New perspective on Time Travel?

dkotschessaa

I blame history/discovery channel for these sorts of misconceptions. In their astrology and cosmology type documentaries they often talk of "seeing into the past" when looking at distant stars, since indeed what we are seeing is something that occurred ages ago, and it is easy to get caught up in the magic of it all, seeing something from so long ago and far away, and convince ourselves that we are actually "seeing into the past."

It's not that the phrase is inaccurate, but that it's not a scientific statement at all, rather a metaphorical or poetic statement about what we are looking at.

But it's merely that a star throws a photon at you and it takes awhile for your eye to catch it. This is no more time travel than, as Zapper said, receiving a letter that was sent which takes you awhile to receive.

When you watch a video on youtube, are you "seeing into the past?"

When you read a novel by Charles Dickens are you "seeing into the past?"

In our imagination it can feel like it. But what we are witnessing is simply the current effect of an even that happened in the past. It's just that this fact is exaggerated by the immense distance of the stars and the immense time that has passed. I think this mystique is not even lost on physicists and astronomers who know very well what is actually happening.

Without this exaggeration the fact that we're not seeing into the past becomes more obvious. If you look at the sun (don't) you're seeing light from 8 minutes ago. This is still pretty remarkable, but perhaps not so much as billions year old light. If you're gazing at the moon you are seeing light from about 1.3 seconds old. If you are looking at a light on a street, a light in your room, your computer monitor or anything at all, you are looking at light that was emitted at some time in the past.

To me the most amazing thing is that we can't see anything BUT old light. Perhaps this is what leads to misconceptions about time travel. But again - it's just that a thing throws a photon at you and it takes some time for your eye to catch it. That's all.

The second mistaken idea was " I mean if we were to create a "machine" to take us instantaneously right at the stars as we perceive them, we would go to a place back in time"

It was answered by Pengwuino:

And Flexgunship:

Which say the same thing, though the first explanation took me a few reads to get what he was saying, as it seemed to imply the future "another X million years." But of course he was referring to the present ("current") light. Sometimes I just read too fast.

I think I've managed to grasp the OPs line of thinking here. "If when we look at stars, we're looking at the past, then by going instantly to the star we will instantly go back to the past."

But I think this should be clear now. "Looking at the past" just means it took your eyes awhile to catch the light that was sent at a later time. Over at the actual star, there is nothing extraordinary happening time-wise. It's just throwing more photons all over the place including in our direction.

By going instantly to the star you'd basically be getting a head start on what we earth-people would see some billion years later. If I send Zapper fan-mail once per day, it takes him 3 days to receive it and he receives one letter per day. If he travels instantly to my location, he may catch me in the midst of writing another letter. Before he can tell me to stop this creepy behavior I have already dropped the letter into a mailbox which zapper cannot get into. After throwing up his arms in frustration, he goes home, and three days later he receives that same letter. At no point did he travel into the past, but it did take him awhile to receive the letter.

Similarly if I travel instantly to a star I will see photons that are being thrown my way, and when I instantly go back home I will see those same photons a few billion years later (when I'm in the ultra-old folks home).

-DaveKA

Pretty Pony

So what ramifications does Quantum Mechanics have on this discussion? Wave-function collapse must happen before our eyes catch the affiliated photons. Since we are always looking into the past, does this mean that we can also change it? For example, imagine the double-slit experiment set up one light-year away from Earth. By observing the interference pattern from this distance, and thus forcing the photons out of a super-position, you are essentially changing the behaviour of photons that existed one full year ago. Now isn't that changing the past?

JaredJames

To be blunt, no.

The photons exist now when we observe them a light year away. They are one year old.

That's like saying the aircraft that travelled from london to new york hasn't aged during the duration of the trip and interfering with it in new york affects it when it was back in london (i detonate a bomb in ny and it explodes on the runway at heathrow).

Dougggggg

Not that I am suggesting it at all possible or anything of the sort, but perhaps some ideas of retrocausality could apply here?

JaredJames

Why?

What makes the OP scenario different to ZapperZ's snail mail one?

dkotschessaa

No, but cool idea. :) Could work in sci-fi!

Again this phrase "looking into the past" is a metaphorical one.

There's a sense in which it's true. According to the table here (we know Wikipedia is completely accurate all the time) the light from an object 1 foot away takes 1 nanosecond to arrive.

So if you're looking at an object 10 feet away, you're seeing what happened 10 ns ago. The closer you get, the closer in real time you get to the object, but since the speed of light is finite, you will never really see the *object* in real time.

However, what you're really seeing is not the object, but the *light* from the object when it hits your eye. That is a current, real time event.* What's happening *now* is that the light is hitting your eyes.

I'm just using a basic thought process here (which I think is pretty logical) and not really thinking about QM. I'm not sure how to describe this in terms of the observer effect. If something is a billion light years away, anything you do to change it is going to take (at least) a billion light years to have any effect, and you won't be able to see the result for another billion years. (Somebody correct me if I'm off here).

-DaveKA


* Yeah, it might take a little bit for your brain to process the light information, and to figure out what the object is, but that's another discussion.

Dougggggg

I was more just trying to play the role of devil's advocate. I think ZapperZ's example was perfect. I just wanted to make sure we at least covered all possible ideas in this concept.

JaredJames

If you change the object a billion light years away (let's say we destroy a star somehow) then it will take a billion years before we see this change reflected in the sky.

However, if we simply 'change the light' as we see it here, a billion light years away from the source, it doesn't affect the source of the light in any way.

Pretty Pony

It was not those photons that I was concerned about; just the ones with the interference pattern on the screen. Perhaps I should use a less ambiguous scenario:
Now imagine a small sampling of Cesium-134 (¹³⁴Cs) floating around in space, one light-year away. With a very powerful telescope, one can change the rate of the radioactive decay by observing it, because of the Quantum Zeno effect. One year ago, this Cesium sample suddenly stops decaying because you are observing it now. Blink, and for about 300-400 milliseconds, it continues to decay. This would be changing the past, no? I'm sure there would be some application of the butterfly effect to this thought experiment, but I don't know a great deal about radioactive decay. Suffice to say that these observations may cause the Cesium to reach its first half-life sometime last September, rather than last July. That, surely, is changing the past? Note that in real time, once you start observing the Cesium, it already reached its first half-life in July. As you observe it, you change this date to sometime in September, thus changing events that have already transpired in the past.

JaredJames

So every time we look up at the stars we extend their life?

I think someone more qualified on the subject should pick things up from here.

apeiron

This is the upshot of delayed choice quantum eraser experiments - probably what you were thinking about here. By looking at ancient star-light, we are indeed collapsing its wave function and so some kind of retrocausality has to be factored into a fuller model of time.

http://en.wikipedia.org/wiki/Wheeler...ice_experiment

The letter analogy works for macroscale classical objects, but QM does say there is more to the story.

Dougggggg

Called it.