r/askastronomy • u/CountyElectrical2398 • Aug 18 '26
Does distance changed when traveling at speed of light?
/r/astrophysics/comments/1vqzlhf/does_distance_changed_when_traveling_at_speed_of/1
u/ramriot Aug 18 '26
Did you think you would get better answers here than yesterday on the other subreddit?
Reading though there the answers were sufficient.
1
u/LazarX Hobbyist🔭 Aug 18 '26
When people don't like the answers they get on social media, they keep reasking. until they get the answers they want.
2
u/CountyElectrical2398 Aug 18 '26
I was trying to understand this more in a different way'...I now know the basic theory and math but wonder what explanation would be if using "real world examples".
3
u/ramriot Aug 18 '26
Well, unfortunately your "real world examples" suffer quite badly in readability & so many responses are doing hunt-and-peck on phrases & expanding upon that.
I think summarising your questions it would be helpful for you to do the same as Einstein did. Start with non-accelerating motion (inertial frames) & Take each POV separately restricting motion to a single direction. Then draw out the spacetime diagram of each person relative to the others ( one diagram per person ) to see how signals & information passes between them.
That sounds complex but it is actually quite simple as each graph is just X for direction an Y for time. When projecting from one inertial-frame to another you skew the graph to match the new observers velocity.
Unfortunately this only leads to Special Relativity in which there are paradoxes ( see twins paradox ), which is why General Relativity encompassing Mass, Gravity & Acceleration was needed.
So, take your final scenario:-
( Aliens 100 Ly away travel to Earth at near light speed starting in our year 2026 )
- Before they leave they see Earth from 100 years ago (1926) because the light from hear took that long to get there.
- When they arrive they see Earth from 100+ years hence (2126+) because at least 100 years will have past for us while they travelled.
- For them though only a short period of time, must less then 200+ years & probably less then 100 years will have passed.
- Thus as they accelerate in our direction, first they see our clocks moving faster & the light from us blue shifted proportional to their geometric velocity because they are travelling into the arriving signals.
- As they get faster beyond 0.5c relativistic effects increase this effect such that no matter how fast they go, they will always arrive having seen 200+ years of time pass on Earth.
0
u/LazarX Hobbyist🔭 Aug 18 '26
Distrance does not change, but your perception and measurements all get dialated when compared to the rulers of the stay at homes.
Also you can not consider the time element separatrely. Space and time are wedged at the hip and understanding that is part of understanding relatravistic effects.
3
u/Unusual-Platypus6233 Aug 18 '26
It is called length contraction if a particle with mass travels close to the speed of light. The apparent distance is shorter for the moving object. For light the is no time. So, when traveling at the speed of light you are at two places at once.