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OK, so here's a question that I've never been able to find a clear answer for (that I understood)... Say I'm sitting on the bridge of a spaceship travelling N-

OK, so here's a question that I've never been able to find a clear answer for (that I understood)... Say I'm sitting on the bridge of a spaceship travelling N-times the speed of light. I'm facing forward. What do I see? Am I blinded because photons from far off stars are hitting my eyes at an increased rate? Also, if I look to the left and the right, and behind me? What do I see ? In Star Trek, it's all stripy-stars, and I'm sure that's not correct.

kxrm 5y ago
That's a good question, I am naive to this as well. However my guess is that if you looked forward you would see a dilation effect. Looking backward, you would see the same, just reversed. If you could go faster than light then looking behind you would be darkness (since light can't catch you). However looking forward, could it be that normal light is dilated to such a degree that we change perspective and can see things normally not in our visual range (like Infrared)?
z2210558 5y ago
IIRC the light coming from in front of you gets blue shifted, each photon increasing in energy, and the count increases because of the geometric changes you see in the video (the angles in front seem to shrink). Light from the rear red shifts (each photon has less energy), and there are fewer as some of the (formerly incident) photons "rotated" to the front. (edit: typo) (edit: didn't read the question closely, this is about approaching c, not exceeding it)
simonh 5y ago
Since you can’t travel faster than light in continuous space time there is no way to answer this. At the speed of light relativistic distortions lead to a singularity, so bye bye spacetime. The one even faint possibility we know of, the Alcubier effect, puts you in a bubble of space time and warps that so it propagates at FTL speeds, but within that space time bubble you are stationary. You wouldn’t see anything outside the bubble though as it’s beyond an extreme distortion of space time that light cannot penetrate.
- Maursault 5y ago
  > Since you can’t travel faster than light The Relativity prohibition is that anything with mass can not travel as fast as light because approaching c, mass increases requiring more and more energy while time slows, such traveling at c increases apparent mass to infinity, requires infinite energy, and time slows to a stop. The same thing could be said for anything moving FTL, as it approaches c, mass increases and time slows to zero. Relativity does not prohibit FTL travel, only travel at c.
  - contravariant 5y ago
    The problem is a bit deeper than that, it doesn't just require infinite energy there simply is no isometry of space that can transform something faster than the speed of light into something slower than the speed of light and vice-versa. So really there's no way to map the laws of physics for something faster than light onto those for something moving slower than light. For similar reasons there are no known elementary particles that can maintain their existence in a superluminal trajectory.
    - simonh 5y ago
      Right, so the maths works for superluminal trajectories, but that doesn’t mean it corresponds to anything physical and to get ‘there’ from ‘here’ you need to get past that singularity.
      - contravariant 5y ago
        Well saying the math 'works' is not quite right. If anything the math doesn't work out because there is no way to transform a normal trajectory into a superluminal one. Sure you can just define a line through space-time that is superluminal. That's basically equivalent to just defining a singular moment in time, the same way that an (inertial) trajectory defines a constant position.
  - deleted 5y ago
    [deleted]
kadoban 5y ago
The question likely makes no sense, you can't travel at N-times the speed of light for N >= 1. If we're wrong and you can, we probably have no idea what it'd look like. You could kind of guess by trying to extend our models out to those speeds, but I think you're going to just find random guesses and formulas that no longer make any sense because you've exceeded the range of values they're defined over.
contravariant 5y ago
I'm not too sure this question is well-posed, there simply isn't an isometry of space that would take a trajectory traveling faster than the speed of light and make it inertial. As such we have no description of the laws of physics that someone would experience on such a trajectory. So how things like red/blue-shifting etc. would work out is simply not knowable. Now if you were just wondering what you'd see if you simply changed position really quickly then you can just imagine putting lots of cameras in a long straight line and triggering them in turn to simulate a superluminal speed then you basically would just see the stars move more quickly than possible. You'd also see time progressing backwards on the stars that you are 'moving' away from and more quickly on the stars that you are 'approaching'. So the stripy stars bit is not really that far off.
- rbanffy 5y ago
  > Now if you were just wondering what you'd see if you simply changed position really quickly then you can just imagine putting lots of cameras in a long straight line and triggering them in turn to simulate a superluminal speed then you basically would just see the stars move more quickly than possible. Or just watch Star Trek, as the iconic “moving through stars” happens throughout the series (mostly TOS, as others added different effects)
superposeur 5y ago
Assuming N<1, you see a scrunched up, blue-shifted version of the night sky in the forward direction and very little in the aft direction... this is a combination of the fact that the stars at the various grid points where the stars are now are truly Lorentz-transformed to be more numerous in the forward direction + the usual "aberration" effect accounting for the fact that you are seeing the stars at the retarded time where they were when their light was emitted, not right now. See: https://math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spaceship.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Spacesh...
- WorkLobster 5y ago
  One thing I don't understand about what this page seems to suggest: shouldn't there be a bright ring of starlight at some non-zero angle away from dead ahead? Given a finite collection of objects out to a certain radius (stars), relativistic length contraction will compress it along the direction of travel, so an observer looking out from the centre should see the density increase to a maximum when perpendicular to the contracted direction (in a way that's sort of the opposite of synchrotron radiation ending up tightly directed forward and backward). I guess the aberration described in your link will bend this fore-wards from the perpendicular, but it seems like it should still be visible.
  - mr_toad 5y ago
    Some science fiction describes the phenomena as a starbow.
  - superposeur 5y ago
    If you travel through a localized clump of stars then yes, as you describe, the clump would be pancake-shaped. However, the Lorentz transform of an infinite lattice is another infinite lattice, just one in which the spacing between stars in one of the three directions is smaller. So, in particular, the average density everywhere remains uniform (no matter where you are situated within the lattice). The reason you don’t see an isotropic distribution of light from this uniform density is the distortion due to aberration + the synchrotron effect you mention (which makes the stars in the forward direction brighter). So, in conclusion no “critical angle” but the dots of light appear more densely concentrated toward front and they are brighter, bluer.
EugeneOZ 5y ago
N>1 - you see the past, N<0 - you’ll see the future. It's simple.
- drran 5y ago
  We always see the past. N<0 makes no physical sense.
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).
drran 5y ago
Looking forward, you will see light with wave length contracted or frequency multiplied by factor N+1, e.g. at N=1 front light will have wave length reduced/ frequency increased by 2x. Looking backward or left/right, you will see nothing (darkness), because these photons have no chance to hit you. It will be more like a Pac-Man game: chose a photon, hit it, consume it, then move forward to a next photon.
nwallin 5y ago
You cannot travel at or faster than the speed of light. I know this isn't satisfying, but that's why you've never had a clear answer. As you approach the speed of light, (.9c, .99c, .999c...) two things happen. 1. Everything gets blue shifted. Light that would normally be coming at you in pretty shades of blue, green, and red become shades of ultra violet, then shades of x-rays, then shades of gamma rays. 2. More and more light arrives. As a result of length contraction/time dilation, you might get a century's worth of starlight in a second. Depending on how fast you're going, the light coming in from in front of you might be extremely dangerous. Even a dim, cool Sun-like star might kill you with intense high energy gamma rays. Left and right of you things would appear fairly normal. Behind you, you'd have the opposite effect of what you see in front of you. Everything would be redshifted, and much lower intensity.
- salawat 5y ago
  To ground the intuition with the amount of starlight, imagine all the light in the universe is a bunch of falling rain, and your rocket is barrelling through the entire volume of it at .99c. It's a lotta light.
chadcmulligan 5y ago
Rather than the speed of light, call it the fastest speed any thing can travel. Light travels at that speed.
keithnz 5y ago
you can't go faster. I know a lot of people find this video helpful in conceptualizing things. https://www.youtube.com/watch?v=au0QJYISe4c&list=FLZv-uS789BRpjAEunZn1zNg&index=1&t=229s https://www.youtube.com/watch?v=au0QJYISe4c&list=FLZv-uS789B...

kxrm 5y ago

That's a good question, I am naive to this as well. However my guess is that if you looked forward you would see a dilation effect. Looking backward, you would see the same, just reversed. If you could go faster than light then looking behind you would be darkness (since light can't catch you). However looking forward, could it be that normal light is dilated to such a degree that we change perspective and can see things normally not in our visual range (like Infrared)?

z2210558 5y ago

IIRC the light coming from in front of you gets blue shifted, each photon increasing in energy, and the count increases because of the geometric changes you see in the video (the angles in front seem to shrink). Light from the rear red shifts (each photon has less energy), and there are fewer as some of the (formerly incident) photons "rotated" to the front. (edit: typo) (edit: didn't read the question closely, this is about approaching c, not exceeding it)

simonh 5y ago

Since you can’t travel faster than light in continuous space time there is no way to answer this. At the speed of light relativistic distortions lead to a singularity, so bye bye spacetime. The one even faint possibility we know of, the Alcubier effect, puts you in a bubble of space time and warps that so it propagates at FTL speeds, but within that space time bubble you are stationary. You wouldn’t see anything outside the bubble though as it’s beyond an extreme distortion of space time that light cannot penetrate.

Maursault 5y ago
> Since you can’t travel faster than light The Relativity prohibition is that anything with mass can not travel as fast as light because approaching c, mass increases requiring more and more energy while time slows, such traveling at c increases apparent mass to infinity, requires infinite energy, and time slows to a stop. The same thing could be said for anything moving FTL, as it approaches c, mass increases and time slows to zero. Relativity does not prohibit FTL travel, only travel at c.
- contravariant 5y ago
  The problem is a bit deeper than that, it doesn't just require infinite energy there simply is no isometry of space that can transform something faster than the speed of light into something slower than the speed of light and vice-versa. So really there's no way to map the laws of physics for something faster than light onto those for something moving slower than light. For similar reasons there are no known elementary particles that can maintain their existence in a superluminal trajectory.
  - simonh 5y ago
    Right, so the maths works for superluminal trajectories, but that doesn’t mean it corresponds to anything physical and to get ‘there’ from ‘here’ you need to get past that singularity.
    - contravariant 5y ago
      Well saying the math 'works' is not quite right. If anything the math doesn't work out because there is no way to transform a normal trajectory into a superluminal one. Sure you can just define a line through space-time that is superluminal. That's basically equivalent to just defining a singular moment in time, the same way that an (inertial) trajectory defines a constant position.
- deleted 5y ago
  [deleted]

kadoban 5y ago

The question likely makes no sense, you can't travel at N-times the speed of light for N >= 1. If we're wrong and you can, we probably have no idea what it'd look like. You could kind of guess by trying to extend our models out to those speeds, but I think you're going to just find random guesses and formulas that no longer make any sense because you've exceeded the range of values they're defined over.

contravariant 5y ago

I'm not too sure this question is well-posed, there simply isn't an isometry of space that would take a trajectory traveling faster than the speed of light and make it inertial. As such we have no description of the laws of physics that someone would experience on such a trajectory. So how things like red/blue-shifting etc. would work out is simply not knowable. Now if you were just wondering what you'd see if you simply changed position really quickly then you can just imagine putting lots of cameras in a long straight line and triggering them in turn to simulate a superluminal speed then you basically would just see the stars move more quickly than possible. You'd also see time progressing backwards on the stars that you are 'moving' away from and more quickly on the stars that you are 'approaching'. So the stripy stars bit is not really that far off.

rbanffy 5y ago
> Now if you were just wondering what you'd see if you simply changed position really quickly then you can just imagine putting lots of cameras in a long straight line and triggering them in turn to simulate a superluminal speed then you basically would just see the stars move more quickly than possible. Or just watch Star Trek, as the iconic “moving through stars” happens throughout the series (mostly TOS, as others added different effects)

superposeur 5y ago

Assuming N<1, you see a scrunched up, blue-shifted version of the night sky in the forward direction and very little in the aft direction... this is a combination of the fact that the stars at the various grid points where the stars are now are truly Lorentz-transformed to be more numerous in the forward direction + the usual "aberration" effect accounting for the fact that you are seeing the stars at the retarded time where they were when their light was emitted, not right now. See: https://math.ucr.edu/home/baez/physics/Relativity/SR/Spaceship/spaceship.html https://math.ucr.edu/home/baez/physics/Relativity/SR/Spacesh...

WorkLobster 5y ago
One thing I don't understand about what this page seems to suggest: shouldn't there be a bright ring of starlight at some non-zero angle away from dead ahead? Given a finite collection of objects out to a certain radius (stars), relativistic length contraction will compress it along the direction of travel, so an observer looking out from the centre should see the density increase to a maximum when perpendicular to the contracted direction (in a way that's sort of the opposite of synchrotron radiation ending up tightly directed forward and backward). I guess the aberration described in your link will bend this fore-wards from the perpendicular, but it seems like it should still be visible.
- mr_toad 5y ago
  Some science fiction describes the phenomena as a starbow.
- superposeur 5y ago
  If you travel through a localized clump of stars then yes, as you describe, the clump would be pancake-shaped. However, the Lorentz transform of an infinite lattice is another infinite lattice, just one in which the spacing between stars in one of the three directions is smaller. So, in particular, the average density everywhere remains uniform (no matter where you are situated within the lattice). The reason you don’t see an isotropic distribution of light from this uniform density is the distortion due to aberration + the synchrotron effect you mention (which makes the stars in the forward direction brighter). So, in conclusion no “critical angle” but the dots of light appear more densely concentrated toward front and they are brighter, bluer.

EugeneOZ 5y ago

N>1 - you see the past, N<0 - you’ll see the future. It's simple.

drran 5y ago
We always see the past. N<0 makes no physical sense.

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).

drran 5y ago

Looking forward, you will see light with wave length contracted or frequency multiplied by factor N+1, e.g. at N=1 front light will have wave length reduced/ frequency increased by 2x. Looking backward or left/right, you will see nothing (darkness), because these photons have no chance to hit you. It will be more like a Pac-Man game: chose a photon, hit it, consume it, then move forward to a next photon.

nwallin 5y ago

You cannot travel at or faster than the speed of light. I know this isn't satisfying, but that's why you've never had a clear answer. As you approach the speed of light, (.9c, .99c, .999c...) two things happen. 1. Everything gets blue shifted. Light that would normally be coming at you in pretty shades of blue, green, and red become shades of ultra violet, then shades of x-rays, then shades of gamma rays. 2. More and more light arrives. As a result of length contraction/time dilation, you might get a century's worth of starlight in a second. Depending on how fast you're going, the light coming in from in front of you might be extremely dangerous. Even a dim, cool Sun-like star might kill you with intense high energy gamma rays. Left and right of you things would appear fairly normal. Behind you, you'd have the opposite effect of what you see in front of you. Everything would be redshifted, and much lower intensity.

salawat 5y ago
To ground the intuition with the amount of starlight, imagine all the light in the universe is a bunch of falling rain, and your rocket is barrelling through the entire volume of it at .99c. It's a lotta light.

chadcmulligan 5y ago

Rather than the speed of light, call it the fastest speed any thing can travel. Light travels at that speed.

keithnz 5y ago

you can't go faster. I know a lot of people find this video helpful in conceptualizing things. https://www.youtube.com/watch?v=au0QJYISe4c&list=FLZv-uS789BRpjAEunZn1zNg&index=1&t=229s https://www.youtube.com/watch?v=au0QJYISe4c&list=FLZv-uS789B...