8 ms·

Ways to travel at nearly the speed of light

134 points by dnetesn 7y ago

NotSammyHagar 7y ago
The easy way to achieve this is get the other party to travel toward you at .99c. Slightly more seriously, it's always interesting to think about how you can have the universe expanding faster than c, and so you can't see it all. You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c).
- IIAOPSW 7y ago
  >You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c). In neither your reference frame or the other person's will it appear to be true that your closing speed is faster than c. However due to the speed of light delay you may perceive the other person as going much faster than c, but this is strictly an illusion. For example, if I were watching a near-light speed ship approaching earth in a telescope, I might see it begin its journey and then see it arrive a few seconds later and my monkey brain would conclude it made the whole trip in the time I was watching it. In reality the light from when it started the trip is a few years old.
  - rightbyte 7y ago
    Would you expect a 0.99c spaceship to be "brighter" in the telescope than a 0.01c? I mean, the average power of the bouncing light on it should be higher, right? Since it's gonna be years of light in a shorter time.
    - deleted 7y ago
      [deleted]
    - jcims 7y ago
      There would be a doppler blueshift out of the visible spectrum i believe, but quantity wouldn’t change much IIRC from this Myhrthbusters episode https://youtu.be/HtbJbi6Sswg https://youtu.be/HtbJbi6Sswg
      - zamadatix 7y ago
        Note that Mythbusters video was revisited later in the series and the claims reversed: https://mythresults.com/episode38 https://mythresults.com/episode38 It's not an equivalent question either, the question is intensity not sum.
        jcims 7y ago
        Very cool, need to check that out. However, isn't intensity equal to sum when it comes to light? At least in the visible spectrum, more energy just shifts the wavelength...perceived intensity comes from photon count.
        zamadatix 7y ago
        Photons sent on a 5 year near c journey would arrive over e.g. 5 days instead of 5 years, hence the intensity. I.e. it doesn't take 5 years for the light you emitted 5 light days ago to arrive but the light you emitted 5 years ago may only be 5 light day ahead.
    - zamadatix 7y ago
      Yes due to the reasons you are thinking but also yes for a more interesting reason, relativistic beaming results in more of the light being aimed in the direction of motion meaning more total light will also be delivered.
  - _Microft 7y ago
    Here's an example with numbers to illustrate what is happening: Let us say that a ship starts at a distance of 100 ly (lightyears) and moves with 0.95c (speed of light) for all of the distance toward the observer through empty space. Light will need 100 years to cover the distance. The ship will need ~ 105.26 years (= 100ly/0.95c). The observer can not know that the ship launched before the 100 years passed that it took for light (or any other signal!) to arrive. So it looks like that only 5.26 years later the ship arrives, making it appear that it only took 5.26 years for a distance of 100 ly which appears like a speed of 19 times the speed of light (= 100 ly / 5.26 years).
- adtac 7y ago
  >You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c). Not true. At relativistic speeds, relative velocity is not a simple addition: https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity https://en.wikipedia.org/wiki/Velocity-addition_formula#Spec... In your example where both parties are moving at 0.6c towards each other, each will perceive the other to be moving at ~0.88c.
  - pfortuny 7y ago
    But what about a third observer? We very much make electrons collide...
    - ViViDboarder 7y ago
      Yes, but when they collide their relative velocities to each other do not exceed c.
cheerlessbog 7y ago
Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler [1]. This neglects the expansion of the universe, the health and safety effects of traveling near to the speed of light, and the fact that we do not know how to do this (you would not be able to bring your own fuel). Nevertheless we do not know why it should be physically impossible. [1] https://en.m.wikipedia.org/wiki/Space_travel_using_constant_acceleration?wprov=sfti1 https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...
- olooney 7y ago
  > you would not be able to bring your own fuel I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem. For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations: https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation https://en.wikipedia.org/wiki/Relativistic_rocket#Relativist... Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics. https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities. https://en.wikipedia.org/wiki/Breakthrough_Starshot https://en.wikipedia.org/wiki/Breakthrough_Starshot I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have. So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!
lucidguppy 7y ago
What I always found interesting was you can go as fast as you want - to you, going from here to alpha centauri could take just weeks for you if you had enough energy - it's just that time would speed up around you so no matter what everyone else would see you get there in 4.5 - 5 years. And if I'm wrong, I apologize and would gladly hear the correct answer.
- pixelHD 7y ago
  Isn’t it the opposite - time slows down for you when you move at relativistic speeds, and hence you end up getting to alpha Centauri in 5years w.r.t clocks at Centauri or earth?
  - zamadatix 7y ago
    An object's relative speed to itself is always 0 (by definition) and so the passage of time is always constant for every object. As you can guess by the name of the theory there is also no "universal clock" or "0 speed" in the universe to compare against, it's only possible to measure speed and the flow of time relatively between objects. For these reasons "time slows down for you when you move at relativistic speeds" is really a bad way to think of it (and not how the math of the theory actually works). It's much more proper to say things in the form "To <observer a> it appeared as if the clock of <observer b> was <sped up|slowed down>". E.g.: To the traveler it appeared as if the clock of Alpha Centauri was sped up. To Alpha Centauri it appeared as if the clock of the traveler was slowed down.
  - dumbfoundded 7y ago
    It's pretty confusing but essentially, as you travel faster, your clock slows down arbitrarily. In the most extreme case, at the speed of light, your clock stops. For a photon, it emerges and appears at its final location instantly, even if those distances are millions of lightyears apart. So if you travel to Centauri from Earth, the fastest you get can get there is 5 years as seen from Earth but to the actual spaceship, it could perceive any arbitrarily small change in time.
saagarjha 7y ago
> Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light. Or that, but with a dozen more 9s in it: https://en.wikipedia.org/wiki/Oh-My-God_particle https://en.wikipedia.org/wiki/Oh-My-God_particle
mLuby 7y ago
Shielding at the speed of light is an interesting challenge. The beginning of Alastair Reynolds's short story "Weather" (in "Galactic North") addresses this.

134 points by dnetesn 7y ago

The easy way to achieve this is get the other party to travel toward you at .99c. Slightly more seriously, it's always interesting to think about how you can have the universe expanding faster than c, and so you can't see it all. You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c).

IIAOPSW 7y ago
>You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c). In neither your reference frame or the other person's will it appear to be true that your closing speed is faster than c. However due to the speed of light delay you may perceive the other person as going much faster than c, but this is strictly an illusion. For example, if I were watching a near-light speed ship approaching earth in a telescope, I might see it begin its journey and then see it arrive a few seconds later and my monkey brain would conclude it made the whole trip in the time I was watching it. In reality the light from when it started the trip is a few years old.
- rightbyte 7y ago
  Would you expect a 0.99c spaceship to be "brighter" in the telescope than a 0.01c? I mean, the average power of the bouncing light on it should be higher, right? Since it's gonna be years of light in a shorter time.
  - deleted 7y ago
    [deleted]
  - jcims 7y ago
    There would be a doppler blueshift out of the visible spectrum i believe, but quantity wouldn’t change much IIRC from this Myhrthbusters episode https://youtu.be/HtbJbi6Sswg https://youtu.be/HtbJbi6Sswg
    - zamadatix 7y ago
      Note that Mythbusters video was revisited later in the series and the claims reversed: https://mythresults.com/episode38 https://mythresults.com/episode38 It's not an equivalent question either, the question is intensity not sum.
      - jcims 7y ago
        Very cool, need to check that out. However, isn't intensity equal to sum when it comes to light? At least in the visible spectrum, more energy just shifts the wavelength...perceived intensity comes from photon count.
        zamadatix 7y ago
        Photons sent on a 5 year near c journey would arrive over e.g. 5 days instead of 5 years, hence the intensity. I.e. it doesn't take 5 years for the light you emitted 5 light days ago to arrive but the light you emitted 5 years ago may only be 5 light day ahead.
  - zamadatix 7y ago
    Yes due to the reasons you are thinking but also yes for a more interesting reason, relativistic beaming results in more of the light being aimed in the direction of motion meaning more total light will also be delivered.
- _Microft 7y ago
  Here's an example with numbers to illustrate what is happening: Let us say that a ship starts at a distance of 100 ly (lightyears) and moves with 0.95c (speed of light) for all of the distance toward the observer through empty space. Light will need 100 years to cover the distance. The ship will need ~ 105.26 years (= 100ly/0.95c). The observer can not know that the ship launched before the 100 years passed that it took for light (or any other signal!) to arrive. So it looks like that only 5.26 years later the ship arrives, making it appear that it only took 5.26 years for a distance of 100 ly which appears like a speed of 19 times the speed of light (= 100 ly / 5.26 years).
adtac 7y ago
>You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c). Not true. At relativistic speeds, relative velocity is not a simple addition: https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity https://en.wikipedia.org/wiki/Velocity-addition_formula#Spec... In your example where both parties are moving at 0.6c towards each other, each will perceive the other to be moving at ~0.88c.
- pfortuny 7y ago
  But what about a third observer? We very much make electrons collide...
  - ViViDboarder 7y ago
    Yes, but when they collide their relative velocities to each other do not exceed c.

cheerlessbog 7y ago

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler [1]. This neglects the expansion of the universe, the health and safety effects of traveling near to the speed of light, and the fact that we do not know how to do this (you would not be able to bring your own fuel). Nevertheless we do not know why it should be physically impossible. [1] https://en.m.wikipedia.org/wiki/Space_travel_using_constant_acceleration?wprov=sfti1 https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...

olooney 7y ago
> you would not be able to bring your own fuel I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem. For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations: https://en.wikipedia.org/wiki/Relativistic_rocket#Relativistic_rocket_equation https://en.wikipedia.org/wiki/Relativistic_rocket#Relativist... Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics. https://en.wikipedia.org/wiki/Bussard_ramjet https://en.wikipedia.org/wiki/Bussard_ramjet Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities. https://en.wikipedia.org/wiki/Breakthrough_Starshot https://en.wikipedia.org/wiki/Breakthrough_Starshot I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have. So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!

lucidguppy 7y ago

What I always found interesting was you can go as fast as you want - to you, going from here to alpha centauri could take just weeks for you if you had enough energy - it's just that time would speed up around you so no matter what everyone else would see you get there in 4.5 - 5 years. And if I'm wrong, I apologize and would gladly hear the correct answer.

pixelHD 7y ago
Isn’t it the opposite - time slows down for you when you move at relativistic speeds, and hence you end up getting to alpha Centauri in 5years w.r.t clocks at Centauri or earth?
- zamadatix 7y ago
  An object's relative speed to itself is always 0 (by definition) and so the passage of time is always constant for every object. As you can guess by the name of the theory there is also no "universal clock" or "0 speed" in the universe to compare against, it's only possible to measure speed and the flow of time relatively between objects. For these reasons "time slows down for you when you move at relativistic speeds" is really a bad way to think of it (and not how the math of the theory actually works). It's much more proper to say things in the form "To <observer a> it appeared as if the clock of <observer b> was <sped up|slowed down>". E.g.: To the traveler it appeared as if the clock of Alpha Centauri was sped up. To Alpha Centauri it appeared as if the clock of the traveler was slowed down.
- dumbfoundded 7y ago
  It's pretty confusing but essentially, as you travel faster, your clock slows down arbitrarily. In the most extreme case, at the speed of light, your clock stops. For a photon, it emerges and appears at its final location instantly, even if those distances are millions of lightyears apart. So if you travel to Centauri from Earth, the fastest you get can get there is 5 years as seen from Earth but to the actual spaceship, it could perceive any arbitrarily small change in time.

saagarjha 7y ago

> Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light. Or that, but with a dozen more 9s in it: https://en.wikipedia.org/wiki/Oh-My-God_particle https://en.wikipedia.org/wiki/Oh-My-God_particle

mLuby 7y ago

Shielding at the speed of light is an interesting challenge. The beginning of Alastair Reynolds's short story "Weather" (in "Galactic North") addresses this.