Time

[strontium]

Let's say hypothetically two people on Earth are born on the same day at the same time down to the second, and one of them becomes an astronaut. He leaves Earth and launches into space. He lives in orbit on the ISS for one revolution of the Earth around the Sun, while his counterpart remains on the surface of the Earth. Upon returning to the Earth, the astronaut is now older than his surface-dwelling counterpart, even though they were born on the same day at the same time and have experienced the same number of revolutions of the Earth around the Sun. In fact, it is not just the astronaut who is older. If we assume that when he launched into space the Universe was 13.8 billion years old, when he returns to Earth, his very Universe itself is now older than his Earthly counterpart's, even though they reside on the same planet at the same time. So the question is, what does it mean in terms of time to say one year has passed? In terms of time, it means nothing. There is no way to objectively quantify the passage of time, because time is relative. Therefore to claim the Universe is 13.8 billion years old is meaningless, because there is no such thing as an objectively valid standardized year in terms of time.

[beecee]

Firstly, I'm pretty sure its the Astronaut that is younger then the stay at home twin. And each twin sees time passing as per 1 second per second within his own frame. It is only when the astronaut returns to Earth that any age difference is noticed.

[strontium]

6 minutes ago, beecee said:

Firstly, I'm pretty sure its the Astronaut that is younger then the stay at home twin. And each twin sees time passing as per 1 second per second within his own frame. It is only when the astronaut returns to Earth that any age difference is noticed.

You are right that I got that backwards. The astronaut ages .007 seconds less for every second his counterpart on Earth's surface ages.

The point I was making, however, is that revolutions of the Earth around the Sun don't correlate to a precise passage of time. So a year in orbit on the ISS is 31,315,248 seconds, while a year on the surface is 31,536,000 seconds (if I did the math right). The point being, it is meaningless in terms of time to claim the Universe is 13.8 billion years old. From the surface dweller's perspective, that would mean the Universe is approximately 4.351968×10^16 seconds old, but from the astronaut's perspective, 4.321504224×10^16 seconds old.

There are an infinite variety of perspectives, each of which will lead to a unique conclusion regarding the age of the Earth, or of the Universe, depending on how you measure it.

[Genady]

1 hour ago, strontium said:

You are right that I got that backwards. The astronaut ages .007 seconds less for every second his counterpart on Earth's surface ages.

The point I was making, however, is that revolutions of the Earth around the Sun don't correlate to a precise passage of time. So a year in orbit on the ISS is 31,315,248 seconds, while a year on the surface is 31,536,000 seconds (if I did the math right). The point being, it is meaningless in terms of time to claim the Universe is 13.8 billion years old. From the surface dweller's perspective, that would mean the Universe is approximately 4.351968×10^16 seconds old, but from the astronaut's perspective, 4.321504224×10^16 seconds old.

There are an infinite variety of perspectives, each of which will lead to a unique conclusion regarding the age of the Earth, or of the Universe, depending on how you measure it.

The age of the universe in cosmology is referred to in the comoving reference frame, which would be time of the observer who is at rest relative to CMB.