12 ms·

This echoes a thought I had recently -- stars traveling at relativistic speeds should look deceptively young due to time dilation. But while this is certainly

by klyrs 2y ago

This echoes a thought I had recently -- stars traveling at relativistic speeds should look deceptively young due to time dilation. But while this is certainly speedy, mere hundreds of km/s isn't enough to significantly prolong the observed lifetime of a star.

malfist 2y ago
> should look deceptively young due to time dilation That's not how we date stars. We typically date the star by it's metallic content. More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.
- Starlevel004 2y ago
  > More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars. It's actually the exact opposite, but yes.
  - malfist 2y ago
    Yes thank you, got it backwards
- pfdietz 2y ago
  There's some weird caveats, like so-called "Lead Stars". These are aged stars with very large amounts of lead relative to other heavy elements. This happens because lead is the end nuclear of the so-called s-process, where neutrons are captured at a slow rate in large stars whose internal processes produce some neutrons by (alpha,n) reactions. In lead stars, there are so few seed nuclei (those metals) that each of the seed nuclei that are there can capture many more neutrons.
- jddj 2y ago
  Does it matter in this context whether it's metals or tree rings or wrinkles around the eyes? It's all time, right?
  - klyrs 2y ago
    > It's all time, right? Sure, but we can measure time dilation between the peak of a mountain and the base of a mountain, due to the differing velocities. Time is relative!
pdonis 2y ago
> stars traveling at relativistic speeds So far, to my knowledge, we have not observed any. As you note, hundreds of km/s is way too small to have any appreciable effect.
- ChrisGranger 2y ago
  "The fastest star ever seen is moving at 8% the speed of light" https://www.space.com/fastest-star-ever-moves-8-percent-light-speed.html https://www.space.com/fastest-star-ever-moves-8-percent-ligh...
  - AnimalMuppet 2y ago
    0.08c translates into a gamma of 1.0032. That's not enough for relativity to be doing much...
    - bloopernova 2y ago
      How much relativity effects occur at 0.2c? A sci-fi book series I'm reading has warships unable to hit other ships if their relative velocity is above 0.2c. I was wondering if that was realistic.
      - klyrs 2y ago
        I found this to help me not sound completely ignorant in this thread https://www.omnicalculator.com/physics/time-dilation https://www.omnicalculator.com/physics/time-dilation
      - luyu_wu 2y ago
        As the lorentz factor calculated above, not much! Time would only be dilated by that much, and length by the the same amount. It'd definetly be hard to hit something at that speed, but not due to relativity!
      - pdonis 2y ago
        The gamma factor at 0.2c is only 1.02, which isn't much of an effect. The main difficulty with aiming at something moving that fast wouldn't be time dilation, it would be light travel time delay. For example, suppose the other ship is one light-second away from yours and is moving at 0.2c. You're seeing the other ship where it was one second ago, and in that time it has moved 1/5 of a light second. Plus, supposing you're firing a laser at it, the laser will take another second to get to the target, so you have to aim 2/5 of a light second away from where you're seeing the other ship. And during that time the other ship could change direction and spoil your aiming calculation. The sci-fi book series probably is making some assumptions about how fast the ships can change direction, how fast and accurately they can aim, and typical distances between ships during combat to come up with the 0.2c number.
chasil 2y ago
The stars that are going beyond our cosmological horizon are effectively traveling away from us faster than their light towards us. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon "Or, more precisely, there are events that are spatially separated for a certain frame of reference happening simultaneously with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past. "While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us."

by klyrs 2y ago

malfist 2y ago

> should look deceptively young due to time dilation That's not how we date stars. We typically date the star by it's metallic content. More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars.

Starlevel004 2y ago
> More non-hydrogen elements in it's spectrograph, then we know it's an older generation of stars. It's actually the exact opposite, but yes.
- malfist 2y ago
  Yes thank you, got it backwards
pfdietz 2y ago
There's some weird caveats, like so-called "Lead Stars". These are aged stars with very large amounts of lead relative to other heavy elements. This happens because lead is the end nuclear of the so-called s-process, where neutrons are captured at a slow rate in large stars whose internal processes produce some neutrons by (alpha,n) reactions. In lead stars, there are so few seed nuclei (those metals) that each of the seed nuclei that are there can capture many more neutrons.
jddj 2y ago
Does it matter in this context whether it's metals or tree rings or wrinkles around the eyes? It's all time, right?
- klyrs 2y ago
  > It's all time, right? Sure, but we can measure time dilation between the peak of a mountain and the base of a mountain, due to the differing velocities. Time is relative!

pdonis 2y ago

> stars traveling at relativistic speeds So far, to my knowledge, we have not observed any. As you note, hundreds of km/s is way too small to have any appreciable effect.

ChrisGranger 2y ago
"The fastest star ever seen is moving at 8% the speed of light" https://www.space.com/fastest-star-ever-moves-8-percent-light-speed.html https://www.space.com/fastest-star-ever-moves-8-percent-ligh...
- AnimalMuppet 2y ago
  0.08c translates into a gamma of 1.0032. That's not enough for relativity to be doing much...
  - bloopernova 2y ago
    How much relativity effects occur at 0.2c? A sci-fi book series I'm reading has warships unable to hit other ships if their relative velocity is above 0.2c. I was wondering if that was realistic.
    - klyrs 2y ago
      I found this to help me not sound completely ignorant in this thread https://www.omnicalculator.com/physics/time-dilation https://www.omnicalculator.com/physics/time-dilation
    - luyu_wu 2y ago
      As the lorentz factor calculated above, not much! Time would only be dilated by that much, and length by the the same amount. It'd definetly be hard to hit something at that speed, but not due to relativity!
    - pdonis 2y ago
      The gamma factor at 0.2c is only 1.02, which isn't much of an effect. The main difficulty with aiming at something moving that fast wouldn't be time dilation, it would be light travel time delay. For example, suppose the other ship is one light-second away from yours and is moving at 0.2c. You're seeing the other ship where it was one second ago, and in that time it has moved 1/5 of a light second. Plus, supposing you're firing a laser at it, the laser will take another second to get to the target, so you have to aim 2/5 of a light second away from where you're seeing the other ship. And during that time the other ship could change direction and spoil your aiming calculation. The sci-fi book series probably is making some assumptions about how fast the ships can change direction, how fast and accurately they can aim, and typical distances between ships during combat to come up with the 0.2c number.

chasil 2y ago

The stars that are going beyond our cosmological horizon are effectively traveling away from us faster than their light towards us. https://en.wikipedia.org/wiki/Cosmological_horizon https://en.wikipedia.org/wiki/Cosmological_horizon "Or, more precisely, there are events that are spatially separated for a certain frame of reference happening simultaneously with the event occurring right now for which no signal will ever reach us, even though we can observe events that occurred at the same location in space that happened in the distant past. "While we will continue to receive signals from this location in space, even if we wait an infinite amount of time, a signal that left from that location today will never reach us."