Recent comments in /f/askscience

Weed_O_Whirler t1_j8f80cw wrote

Reply to comment by Interesting-Month-56 in In the twin paradox, what happens if the travelling twin never U-turn to get back to earth? (explanation in the post) by PoufPoal

> if you choose the right reference frame, it would seem that the return trip requires the twin to experience negative acceleration.

While in relativity velocity is relative, accelerations are not. Accelerations can be measured and felt. Easiest example. You're driving along a perfectly flat road in the back of a windowless truck. There is no experiment you could do to determine what speed you're going. All speed feel the same. But if the truck accelerated- sped up, slowed down or even turned, you would notice.

15

shimadon t1_j8f5l6w wrote

Reply to comment by peselev in In the twin paradox, what happens if the travelling twin never U-turn to get back to earth? (explanation in the post) by PoufPoal

That's not the paradox. The paradox is that if each twin point of view is relative, then relative to the twin that is on the space ship, it's actually the earth twin that is traveling. So the spaceship twin should be older, not the earth twin. But it's not the case, and this is the paradox.

5

shimadon t1_j8f4fnn wrote

Reply to comment by Aseyhe in In the twin paradox, what happens if the travelling twin never U-turn to get back to earth? (explanation in the post) by PoufPoal

That's a very good answer. Everyone talks about acceleration all the time, but it's really the length of space time path. For example: you can have a situation in which one twin is in a space ship orbiting earth, and the other twin is in a space ship which is hovering above the surface, canceling gravity with a constant force upwards by its thrusters. In this case, the twin who is moving in orbit will be younger, but he is in constant free fall! It's the twin who hovering which will be older, but he was the one who was subjective constantly to an external force! So it's not acceleration, it's space time length...

7

Onetap1 t1_j8et0de wrote

Reply to What do bacteria living in mechanical ventilation feed on ? by malahchi

They don't live in mechanical ventilation. They live in tepid water which has nutrients. Anything with warm water is hazardous.

The first case, where it was identified, killed a lot of US Legionnaires (armed forces veterans) at a convention at a hotel in Philadelphia. The source was an evaporative cooling tower, which uses evaporating water to discard waste heat from the air conditioning systems.

The thing about cooling towers is that the water is recycled from a pond in the base of the tower. It collects insects, dust leaves, limescale, etc, nutrients for bacteria. And the water is warm. The bacteria was distributed on the water droplets leaving the tower.

https://en.wikipedia.org/wiki/Legionnaires%27_disease#Epidemiology

516

peselev t1_j8esdxt wrote

Reply to In the twin paradox, what happens if the travelling twin never U-turn to get back to earth? (explanation in the post) by PoufPoal

The one who hops into the rocket and leaves, has to accelerate (to gain speed). Because one frame experienced the acceleration, and the other frame didn't, the frames are not symmetrical. The U-turn is not the only element that requires acceleration (and deceleration upon landing)

The paradox itself was not that the twin's experiences are different, rather than the one who traveled more, aged less.

At least, this is what I remember from my university years.

1

supapoopascoopa t1_j8er07o wrote

Reply to What do bacteria living in mechanical ventilation feed on ? by malahchi

There is a lot of water in mechanical ventilation circuits due to condensation from air exhalation from the lungs. In fact many ventilators add humidification to decrease this water loss, which further saturates the circuit.

Warm water will turn into an ecosystem if left undisturbed, especially when exposed to oropharyngeal secretions dripping around an endotracheal tube cuff.

49