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What would be the one thing that would have the greatest mass in the universe?

What is the relationship between mass and speed?

Would this then make light the heaviest as it is the fastest?

The heaviest object in the universe is the universe itself.

Fact is that we have no means of measuring mass, only of comparing masses.

However, just for fun, let's considerm = m_{0}/√(1 - v^{2}/c^{2})which may be familiar.

We know that distant bodies in the universe are retreating from us, and thus us from them, at an accelerating rate.

Now consider planet X with an orbiting moon, rushing away from us at a finite and increasing v.

Since the mass of both planet and moon will be increasing, so will their mutual gravitational force. Describe the change in the moon's orbit.

Note that "from the viewpoint of a stationary observer on earth" is irrelevant: if you are standing on X, is the moon getting closer or further away?

Peter: By "us" I mean you and me standing on Earth, and observing planet X moving away from us at a considerable speed and distance.It's a rhetorical question for the time being but if you have any ideas about the solution, I'd be interested in them. For simplicity we can assume a circular orbit. I don't think we need to consider the small change of velocity when the moon M of X approaches us or recedes - it's all a very long way away and X is receding very fast compared with the orbital velocity of M.I'm not going to lose any sleep over this, but it would make an interesting interview question!