Gravity is different

  • Thread starter Lapidus
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  • #1
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From Greene's pop science book 'The hidden reality', I got this:

What’s vital to realize is that the gravitational field’s energy can be arbitrarily negative. Consider two particles falling toward each other under their mutual
gravitational attraction. Gravity coaxes the particles to approach each other
faster and faster, and as they do, their kinetic energy gets ever more positive.
The gravitational field can supply the particles with such positive energy
because gravity can draw down its own energy reserve, which becomes
arbitrarily negative in the process: the closer the particles approach each other,
the more negative the gravitational energy becomes (equivalently, the more
positive the energy you’d need to inject to overcome the force of gravity and
separate the particles once again). Gravity is thus like a bank that has a
bottomless credit line and so can lend endless amounts of money; the
gravitational field can supply endless amounts of energy because its own
energy can become ever more negative.
Could someone explain that I bit further? Why and how is in the gravitational field an arbitrarily large amount of energy stored?

What about the potential energy of the two approaching particles, does it not decrease, while the kinetic energy increases?

Greene says this arbitrary gravitational energy supplied the early universe the necessary energy to inflationary expand. How?

thank you
 

Answers and Replies

  • #2
Simon Bridge
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As the kinetic energy increases, potential energy decreases.
Since it starts out at zero, then potential energy goes negative. This is what he's talking about.

Classically: if two point masses can get arbitrarily close to each other then the energy gets arbitrarily negative. Usually real objects hit each other's surface before that happens.

This is because gravitational potential energy is the work done to separate masses by infinite distance.

One of the puzzles of the formation of the Universe is where all this matter came from - it cannot come from nothing and each gram accounts for a great deal of rest-mass-energy.

One of the arguments is that when you get matter you also get gravity and thus gravitational potential energy. The energy of matter is positive and the energy of gravity is negative - sum them up to get zero. So everything can come from nothing!

But you should be careful of pop-science descriptions - the authors tend to like to blow your mind at the expense of being accurate. It's supposed to be for stimulating your mind but not for drawing conclusions.
 
  • #4
343
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As the kinetic energy increases, potential energy decreases.
I see, thanks
 

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