6 ms·

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 lif

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.

amne 1y 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.
- Sharlin 1y 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.
  - amelius 1y ago
    What are the chances of hitting a small meteorite or part of it, traveling now at relativistic speeds wrt you?
    - kqr 1y ago
      Extremely low. Space is very empty.
      - brazzy 1y ago
        But it's also very big, and GP doesn't even specify how far of a trip they're asking about nor how small a meteorite. "Extremely" and "very" don't cut it here. This is beyond the human ability to guess. You'd actually do at least some back-of-the-napkin math to give a real answer, and with a far enough trip, the answer may well become "Almost 100%".
        wat10000 1y ago
        And at a high enough speed, the impacts from the ~1 hydrogen atom per cm^3 in interstellar space become a major problem.
        amelius 1y ago
        How far a trip: maybe start with the nearest star.
      - jandrese 1y ago
        It's one of those cases where you have very small numbers multiplied by very large ones. The actual risk is hard to intuit because there are so many orders of magnitude involved in both directions. In any case it's probably a moot concern as long as we are living under the twin tyrannies of Newtons Third Law and the Rocket Equation. Building a rocket that can accelerate constantly and noticeably for weeks, months, or even years on end in order to accelerate up to a velocity where Relativity starts to matter requires an absurdly large rocket. Like converting the mass of Jupiter into rocket fuel to make it to the next habitable solar system in a couple of centuries level of craziness.
      - Sharlin 1y ago
        Micrometer-scale dust particles would in fact hit you all the time. And they’d absolutely mess up your ship over time without a lot of shielding.
    - TheOtherHobbes 1y 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.
      - southernplaces7 1y 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.
    - Sharlin 1y 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.
      - folli 1y ago
        Why ice?
        Sharlin 1y ago
        Mostly that it's plentiful, ablative, expendable, plus good radiation shielding (yeah, cosmic ray protons are really going to mess things up at relativistic speeds too unless there's enough mass to stop them).
    - rnjesus 1y ago
      zero if you hit that spice first
  - seanw444 1y 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?
    - IAmBroom 1y ago
      Yes. To some degree.
    - doph 1y 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.
thombat 1y 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.
causal 1y 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.
- ryandrake 1y 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.
  - TimTheTinker 1y 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.
    - widforss 1y ago
      And back?
      - TimTheTinker 1y ago
        No, it would direct the photons out the back of the space craft, producing a beam intense enough to vaporize any solids or liquids close by. This beam would ideally be able to provide perhaps 1G or so of continuous acceleration.
    - lukan 1y 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.
  - aledalgrande 1y ago
mock-possum 1y 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.
- zwily 1y ago
  Read up on time dilation and special relativity. Time absolutely does pass slower for you as you accelerate.
  - IAmBroom 1y 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.
    - mr_toad 1y ago
      The journey will take 10,000 years for an external stationary observer and about 8695 years for the pilot.
- ghosty141 1y 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.
danudey 1y 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.
- mr_toad 1y 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.
eslaught 1y 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?
- Scarblac 1y ago
  According to the first Google hit, https://scitechdaily.com/nasas-parker-solar-probe-the-fastest-spacecraft-ever-just-skimmed-the-sun-at-430000-mph/ https://scitechdaily.com/nasas-parker-solar-probe-the-fastes... , the fastest we've made so far went 430k mph (falling towards the sun), or about 0.064% of c. Good guess.
- anon_cow1111 1y ago
  We have a number! Around 0.1c maximum and unsurprisingly it involves using nuclear bombs to push yourself. https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
- btilly 1y ago
  If you enjoy such questions, I highly recommend https://www.amazon.com/Indistinguishable-Magic-Robert-L-Forward/dp/0671876864 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.
  - DiggyJohnson 1y ago
    Slow down by pulling on the fishing line tied to the back of it, carefully.
    - btilly 1y ago
nilamo 1y 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.
- tanewishly 1y 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.
  - lukan 1y 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.
chrisweekly 1y ago
> "A photon instantly travels to the end of time" Please explain this. TIA
- tridentboy 1y 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.
  - chrisweekly 1y ago
    thanks, great explanation
  - dbetteridge 1y 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.
    - svachalek 1y 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.

amne 1y 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.

Sharlin 1y 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.
- amelius 1y ago
  What are the chances of hitting a small meteorite or part of it, traveling now at relativistic speeds wrt you?
  - kqr 1y ago
    Extremely low. Space is very empty.
    - brazzy 1y ago
      But it's also very big, and GP doesn't even specify how far of a trip they're asking about nor how small a meteorite. "Extremely" and "very" don't cut it here. This is beyond the human ability to guess. You'd actually do at least some back-of-the-napkin math to give a real answer, and with a far enough trip, the answer may well become "Almost 100%".
      - wat10000 1y ago
        And at a high enough speed, the impacts from the ~1 hydrogen atom per cm^3 in interstellar space become a major problem.
      - amelius 1y ago
        How far a trip: maybe start with the nearest star.
    - jandrese 1y ago
      It's one of those cases where you have very small numbers multiplied by very large ones. The actual risk is hard to intuit because there are so many orders of magnitude involved in both directions. In any case it's probably a moot concern as long as we are living under the twin tyrannies of Newtons Third Law and the Rocket Equation. Building a rocket that can accelerate constantly and noticeably for weeks, months, or even years on end in order to accelerate up to a velocity where Relativity starts to matter requires an absurdly large rocket. Like converting the mass of Jupiter into rocket fuel to make it to the next habitable solar system in a couple of centuries level of craziness.
    - Sharlin 1y ago
      Micrometer-scale dust particles would in fact hit you all the time. And they’d absolutely mess up your ship over time without a lot of shielding.
  - TheOtherHobbes 1y 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.
    - southernplaces7 1y 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.
  - Sharlin 1y 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.
    - folli 1y ago
      Why ice?
      - Sharlin 1y ago
        Mostly that it's plentiful, ablative, expendable, plus good radiation shielding (yeah, cosmic ray protons are really going to mess things up at relativistic speeds too unless there's enough mass to stop them).
  - rnjesus 1y ago
    zero if you hit that spice first
- seanw444 1y 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?
  - IAmBroom 1y ago
    Yes. To some degree.
  - doph 1y 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.

thombat 1y 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.

causal 1y 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.

ryandrake 1y 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.
- TimTheTinker 1y 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.
  - widforss 1y ago
    And back?
    - TimTheTinker 1y ago
      No, it would direct the photons out the back of the space craft, producing a beam intense enough to vaporize any solids or liquids close by. This beam would ideally be able to provide perhaps 1G or so of continuous acceleration.
  - lukan 1y 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.
- aledalgrande 1y ago

mock-possum 1y 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.

zwily 1y ago
Read up on time dilation and special relativity. Time absolutely does pass slower for you as you accelerate.
- IAmBroom 1y 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.
  - mr_toad 1y ago
    The journey will take 10,000 years for an external stationary observer and about 8695 years for the pilot.
ghosty141 1y 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.

danudey 1y 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.

mr_toad 1y 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.

eslaught 1y 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?

Scarblac 1y ago
According to the first Google hit, https://scitechdaily.com/nasas-parker-solar-probe-the-fastest-spacecraft-ever-just-skimmed-the-sun-at-430000-mph/ https://scitechdaily.com/nasas-parker-solar-probe-the-fastes... , the fastest we've made so far went 430k mph (falling towards the sun), or about 0.064% of c. Good guess.
anon_cow1111 1y ago
We have a number! Around 0.1c maximum and unsurprisingly it involves using nuclear bombs to push yourself. https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
btilly 1y ago
If you enjoy such questions, I highly recommend https://www.amazon.com/Indistinguishable-Magic-Robert-L-Forward/dp/0671876864 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.
- DiggyJohnson 1y ago
  Slow down by pulling on the fishing line tied to the back of it, carefully.
  - btilly 1y ago

nilamo 1y 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.

tanewishly 1y 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.
- lukan 1y 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.

chrisweekly 1y ago

> "A photon instantly travels to the end of time" Please explain this. TIA

tridentboy 1y 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.
- chrisweekly 1y ago
  thanks, great explanation
- dbetteridge 1y 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.
  - svachalek 1y 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.