Nope, still c.
4 Likes
Witch! (plus two more characters)
1 Like
Are you sure? I stand with my arms stretched out, a flashlight in each hand, the flashlights pointing in opposite directions, and turn them on simultaneously. From my perspective, the photons of one beam are moving away from the photons in the other at 2c, surely?
Physics gets really odd when things are travelling at speeds close to c.
3 Likes
All I know is nothing can go faster than C, just infinitely closer to it.
I appreciate that, and I understand that Newtonian addition of velocities doesn’t apply with speeds a significant proportion of the speed of light.
Nevertheless: I have two flashlights, or two lasers if you prefer, pointing in opposite directions, each at a different humungous space mirror magically parked one-light second away. I fire both lasers simultaneously.
- When do I see the flashes in the mirrors (using my twinset of superduper telescopes)?
- Do I get a different answer if I only fire one laser?
1 Like
In this case, you will see the flashes after a certain time, but if someone was travelling out and back along with the photons, they would disagree with you about how much time elapsed since they set out, and they may also disagree that the two light flashes got back at the same time.
1 Like
Would you not have to all arrive at the same place at the same time to argue about it, thus obviating any disagreements?
Absolutely, no disagreement about that. But the post I was responding to posited that (if I understood correctly) that:
- if you had something going at c in one direction
- and something else going at c in the opposite direction
- each of them would record their speed of separation as c (not 2c)
- as would any third-party observer.
I’m fine with #3. I’m dubious about #4 .
2 Likes
I think it boils down to how you are defining “the speed of separation” here. Each photon will be measured as travelling away from the observer at c, regardless of the position or velocity of the observer.
1 Like
I’m defining it as the rate at which the distance between the two increases.
This is how I best read it explained (though apologies to any actual physicists if I mangle the details):
Alice likes to sit by the railway watching trains go by. Bob likes to ride on the roof of trains, always sitting exactly in the middle of the car’s roof.
One day, Bob’s train goes past while Alice is watching. They high-five each other as they pass (presumably Alice is on a gantry or something). A split-second later, Alice sees lightning strike each end of Bob’s car simultaneously. Thanks to her preternatural sense of time, she is able to calculate that this means that both strikes occurred at the exact instant she and Bob high-fived. She also knows that Bob would have seen the flash of the strike at the front of the car before that at the back, because by the time she saw the flashes, Bob would have moved slightly in the train’s direction of travel: the front flash would pass Bob on its way to Alice, while the rear flash would pass Alice on its way to Bob.
Now, let’s look at it from Bob’s point of view. Under relativity, it’s just as valid to say that Bob and his train are stationary and Alice and the station are shooting backwards as it is to say that Bob/the train are moving and Alice is stationary. We know that Bob sees the front flash first. As the front and the rear of the car are exactly the same distance apart, and as the speed of light is always the same, this means that from Bob’s perspective, lightning strikes the front of the car first.
Alice and Bob can meet up later and compare notes, but they are never going to be able to decide who’s “really” right about whether the strikes were simultaneous or not. Both can prove their versions of events. Both are right. Simultaneity and order of events are relative.
4 Likes
Nope, that’s not how Special Relativity works. You observe the photons leaving you at the speed of light. So does an observer on the ground. Spatial axes go all wonky, among other effects, when you get near to lightspeed.
3 Likes
To everyone learning about this for the first time, and feeling like their brain is breaking: Hello and welcome to physics!
Tomorrow we’ll talk about quantum physics, which makes all this look downright sensible. 
3 Likes
Recall that the observer was already going c. So yeah, but the photons and the observer would both be going c in the same direction.
(It’s not possible the physical observer is going c. As I pointed out, I glossed over that. Sometimes it’s worthwhile to use a hypothetical like that to better understand the principles. Although arguably I should have just said, “Impossible question.”)
All of this got me wondering whether communication through quantum entanglement could break the speed of light (like it does in sci-fi books). Apparently…not. 
Good article: https://www.forbes.com/sites/chadorzel/2016/05/04/the-real-reasons-quantum-entanglement-doesnt-allow-faster-than-light-communication
This topic was automatically closed after 5 days. New replies are no longer allowed.
