B New Paradox Discovered, I Think

[PeterDonis]

If we are observing the alien on the planet, and it points a flashlight at us and flicks it on. We would see the light turn on wouldn't we?

Yes, after the light has time to travel to us. If the alien is 2 million light years away, then we see the flashlight go on 2 million years after the alien turns it on.

Meaning the time from the flashlight turning on and the photon being emitted,to the time we see that photon, would be zero.

No. This has already been explained to you more than once.

Again I'm speaking purely from our perspective on earth. I understand that the actual time it took the photon to get here is 2 million years.

The 2 million years is from "our perspective on earth". The "perspective" in which we calculate a speed much faster than $c$ for the ship and an infinite speed for the first light ray we see is not "our perspective on earth". It is not an inertial frame in which earth is at rest. It is something different.

what I'm saying is purely from what we are visually able to see through our telescope and how things appear to be.

No, it isn't. When you see the alien's flashlight turn on, purely from that information alone, you don't know how long it took the light you are seeing to reach you. You can't say "the light took zero time to reach me" just from looking at the light. The light is not telling you that. You have to adopt a particular interpretation of what you are seeing, and we have been trying to explain to you why that particular interpretation is not a good one for you to use.

It seems like you're saying that my description of seeing the light move instantly from the flashlight to us here on earth is correct but only for the first ray.

For the interpretation that calculates a "speed" for the ship that is much faster than $c$, yes, because you are using the time elapsed on your clock between the arrival of the first light ray (the one from the launch of the ship) and the ship. So you would have to apply the same reasoning to other light rays that arrive after the first one--which means their speed is not infinite.

Please note, again, that I am not recommending this interpretation. Indeed, I am trying to show you how confusing it is by pointing out implications of it that astronomers who use it don't normally discuss.

I may not be understanding the terminology correctly but isn't a single "light ray" the same thing as a photon?

No. I already explained this.

What would we actually see while observing all of this?

I already explained that too. See post #68. To restate the "what would we actually see" part of that: we see the light ray emitted when the ship launches from Andromeda. Over the next 10.5 minutes we then see all of the light rays emitted by the ship during its journey, in order. At the end of that 10.5 minutes, the ship arrives.

 

[DAH]

I like to read threads like this one and they seem to get a lot of attention from the regulars here as well. I think the OP is getting confused with the difference between seeing events and observing events. Seeing an event is when the light from that event reaches your eye or telescope in this case, however in special relativity its usually more about observing events in spacetime, and you assign coordinates to when those events actually happened. So in this case, when you see the launch of the ship, the time coordinate would be T minus 2 million years. Then 10 minutes later when the ship arrives on Earth, the time is T = 10 min, so the total travel time as observed from Earth is just over 2 million years.

The seeing events is a totally different thing where you would have to account for a huge Doppler factor and apparent superluminal speeds many times the speed of light.

 

[Somoth Ergai]

I like to read threads like this one and they seem to get a lot of attention from the regulars here as well. I think the OP is getting confused with the difference between seeing events and observing events.

Yes, after the light has time to travel to us. If the alien is 2 million light years away, then we see the flashlight go on 2 million years after the alien turns it on.

This is the crux. What do you mean by observe? For a scientific observation, we make our very best estimation (measurement) of an event. It does not necessarilly correspond to what we see with our eyeballs.

I "see" what you all mean now. and I have in fact been way too sloppy with my terminology. It's even worse than that actually because I know what "observe" means in the context of making measurements. So I should have done better and been more careful with the words I was using. I do apologize for that and I'll try to be better about how I use terms in the future.

In nearly every case where I have used the word "observe" I meant it the way laymen mean it. Probably because I am one myself. as in, if i set up a video camera behind the viewing screen of our telescope and recorded these events and watched it back, what would it look like?

I think a great deal of confusion from both sides of this discussion, admittedly my fault, has been from thinking I meant something different than I did. I believe I do understand the primary errors in my initial post with my failing to understand how the doppler effect would work with this hypothetical recording of the event.

What I have been repeatedly trying and failing to understand now, I think due to my improper use of terms, is indeed what we would see on this video and why.

I already explained that too. See post #68. To restate the "what would we actually see" part of that: we see the light ray emitted when the ship launches from Andromeda. Over the next 10.5 minutes we then see all of the light rays emitted by the ship during its journey, in order. At the end of that 10.5 minutes, the ship arrives.

I appreciate your repeated attempts to explain this and the patience it must take to keep trying still. and I'm sorry I I'm having such a hard time understanding. But, to me, even this explanation is still not answering the primary question I have been trying to get a clear answer to. You said

"Over the next 10.5 minutes we then see all of the light rays emitted by the ship during its journey, in order. At the end of that 10.5 minutes, the ship arrives."

the implication I get from that, is that if we were watching the above described video that 1) we know how far away the planet is (2 million light years). 2) we know that anything traveling at c from that planet to earth will take 2 million years to get from the point of origin to us. and that 3) if we are watching a ship make that same trip in 10 minutes it must "look" like it moved faster than c.

if all of the light rays emitted by the ship (2 million years worth of light information) arrive, one after the other, in the span of 10.5 minutes, then I simply can't understand how it could be the case that the image we see of the trip doesn't appear to be hyper sped up far far beyond c.

Again, I understand that it is not the case that the ship did not, in actual fact, move faster than light at any point along its journey. It is simply due to the doppler effect bunching up all of the light rays so that in effect they arrive all at once which leads to an image that appears to be in fast forward.

the side point I have been trying to grasp, and as others have pointed out appears to be a tautology. Is this. suppose on our hypothetical recording we watch any other normal object, traveling at say, 1000 Mph in a straight line from the alien planet to earth. we are able to watch the object the entire time along its journey. and we will be able to say that the total time of travel (on the recording) the object took X amount of time to get from that point of origin to earth.

Suppose the object is traveling much faster. Say, .99999999999 c. fast enough for the Doppler effect to be significant. The object will still take a certain amount of time to get from point A to Point B. but because of the huge doppler effect the time that it takes will be vastly shorter compared to how fast the physical ship was actually moving.

Meaning that while the "actual" trip took 2 million years and 10.5 minutes. the trip on our recording will only take that 10.5 minutes and thus appear super sped up. giving the appearance of the ship crossing the distance faster than c.

It seems to me that the faster and faster (or more precisely the closer and closer to c) an object moves through space the shorter and shorter it's travel time appears to be on our recording.

Suppose now that the object is a pulse of light coming from a flashlight on this planet 2 million light years away. being that the object is now traveling at c the apparent travel time on our recording should now be zero.

I have tried my very best to avoid using terms in ways I don't mean. I sincerely apologize if i'm still being unclear in what answers i'm actually trying to get. And I appreciate the continued patience everyone has trying to hammer knowledge into me. It's like trying to tattoo a diamond I know. I'm sorry.

 

[PeterDonis]

"Over the next 10.5 minutes we then see all of the light rays emitted by the ship during its journey, in order. At the end of that 10.5 minutes, the ship arrives."

the implication I get from that, is that if we were watching the above described video that 1) we know how far away the planet is (2 million light years). 2) we know that anything traveling at c from that planet to earth will take 2 million years to get from the point of origin to us. and that 3) if we are watching a ship make that same trip in 10 minutes it must "look" like it moved faster than c.

I have already said, multiple times, that this is an interpretation of what you are seeing that you can choose to adopt. You just have to be willing to accept what comes with it.

if all of the light rays emitted by the ship (2 million years worth of light information) arrive, one after the other, in the span of 10.5 minutes, then I simply can't understand how it could be the case that the image we see of the trip doesn't appear to be hyper sped up far far beyond c.

Sure, because you are interpreting "2 million light-years divided by 10.5 minutes" as the apparent speed of the ship. But the "video" you are seeing only gives you the 10.5 minutes (since that's how long it takes you to watch it). It doesn't give you the 2 million light years. You got that from somewhere else, and decided to use it to interpret what you are seeing in the video. You can choose to do that, but nothing forces you to. And, as above, if you do choose to do it, you have to accept what comes with it.

It seems to me that the faster and faster (or more precisely the closer and closer to c) an object moves through space the shorter and shorter it's travel time appears to be on our recording.

Yes.

Suppose now that the object is a pulse of light coming from a flashlight on this planet 2 million light years away. being that the object is now traveling at c the apparent travel time on our recording should now be zero.

Yes.

But note that in the last two statements I quoted, you only made statements about the "apparent travel time", i.e., the time in between the first light ray arriving at your location, and the object itself (which in your second example above is the first light ray) arriving at your location. And those statements are direct observations involving the light you are seeing; you can directly read off the elapsed time on your clock during the "video" (in your second example above that time is zero).

You said nothing at all about "apparent speed" in the above quotes. And you don't have to. Nothing forces you to take the observations you describe and conclude anything about "apparent speed".

If you do want to conclude anything about apparent speed, then, as I have already said, you need to bring in other information that is not directly observable in the light you are seeing, information which gives you a distance, which you then divide by your "apparent time" to obtain an "apparent speed". And you have chosen to do that in a way which makes the object in your first example above have an "apparent speed" much faster than $c$, and the light ray in your second example above have an infinite "apparent speed". And if all that seems ok to you, then sure, go ahead and interpret things that way.

But if it doesn't seem ok to you (and why else would you be belaboring this?), then the only solution to that problem is to stop interpreting things that way. In other words, stop insisting on dividing a distance you obtained some other way by the time it takes you to watch a video of a trip, and calling that the "apparent speed". This isn't something you're going to figure out by asking questions or looking at equations. It's a choice you have to decide to make if you don't like the implications of the alternative.

 

[Tom.G]

Asking Google to define apparent:

ap·par·ent​

/əˈperənt/

adjective
clearly visible or understood; obvious.
"it became apparent that he was talented"

Going back to the flashlight on the distant planet scenario. Two flashes are seen here 10.5 minutes apart.

With no other information, how is the choice made between:
1) the flashlight was turned On and Off two times 10.5 minutes apart
2) the flashlight was turned On and Off once at the distant planet,

then, traveling close to the speed of light, was turned On and Off at 1000 miles from the same observer​

Asking Google to define apparent:

ap·par·ent​

/əˈperənt/

adjective
clearly visible or understood; obvious.
"it became apparent that he was talented"

Much of the discussion about apparent sure looks like differing interpretations of the word between the popular 'common usage' and a different specific interpretation in the astrophysics community. Is it worth anyones time to clearly explain that difference, if any?

Cheers,
Tom

 

[Dale]

I simply can't understand how it could be the case that the image we see of the trip doesn't appear to be hyper sped up far far beyond c.

It does “appear” to be hyper sped up, far far beyond $c$.

It seems to me that the faster and faster (or more precisely the closer and closer to c) an object moves through space the shorter and shorter it's travel time appears to be on our recording.

Yes. (For objects moving towards us)

There is nothing in physics that constrains it to “appear” to travel at less than $c$.

 

[PeroK]

The miracle of the Doppler effect!

 

[Orodruin]

The miracle of the Doppler effect!

... which is also not a necessarily something related to special relativity. Sound exhibits this behaviour too!

... as mentioned many times over ...

 

[PeroK]

... which is also not a necessarily something related to special relativity. Sound exhibits this behaviour too!

And bricks! Before I started university I got a summer job at a brick works. One of the jobs was to stack the "green" bricks onto a pallet to be fired. The bricks would come along a conveyor belt about 3m long. You would walk towards the machine, so the bricks came quicker, then walk back to the far end of the belt and rest your arms for a few seconds before repeating. It certainly seemed less monotonous that way.