Comment by raxxorraxor

Comment by raxxorraxor 4 days ago

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Not out of reach if you get very close to light speed. Time would advance very slowly for you, so counterintuitively it is possible to travel 5000ly in your life time.

Although for everyone else at least 5000 years will pass, so better say goodbye to family and friend.

Hm, not sure if that is really less depressing...

Also light isn't slow. A photon instantly travels to the end of time and yet it still takes a few minutes from the surface of the sun to us. Or about 100000 years from the center of the sun to its surface.

causal 4 days ago

Yeah if you have a body that can tolerate sudden jumps between reference frames you could pretty much explore the entire galaxy trivially, so long as you don't mind that few places will stay the same long enough to visit twice.

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  • ryandrake 4 days ago

    You wouldn't need a sudden jump. If you had a rocket that accelerated at a pleasant 1G forever, you could reach and stop at the center of the milky way in about 20 (your time) years, and you could reach and stop at the Andromeda galaxy in about 28 years. Play around with some of the online space travel relativity calculators--it's wild!

    Of course building and fueling such a rocket is what's totally out of reach.

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    • TimTheTinker 4 days ago

      > Of course building and fueling such a rocket is what's totally out of reach.

      We'd need a device that could efficiently transform several kg of matter to photons.

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      • lukan 3 days ago

        Also some kind of a energy shield. Space is pretty empty, but if you go fast enough, you will still hit lots of non empty space.

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    • aledalgrande 4 days ago

      Is there drag in space? I.e. would you need increasing energy to accelerate at a constant rate as the speed goes up?

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      • ryandrake 4 days ago

        With a traditional rocket, I believe you'd need decreasing energy to maintain the same acceleration as the flight progressed, since you are carrying along with you and burning the fuel, and so the total mass (payload + fuel) that needs to be accelerated is constantly decreasing.

        Of course there's the pesky problem that for every N kg of mass you want to accelerate at 1G for that kind of a trip, you're probably going to need somewhere on the order of N billion kg of fuel to burn.

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      • [removed] 3 days ago
        [deleted]
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      • dmoy 4 days ago

        I guess one assumes that whatever system prevents you from getting hulled by space dust also removes the drag from the equation?

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nilamo 4 days ago

> Hm, not sure if that is really less depressing...

A starship capable of such a journey is surely large enough to bring all your friends and family along, colony-ship style.

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  • tanewishly 4 days ago

    We're already on that starship. Our engine is about 8 lightminutes away. All we need is to figure out how to steer this thing - and how to not wreck it while en route.

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    • lukan 3 days ago

      I would prefer the concept of people building an artificial planet/asteroid/spaceship for a starship, instead of messing with our star system. But luckily that debate is some years away and currently we cannot even figure out, how to deal with some increased CO2 levels.

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amne 4 days ago

How would that feel as a traveler? Does all motion slow down to a crawl, all sub-atomic particles just "freeze" and essentially your thoughts and body aging too? So it would seem like you got there in an instant?

For sure you're not just sitting there watching people get born, live and die in second and shrugging your shoulders.

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  • Sharlin 4 days ago

    You’d feel nothing out of the ordinary whatsoever. The starscape outside the ship would look strange though, shrinking into a small, blueshifted patch of sky straight ahead, while stars behind you would redshift out of the visible range. Everything moving at very low speeds relative to you would indeed appear to happen really fast.

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    • amelius 4 days ago

      What are the chances of hitting a small meteorite or part of it, traveling now at relativistic speeds wrt you?

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      • TheOtherHobbes 4 days ago

        There's about one particle of dust per million cubic metres. c is about 300 million metres/second. So even at 0.5c that's still a lot of particle collisions per second, each having significant kinetic energy.

        Basically it would be like flying through explosive sandpaper. Each dust particle would be reduced to plasma, which creates problems of its own.

        If you're accelerating there's also the Unruh Effect, which will raise the perceived temperature. By a lot.

        There's no way to make this work with any kind of engineering we know about today.

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        • southernplaces7 4 days ago

          The Unruh effect is theoretical, and no evidence at all has ever been found that it's real. It literally exists as nothing more than a hypothetical mathematical model, that also happens to be debated by others who know enough to effectively debate it, and disagree.

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      • Sharlin 4 days ago

        Micrometer-scale specks of dust would hit you like they were armor piercing tank gun rounds. The usual shielding proposed is ice. Lots of ice.

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      • kqr 4 days ago

        Extremely low. Space is very empty.

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    • seanw444 4 days ago

      If the light behind you redshifts out of the visible spectrum, would the light in front of you blueshift into dangerous territory? X-rays, gamma rays, etc?

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      • doph 4 days ago

        Yes, and this provides a nice intuition about the relation of wavelength to energy. But x and γ wavelengths are several oom shorter than visible light, so you'd have to be traveling at very close to c to experience that amount of Doppler shift.

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danudey 4 days ago

It depends on acceleration though. If acceleration and deceleration take long enough, it could take an entire generation to get up to a fast enough speed that relativistic effects make any difference, and another generation to slow down enough to interact with anything you might see.

Plus if you're traveling at near light speed, running into any matter at all would be pretty devastating for whatever craft you're in.

Edit: someone further down claimed that the math says that accelerating at 1G would get you to 0.1c in a month, so that's actually not that bad all in all. I still maintain that hitting any matter at those speeds might be unpleasant.

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  • mr_toad 4 days ago

    > that accelerating at 1G would get you to 0.1c in a month

    Minor problem is that we don’t have any technology that’s close to capable of that. And at 0.1c relativistic effects are barely noticeable.

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thombat 4 days ago

But unless you have a way of slowing down again you'll never see anything of your destination, just the briefest of flares of light as you sail past. And if you do have a way that involves anything like physics that we recognise, you've brought along a huge rest mass that then got accelerated to near light speed. Probably your civilization needs to be approaching Kardashev Level 2 to pull this off.

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eslaught 4 days ago

One thing I've always wondered is what fraction of c is actually realistically achievable with current technologies? (Maybe with scenarios for manned/unmanned spacecraft.)

Like are we at 0.1% or 0.01% or more orders of magnitude off?

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  • btilly 4 days ago

    If you enjoy such questions, I highly recommend https://www.amazon.com/Indistinguishable-Magic-Robert-L-Forw....

    The best speed for interstellar travel with technologies that current theory says should be within our reach can be achieved with a vehicle with a light sail pushed by a giant laser, that is powered by solar power. There is even a way to brake it when it reaches the target star. I forget what the predicted velocity was though.

    This technology is basically the same as one that the Moties developed in the story, The Mote in God's Eye.

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    • DiggyJohnson 3 days ago

      Slow down by pulling on the fishing line tied to the back of it, carefully.

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      • btilly 3 days ago

        It slows down by releasing a large light sail in front of it, designed to reflect light back to a much smaller light sail behind it. The laser then pushes the large sail away, and as the sail goes it pushes the smaller sail (and ship) back. This leaves the ship at moderate speed relative to the new star, and a large sail traveling very, very quickly beyond it.

        We do not yet have this technology. But we can show that it is plausible.

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chrisweekly 4 days ago

> "A photon instantly travels to the end of time"

Please explain this. TIA

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  • 542354234235 17 hours ago

    A very layman’s explanation I read a long time ago. Imagine you are in a car driving in a big open desert. You can drive in any direction you want. You can drive East or South or West or North, or some degree between them. But the more you travel East, the less you can travel North or South. The more you travel North, the less you can travel East or West. If you drive completely North, you aren’t traveling at all in the East/West direction.

    Spacetime is like that, except North/South is space, and East/West is time. The more you travel through the spatial dimension (the faster you go) the less you travel through the time dimension (the less time passes for you). Photons are traveling completely through the spatial dimension (North/South) and so aren’t moving through the time (East/West) dimension at all, so from a photon’s perspective (if they had perception) no time ever passes so they would zip around “instantly” and never “experience” anything.

    It is also why the speed of light is the fastest you can go. Once you are going 100% North, you cannot be going any more North.

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  • tridentboy 4 days ago

    First set gamma as being 1/sqrt(1-v^2/c^2), with "c" being the speed of light. The factor for time dilation and distance contraction in special relativity is gamma and 1/gamma respectively.

    That means that when you get to speeds equal to c, your time runs infinitely slower and the distances are infinitely shorter. So if your clock is infinitely slower, so every travel at "c" speeds means that no time passes for you. And if your distances are infinitely shorter, all travels at "c" speeds cover any distance as immediate. So you could reach every point of the universe as if it was immediately closer and in no time at all.

    So in the frame of reference of the photon, the moment it is created it has already reached its destination, be it wherever it is on the universe.

    Of course we can never reach "c" as beings with mass, but we can get closer to that. So for example if you get to 99.99999999999999% of the speed of light, you could travel a distance of 54,794,520 ly and only one year would pass to you, while 54,794,520 years would pass on earth.

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    • dbetteridge 3 days ago

      Follow up question from someone who's mostly forgotten his university physics.

      Do photons actually exist, in the traditional sense of physical matter.

      Or are they just a convenient short hand to describe the transfer of energy via waves in the fabric or space time, if they dont experience the universe when passing through it but only when interacting with matter and matters "dents" in space-time.

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      • svachalek 3 days ago

        As a non-physicist, my understanding is that they actually exist, but can't be thought of as flying around like ping pong balls. I think it's one of those things that comes down to interpretation though, where the math is very clear but how you think of what it "means" lies beyond science.

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        • dbetteridge 2 days ago

          Went on a slight rabbit hole reading after posting this and seems like we're all just excitations in a stack of quantum fields :-D

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mock-possum 4 days ago

That doesn’t make sense - if you were traveling at the speed of light, it would take you 5000 years to travel 5000ly - longer if you were just ‘very close’ to C. Time wouldn’t advance slowly for you, it wouldn’t advance perceptively different at all - you’d still live every second of those 5000 years.

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  • ghosty141 4 days ago

    I dont think you are right. Light for example doesnt perceive time at all. From the photons point of view it never aged even a microsecond while it traveled lightyears. Time is relative too so from our POV 1 year passed when a photon traveled 1 ly, but for the photon no time passed.

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  • zwily 4 days ago

    Read up on time dilation and special relativity. Time absolutely does pass slower for you as you accelerate.

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    • IAmBroom 4 days ago

      You two are talking about different meanings of "time".

      Traveling 5,000 LY at 0.5 c will cause you the spaceship pilot to age 20,000 years. It's non-relativistic, inside that inertial frame. Clock second hands still sweep slow but noticeable circles.

      Meanwhile, everyone outside of the spaceship is happening FAST, by your observations. You'll see stars turn red and go supernova.

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      • mr_toad 4 days ago

        The journey will take 10,000 years for an external stationary observer and about 8695 years for the pilot.

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