# I Is there a terminal velocity for expansion in a vacuum?

1. Apr 1, 2017

### DWT

For this question imagine an balloon that will never pop and contains infinite mass. This balloon materializes inside a perfect vacuum with no boundary.

Is there a terminal velocity of the expansion of the balloon?

If so, does the balloon gradually accelerate to this velocity or does it hit it immediately?

If there is no terminal velocity then would the balloon accelerate to light speed and potentially beyond?

2. Apr 1, 2017

### Staff: Mentor

Assuming infinite mass is not reality. But if there is enough mass, it won't expand at all. It will collapse into a black hole.

If there is not enough mass for that, but if the skin of the balloon is elastic, expansion will stop when gas pressure balances elastic forces from the balloon.

If there is no balloon skin and not enough mass for a black hole, expanding gasses will stop accelerating when the molecules get too far away from each other to interact, so yes there would be a terminal velocity.

3. Apr 1, 2017

### DWT

Infinite was an incorrect description sorry, I meant constant mass to volume. As the balloon expands mass materializes to keep a constant volume to mass ratio. As for the skin on the balloon lets say there is no elastic force or it also materializes as the balloon grows.

4. Apr 1, 2017

### Staff: Mentor

Physics deals with reality, you can't just wave your hands to ignore reality So no answer for that question as posed.

But you may have a real physics question. Can you ask it in a simple way to phrase it without the impossible balloon analogy?

5. Apr 1, 2017

### DrStupid

Do you mean something like a multi-charge gun? What is the velocity of the additional mass? Is it already co-moving with the balloon or does it start at rest?

6. Apr 1, 2017

### Staff: Mentor

@DWT , are you trying to construct a scenario where the gas experiences constant accelerating force?

7. Apr 1, 2017

### DWT

I guess my question may be as follows;

Is the expansion velocity of an object in a vacuum reliant on the energy contained within the expanding object (i will call this the pushing force) or does the vacuum have a "pulling force" on the object.

8. Apr 1, 2017

### Staff: Mentor

The vacuum has no pulling force.

In our day to day experience it feels as if it does - put your hand over the end of a vacuum hose, and it will certainly feel like the vacuum is trying to pull your hand in - but in fact it's the air pressure on the other side trying to push your hand in. In your expanding gas cloud, the pressure falls as the cloud expands, and when the expansion stops when the pressure becomes small enough that it can no longer force the gas farther out.

9. Apr 1, 2017