4 ms·

You cannot go faster than the speed of light. But what you can do is imagine what would happen if you reached the speed of light. Length contraction means tha

by db48x 5y ago

You cannot go faster than the speed of light. But what you can do is imagine what would happen if you reached the speed of light. Length contraction means that while you and your ship appear to be the normal size, the universe around you shrinks along the direction of travel. The faster you go, the less distance there is in front and behind you. At the speed of light, the entire width of the universe shrinks to zero. Also, you are at the same time experiencing time dilation. Although time on board your ship advances at the normal rate, time outside the ship appears to slow down. When you reach the speed of light, the rate of time passing outside the ship goes completely to zero. Together these mean that the universe outside your ship effectively vanishes! It occupies no volume, and has no events in it. At the speed of light, your current position and your destination are the _same place_, because there is no distance and no time separating them. This is why you cannot go faster than the speed of light. There aren’t any speeds faster than that. This video takes a round–about route to get there, but it has a nice visualization: https://www.youtube.com/watch?v=HU6t8QvGZmA https://www.youtube.com/watch?v=HU6t8QvGZmA

rland 5y ago
What made it click for me was that the speed of light isn't just the speed of light, but actually the speed of information/causality. So if you're traveling at the speed of causality, everything you "hit" or go past happens instantly, because you're traveling "with" causality/information itself.
trixie_ 5y ago
With a ship going the speed of light you could travel from galaxy to galaxy instantaneously. Zero distance. Though time on the other hand... If you travel to a star 4 light years away and then back, what would be no time for you, everyone on Earth would be 8 years older.
- CydeWeys 5y ago
  It ends up being worse than that once you plug in general relativity. You're only using special relativity, but that doesn't hold when acceleration is involved (as would be required to go from 0 to 0.99999999c).
  - tshaddox 5y ago
    It’s SR pretty much fine for accelerations of fairly small masses (like a spaceship that’s much smaller than, say, a planet)?
  - perl4ever 5y ago
    "It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames, but can still deal with accelerating frames. And accelerating objects can be dealt with without even calling upon accelerating frames. This idea that special relativity cannot handle acceleration or accelerated frames often comes up in the context of the twin paradox, when people claim that it can only be resolved in general relativity because of the acceleration present. Their claim is wrong." https://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html https://math.ucr.edu/home/baez/physics/Relativity/SR/acceler...
    - CydeWeys 5y ago
      Interesting, but the point remains that the answer is different from the 8 years mentioned given that you do have to accelerate up to near-c (at gees the human body can survive in) and then decelerate from it. You can't just instantly go up to near-c and then back down to zero. Whether it's special/general relativity that's necessary for this calculation, it's a materially different result.
deleted 5y ago
[deleted]
femto 5y ago
Is this the foundation for the "holographic" description of the universe? Photons, traveling at the speed of light, effectively don't see one of the dimensions, so can be described using "N-1" dimensions. (Edit: Or does zero rest mass make them a special case?) Can someone also confirm whether the following thinking is valid? The expansion of space means that the edge of the observable universe appears to be receding at the speed of light. As such, it has "N-1" dimensions and that is where the idea of a holographic boundary comes from? The same reasoning applies to the "N-1" dimensions of the event horizon in a black hole?
- db48x 5y ago
  No, that comes out of quantum mechanics rather than relativity.
dragosmocrii 5y ago
Travelling in a ship at 0.95c, what is the danger of hitting anything on the way? Is it possible to dodge anything at all?
- db48x 5y ago
  Probably quite risky. You have little reaction time, every maneuver is 10× more expensive because your effective mass has gone up so much, and even collisions with small objects will release fantastic amounts of energy.
  - rbanffy 5y ago
    You don’t perceive the mass from inside the ship as its engines and attitude control are not relativistic in relation to you. You’ll see the object approaching at a relativistic speed, however, and it will release a very non Newtonian amount of energy if it hits.
    - db48x 5y ago
      You will definitely notice that it takes 10× more propellant and 10× more time to complete a maneuver.
      - rbanffy 5y ago
        I don't think you will - need to do the math though - but my impression is that the distorted time and space from the ship's frame of reference will account for the observed energy expended.
mysterypie 5y ago
> The faster you go, the less distance there is in front and behind you. At the speed of light, the entire width of the universe shrinks to zero. Since you could get to anywhere in the universe almost instantly by travelling sufficiently close to the speed of light, why do many science fiction writers and science fiction movies want to introduce faster than light travel? It doesn’t seem necessary; just tell the readers that the spaceship was travelling at 0.99999c to get to the other size of the galaxy in a week or at 0.99999999c to get there in a minute. (I didn’t do the calculation, but you can imagine what I mean.) I’m going to guess that it’s easier to move the story along by saying “faster than light” than it is to explain the above. Most people would think the author made a goof by saying that something travelled 100,000 light years in less than 100,000 years if travelling at less than the speed of light. (Talking from the traveller’s perspective of course.)
- visarga 5y ago
  From an outside point of view the ship takes 53K years to cross our galaxy, from inside it looks like an instant. Time flows at different speeds.
  - WitCanStain 5y ago
    Star Wars would have been a very different story if the Rebels had arrived at the Death Star 10k years after everyone aboard had died.
    - rbanffy 5y ago
      OTOH, by the time the Death Star arrives at Yavin, the Rebel civilisation will have collapsed a couple times and won’t even remember why they even got there in the first place.
- t8sr 5y ago
  Ursula Le Guin's Hainish Cycle explores how an interstellar society would function if people could travel this way: an instant for the traveler, but decades back home. Despite glossing over all the technobabble that other scifi finds so fascinating, these books, more than any others I've read, show just how great the distances are between stars.

by db48x 5y ago

rland 5y ago

What made it click for me was that the speed of light isn't just the speed of light, but actually the speed of information/causality. So if you're traveling at the speed of causality, everything you "hit" or go past happens instantly, because you're traveling "with" causality/information itself.

trixie_ 5y ago

With a ship going the speed of light you could travel from galaxy to galaxy instantaneously. Zero distance. Though time on the other hand... If you travel to a star 4 light years away and then back, what would be no time for you, everyone on Earth would be 8 years older.

CydeWeys 5y ago
It ends up being worse than that once you plug in general relativity. You're only using special relativity, but that doesn't hold when acceleration is involved (as would be required to go from 0 to 0.99999999c).
- tshaddox 5y ago
  It’s SR pretty much fine for accelerations of fairly small masses (like a spaceship that’s much smaller than, say, a planet)?
- perl4ever 5y ago
  "It's a common misconception that special relativity cannot handle accelerating objects or accelerating reference frames. Sometimes it's claimed that general relativity is required for these situations, the reason being given that special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames, but can still deal with accelerating frames. And accelerating objects can be dealt with without even calling upon accelerating frames. This idea that special relativity cannot handle acceleration or accelerated frames often comes up in the context of the twin paradox, when people claim that it can only be resolved in general relativity because of the acceleration present. Their claim is wrong." https://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html https://math.ucr.edu/home/baez/physics/Relativity/SR/acceler...
  - CydeWeys 5y ago
    Interesting, but the point remains that the answer is different from the 8 years mentioned given that you do have to accelerate up to near-c (at gees the human body can survive in) and then decelerate from it. You can't just instantly go up to near-c and then back down to zero. Whether it's special/general relativity that's necessary for this calculation, it's a materially different result.

deleted 5y ago

[deleted]

femto 5y ago

Is this the foundation for the "holographic" description of the universe? Photons, traveling at the speed of light, effectively don't see one of the dimensions, so can be described using "N-1" dimensions. (Edit: Or does zero rest mass make them a special case?) Can someone also confirm whether the following thinking is valid? The expansion of space means that the edge of the observable universe appears to be receding at the speed of light. As such, it has "N-1" dimensions and that is where the idea of a holographic boundary comes from? The same reasoning applies to the "N-1" dimensions of the event horizon in a black hole?

db48x 5y ago
No, that comes out of quantum mechanics rather than relativity.

dragosmocrii 5y ago

Travelling in a ship at 0.95c, what is the danger of hitting anything on the way? Is it possible to dodge anything at all?

db48x 5y ago
Probably quite risky. You have little reaction time, every maneuver is 10× more expensive because your effective mass has gone up so much, and even collisions with small objects will release fantastic amounts of energy.
- rbanffy 5y ago
  You don’t perceive the mass from inside the ship as its engines and attitude control are not relativistic in relation to you. You’ll see the object approaching at a relativistic speed, however, and it will release a very non Newtonian amount of energy if it hits.
  - db48x 5y ago
    You will definitely notice that it takes 10× more propellant and 10× more time to complete a maneuver.
    - rbanffy 5y ago
      I don't think you will - need to do the math though - but my impression is that the distorted time and space from the ship's frame of reference will account for the observed energy expended.

mysterypie 5y ago

> The faster you go, the less distance there is in front and behind you. At the speed of light, the entire width of the universe shrinks to zero. Since you could get to anywhere in the universe almost instantly by travelling sufficiently close to the speed of light, why do many science fiction writers and science fiction movies want to introduce faster than light travel? It doesn’t seem necessary; just tell the readers that the spaceship was travelling at 0.99999c to get to the other size of the galaxy in a week or at 0.99999999c to get there in a minute. (I didn’t do the calculation, but you can imagine what I mean.) I’m going to guess that it’s easier to move the story along by saying “faster than light” than it is to explain the above. Most people would think the author made a goof by saying that something travelled 100,000 light years in less than 100,000 years if travelling at less than the speed of light. (Talking from the traveller’s perspective of course.)

visarga 5y ago
From an outside point of view the ship takes 53K years to cross our galaxy, from inside it looks like an instant. Time flows at different speeds.
- WitCanStain 5y ago
  Star Wars would have been a very different story if the Rebels had arrived at the Death Star 10k years after everyone aboard had died.
  - rbanffy 5y ago
    OTOH, by the time the Death Star arrives at Yavin, the Rebel civilisation will have collapsed a couple times and won’t even remember why they even got there in the first place.
t8sr 5y ago
Ursula Le Guin's Hainish Cycle explores how an interstellar society would function if people could travel this way: an instant for the traveler, but decades back home. Despite glossing over all the technobabble that other scifi finds so fascinating, these books, more than any others I've read, show just how great the distances are between stars.