Speed of Light question

Factual Questions
1

I think I have this right, but I feel like I have this wrong, and I’d sure appreciate some help in finding out which it is.

As I understand it, if you shoot a beam of light from the Earth to the Moon such that it’ll reflect back to the Earth, the round trip takes a little over two-and-a-half seconds: schoolkids can do the math on how long it should take to get there and bounce back, and could even run the experiment and time it.

Scenario #1: you and I visit a crackpot inventor who’s tinkering with something in his backyard. He explains that it’s set up to bounce light off the Moon and back in the expected two seconds and change; he demonstrates it for us; it does. He then fiddles with the settings and says he can send something that’ll reach the Moon and bounce back faster than that; he demonstrates; it takes two seconds flat. He then announces that he invited us here today because he thinks he can finally break the one-second mark; he fiddles with it a bit more, and then apparently sends a beam that bounces back in maybe half a second.

“Wow,” I say. “Do you think you’ll ever get below zero?”

“Uh, no,” he says. “What a stupid question.”

“But what you’ve done is faster than light,” I say.

“Yeah, but I can’t do faster than instantaneous,” he says.

Scenario #2: you and I visit a crackpot inventor; he does the two-seconds-and-change setup; he tells us he can make some quick adjustments and bounce something off the Moon and back (a) in two seconds flat, or, if he makes some further adjustments, (b) in negative two seconds flat: faster than instantaneous, two seconds before he sends it.

My question: given our current understanding of physics, is Scenario #1 believed to be exactly as impossible as Scenario #2? Are both hypotheticals, at present, ruled out for the same reason?

2

If Special Relativity is correct, then both situations are equally impossible, and SR is, in some sense, the most thoroughly-tested theory in all of science, and has passed all of its tests. In fact, when people say that FTL is impossible based on what we know of physics, the reasoning is actually “According to SR, FTL is exactly equivalent to reverse causality, and of course reverse causality is impossible, and therefore so is FTL”.

However, it is always possible that SR is incorrect in some subtle and difficult-to-probe way, in which case those two scenarios might or might not be equivalent.

3

Thanks! Yeah, that was my understanding; I’d pretty much heard the first part, and heard the second part — and thought it made sense that both follow from the same explanation — but I don’t recall having ever heard (a) both scenarios discussed in the same breath, let alone hearing it explicitly get spelled out (b) that, at present, either would be just as big a conceptual hurdle as the other, and for the same reason.

4

never mind

5

Well, I learned some years ago about retrocausality and the weak measurement experiments that can show changes in measurements at t1 due to pre- and post-selection and measurements at t0 and t2 (t0 is before, and t2 is after t1). So perhaps the crackpot has developed an experimental method for the (presumably entangled) photons he is bouncing off the moon?

6

To go all Star Trek, what if the scientist is using tachyons. Would #1 be possible but #2 not?

7

I had a physics instructor who answered all questions of this nature politely saying, “Nothing can travel faster than light. Therefor I refuse to entertain the question”.

8

One way of expressing the equivalence is that if either situation occurred, aome observer travelling slower than the speed of light would see that situation as the other one

9

“Oh, would some Power the gift give us
To see ourselves as others see us!”

10

“I saw that coming before you said it.”
:wink:

11

Not sure that’s a good idea. Plenty of self-hate in the world already. :wink:

12

The most mind warping part of relativity is that there’s no such thing as simultaneity. If two events don’t happen at the same place at the same time, the time they happen will be different times for different observers (a side effect of the way time and distance change for different relative velocities)

13

Actually it would take about four seconds, because it’s two light-seconds going in one direction.

ETA: okay just checked—1.3 seconds from earth to moon, so about 2.6 seconds round trip. Nothing to see here. And for what it’s worth, ye canna change the laws of physics, laddie. Neither scenario is doable, but ESPECIALLY the second one. IMHO.

14

Use of Thiotimoline has been suggested as a way of creating a cascading telechronic battery that could create causality violations of even days in length. The applications are clearly boundless.

15

Did you have Dr. Morrow for physics 1300 at UALR?

16

The elementary school I went to was OLR. Close enough?

17

Wow, small world.

18

But, see, what you mention in passing there at the end is the part that interests me: that, in terms of being doable, one scenario is ESPECIALLY problematic — in all caps, as you’d just put it. Which, intuitively, seems right: that reducing the time by, oh, roughly half a second, or by two seconds, well, uh, that’s not doable, you’d say with a shrug and no capitalization — but reducing it by three or four seconds, you hasten to add, is ESPECIALLY not doable.

And my question is: is it? Is that second scenario ESPECIALLY not doable, compared to the first? Or is it, as Chronos said, that, no, “both situations are equally impossible” according to “the most thoroughly-tested theory in all of science,” even?

That it is, in effect, the same proposition in each scenario?

19

Both scenarios are equally impossible because they are essentially the same. If a signal could be sent faster than light, some observers (although not the sender) would see the signal received before it was sent.

20

I think it’s a little like watching a hockey game live or the same hockey game on two second delay. It’s just the transmission/reception part that affects the timing. Game is still the same. But to have an echo of an event that hasn’t happened yet? I see that as a bigger stretch than just improving transmission to NOW. When we look at the sun, we’re always looking 8 light-minutes into the past. Always have been, but we don’t think about it. When we see that particular solar flare, it actually happened 8 minutes ago. But it’s always been that way so we don’t think about it unless we make a point of thinking about it. Kind of like watching hockey delayed by 8 minutes. But an event that hasn’t happened yet? Well, with solar flares, one looks pretty much the same as any other in my experience, so I probably wouldn’t notice. But in terms of a particular one-time event, I’m not sure how you create an echo, unless time can work backwards. Ever see Tenet?