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I'm not sure about this but I guess there must be negative masses in the universe because of the symmetry. If the gravity is one of the main forces in nature it must has negatives mass to be able to uniform with other forces. I think the m^2 in the relative energy E^2=P^2c^2+(m^2)c^4 refer to it and it must corrected that E=|m|c^2

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marked as duplicate by Ben Crowell, Waffle's Crazy Peanut, Qmechanic May 19 '13 at 18:16

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

And negative masses would attract each other, but repel positive masses, right? That's why they are somewhere far :) – Juris May 19 '13 at 13:40
Yes exactly I thought more about that and i think there must be two universes with positive and negative mass. – Hamid Ghaffari May 19 '13 at 13:42
Physics become interesting when you consider exotic and scifi-like things. Even Newton's second law becomes funny if you consider negative masses :) – Juris May 19 '13 at 14:08
Yeah (as QMechanic says) ... It's a duplicate to most of the answers here..! You should do a search ;-) – Waffle's Crazy Peanut May 19 '13 at 15:50

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All this time, the field of physics did not seriously consider the possibility of existence of negative mass (energy) in a general state. The standard explanation of negative mass is that the state of low energy is stable when a negative energy level exists and that the lowest state of energy is minus infinity. Thus, this means that all positive mass emits energy and this will be transitioned to the energy level of minus infinity and the universe will collapse.

However, at the present, our universe exists without collapsing, so the explanation for this has become strong proof of the nonexistence of the negative energy(mass) and negative energy level of. Thus, we have considered this to be obvious common sense and have taught this to students.

At the center of this background, there is the fundamental prionciple(proposition) that “State of low energy is stable".

But, this principle is an incomplete, and that it is stable at a low energy state in the case of positive mass. However, it is stable at a high energy state in the case of negative mass.

Due to this, “the problem of the transition of the energy level of minus infinity" does not occur, and thus positive energy(mass) and negative energy(mass) can exist in the same space-time. Moreover, negative energy(mass) provides a qualitative explanation for dark matter and dark energy, which are the biggest issues posed to cosmology at the present. <== View to Fig.

The acceleration of negative mass is opposite to the direction of force. Therefore, the negative mass has harmonic oscillation at the maximum point and it is also stable at the maximum point.

Motion of the negative mass :

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