# All Questions

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### Gravitation and equilibrium (…and similar problems) [closed]

One problem that I'm having trouble with (as opposed to the other): The Messenger is a probe that orbits Mercury $700 \rm km$ from the surface. What is the tangential velocity it should be ...
1k views

### What is a mass gap?

I read that Quantum Chromodynamics is a theory with a mass gap. What is a mass gap in laymen terms? Why some theories have it? Which theories does not have it? Note: I searched for mass gap before ...
340 views

### Why is plasma the highest “state” of matter?

As far as solid, liquid, gas, plasma go, why is plasma the highest state? Are there any other states of matter?
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### Name of the process that causes ice to melt under pressure and refreeze after

I remember a physics experiment from school where a wire with weights on each end was placed over a block of ice. The pressure of the wire caused the ice to melt underneath and refreeze once the ...
227 views

### What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe's expansion?

Brian Greene in this TED talk about possible multiverse, claims tomwards the end (At around 18:00 mark) this statement. 'Because the expansion is speeding up, in the very far future, those galaxies ...
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### What does “nearly infinite mass” mean?

I am sure this is a silly question, but I was reading something that described the pre big-bang universe as having "nearly infinite mass." How can something be "nearly" infinite? The term seems to ...
344 views

### What does AdS/CFT have to say about quantum gravity in our world?

The Ads side of the AdS/CFT correspondence is a model of quantum gravity in 5 dimensional antidesitter space. What can it say about quantum gravity in our 4-spacetime dimensions? Or is it just a toy ...