4 ms·

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 ha

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.

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.