I was wondering if an object that changes its own mass could exist. But let's try not to fill it up with air or other gas so it feels "lighter".

That way you could protect things from being stolen if the weight increased A LOT, removing usability from it.

  1. You could apply that to a bike or a scooter being way heavier and really hard to navigate (and you would need something like a key to switch between the two states Heavy/Light).
  2. You could use this to do a magic trick with like a Thor hammer being super heavy and all of the sudden you change it's mass and tell a random guy to pick it up and it would be possible for him since it's now a bearable weight like a few kg when it was impossible to lift it up more than a few cm before because it's really heavy, but not insanely heavy since it's not that big. (Or maybe it would indeed be insanely heavy thanks to your solution.)

Would this be possible?

First, something that changes its own mass.

Then, a change that is significant enough to feel.

And, finally, if not possible by itself, maybe some external ways (not too obvious for people around the object) to change mass (both ways) that could be used for the Thor hammer trick because this would be so awesome.

  • 1
    $\begingroup$ IRL, we don't change the mass of an object, we tend to chain the object to the ground. Or something that's reasonably well fixed to the ground like a bike rack. You should be able to use this with any object, not just bicycles or motorcycles - even mythical hammers. Less cool but it could be funny having Thor try to find where to lock Mjölnir when he needs to leave it. $\endgroup$
    – VLAZ
    May 28, 2019 at 11:13
  • 5
    $\begingroup$ Not entirely serious, but I've changed my mass by -55kg over the past two years... $\endgroup$ May 28, 2019 at 13:40
  • $\begingroup$ I don't have the science to write a good answer, but could this be done by spinning 2 gyroscopes in opposite directions and thus storing enough energy in them to make the object virtually immovable? $\endgroup$
    – Andrey
    May 28, 2019 at 15:32
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    $\begingroup$ A child (or dog) that does not want to be picked up seems to be much more massive than a happy child (or dog). $\endgroup$
    – user535733
    May 28, 2019 at 21:35
  • $\begingroup$ Yes, dogs have a special power. They are able to convert a fraction of their mass to Dogonium. I have noticed that my brother's staffy can increase his weight at least 3 fold this way. I believe it occurs spontaneously when they are sleeping on your feet. $\endgroup$
    – pHred
    Jun 1, 2019 at 14:21

2 Answers 2


At the elemental level

As it stands at the moment, no element can change its own mass. now this is absolutely true, but also a little false, highly radioactive materials that are decaying into more stable isotopes do technically lose a little mass in the process, however we're talking very very small amounts 0.0001% for example (This is generalization and not a specific element)

And this change is only 1 way... there is no known physical mechanic that could change this at will by any significant amount in a short space of time. Not at least one you'd like to hold in your hand without the risk of it turning black and falling off...

And there is no way to have it gain mass of its own accord.

On a more physical level

On a larger scale level then it is simply a matter of adding or subtracting mass to/from around it. whenever you eat or drink something you change your mass, and when you visit the toilet you change it again. and even if you don't eat, drink or visit the facilitates you change your mass very slowly by expelling CO2.

But all of this requires something to be added or taken away... so alas its not really possible


Magnets!!! depending on your surrounding environment, you could have a powerful elctro magnet, which could either attract or repel a wall or flooring, this has actually been done with a replica Thor's hammer, they used a a fingerprint reader to make sure the person was "worthy"

This would not effect the mass in any way, but would at least give the illusion of increased weight

  • 1
    $\begingroup$ Magnets wouldn't affect the mass, only the apparent weight. When talking about something like a motorbike/scooter as per the example 1 in the question, it is mass rather than weight that matters. However Thor's hammer can be done with magnets. $\endgroup$
    – Separatrix
    May 28, 2019 at 11:13
  • $\begingroup$ @Separatrix, Sorry, i should have put weight in there, i didn't mean the magnets would have changed the mass but could have given the illusion of increased weight $\endgroup$ May 28, 2019 at 12:09

Since the question asks for "science" and "reality check", we can ignore the details of what's being asked and simply look at the implications of what is being asked.

We could build a device that:

  • Lifts an object to some height.
  • Allows the object to fall, while pulling on an attached chain.
  • Uses the tension on the chain to power some other device.

Such devices already exist (e.g. how grandfather clocks are powered).

Now suppose that it were possible to change the mass of the object:

  • Reduce it to minimum mass before lifting it, to minimize the energy required.
  • Increase it to maximum mass before dropping it, to maximize the energy produced.

The net effect is that more energy would be produced than is used. Some of the produced energy could be fed back and used to power the system and the rest is available to use for whatever you want.

This is basically not only a perpetual motion machine, but one that produces extra energy from nowhere.

For science and reality, this is simply not possible.

The only way it could be possible is if the (otherwise false) assumption of the ability to change mass also required the addition or release of large amounts of energy. That is, to make it heavier, you would have to add a lot of energy to it, and to make it lighter, you'd have to remove a lot of energy from it.

Here, "a lot" is a vast understatement.

Einstein has already provided an equation making it easy to calculate the amount of energy required: E=mc². The amount of energy involved in even a very small change of mass is what produces nuclear bomb explosions.

The "Little Boy" bomb, which destroyed Hiroshima and 100,000 people, converted about 1 gram of mass into energy. That's the mass of a small paperclip. See "Hydrogen Bomb Mass to Energy?" for more details.

So, even if it were possible, it certainly wouldn't be practical.


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