium either reduced or partially neutralized the Higgs, perhaps a decoupling of the Higgs field then the mass might be reduced. The question about their chemical properties might be tricky. How would chemicals with lower mass behave? Plus one for Higgs. $\endgroup$
    – a4android
    Jan 30, 2019 at 11:49
  • $\begingroup$ @a4android: actually the chemistry is affected too, thanks for noting, updated the answer. $\endgroup$
    – avek
    Jan 30, 2019 at 12:39
  • $\begingroup$ Yes, I'm aware of "cold fusion" which is doable with muons instead of electrons. Interesting insight - that is - that we can think of changing mass disproportionally. But my guess is of course that such a transformation would utterly and irreversibly destroy the whole thing which is undesirable of course. $\endgroup$
    – Alma Do
    Jan 30, 2019 at 12:45

We have two laws of physics which, at the moment, forbid what you want from happening:

  • conservation of energy
  • Einstein relation $E=mc^2$

The only transformations which change the mass involved are nuclear reactions, where mass is converted into energy. If you rule out destroying mass you are ruling out nuclear reactions, so the conservation of energy actually becomes conservation of mass. So, mass cannot be created or destroyed.

  • $\begingroup$ It's not true in general. What you say is just an approximation used in the chemistry. In general, the mass isn't conserved at all. If you see some object moving and try to observe its mass mechanically, it will grow with the object's speed, becoming larger and larger as the speed approaches the speed of light. That 's directly related to E=mc^2, by the way. And the conservation of energy only applies to a closed system as a whole. Just make the energy go into some other place, and nothing stops it from leaving the form of mass and taking some other form in some other place. $\endgroup$
    – avek
    Jan 30, 2019 at 11:14
  • $\begingroup$ @avek: Energy is the same as (inertial) mass. It cannot take another form, it is the same thing. $\endgroup$
    – AlexP
    Jan 30, 2019 at 12:12
  • $\begingroup$ @AlexP: Full energy of a single body is essentially the same as its inertial mass, yes. But a single body isn't typically a closed system, it interacts, so that energy (and inertial mass) is not conserved. It also includes energies in different forms, like the internal energy, the energy interaction with other bodies and the body's kinetic energy, all usually changing. For instance, if it heats up the air around itself, it loses some of its energy (and mass!) completely, that goes into air's internal energy, which is a different form in different place. Why do you say it can't happen? $\endgroup$
    – avek
    Jan 30, 2019 at 12:22
  • $\begingroup$ So, practically, if I'd like to reduce mass, I need to allow the energy to be released? Which would imply that for significant mass changes (say, 50% of original mass) it would be required to outburst great amounts of energy making it very likely to simply destroy the whole thing in the process? $\endgroup$
    – Alma Do
    Jan 30, 2019 at 13:07
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    $\begingroup$ @Soan: You're wrong. Never do equations this way. 4=2+2, so by your method sqrt(4)=sqrt(2)+sqrt(2)=2*sqrt(2), but it's really just 2. $\endgroup$
    – avek
    Jan 30, 2019 at 17:55

This happens routinely, but the changes in the mass are too small to measure unless you're looking at nuclear reactions, and even then the changes are around the less than 1% scale.

For example consider combining an electron and a proton to make a hydrogen atom. If you start with the electron and proton far apart then their mutual electrostatic attraction will make them accelerate towards each other. The trouble is that when they meet they'll be travelling fast - too fast to form an atom - and they'll just fly apart again.

To form an atom you have to remove their energy of motion, or to be specific you have to remove 13.6 electron volts worth of energy. But Einstein's famous equation E = mc² tells us that removing that energy is the same as removing mass. And indeed if you measure the mass of a hydrogen atom very carefully you find it is less that the mass of the electron plus the mass of a proton. The difference is that 13.6eV of energy we removed divided by .

It's generally true that any bound system has a mass smaller than the masses of its constituents. This is called the mass deficit. So when you mix your materials X and Y there will in general be a change in the total mass. If the heat of mixing is H then the mass will change by H/c². Heats of mixing can be negative or positive so the mass could increase or decrease.

But I must emphasise that these changes are tiny. The problem is that you can only reduce the mass of your XY mixture by taking energy out of it, and a small change in mass produces a large amount of energy. This is of course the source of the energy in nuclear bombs, and any significant change in the mass of your XY mixture would produce the same sort of bang that a nuclear bomb does.

There isn't any way round this. Assuming you start with a certain total number of electrons, protons and neutrons and end with the same total number of electrons, protons and neutron the total mass can only be changed by the (small) alterations in their binding energies.


I got your negative mass right here, in this chunk of

Negative Matter


In theoretical physics, negative mass is matter whose mass is of opposite sign to the mass of normal matter, e.g. −1 kg.[1][2] Such matter would violate one or more energy conditions and show some strange properties, stemming from the ambiguity as to whether attraction should refer to force or the oppositely oriented acceleration for negative mass.

I yanked it out of my Alcubierre drive to show it to you. Here on Earth you can hang onto it because the gravity of the planet outweighs the repulsion by the negmass, but it weighs less than anything else of the same volume, including hydrogen. If you want to grind it up fine and mix it with normal matter, that should accomplish your goal of reducing the mass of the final gemisch.

You can read more about the weird properties of (as of yet hypothetical, but theoretically possible) negative matter here: Negative Matter Propulsion.

  • 2
    $\begingroup$ Interesting. Reading into the links from that article, I found out that the general version is not $E=mc^2$ nor $E=pc+mc^2$, but rather $E^2=(pc)^2+(mc)^2$. That general version admits negative mass without requiring defining something ugly like imaginary energy. 'Course we haven't seen anything with negative mass yet, but its fascinating that the equations don't forbid it. $\endgroup$
    – Cort Ammon
    Jan 30, 2019 at 15:06
  • $\begingroup$ @CortAmmon: E=mc^2 is technically always correct, but uses a special notion of inertial mass that's rarely used in physics. The proper formula for the usual (rest) mass if $E^2=(mc^2)^2 + (pc)^2$, as posted here: hyperphysics.phy-astr.gsu.edu/hbase/Relativ/releng.html, for example. It's better to check this formulas by putting units into them. $\endgroup$
    – avek
    Jan 30, 2019 at 18:08
  • $\begingroup$ @Willk: The OP has specifically stated that his Unobtainium is composed of known particles. No particle of the Standard Model has a negative mass under any known circumstances. It remains to be seen if excitons from the experiment by the Rochester’s Institute of Optics count; they are not real particles, those were just getting heavier in the experiment. While cool, I don't think that negative mass fits here. $\endgroup$
    – avek
    Jan 30, 2019 at 18:14
  • $\begingroup$ @avek - I figured just as antimatter versions of any matter particles can theoretically exist, so too negative matter would mirror the normal positive matter particles. You are right that it is more theoretical than other answers so far posted but also the one which actually is lumps of stuff with negative mass. $\endgroup$
    – Willk
    Jan 30, 2019 at 18:58
  • $\begingroup$ Well, lots of concepts in physics were "theoretical" and borderline "unbelievable" back then... until it was all confirmed and those strange conjectures became everyday life knowledge. This is an interesting insight $\endgroup$
    – Alma Do
    Jan 31, 2019 at 10:24

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