# Inertial frame of reference of light speed (And beyond)

• Eric B
In summary: It's like parallel universes all collapsing into each other as you approach the speed of light.In summary, at the speed of light, the entire universe is flattened down to zero, and you see yourself passing through a "flatland" having only height and width, and no depth.
Eric B
Hi All! I found discussions on this here while researching the subject to see if anyone else has attempted to explore the subject. I saw that several people asked questions pertaining to this, but the question was never really answered. It seemed unanswerable.

While many discussions of relativity will discuss the increasing disparity between inertial frames of reference as one approaches the speed of light, one thing I've never seen anywhere, is what it is like AT the speed of light.

Since you cannot reach the speed of light through acceleration, we will have to imagine a hypothetical and basically supernatural situation where people could be "beamed" whole at the speed of light. (sort of like we imagine that matter could move faster than light; but only if it always moves faster than light. i.e "tachyons").

Extending it to the hypothetical limit of what happens as you approach the speed of light, the entire universe, in the direction you are traveling, flattens down to ZERO. People often describing it as extending in that dimension to be "everywhere at once". That is true only in a limited sense. It's not you who are extending, it is that dimension of space that shrinks. You're still in one point at a time, but the points have all compacted together.
So the traveller basically sees himself passing through a "Flatland" having only height and width, and no depth. Its time is also frozen in one point (The point in which he was "emitted").

Really, because all three of his proper space dimensions are unchanged to him, and volume is h, w and d multiplied together, then the universe will be h × w × 0, which reduces the entire volume of the universe to zero. The universe basically doesn't exist from that frame of reference!
Even though a photon can be deflected, changing direction, in our coordinate time; in its proper time, it has simply passed through an infinitely thin membrane. While photons do exist in our coordinate time, their "proper" frame of reference does NOT exist. It is flattened down to zero in the direction it is travelling.

Since the "forward" direction being traveled is still the same in proper time, yet the coordinate universe has been flattened down to zero in that same direction, then what in heavens is this "forward" dimension, (as well as "backward")? That is really the central question here. The answer is a bit shocing, but it should figure!
It comprises a new space dimension perpendicular to all three of the coordinate universe's dimensions! So one dimension of space has flattened down, and been replaced by another!

Let's remember the primary rule of general relativity. In that direction, he [the "c" traveller] will by default see himself at rest, just like all other observers.
In his frame of reference, a beam of light would still move forward at c. Coordinate time observers would "see" an infinitely flat ship (or whatever), frozen in time, moving at c.
However, in proper time, any light that moves past him at c, is really moving in a new dimension not shared by coordinate space, and the coordinate dimension the photon is traveling is perpendicular to all the dimensions that can be can measured.

This is interesting, as M-Theory now proposes a superspace consisting of a large fourth spatial dimension in addition to the six compacted ones. But, IIRC, it is only gravitons whose strings are free to move in this dimension; I don't think electromagnetism. (All other strings are believed to be attached to the three-brane we know as space, and thus limited to 3D). Still, since these hypothetical gravitons are also said to move at c, then perhaps that is the reason they have access to a large additional dimension.

That's how a paradox I noted was resolved as I pondered this question for years. I had not before completed the thoughts on this, because of being stumped by what it would mean for c to still hold true in the proper time frame of reference of c, while still being c in our coordinate time. "Where" would the light in the c frame be "going"? This explanation answers that!

Also, suspecting that if one space dimension begins flattening down as you approach c, then I wondered if time would then become a new space dimension (since time and space are said to be "exchanged" at or beyond the speed of light; and like inside the Schwarzschild radius of a black hole), and would you see four space dimensions (h, w, and d and t partially flattened with one shrinking and the other unfurling) as you get closer to c? I couldn't make sense of it.
But then I remembered that you really can't accelerate to the speed of light. So as you are accelerating, you see the coordinate depth dimension shrink, while coordinate time slows down. All they'll do is continue to shrink, forever, down to fractions of spacelike units, but never reaching 0. You're not "getting [any] closer" to the speed of light (as I mistakenly was assuming), so you don't encounter any "inbetween" state with a partially extended extra dimension!
You're always in your own proper spacetime, where the three dimensions plus time are always the same as they always were. It's the coordinate spacetime you see collapse, and the new dimension doesn't appear as such until the old space has collapsed completely to zero, and that only occurs AT the speed of light.
And again, it is really nothing more than your familiar old back/forth dimension, which is now no longer part of coordinate space.
(If space is a hypersphere, you'll see it flatten down to a disk. If its infinite, it won't shrink at all; you'll only see the matter in it shrink, until you reach then end of matter's distribution. If it has some sort of "edge", beyond which there is no space, then there will be a shrinking distance you'll move in before you just cease to exist.

To be AT the speed of light is a quantum jump, in which you'll find one dimension completely collapsed, and a new one filling its place in the "ahead/behind" directions.
If you move in that dimension backwards, even close to c, you still do not come "closer" to reentering the old dimension; you are still moving in the new dimension. All of coordinate space has now become the unapproachable c to you!
Likewise, if you move forward, you're not going faster than light. FTL was based on the coordinate frame of reference; and that no longer exists in proper space-time, remember. Your motion in the new dimension has no bearing on the old one.

Now, if space is a hypersphere, it will be an infinitely thin disk that you're perpetually "stuck" going through as you make an infinite number of laps in that direction, in zero proper time, and circumnavigating at 186,000m/s forever in coordinate time. If it has an edge, you'll instantly cease to exist. If its endless, then the distribution of matter (superclusters of galaxies; observable universe, etc) will be infinitely thin, but as for space itself; you'll have a 0×∞ paradox. (Another reason I don't believe in infinite space).
In any scenario, you'll be at the "lightlike infinity" at the boundaries of the Penrose diagram of the universe.

What about other c frames of reference traveling ahead or behind you? Like if you are "emitted" at one point in time, and then another c traveller is "emitted" one second behind you.
If you're traveling less than c, yet accelerating towards c, then in coordinate time, the distance between you approaches 186,000 miles. Yet the proper distance collapses towards zero. You in essence, would occupy the same proper space. However, the proper time approaches infinity! Remember, coordinate time grinds to a halt, so "one second behind you" increases towards forever.

You can see why, by graphing this on the Minkowski diagram. Coordinate space runs along at 0° to the right, while coordinate time is 90° (vertical), and c is 45°. However, proper time collapses towards 45° in both dimensions. As your timelike proper "here" heads outward at a decreasing angle, your spacelike proper "now" moves upward at an increasing angle.
At c, the space and time dimensions have collapsed into the c line at 45°.
So another c traveller emitted one second behind you will be a parallel 45° line that never intersects your line.
—at least not in that coordinate space direction being travelled.
Since the space between you decreases to nothing at the same time, this might balance itself out. How?

What I'm wondering, is that that new "forward" proper space dimension might be what represents coordinate "time". Thus fulfilling the theory of space and time being swapped.
So you might see your "follower" displaced (perhaps at 186,000 miles?) in that new "behind" dimension. If you both head backward approaching the speed of light in proper time, the distance would shrink, and proper time would also slow down.

This should give us an idea of FTL travel, in which coordinate/proper space and time become "imaginary" relative to each other.
Now, the collapsed space and time are not even zero; they are beyond zero. That is, in a multiplicative, not additive sense. It doesn't become simply "negative", which is the additive "beyond zero" realm. (Like take 1 and keep dividing it by finite positive numbers until you reach zero. And then imagine continuing to "past" zero. You can't even reach it in the first place). It is analogous to "greater than infinity". Thus, truly "imaginary".
In the FTL realm, the rules are the same. A beam of light will still pass you at c, but it will be a different dimension from the direction being traveled in coordinate space.
I believe that the so-called "tachyons" will thus not interact with coordinate spacetime at all (as has been speculated). Light speed is like the "bridge" between the sub- and super- luminous realms, where time and space are reversed like FTL, but you can still interact somewhat with the subluminous universe.

On the Minkowski diagram, while FTL's "here" (the actual world line) would lie in its expected spacelike (less than 45°) line; its "now", rather than lying in a timelike orientation, would generate a third dimension of the diagram; sticking out in the Z axis as a second space axis.

So what does everyone think of this?

Time and length cease to have meaning in the limit v→c. In that limit, all time and length intervals shrink to zero. In the rest frame of a photon, the coordinates of any point in the universe at any time in the past, any time in the future is identically zero. That just doesn't make a bit of sense.
This is exactly what I had thought as well. But then the idea of another dimension suddenly occurred to me. The problem is, we're still thinking in terms of the coordinate frame of reference (observers at "rest" relative to us). As I say in there, both frames of reference essentially cease to exist to each other. So in a way, what's being said is true. It's meaningless. —to our frame of reference. But in its own frame of reference, we would be zero as well.

It's sort of like the statement (forgot which book I got it from: Kaku, Rucker, Greene, Kaufmann, etc) that tachyons, if they exist, occupy a sort of ghost world where time and space are interchanged. So then luxons (light, etc), are sort of on the boundary of this, where they can interact with the universe, but their frames of reference are zero to us.

I think the theorists should give an additional space dimension (which is appears no one has ever thought of regarding the c frame of reference) some consideration. Especially, once again, since M-Theory is proposing an additional large dimension.

As for the light cone coordinates, I don't fully understand all of that, but right away, it has a diagram of two light rays heading away from each other (X-, X+; forming "the cone"), and mentions the ideas of one being a new time or space coordinate relative to the other, and that in either case, it wouldn't be ordinary. This may well correspond to what I'm suggesting! Obviously, any new dimension would not be one of ours, since ours would have collapsed down to zero relative to the light cone coordinate.

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Eric B said:
It's meaningless. —to our frame of reference.
Not just ours. It is meaningless to all inertial frames.

Well, all inertial frames that are not c. Anything under c, (and in any direction), would still experience the same coordinate three space dimensions and time (though contracted if they are moving at a relativistic speed past another observer). They would still be more aligned with what I was calling "our" frame of reference, or coordinate time; having real, positive (though perhaps decreasing) space and time values.
"Meaningless" in that case would just be another way of representing "collapsed to zero", in which case it is in fact, meaningless in any practical sense. But that doesn't mean it doesn't exist in any frame of reference; at least hypothetically.

Eric B said:
Well, all inertial frames that are not c.
That is all of them.

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But that seems to be if you presuppose that there can be no c frame. And that seems to stem from the apparent paradoxes that result apart from the existence of a new dimension.

Eric B said:
that doesn't mean it doesn't exist in any frame of reference; at least hypothetically.
It can't exist in any inertial frame of reference, not even hypothetically. It is a self-contradiction. By definition light goes at c in any inertial frame, and by definition anything goes at 0 in it's rest frame. So the idea of an inertial frame of light is self contradictory. Throwing in the word "hypothetically" doesn't throw out logic and the requirement for self-consistency.

I believe it is a self-contradiction only when you look at it from a less than c frame of reference, and the coordinate spacetime dimensions. (Which is what I believe the FAQ is doing). There would obviously be some sort of total disconnect from any less than c frame.

The way light can both be at rest and move at c at the same time is that time completely stops from one such frame of reference to the other frame, so in zero time, all speeds are the same: zero. The very concepts of "motion" and "rest" become indistinguishble (to the other frame, that is). Think about what "frozen in time" would mean. Anything "moving" is now just as frozen as any original "coordinate" standard.
So likewise, what one sees as the c speed moving past himself, can include many different relative speeds frozen in time.

In the c frame of reference, you would see that you were really at rest, and that there was still a c that moved at the same speed, in a new dimension. But to us, all of this would be an infinitely flat completely time-frozen perspective moving past us at c (yet with everything in it appearing to move at zero relative to each other).

So in a c frame (which again, is relative to us), the observer (light itself cannot observe) would see himself at rest, while light still moves at its normal speed, but in a dimension different from ours.
And again, that paper the other person linked to appears to support this to some extent. And also, that theorists do allow the possibility of tachyons existing, but that they likely would occupy, essentially, another kind of spacetime dimension. This would be the same dynamic.

Eric B said:
I believe ...

Please stop speculating. It is contrary to the forum rules which you agreed to when you signed up.

If you believe that the concept of an inertial rest frame for light is scientifically well-founded then please provide a mainstream scientific reference supporting the idea.

It must be me but I cannot find anything related to physics in this topic. Anything with rest mass cannot travel at the speed of light. I do not see the point in speculating about what if it could, it can't, end of argument.

As both atyy and DaleSpam already said, there is no Lorentz inertial frame at c. It is a meaningless and self-contradictory concept. Read the FAQ. The FAQ exists to forestall nonsense discussions.

## 1. What is an inertial frame of reference?

An inertial frame of reference is a coordinate system that is at rest or moving at a constant velocity. In this frame, Newton's laws of motion hold true.

## 2. How does the speed of light play a role in an inertial frame of reference?

The speed of light is a fundamental constant in the universe and is the same in all inertial frames of reference. This means that no matter how fast an observer is moving, they will always measure the speed of light to be the same value.

## 3. What happens when an object approaches the speed of light in an inertial frame of reference?

As an object approaches the speed of light, its mass increases and time slows down. This is known as time dilation and is a consequence of Einstein's theory of relativity. The object also becomes more and more difficult to accelerate.

## 4. Is there a limit to the speed at which an object can travel in an inertial frame of reference?

According to the theory of relativity, the speed of light is the maximum speed at which any object can travel in an inertial frame of reference. This limit applies to all objects, regardless of their mass or energy.

## 5. Can an object travel faster than the speed of light in an inertial frame of reference?

No, it is not possible for an object to travel faster than the speed of light in any frame of reference, including an inertial one. This is a fundamental law of physics and has been supported by numerous experiments and observations.

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