# If it is physically possible to exceed the velocity in which the universe is expanding, can we move forward in time?

Nov 5, 2016

It is nothing to do with the rate of expansion of the universe, but it is perfectly possible to travel forward in time.

#### Explanation:

It is nothing to do with the rate of expansion of the universe and everything to do with the speed of light.

We are travelling forward in time all of the time, but the time experienced is relative to your frame of reference. As a result, it is possible (in fact unavoidable) that we experience different lengths of time according to how we move around.

Most of the time, this time dilation is too small to be easily measurable, but there are some scenarios in which it is much more noticable.

For example, we detect far more muons at the Earth's surface coming from cosmic rays than we would expect. Muons have a half life of about $1.56$ microseconds, but travel at about $0.9 c$. As a result, they appear to have a half life about $5$ times as long from the perspective of a ground observer. We can explain this in terms of time dilation by a factor of $5$. From the muon's perspective they have a perfectly normal half life, but they experience length contraction by about a factor of $5$.

If we had a spaceship that could sustain a $1 g$ ($9.8 m {s}^{- 2}$) acceleration or deceleration for extended periods, then it would be possible for a traveller to make the journey to the Andromedra galaxy ($2.5$ million light years away) and back in about $56$ years - ship time. When they returned they would find that $5$ million years had passed.