
Ask HN: Best explanation of time dilation? - ssijak
Hey HN, has anybody struggled to really grok time dilation like me but succeeded? If yes, what was the ultimate resource that helped you do it?<p>I am a Software Engineer and really into astronomy for the whole my life, but for the life of me can`t really understand time dilation. For example :<p>- If you travel at the speed of light (impossible but for the sake of argument), does time &quot;stops&quot; for you in the sense that there is no motion in you, your atoms are &quot;frozen&quot;?<p>- Same applies when you are near the ultra massive object, but I can`t understand why would mass impact it in that way. I understand it bends spacetime, but why would it slow time by much and will it be the same &quot;slowness as in the first case?<p>- If you are the only object in the universe and there is no reference point, do same logic&#x2F;laws apply? If you speed up, accelerate, does anything change related to &quot;time&quot;. My logical brain says yes, but reading all these explanations about time dilation makes me wondering.<p>- All the examples focus on the points in local time where observers &quot;see&quot; an event (light from the other source coming), but I can`t transfer that into different &quot;speed&quot; of local time (related to the first point)<p>Yeah, enough confusion for now.. I won`t even go to the quantum world now..
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gus_massa
You are triying to understand too much in a single bite. Just try to
understand the first item for now.

Forget the 100%c case. It's not physical so you end up with metaphors that you
can't prove or disproof. Analyze the 99%c, 99.9%c, 99.99%c cases.

The easiest example is a "space ship" that has a "clock" that use the time
that a light ray use to travel between two mirrors that are parallel to the
speed, and the ray is perpendicular. I was going to write the details, but
Wikipedia has a very clear explanation.
[https://en.wikipedia.org/wiki/Time_dilation#Simple_inference...](https://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_velocity_time_dilation)
Go and read the subsection, I'll be waiting ...

waiting ...

waiting ...

Good. The important part is that ideal experiment is not the cause of the time
dilation. The time dilation is a property of the universe, or the space-time,
or whatever. So any experiment will have the same result.

This experiment is nice because the calculations are very simple. If you put
the mirrors perpendicular to the speed and the light ray travel forwards and
backwards, the calculations are much more complicated but you get the same
result. If the mirrors are parallels but in other direction (skewed to the
speed) the calculations are horrible, but you will get the same result. (I
never tried, it's too difficult.)

You can use a mechanical clock, a chemical clock, a biological clock (How long
does this rats live?) All of them will give the same result. It's not a
property of this ideal clock. It's a property of the universe.

Read all the Wikipedia article to get more details.

About point 2 and 3: Good questions for another day.

About point 4: If someone travels inside the rocket with the two mirrors and
the light ray, the astronaut will not notice any strange thing. No time
dilation. The two-mirrors+light clock will work as usual. The mechanical clock
will work as usual. Chemical reactions and rat will work as usual. The heart,
brain will work as usual. But if the astronaut look at the window, all the
clocks in the Earth will look slower.

But if you are in the Earth, the clocks in the Earth will be normal but all
the clocks will look slower.

More details in the same Wikipedia article
[https://en.wikipedia.org/wiki/Time_dilation#Reciprocity](https://en.wikipedia.org/wiki/Time_dilation#Reciprocity)

