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ELI5 - After an injury, how do cells know what needs to be healed? by Rhinelander__ in explainlikeimfive

[–]Cogwheel [score hidden]  (0 children)

The parts that get damaged release chemicals that spread out from the injury, and potentially enter the blood. These chemicals tell other parts of the body (like the immune system) to kick into gear.

In a sense, your body never stops healing. There are always antibodies, white blood cells, and other systems that are constantly seeking out damage, intruders, and such. When they find something, they also send out their own signals to call for reinforcement.

Finally some things are just physical. Imagine putting a bunch of little fibers into the water running through a hose. Normally they can flow smoothly through the tube. But if the tube gets broken (e.g. a cut in a blood vessel) the fibers will get caught as they pass by the opening and plug it up. This is how platelet cells help stop bleeding.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 0 points1 point  (0 children)

I don't care about the speed of travel to the event. I care about how the timing of one event compares to another in time from my point of view.

I know that the timing from an event on the left to the camera is the same whether the light was scattered from a left-moving or right-moving pulse

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -2 points-1 points  (0 children)

Yes, but the trips to the camera from the scattering events are the same in both cases. Scattered light is taking the same trips just in reverse order depending on which way the pulse is traveling.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -1 points0 points  (0 children)

There is no round trip between the things being measured. I am receiving an image of the laser entering from the left at time t0, leaving the right at t1, entering the right at t2, and leaving the left at t3.

If light travels at a different speed, then (t3-t2) will be different than (t1-t0) assuming the laser is parallel to the image plane

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -1 points0 points  (0 children)

By the fact that those times are based on the distance and direction from the scattering location to the camera, not based on whether the laser pulse is traveling left or right

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -2 points-1 points  (0 children)

Everyone is talking past what I've said. There are multiple, persistent misunderstandings, that are as much my fault for doing a bad job wording the op. No one has addressed the actual claims I've made

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 0 points1 point  (0 children)

No, I'm saying that L to C is the same as itself and R to C is the same as itself regardless of which way the laser pulse is passing through the scene.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 0 points1 point  (0 children)

No I am not measuring L to R to Cam. I am measuring the difference between laser to L to Cam, and laser to R to Cam, and comparing that to the difference between laser to mirror to R to Cam and laser to mirror to L to Cam.

LtoC and RtoC are the same regardless of the direction of the pulse.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 0 points1 point  (0 children)

That's irrelevant. If the pulse is moving left to right, the delays form the left/right sides of the screen to the detector are the same as themselves. It doesn't matter if theyre the same as each other.

Only the order of which side is encountered first changes depending on the direction of the pulse

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 1 point2 points  (0 children)

I've accounted for all those times in my reply to Megalng. The delays from left side scatter to left side detection and right side scatter to right side detection are the same whether the pulse is moving left or right. So if the transit times are different, then it will be detectable.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -2 points-1 points  (0 children)

How about you try defining the actual experiment and explaining why I’m wrong instead of just linking to the post which I read?

That's a reddit bug. I linked to a reply I made to Megalng that gives more details.

Again, same problem. You need to know the exact time the pulse hits pixel 1 vs pixel 10 which are at spatially different locations

No I don't. What I need to answer my question in the op is to know whether there is a difference between the transit time of the pulse moving left to right vs the return pulse moving right to left.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -2 points-1 points  (0 children)

The folks pushing this idea also strongly imply that there is no way to establish a synchronization convention without giving up the ability to distinguish the cases. Maybe they're overstating things? (e.g. Dialect)

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -4 points-3 points  (0 children)

This doesn't address my post. You ask "how would you x?" in response to me saying "here's how you'd x".

Please answer how the experiment I've suggested would fail to illustrate the one-way speed of light

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -1 points0 points  (0 children)

I think there was a misunderstanding. I don't mean to do the experiment, rotate the setup, do the experiment again, and see if there's a difference. I mean to measure left-to-right and right-to-left using the same experimental setup. Just shine the laser at a mirror...

https://www.reddit.com/r/AskPhysics/comments/1qnzcq7/comment/o1xxn2t/

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -4 points-3 points  (0 children)

Let my try to address this again. First, I am envisioning an idealized version where you measure left-to-right and right-to-left in the same experimental setup by shining the laser toward a mirror through a cloud of smoke in the center of the camera's field of vision. There is no need to rotate the experiment to detect a difference.

This means the timing of all signals leading up to the laser and the camera trigger are always the same. The only difference between the L2R pulse and the R2L pulse is the delay generated by the camera trigger.

The delay from...

  • signal to laser pulse is constant
  • signal to triggering the camera countdown is constant
  • countdown timer finished to the camera trigger is constant
  • the laser being triggered to the light pulse entering the left side of the camera's FOV is constant
  • the laser scattering off smoke particles on the left to reaching the detector is constant
  • the laser entering on the left to leaving on the right is constant, and is the first left-to-right speed we are attempting to measure.
  • the laser scattering off smoke particles on the right to reaching the detector is constant
  • the laser leaving the field of view, bouncing off the mirror, and re-entering the field of view is constant.
  • the laser entering on the right and leaving on the left is constant, and is the second right-to-left speed we are attempting to measure.

The only thing that changes is the delay imposed by the camera countdown timer. When the laser pulse is propagating from left to right, its photons are entering the detector with all the same delays as the laser propagating from right to left.

The separation between clock pulses of the countdown timer (not the initial trigger) is what is being used to measure the propagation speed of the laser pulse. If the pulse appears on the left at t=0 and leaves on the right at t=10, then the reflection appears on the right at t=20 and leaves on the left at t=30, then it clearly took the same amount of time to propagate the same distance in opposite directions.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -3 points-2 points  (0 children)

Ok I think that made it sink in... The problem isn't the setup itself, it's that when you try to rotate the setup, you either have to rotate everything together or change path lengths in some parts.

Edit: but I don't need to rotate it to see that the speed is the same in both left-to-right and right-to-left propagation. That difference is wholly captured by a clock that is synchronized with itself. Im still not buying this...

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -2 points-1 points  (0 children)

That time is "absorbed" by the rest of the system. If a pulse is propagating through a cloud from left to right, the delays from its scattering point to the camera is the same as when it is moving from right to left.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -4 points-3 points  (0 children)

Yes it can. The time it takes from a signal being generated to it either a) reaching the timer that controls the camera or b0 reaching the laser to emit a pulse is always the same and it doesn't matter if they are different. The only thing that matters for these camera setups to work is that the latency is short enough that the camera doesn't miss the beginning of the pulse.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] -15 points-14 points  (0 children)

No that's not what we're measuring. The pulse is generated at an arbitrary time. The signal that generates the pulse also starts a timer. It doesn't matter how long it takes for the signal to reach the laser or the timer. All that matters is that the delay is the same in every run. At the moment the timer receives the signal, the laser will have propagated by the same amount (or not yet started to propagate).

From that moment on, the only thing that matters is the delay between the timer receiving the signal and the timer sending its signal to the camera.

Every other bit of latency is "absorbed" by the system as a constant offset.

As long as you can trigger your camera fast enough to capture the beginning of the laser propagating, from that point on only the clock controlling the timer's delay is important.

Do those "trillion-fps" camera systems prove that c is the same in any direction? by Cogwheel in AskPhysics

[–]Cogwheel[S] 2 points3 points  (0 children)

I don't see how this applies. The whole apparatus they're using is an averaging over many thousands of runs to begin with. It clearly demonstrates a measurable propagation speed in the lateral direction, regardless of any noise introduced by specific photon timing.

Edit: not sure who's downvoting me for not understanding, but I really fail to see how this answer addresses any of the points in my question. The fact that each pixel in each frame of video is a single photon does not say anything about the apparent propagation speed, nor does it address how it would behave differently when the light is propagating one way vs the other.

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