A simple physics question …

[Zermelo]

OK, since you raised potential energy, I'll ask one that's been bugging me for a while.

In school, we started off by calculating gravitational potential energy purely in terms of differences and changes. Then we progressed to situations where energy is converted between kinetic and potential forms - again, thinking solely in terms of change. Finally, my physics teacher said something like "well, it would be a lot more convenient if we could assign actual numbers to the potential energy of objects in situ, not just to the change in energy. Since any scale we choose is arbitrary, scientists decided to assign a value of zero to an object at infinity (compared with the reference object). This means that the gravitational potential energy of all objects is negative, but don't worry about that; all we ever do is use the numbers to calculate differences." (this was in the old 'O' level syllabus; I'm sure GCSE has made some changes).

Fast forward a few years, and I read an article that explained the apparent creation of mass/energy in the big bang as follows: if you add it all up, the total (positive) mass/energy is equal and opposite to the total negative gravitational potential energy. Therefore, the total energy is zero, so the entire universe was created from nothing, and will return to nothing. Leaving aside the possibility of an ever-expaninding universe, or the possibility that the numbers might actually not balance, it's the principle being used here that I have trouble with. It only works if you assign negative values to the potential energy. It's the absolute values, rather than the differences, that are important. My physics teacher lied to me, and I suspect he's no longer around to explain. Can anyone else?

[andrew s]

@Zermelo any place in physics you see "potential" it means a difference e.g. potential energy or potential difference in voltage.  In which case the zero point is totally arbitrary and has no physical meaning.

I suspect the sum of mass / energy in the Universe was done prior to the discovery of dark energy and dark matter. I am not sure where that leaves us. In reality I don't think we know if there is a net balance. In GR energy is not even conserved as you don't have time reversal symmetry.  Indeed the current best cosmological model is not time reversible either. 

Regards Andrew 

[iantaylor2uk]

There were no particles with mass straight after the big bang. Once particles with mass appeared, gravity could attract other particles. If you say that the potential energy of something in empty space where there is no gravity is zero, then if you imagine something on the earth, you have to put lots of (kinetic) energy to get it into this position. Therefore you have to add energy to get something to a position where the total energy is zero, so to make this work the potential energy must be negative.

[andrew s]

8 minutes ago, iantaylor2uk said:

There were no particles with mass straight after the big bang. Once particles with mass appeared, gravity could attract other particles. If you say that the potential energy of something in empty space where there is no gravity is zero, then if you imagine something on the earth, you have to put lots of (kinetic) energy to get it into this position. Therefore you have to add energy to get something to a position where the total energy is zero, so to make this work the potential energy must be negative.

Gravity does not just come from mass. In GR it comes from both the flow  and density of energy (which includes mass).  

The almost perfect uniformity of the CMB shows it was very evenly spread out at the beginning. 

Regards Andrew 

Edited August 21, 2022 by andrew s

[iantaylor2uk]

You don't need GR to model how to launch something from the earth into a position in space where gravity is negligible, Newtonian physics is good enough.

It is an assumption that the energy of something in empty space (that is not moving relative to you) is zero, and so this is why the gravitational potential energy is negative.

[cloudsweeper]

The whole mass/energy/gravity/time thing is clearly very complex and non-intuitive.

For example, a Youtube video asserted that mass bends time, and bent time causes gravity.

None of it rests easy with me, but that is of course my failing.  I thought I understood what mass was, but even that is in question now.  

Good to hear all the contributions, though!

Doug.

[andrew s]

13 minutes ago, iantaylor2uk said:

You don't need GR to model how to launch something from the earth into a position in space where gravity is negligible, Newtonian physics is good enough.

It is an assumption that the energy of something in empty space (that is not moving relative to you) is zero, and so this is why the gravitational potential energy is negative.

Yes it's a convention it has no physical meaning it could have been chosen differently. 

I was commenting on your big bang point. There was always gravity it did not need to wait for the Higgs mechanisms to switch on.

Regards Andrew 

[billhinge]

8 minutes ago, cloudsweeper said:

The whole mass/energy/gravity/time thing is clearly very complex and non-intuitive.

For example, a Youtube video asserted that mass bends time, and bent time causes gravity.

None of it rests easy with me, but that is of course my failing.  I thought I understood what mass was, but even that is in question now.  

Good to hear all the contributions, though!

Doug.

gravity on the ISS is actually 89% of the surface gravity, it just appears to lack gravity due to free-fall .

We feel the moon's gravity and vice versa in tides

I think the problem is the mixing of newtonian and relativistic concepts of energy. In GR there is no clearly defined concept of potential energy . The previous question is then answered by the person lifting the weight gets hot through doing work

The problem with learning is the analogies we get taught initially  get engrained so it becomes difficult to shake. Took me ages to shake the newtonian model out of my head, same with covariant/contravariant vectors (at school you get taught euclidiean geometry and vectors look like A=5*i + 6*y + 7*z etc and the basis vectors i, j, k are at right angles to each other) and they don't even mention tensors other than pretend they are funny matrices

'A Youtube video asserted that mass bends time, and bent time causes gravity.'  Thats a good one isn't it , think its semantics 😉 - causes gravity? or causes to fall etc ? 

[iantaylor2uk]

48 minutes ago, andrew s said:

Yes it's a convention it has no physical meaning it could have been chosen differently. 

I was commenting on your big bang point. There was always gravity it did not need to wait for the Higgs mechanisms to switch on.

Regards Andrew 

I think this is debatable - most modern theories of everything believe space-time is emergent, so there could have been a time at the start of the big bang when space-time did not exist.

[vlaiv]

4 minutes ago, iantaylor2uk said:

so there could have been a time at the start of the big bang when space-time did not exist.

 

[andrew s]

23 minutes ago, iantaylor2uk said:

I think this is debatable - most modern theories of everything believe space-time is emergent, so there could have been a time at the start of the big bang when space-time did not exist.

Since we have no evidence for any theory of everything I will pass on this debate. 

Regards Andrew