md65536

A nuclear fission reactor.

Naw.

Maybe time for a clue: The meaning of the words danger and stronger, as well as the answer, don't matter. The specific meaning of the words 'stem' and 'correspond' are important.

Suppose a traveller has slowly approached a 3-solar mass non-rotating black hole, & is now hovering near it at 3 times the Schwarzchild radius. He now commits suicide by switching his engines back on so that he begins to slowly drift towards the black hole. How many minutes of his time pass before he hits the singularity? How many kilometres in his reference frame will he have travelled? Or are black holes infinitely many kilometres deep?

http://www.youtube.com/watch?v=3a2HqtH4aiE

There's a clock on the bottom right of the video, but I'm not sure exactly what it means.

According to our poor traveller, he hasn't moved at all, so no distance.

As for the time according to the traveller, or proper time, it wouldn't be very long. I don't understand GR well enough to tell you exactly how long, but it'd be roughly the same as falling into a star of similar mass.

I don't think this is true, because space would appear to warp severely... so I think that the distance to the center of the black hole would change as you fell in???

Some thoughts:

According to the traveler, time (in local space) will tick at a normal rate.

However, with severe enough gravity (or is it gravitational gradient?), "local space" will become smaller and smaller, so that at some point your ship is not entirely in local "flat" space. Eventually your body won't be in local space, meaning that different parts of you will be experiencing time at different rates relative to other parts of you. You'd probably be dead anyway from tidal forces (http://en.wikipedia.org/wiki/Spaghettification). But if you imagine it as a point observer, time would pass at a normal rate.

I don't think anyone knows what would happen inside a black hole especially regarding a theoretical singularity (like whether it can become "infinitely many kilometers deep" when approaching it or whatever) so I think any answer would be speculative.

A lot of people find this surprising but in GR gravity points at the instantaneous position of the source, rather than the retarded, light-delayed position, even though gravity propagates at c.

This certainly adds some complicated details and proves to me yet again that my understanding of relativity is inadequate. I'm not sure now what of my previous posts can be salvaged.

Note that this can only be true when the instantaneous position of the gravitational mass (sun) is known from the delayed light. Otherwise it is a violation of causality. This is just another way of saying:

This extrapolation is exact except when there is a change in acceleration, in which case the extrapolation "misses" and leads to a loss of energy/momentum in the form of gravitational radiation.

In light of these details, I'd like to change my answer:

- Ignore anything I've said.

- Assume no acceleration (at least until the thought experiment makes complete sense with constant velocity)

- Consider the frame of reference where the 2 objects are relatively at rest, which will be the simplest perspective.

- All other frames of reference will observe something that is consistent with that.

Clearly, in the rest frame the only gravitational attraction of the objects would be directly towards each other.

Haha, a guy working at cern just read this and went oh sh**!

I really don't think it'll be something "easy and obvious". This is a "large group" of physicists working for a few years on this. They're not idiots. They didn't spend an hour thinking about it only to say "Well I'm stumped. Maybe the internet will know!"

That does not rule out something simple and obvious. Science isn't about a fixed set of knowledge; everything that is scientifically obvious today was missed consistently up to some point in history.

In some science blogs about the story, I'm seeing a kind of hostility toward the scientists, such as here: http://motls.blogspot.com/2011/09/italian-out-of-tune-superluminal.html

Some people appear to feel that relativity is such a crucial part of their understanding that a challenge of it feels like a personal attack (as I tend to feel about causality), and that any challenging evidence must necessarily be wrong and thus the experimenters must have made a mistake and thus they are incompetent retards.

This is a typical view of science these days, due to politics or lack of understanding or whatever it is... there's this general idea that scientists don't know what they're doing, or that someone "off the street" -- whose knowledge of the experiment or the science behind it might consist of "I read about it in a news story and then looked it up in wikipedia" -- is just as knowledgeable about the science as the experimenters, and likely to think of something "obvious" that the experimenters missed.

It could be. No one person will think of everything. The entire "large group" of scientists could all have overlooked something. It's likely a systematic error; it could be a simple one. I'm not going to assume it though. I think they know what they're doing.

There's some interesting info on the story here: http://scienceblogs.com/startswithabang/2011/09/this_extraordinary_claim_requi.php

(*SBIS; 170 and WAIS 178.) So he intentionally exaggerated the above as a 200 IQ. As a psychologist I can only assume that he was misrepresenting my IQ intentionally to influence readers to believe that I am a liar.

I searched for those values to ascertain their meaning and I came across another user on various forums who has the exact same IQ scores.

This other user has been banned from scienceforums.COM, sciencechatforum.com aka philosophychatforum.com, and possibly the Genius Forums and myspace.

I found this post from a staff member of one of the sites especially interesting:

The fact that you have multiple "stalkers" roaming the Internet, warning the innocent of your checkered past is not at all surprising. I'm even considering the possibility of becoming a "stalker" myself, such as the one who informed us of you. I would certainly have the satisfaction of knowing I was helping to build a better world, opposing the agents of superstition, ignorance and delusion. I would also like to put together a "training" presentation that I can take from website to website, showing the newbies and the innocent how to spot TWATs -- Trolls, Whackos and Thumpers. I have a vision that in a few years, I might help educate the SCILLs (Scientifically Curious, Intelligent and Logically Lucid) to recognize the signature behavior of TWATs, and to band together so that no foothold is allowed for their contagion.

This other user seems to roam from site to site over the years, pissing off a lot of people trying to "enlighten" them with the same crap over and over, wasting a lot of people's time getting into similar closed-minded arguments. He, too, is also a psychotherapist, but only with "a private practice by word of mouth with no advertising and no listing", so I assume no license or doctoral training.

My point is that high IQ is not credentials.

The abstract says, "The measurement is based on high-statistics data taken by OPERA in the years 2009, 2010 and 2011."

My guess is that they didn't measure any individual neutrinos traveling at v > c, but can infer that they did from a distribution of a large number of neutrino measurements.

Unless it turns out to be measurement error, I speculate that this is some kind of "entanglement" effect between the neutrinos.

If so, this would NOT violate SR, for the same reasons that the results of Delayed choice quantum eraser experiments do not violate SR.

In the quantum eraser experiment, results collected after a large number of photon events show an interference pattern, which indicates information having traveled faster than light.

However, the information is only available in the form of the interference pattern, which cannot be determined by any single photon event. The information is only extractable after the fact. There is no useful information that can possibly be transmitted faster than light. Causality is not violated. SR survives.

I'm willing to bet that similar results will be found (because "measurement errors" is boring and I'm an optimist! )...

- The explanation will be "quantum weirdness".

- It'll be proven that there is no way to use it to send information faster than light and thus causality remains intact.

- News stories will explain it like "The scientists say that this is allowed in relativity and Einstein's theory remains on solid ground... for now."

if I was spot on, it was accidental or misinterpreted.

 

I think the double takes were inappropriate and the first takes were probably better.

 

My "Huh?" was not intended to be "that's the way it is. Right?" It was "this makes no sense to me, I am bewildered, and questioning an unbelievable, impossible thing, that does not add up."

 

I wish the double takes were appropriate, and an indication that yes indeed TAR is finally getting it.

You took some intuitive assumptions based on observations, and you logically deduced some conclusions that were not intuitive.

Yes, it felt like you were arguing that this doesn't make immediate intuitive sense... but -- refreshingly for this thread -- you didn't actually say "Therefore it must not be true." This is probably the reason for the double-takes.

Einstein and many others probably went through the same thing, accidentally deducing something puzzling, and then figuring it out instead of denying it. To figure out the puzzle often requires trusting the logical deductions, over preconceived intuitive ideas or beliefs. If the solution to every puzzle was intuitive and something you already knew, you wouldn't learn anything new from it.

Wait, if those like, calabi-yao manifolds are always fixed, what's actually expanding when the fabric of space expands, and why do some scientists suspect the possibility of a "big rip"?

I'm going to take a stab at this to see how well I get this.

I would not consider this post as an answer until it is approved or corrected by someone who knows!

The expansion is equivalent to the larger spacetime intervals between pairs of objects on a Cauchy surface, relative to the spacetime intervals of the same objects on past Cauchy surfaces.

If you imagine a plane in space-time that is the set of all points where t=0, for all clocks that have been synchronized to some arbitrary single clock that has arbitrarily been set to 0 (sorry if I'm unnecessarily obfuscating this!), then you'll be imagining one possible Cauchy surface.

Now if you sweep this plane forward through time*, you'll be sweeping it through the fixed spacetime manifold. The spacetime intervals between expanding regions will be larger on subsequent future Cauchy surfaces that you sweep through.

* My guess is that there are many ways to do this, and some ways but not all will result in new Cauchy surfaces.

Speed of light however tells us something. Not sure what, but if a second is not always a second, and a meter is not always a meter, then it can't tell us much, because it would not be an invariant speed. If a second was shorter than a second and light traveled 300,000,000 meters in that time, then it would be going faster than our invariant speed. Or if light covered the distance between here and a spot 300,000,000 meters away, but it took more or less than a second to make the trip, then again, our invariant speed would be not be invariant.

 

The consequences of relativity seem to be saying to me that light can take half a second to make a 300,000,000 meter trip, as long as the distance it traveled was actually shortened to 150,000,000 meters. Huh?

Jesus! I must have been more than half asleep last night Spot on, Tar

Iggy, I think you were fully awake when you wrote the previous "That's not right."

tar, the second paragraph that you wrote is... fine... but the first paragraph is more confusing than good. Whether a statement like "a meter is not always a meter" is right or wrong depends on how you interpret it, and whether the two "meters" refer to separate things. Wrong: A meter is by definition a meter. Right: One's measure of a meter's distance is not always the same as another's measure of the space. I don't think I can explain what I mean in a helpful way without explaining relativity and I don't think I'd be any good at that. I think that understanding relativity first, and then understanding the meaning of length contraction, is better than trying to grasp relativity by first figuring out the meaning of length contraction.

But this is all "learn SR... trust us it makes sense in the end... mostly" which can only be experienced for yourself.

Philosophically, I think that what this all says is this: If you think about how you know and understand the concept of distance, what is it ultimately based on? Is it the length of a stick or the diameter of the Earth? If so then is it because there are a certain number of molecules lined up side-by-side in those lengths, and each molecule has a certain size and spacing? How do you intuitively know that what you consider the definition of a meter will not vary? I would argue that our intuitive understanding of distance comes from the speed of light. Since anything we've ever observed or measured in history was done with an invariant speed of light, we intuitively have a very consistent perception of distance.

The caveat is that distance must adhere to the rules of light, rather than the number of molecules in a stick etc.

Edit: Okay so I can't read. tar actually your first paragraph is a good example of how treating distance and time inconsistently leads to contradictions, which leads to the correct conclusion in the second paragraph.

Just as the pancake does not actually change its reality, neither does earth. Simply the three dimensional piece that we consider to be 'now'.

All of our interactions are based on a 3d slice of the 4d object, so this is what we perceive at any given time.

This is what I've been saying throughout this whole thread. Where does that leave the "length contracted" earth of 1000 mile diameter, as seen from a near lightspeed fly-by FOR, as posited by Cap 'n R?

I think I understand now. Just as the 3d shape of a pancake doesn't change depending on its 2d representation (which can change as you turn it), the 4d shape of the Earth doesn't change when its 3d shape is affected by length contraction. I didn't realize that's what you were saying throughout the thread.

You missed the point that realism supports the general (non specific) proposition that "the world" in general is real, as it is, regardless of how we observe or measurement.

If you say that some of "the world's" properties are intrinsic, again, "good for you." You are a realist regarding those properties.

If you say some properties of "the world" are not intrinsic, then you are an idealist regarding those properties, because they will depend, for their pseudo-reality on the FOR from which they are measured.

Clear enough?

owl, I dare say you've won this discussion, using a technique that I've found very useful over the last few years called "proof by redefinition".

I still think that there are some contradictions and inconsistencies to work out, but basically I accept that your own definition of realism and your own definition of distance deny the reality (as you define it) of your definition of relativity.

Thanks for the link. I'll have to read more of it.

How do you mean "the two velocities"? If everything is from O's perspective then the velocity of O is zero and the velocity of P is some value [latex]v[/latex].

The two velocities would be P's change in O's measurement of distance divided by O's delta time, and P's change in P's measurement of distance divided by P's delta time.

I guess that's the same as "velocity measured in O's frame" and "velocity measured in P's frame".

I guess that any observer would agree on what P measures...

and trying to express this from O's perspective is an unnecessary complication?

I don't know of any name. I agree it would be mixing frames, and it could have values greater than c so probably is avoided.

Good point. It's not quite a velocity at all. If it has no name I'll just refer to its description; it represents the change in rest distance per unit of relativistic time. But this isn't a robust definition so I'll have to be careful!

Suppose we're considering the velocity of a point P relative to an observer O.

P's velocity can be expressed as a change in the distance to P as measured by O, divided by the change in time as measured by O.

It can also be expressed as a change in the distance measured by P, divided by the change in time of P.

These two velocities are the same value; the speed that O measures P approaching is the same speed that P measures O approaching.

If we're talking about everything from O's perspective, do these 2 velocities have different names? That is "change in locally defined distance over local time" vs "change in remotely defined distance over remotely defined time"?

There is also the idea of dividing O's distance by the change in P's clock. Does this value have a name?

It usually comes up when one mixes frames, or tries to calculate P's velocity in terms of rest distance instead of relativistic distance.

"rest velocity" comes to mind but that term is obviously nonsensical. Does this "invalid velocity" value have a practical application other than in mistakes? I'd like to refer to it as a useful value.

Thanks.

What is true is that neither are in the forward light cone of the other. If you go far enough back in time you may and probably will find a light cone that contains both points. In special relativity you will be able to do this. When you throw in the big bang then a lot depends on the nature of the associated singularity.

Thanks, this helps.

Is it fair to say that causality is simple in SR (any two causally related events are within each other's light cone (one's future and the other's past cone unless the events share a point in spacetime)), but that it's not always that simple with GR?

Well I hate to disagree with the good Doctor, but its my understanding that the post-inflation universe has causally disconnected observable universes, which, because of the finite speed of light, cannot freely pass information to each other.

My feeble understanding of inflation is that just because 2 regions are causally disconnected now doesn't mean they always were.

As with Guth's pre-inflation period that you mention, if regions are close enough together for enough time, they can share information, and still then be separated by inflation faster than c and become causally disconnected.

So I guess that without Guth's inflation, this would not happen. Even if these regions were once a fraction of a meter away from each other in the first conceivable moments after the big bang, inflation separated them early enough and fast enough that light didn't have enough time to cross even those small distances.

This would allow those 2 disconnected regions to "never" (in all well-defined time) have been causally connected with a common causal parent event, and yet still have a common cause in the BB singularity.

No single light cone can contain any two spacelike separated points. This has nothing to do with inflation. So, there are lots of such points -- the tip of your nose and the tip of your right index finger at any single moment in time for instance.

Yes, but I would say that the tip of my nose and the tip of my right index finger at a single moment one second after noon are within the light cone of my belly button at noon.

Am I not using the terms correctly?

I guess what you're saying is that 2 objects might be separated by a time-like interval at one pair of times, and be separated by a space-like interval at another. It doesn't require inflation or even for the objects to move. My fingertip yesterday and my nose today are connected by a time-like interval, but my fingertip now and nose now are connected by a space-like interval. All these events have fixed coordinates on the spacetime manifold. I was in error in trying to reason about spacetime intervals as somethings that move or evolve in space or time.

Events are spacetime points vand are neither timelike nor spacelike.

[...]

This makes no sense. "Locations" would usually be interpreted as spacelike related points (not joinable by any timelike curve) and hence even in the flat case no two "locations would be in any single light cone.

I'll try to fix my wording...

Can inflation cause a time-like interval between 2 events to become space-like? Or does "the spacetime manifold is fixed" mean that nothing can change whether an interval is space-like, time-like, or light-like?

and

If our universe is flat, is it possible to find 2 points in spacetime such that no single lightcone (of any 3rd spacetime point) contains both, given inflation?

Addendum: Can the BB be considered an event, with spacetime coordinates and a light cone? Treating it as a normal event might be the source of my confusion...

Several things I don't get:

1. Can we say that every event in the universe is causally connected to the big bang?

2. Can inflation cause 2 time-like events to become space-like?

In a flat universe, inflation could cause 2 locations to not be in any single light cone*, right? And thus there is no single event that could be causally related to both.

So does that mean that inflation prevents various locations in the universe from having a common causal source (and thus the answer to question 1 would be "no")?

Or does it mean that inflation allows 2 events that are not in any single light cone, to have a common causal source?

I think what I'm asking is "Does inflation destroy causal relationships, or does it preserve them?"

* If spacetime is open, it might be possible to have an infinite inflating region of space contained within an earlier light cone... if I got that right. See http://edge.org/conversation/next-step-infinity

Events happening right now are simultaneous no matter where they are happening or how long it takes any observer anywhere to see them. Simultaneity in the real cosmos does not depend on the FOR from which events are observed.

That's an interesting conjecture! If you can demonstrate that it's true, then relativity will be in trouble and you'll have a base to begin proving the rest of your statements. All it needs is a bit of evidence or logic to show that it's true.

Meanwhile you have several people trying to prove to you that it's logically not true, given an acceptance of a constant speed of light in all inertial reference frames. I suppose that if you can show that universal simultaneity is true, it should suggest a way to punch a hole in their arguments.

It took 18 pages, but I have a good feeling that we're getting close to the start of a productive conversation! Keep it up! Almost there!

Wow! Someone is really mad at me. Another two demerits! (Is there a limit per post?) I wonder who...

One can only vote once on a particular post. So it is not someone; it is someones. I voted, to show that I wasn't one of the original 2 votes. Also I can only make one negative vote per day... I often find myself faced with a "you have reached your quota of negative votes" message.

I don't think that it indicates that people are "mad at you". I myself try to vote + on any message that is helpful, that makes me "get" something or is enlightening or educational. I try to vote down posts that are negatively helpful or damaging, including misleading arguments, misinformation, spiteful insults, etc.

I think the negative votes are mostly indicative that your students here in this thread are not getting your lesson.

I myself think that you're thisclose to disproving relativity, and dismantling and rewriting all of science as well as philosophy and a lot of history as well. Except of course for just a few questions that remain still unanswered. But I also believe that SR is correct.

See, when a student has already decided that the teacher is wrong, the student is not going to learn.

Even with the unusually low student-to-teacher ratio here in this thread, and the amount of attention devoted to teaching the lesson, and the endless repetition of the lesson, it remains endlessly unlearned. But I wonder, is it rewarding to teach with persistence to a problem student, because the lesson can still benefit others? Or is it foolish to try to teach a student who refuses to accept the lesson?

the ether.

or is it aether

it's "either"

Learn to read the diagram. Try this site: http://www.phy.syr.e...ONE/events.html

I agree that it's helpful to learn about the diagrams in order to understand them.

These concepts aren't going to make sense unless one lets go of some preconceptions from classical physics (including assumption of universality of simultaneity).

Also, it's difficult to learn relativity from scratch by looking at diagrams alone.

With a little understanding of the basics of relativity, the diagrams make a lot more sense. Understanding the diagrams makes some of the concepts of relativity easier to understand. Learning a bit from one helps with the other; they go hand in hand. Without one, the other alone can be quite counterintuitive.

Huh? Negative interval squared (imaginary interval) just means more (real) distance than (real) time. At no point did I suggest distance squared would be negative.

Putting an imaginary number in x, y, or z is has no meaning that I can think of without further context.

Oops I got stuck thinking of real spacetime intervals with imaginary distance. But I've long left behind knowing what I'm talking about, so I'll leave this thread until I figure some of it out.

@moderators: could we move this thread to speculations? Although it's fun to talk about this model of FTL, I don't think it has a place in the physics forum.

I think the thread so far is better here; speculations doesn't tend to be treated very seriously.

The discussion seems to have evolved to "Traveling at the speed of light is prohibited by SR, but what implications does relativity have for hypothetical FTL speeds?" I think this is useful because it shows the problems with the concept within SR, which are not there in classical physics, and would be ignored with open speculation.

It'd do all sorts of weird things, like if you were to watch it, you'd only see it after it'd passed you. Then you'd see it in two places (if you could see it...don't know how it would interact with light), moving away from you in opposite directions.

I don't even know how to think about objects that aren't point-like in this context. Depending on how close 1/v was to 0, they'd do some sort of weird length-dilation until they took up the entire universe (in that direction) for a few moments -- as measured by your currrent frame. What you'd see would be more like the previous paragraph.

Okay this next part belongs in speculations! And I'm in way over my head.

I picture imaginary magnitudes as negative distances. Treating yourself as a point observer, these would be distances with a magnitude that projects "into you", not along normal spatial axises, but along imaginary axises.

My intuition says that even a point particle at an imaginary distance, say equivalent to [math]\sqrt{-d^2}[/math], would take up area in your imaginary field of vision, and look like a spherical shell of radius d at a distance of d (I guess you'd intersect its surface). But uh... it's inverted or something. Anyway it would get bigger the farther it is from you, until it took up half the imaginary universe at a distance of infinity.

Yes, my head exploded, and that was the result.

space-like intervals represent real distances, remember it's [math](\Delta s)^2 = (\Delta (ct))^2 - (\Delta x)^2[/math]

A negative interval squared (imaginary interval) merely represents more distance than time.

So this would work if you were considering a world-line (ie. particle which is 1 dimensional at any given time)

You wouldn't be able to use formulae for things like length contraction and time dilation, but these are simply derived from the Lorentz transform.

The Lorentz factor would be imaginary, as would the result of the Lorentz transform, no?

Do you mean that intermediate imaginary numbers are fine, because the space-time interval can still come out to be real, even with imaginary time and spatial coordinates?

Time dilation and length contraction wouldn't apply, and time paradoxes could be constructed, but the math still works out and describes a situation that can be made sense of, and is compatible with SR even if it corresponds to no observed phenomenon?

No. Realism asserts that physical things do not contract except for obvious natural reasons.

Can this be extended to other properties in general, or does it apply only to length?

Would you say that 'Realism asserts that reality consists of only what is obvious'? Or are there "natural reasons" that are not obvious (just none for length)?

 

If gravity "curves spacetime" then the obvious ontological question is, "What is IT that gravity curves." Obviously the trajectories of objects are curved by gravity. What does "spacetime curvature" add to that fact? (Nothing.)

But if gravity "curves the trajectories of objects" then the obvious ontological question is, "What is IT that gravity curves."

I must be mistaken somewhere, because I thought your argument for why 'GR takes "spacetime" and "makes something of it"' is that GR says that spacetime is curved.

My understanding of that is that if something can be curved, it must be an entity.

You are saying that the trajectories of objects are curved.

Does this mean that the trajectories of objects are entities?

Where have I gone wrong?

As far as I know there is nothing in special relativity which prevents objects that move faster than the speed of light, nor should a physicist balk at seeing imaginary numbers (for one, they come up when solving simple things like a mass-spring-damper system) as long as the observables you predict are real.

I think though the observables would also be imaginary. I think you'd get imaginary distances.

But maybe that itself allowed with GR? And objects inside black hole event horizons??? (It's easy to consider real-world examples that are outside the scope of SR simply by considering gravity. GR doesn't violate SR but it handles situations that SR doesn't.)

[...]to carry information back in time, this would break causality

I always thought that a violation of causality was a violation of SR. Certainly you can get paradoxes (http://en.wikipedia.org/wiki/Special_relativity#Causality_and_prohibition_of_motion_faster_than_light)...

Or can you allow violation of causality without violation of SR through interpretations like alternate realities and MWI?

To me it sounds something like "If I can't catch up to the car in front of me no matter how fast I go, then why don't I just pull in front of them and not worry about catching up?"

And now that I think about it, saying "I don't see how that's possible" is not good evidence that it's impossible. However, you probably wouldn't be able to pass a car that you can't catch, by speeding up, nor would you be able to travel faster than light by speeding up. But if say your car were able to "jump" ahead without changing speed, it might be possible.

But I still don't think it's possible.

If you're only trying to understand observed reality or relativity, I think it's safe and beneficial to assume that causality cannot be violated. If you want to change our understanding of reality, perhaps there will come a reason to reevaluate it.

What about skipping the speed of light entirely? What about going at 99.9999% the speed of light, and then instead of reaching 100%, you just jump to 101% without having to go at the speed of light at all?

One, it's impossible. Two it doesn't make sense.

The Lorentz factor would then have an imaginary number. This doesn't correspond to reality. If you can make sense of how your idea would work, you'd have to describe it with something that is not special relativity.

To me it sounds something like "If I can't catch up to the car in front of me no matter how fast I go, then why don't I just pull in front of them and not worry about catching up?"

If the amount of energy it takes to reach c is infinity, what would it take to go even faster?

Also, why aren't photons subject to all this time dilation stuff? Why don't photons contract infinitely?

Because photons aren't observers in a frame of reference.

Length contraction is described in terms of a particular frame or observer.

In the twin paradox example, a traveler approaching c moving from earth to a mirror 1 LY away would see the distance between earth and mirror contract.

The earthbound twin would be at rest with the earth and mirror and not observe any length contraction between the earth and mirror.

Addendum: Photons are like point-particles without any size. I don't know if it's fair to consider them subject to length contraction, but if you did you might say that they're already infinitely length-contracted, according to any observer. But uh... don't quote me on that! This kind of involves imagining photons as ordinary matter, and will probably lead to more false conclusions than useful ones.

Perhaps it is just better to say: Photons are point-particles and so wouldn't be affected by length contraction.