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desepticon t1_j272gd7 wrote

Reply to comment by Narwhal_Assassin in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

If you shot a neutrino, or whatever, in one direction and then another neutrino in the opposite direction, wouldn't they be going faster than light with respect to each other?

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Narwhal_Assassin t1_j27jbp5 wrote

Nope! Let’s say each neutrino is going 51% the speed of light, in opposite directions. If neutrino A were to look at neutrino B, it would only see B traveling at about 81% the speed of light. B would see A going the same speed, but in the other direction.

Now, if you’re on the ground watching these particles fly, you would see them move apart with the gap between them growing at 102% the speed of light. However, the individual objects would only move at 51% C, so nothing is violating physics

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Raflesia t1_j27izsh wrote

They would be moving away at the speed of light from any reference point. One neutrino would "see" the other moving away at the speed of light.

If you stood between them they would both be moving away at the speed of light from you. If you shot one away and then accelerated to the speed of light in the other direction then that neutrino would still be moving away at the speed of light.

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acrabb3 t1_j28jcvn wrote

How would the neutrinos perceive each other's (and their own) velocity relative to the start point?
The most cohesive answerer I can think of is that they would see the other neutrino as still at the start point, and therefore everything at the start would appear to be frozen in time.
Which makes sense, since no new information could catch up with them without going faster than light

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Raflesia t1_j29lvb8 wrote

Yup, you got it.

My previous answer is technically incorrect in the sense that "objects" at light speed don't really experience time anymore, but people have a tendency to ignore that bit when trying to explain relativity in hypothetical examples.

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jbp216 t1_j27jy00 wrote

Yeah, with those speeds, your pattern recognition of what you would expect to happen breaks down, there’s a reason relativity is so confusing to most people

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