<p>Cross-posted from Physics Stack Exchange - <a href="https://physics.stackexchange.com/questions/652106/formula-to-calculate-round-trip-travel-time-based-on-current-distance-to-far-awa">original</a></p>
<p>The question was closed as HW-like there.</p>
<h3 id="the-premise-rgo3">The Premise</h3>
<p>You travel at a constant velocity towards a far-away galaxy. You then travel back at the same velocity. How long will have passed for the people on Earth? Answers must take into account the expansion of the universe.</p>
<h3 id="definitions-8plv">Definitions:</h3>
<ul>
<li>now is defined as 21st century</li>
</ul>
<h3 id="parameters-smxl">Parameters:</h3>
<ul>
<li>Time from now you will depart (in years)</li>
<li>Distance away that the galaxy is now (in light-years)</li>
<li>Velocity (in % of light-speed)</li>
</ul>
<h3 id="output-sef0">Output:</h3>
<ul>
<li>number of years that will have passed on Earth between the time you depart and the time you get back</li>
</ul>
<p>This was inspired by this question: <a href="https://physics.stackexchange.com/questions/588745/at-50-c-i-go-party-to-ic1101-galaxy-and-back-home-how-many-times-can-i-go-be/">At 50% c, I go party to IC1101 galaxy, and back home. How many times can i go before the expansion of the universe will lock me on one side?</a>
.</p>
<h3 id="significance-k8st">Significance</h3>
<p>This will allow for many different astronomical calculations by moving the elements of the formula around (eg. how long until we won't be able to communicate with a distant galaxy?).</p>

CC BY-SA 4.0
2021-07-19T19:48:28.390Z
Please let me if know if this should be moved to Space Exploration.
trinalbadger587
CC BY-SA 4.0
2021-07-20T21:19:45.787Z
Just a quick note: Cross-posting is discouraged on Astronomy SE.
WarpPrime
CC BY-SA 4.0
2021-07-20T22:10:42.593Z
@fasterthanlight I thought it would be fine as the question is no longer open on Physics SE.
trinalbadger587
<cosmology><expansion>
Formula to calculate round-trip travel time based on current distance to far-away galaxy
trinalbadger587
# Formula to calculate round-trip travel time based on current distance to far-away galaxy

(Asked by: username_0 on 2021-07-19)

Cross-posted from Physics Stack Exchange - [original](https://physics.stackexchange.com/questions/652106/formula-to-calculate-round-trip-travel-time-based-on-current-distance-to-far-awa)

The question was closed as HW-like there.

### The Premise

You travel at a constant velocity towards a far-away galaxy. You then travel back at the same velocity. How long will have passed for the people on Earth? Answers must take into account the expansion of the universe.

### Definitions:

* now is defined as 21st century

### Parameters:

* Time from now you will depart (in years)
* Distance away that the galaxy is now (in light-years)
* Velocity (in % of light-speed)

### Output:

* number of years that will have passed on Earth between the time you depart and the time you get back

This was inspired by this question: [At 50% c, I go party to IC1101 galaxy, and back home. How many times can i go before the expansion of the universe will lock me on one side?](https://physics.stackexchange.com/questions/588745/at-50-c-i-go-party-to-ic1101-galaxy-and-back-home-how-many-times-can-i-go-be/)
.

### Significance

This will allow for many different astronomical calculations by moving the elements of the formula around (eg. how long until we won't be able to communicate with a distant galaxy?).

**Tags:** cosmology, expansion

> **Comments:**
> **username_0**: Please let me if know if this should be moved to Space Exploration.  
> **username_1**: Just a quick note: Cross-posting is discouraged on Astronomy SE.  
> **username_0**: @fasterthanlight I thought it would be fine as the question is no longer open on Physics SE.  

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## 0 answers

