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randomnickname99 t1_j27r5e7 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

Let's say I have two guns that shoot bullets at the speed of light. I simultaneously shoot one to my right and one to my left. If I follow correctly, I can look left or right and see a bullet moving away at the speed of light. But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Here's the part I never understood though. Let's say I was standing directly between two walls that were 600,000 km apart. When I shoot the guns I should be able to see each bullet travel for one second before hitting the wall. But from the bullet's perspective that's impossible, because they would have had to travel apart at 2c to do so. How is that reconciled?

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echohack t1_j282aid wrote

In special relativity, simultaneity depends on your reference frame. In one reference frame, event A can occur before event B, but in another, event A can occur after event B. There is no absolute ordering of events that are separated in space time, unless the events are causally connected.

>But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Additionally, the reference frame of the bullet (photon) is not defined. There is no reference frame where a photon is at rest, so you cannot use special relativity to consider the perspective of the "bullet."

If the bullets were traveling at almost c, each bullet would regard the other bullet as traveling at almost c. You may observe the bullets moving away from each other at almost 2c from your reference frame and hitting the walls simultaneously, but the bullets would not. See closing speeds and other examples of apparent superluminal speeds.

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randomnickname99 t1_j29cbvv wrote

Whoa, that's exactly what I was looking for. Thanks! I didn't realize the order of events could change like that.

The space time diagrams in the link also make it a lot easier to follow.

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Yaver_Mbizi t1_j285j4z wrote

I'm pretty sure "relativity of simultaneity" describes what you're talking about.

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rckrusekontrol t1_j287j5h wrote

There’s more thorough explanations here already, but quite simply- one bullet would not perceive the other hitting its wall at the same time as itself. Remember that to see the bullet hit the wall, the light from the event has to travel to your eyes. You are equidistant. If the bullet had eyes, that light has to travel that extra distance- wall to wall. It would hit the wall, and slightly later would see it’s companion hit it’s wall.

A more mind bending “paradox” is the ladder paradox in which a ladder contracts to fit in a barn too small for it. I can’t explain it better than wiki here.

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