Some time ago I had the following shower thought: "the Speed of Light is pretty slow"
How i got there:
The closest major galaxy to the Milky Way is Andromeda, and is 2.5 million! light-years away. And this is the CLOSEST galaxy, the universe is extremely big.
Of course that as you get closer to C, the traveling object will experience time dilation (relative to observer), so the time passed will be less. At 99.999% C, the traveler would take ~11,000 years to arrive to Andromeda.
So again, even at 99.999% C, 11K years seems like a LONG time to reach even the closest galaxy.
My reasoning was: the speed of light is pretty damn slow.
But then I realized: no, it's not the speed of light that is slow, is my frame of reference.
For us humans, 11,000 years seems like A LONG time, but for the universe is not that long.
The universe's age is estimated to be 13.8 billion years. 11,000 years is 0.0000007971 of the age of the universe.
An average human lives 70 years, 0.0000007971 of that lifespan is approximately ~0.4 hours, or 29 minutes, so it's not that bad.
So yeah, frame of reference matters.
In terms of comparisons to human lifespan, I think this one is interesting:
If you can somehow accelerate/decelerate at a constant human-acceptable 1G, time dilation means almost anywhere is reachable in a human-lifetime.
That coincidence(?) could easily become false if we are accustomed to lower accelerations or lesser lifespans.
Not only that, from light's perspective, it takes no time at all. ;)
From the light's perspective: "what is this.... time you speak of?"
Also: ”Did we just go somewhere?”
It doesn't really make a lot of sense to contextualize a velocity with a distance or a time period.
You think those time periods are large because you see a lot of numbers in the units you chose, or because it is much larger than your lifespan. But in the grand scheme a galaxy is nothing but a spec of dust, 10k years a blink of an eye.
There is no other natural velocity that I'm aware of that we could compare it to, but we can say that the speed of light is the fastest there is, so how can it be slow?
> At 99.999% C, the traveler would take ~11,000 years
if we add more 9s, is it possible to reduce that number to within a human lifespan?
Yeah, it is and the math get's really counterintuitive to grasp since humans (at least me) have trouble thinking in exponential terms.
If you accelerate at 1g constantly for 1y you travel 0.5 light years. You do that for 10.5 years and you reach the center of the milky way. You do that for another 4 (~14 total) years and you are in the Andromeda galaxy, and you do that for another 10 years (~24 years total) and you reach what today is considered the edge of the observable universe.
By the time you get there you are basically traveling at a rounding error from C.
oh, so that’s where all the aliens are … up and cruising around at .99g
> If you accelerate at 1g constantly for 1y you travel 0.5 light years
Can't we accelerate past 1G constantly? Or do we expend so much energy doing it that we can't realistically do it with today's technology?
Sadly there is no realistic approach to get there (even assuming fantasy-levels of technology).
Chemical rockets are currently the only thing that allow sustaining such accelerations briefly for human-size payloads, but the low exhaust velocity and exponential reaction mass requirement make sustaining it for days/months/years completely impossible.
You'd have to supply the energy externally (i.e. some sort of beam propulsion), but getting any significant fraction of g out of such a system (with human-sized payloads) seems unlikely within the next centuries, especially as the distance increases.
> You'd have to supply the energy externally (i.e. some sort of beam propulsion), but getting any significant fraction of g out of such a system (with human-sized payloads) seems unlikely within the next centuries, especially as the distance increases.
Or some form of ram scope, plenty of H everywhere, but those also have the issue of you collecting things while going at relativistic speeds.
We can't do it with any rocket-like propulsive technology. By the time you get anywhere close to c the front of your spacecraft is being abraded into nothing by interstellar gas, larger particles of dust will kill you, and colliding with anything bigger creates an explosion that can be seen for light years. You need to be flying a large asteroid to even think about surviving it - you still won't for long, but you will have time to think about it - and the energy requirements are wildly impractical.
Magic warp bubble tech is the bare minimum, and we're nowhere close to inventing that.
We can easily accelerate past 1g... 1g is just chosen for human comfort, but running any rocket engine continuously for a year requires an impossible amount of fuel.
We need a massive rocket to escape the earths gravity (1g). That takes us minutes and a crap ton of fuel. The issue is to keep doing that with fuel and oxidiser constantly for 1, 2 or 10 or 25 years is a monumental amount of energy, which is a monumental amount of weight, which is a monumental amount of volume... Also you also need the exact same amount of fuel to stop again.... So yeah ... We don't got it yet
We can't necessarily survive that well when under acceleration greater than 9.81 metres per second per second for extended periods of time.
Gravity on Earth varies about .7%.
Increasing the acceleration a little bit more than that (eg to 1%) would make a difference.
https://en.wikipedia.org/wiki/Gravity_of_Earth
> "Increasing the acceleration a little bit more than that (eg to 1%) would make a difference."
Wouldn't that be only a few months on a decades-long journey at 1.01G vs 1G??
> math get's really counterintuitive to grasp
People always say that about speed of light stuff, but I don’t get it. Do you have any more examples of counterintuitive math?
Because what you’re describing is basically the equivalent to compound interest in finance. (e.g. investing $100 at 10% interest over 10 years results in $260)
The proper time (that is, the time the traveler measures on a clock) goes to zero very quickly towards the end. Even with both axes plotted logarithmically, it's just a sudden drop at the end towards light speed.
- [pic](https://home.davidgoffredo.com/hackernews/proper-time-one-li...)
- [plot](https://home.davidgoffredo.com/hackernews/proper-time-one-li...)
Links seem to be broken?
No, but IPv6 only these days. I never set up dynamic DNS.
One million seconds is 11 days, a billion seconds is 31 years.
Thanks! But same here imho:
It all seems quite logical once you put everything into the same unit, I think.Maybe it's just our human calendar/time unit rollercoaster (60*60*24*30*12) that's playing tricks on us here.
Of course it is logical, we are not discussing the math.
But it isn't intuitive, you're not used to numbers that big or that grow that fast.
Few things go from a million to a billion, especially when related to time.
It’s the units that make it seem unintuitive. 12 days is roughly 0.032 years, and 17 minutes is likewise 0.000032 years. It’s relative to how we were taught to view smaller lengths of time. Of course you could argue measuring time by the rotation of heavenly bodies is more intuitive than counting on your fingers for the hours in a day and days in a lunar cycle.
Going from a million to a billion isn't some special relationship. It's just * 1000. It's the same as going 11000 or 101000.
I find it frustrating that people seem to think that someone going from 1M to 1B is somehow different to other numerical operations. It's not. It's 1000 times bigger, it's not a huge deal.
You can but I don't want to see that energy bill
Of course! You can arrive as quickly as you want as the traveler. You can cross the observable universe in a few hours if you add enough 9’s.
Yes. Time dilation (with respect of the stationary observed) is calculated using the Lorentz factor.
At 99.999% of C, the Lorentz factor is ~223.6.
It grows pretty quickly as you add more 9s to the fraction. Every two additional 9s multiply the Lorentz factor by ~10.
So at 0.99999999999 c, it'd be ~223,607x.
2.5M years / 223,607 is: ~11 years
Of course this is all highly theoretical.
https://en.wikipedia.org/wiki/Lorentz_factor
The speeds are theoretical (unachievable), time dilation itself is robust and proved (gps wouldn't work as precisely without accounting for it)
Proper time asymptotically approaches zero here right?
Yes. If you can manage to get arbitrarily close to the speed of light, the amount of perceived time would get arbitrarily close to zero.