Can Mass Be Negative?


Try kicking a ball. A simple exercise. The outcome? Most certainly, the ball would accelerate in the direction to where you kicked it. But what if it doesn’t? What if on kicking it, instead of accelerating in the direction of the force applied, it accelerated in the opposite direction? Impossible? I guess not. This is a characteristic of a body with ‘negative’ mass? Welcome to the weird world of Physics.
Scientists have created a negative mass fluid, but what is the science ?

So, what exactly is negative mass? Well, we know that there are two kinds of charges, positive charge, like proton, and negative charge, like electron. Similarly, scientists thought whether there could be a negative mass. The most basic definition of mass is that ‘it is the amount of matter contained in a body’. (Do not confuse mass with weight, the latter is the force with which the Earth attracts the body and has the dimensions of force, SI unit ‘Newton’). So then is negative mass, um, ‘the absence of matter’. Quite wrong. We don’t know much about its behaviour and characteristics, but I am pretty sure, it isn’t absence of matter, because that would be just vacuum. But most theorists claim that like positive mass has an attractive gravity, negative mass would have a repelling gravity. Another characteristic of negative mass is that when applied with a force, it moves in the opposite direction of the force, like in the hypothetical scenario I mentioned in the first paragraph. This behaviour comes from the second law of motion by Newton, which gave us a formula, relating force(F) applied to the mass of the body(m) and acceleration produced(a). The formula is F= m*a. Simple manipulation gives us, a=F/m . So till now, we assumed mass to be intrinsically positive, and so a push to the left accelerated body of mass m to the left, a positive acceleration. Simple. But if we now say that mass can have a = or – sign, the formula for a is, a= +or-(F/|m|), where the sign depends on m being positive or negative respectively. So, when we are concerned with a negative mass, a force on the left, will result in the body moving right, because the acceleration is negative, i.e., in the opposite direction. (WHAT?!)

Action of force on a negative mass
A diagram to show the effect of force on bodies with postive(+m) and negative mass (-m)

Well, this idea was understandably hard to grab. The characteristics are counter intuitive, but then again intuition is just a conditioning to what we have experienced, and there are a lot of things yet to be experienced for the first time. In the beginning the idea met with a lot of criticism, it was said that it violated the ‘energy condition’ of relativity. The field equations of relativity allow for a number of matter configurations and gravitational fields, but to allow only ‘logical’ solutions to exist, physicists have imposed a certain restriction to what kind of solutions can be allowed, which is called the ‘energy condition’, though this does not mean that a solution not satisfying this condition isn’t mathematically possible. Negative mass was a part of ‘exotic physics’, something that, if not impossible, has no proof of existence. Negatuve matter became a part of even exotic theories, like means to create a wormhole. Like in the space time fabric positive matter creates a valley, and this creates its ‘pulling’ gravitational field, a negative mass should create a ‘hill’ in this fabric, ‘pushing’ away all matter. On July 16, 2004, Soussane Mbacek and Manu Paranjape of Université de Montréal in Canada, proved in a paper published by them that it could be shown that using perfect liquids, negative mass did not violate the energy condition. (They used a non-asymptotically flat DeSitter space). Well, then why aren’t we coming across people buying ‘-10 Kg of flour’ or even using the notion of sign, + and -, before mentioning mass? Because no one has actually seen negative mass around us, until now.

Negative mass and postive mass effect on spacetime
Effect of mass on the space-time fabric depending on its sign.

A few weeks ago, a team headed by Dr. Peter Engels, from Washington State University, invented the first known sample of negative mass, a stable negative mass liquid. Their setup involved cooling rubidium atoms to an incredibly low temperature, just fractions above absolute zero, or 0K, or -273.15° Celcius. The world and physics at this temperature is inherently weird, and you are advised to leave your intuitions and common-sense at the entry. At this temperature, the atoms behave as what is called a Bose-Einstein condensate, and obey the laws of quantum ‘weird’ mechanics. The atoms move in unison, and create what is called a superfluid, a fluid which has no friction or viscosity in it. You swirl it, and it will continue swirling forever. Now what they did was while they used one set of lasers to maintain the temperature of 0K, and hold the superfluid in a ‘bowl’ of electromagnetic field, so it doesn’t escape, they used another set of lasers to flip the spins of the atoms by kicking them back and forth, and as soon as there was a crack in this bowl of electromagnetic field, the rubidium superfluid escaped, and expanded outwards, with some of it behaving like negative matter. The scientists noted that on being pushed, they moved back!!!!! Just as predicted for negative mass to do. This can help us study negative mass more widely, and perhaps understand some strange behaviour of black holes, and neutron stars. This is what I love about science, sitting here on Earth, studying a sample of rubidium atoms, we can unlock secrets of the grand Universe outside, and understand objects as massive as black holes and neutron stars.

Prof. Peter Engels

Negative mass footballs might still be a thing of science fiction (not impossible though, so just stay optimistic), but this was definitely a big step. The current advancements make me so happy and excited, so much is being unraveled and so much is yet to be known, and every new discovery and invention seems to have jumped out of fairy tales and science fiction. So, you might probably want to start putting a sign before you mention the mass of something. But seriously, the ‘absence of matter’ and ‘negative gravity’ are actually some mind boggling thoughts. Till the next strange science concept, Happy Reading!

-The Cosmogasmic Person.

PS: I have been a bit unactive, and will be so for these couple months. My second year finals are here, and though I have so many thoughts and ideas and theories to share and talk about here, its really tough finding out time between studying and procastinating the former. But I will try my best to do a couple of posts a week. And, thanks for stopping by, and reading & supporting my blog. Cheers!


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