Can an imminent laser strike ever be observable?

[davidjoe]

I’m playing by the rules, Hutch. I’m not suggesting an object reach or exceed light speed.

I’m simply examining the constituent components of relativity.

Relativity does not require suspension of any logical precepts, to my knowledge.

Without reaching the speed of light, by only the slimmest of margins, your speed of C minus one iota, is NOT a constant in all reference frames, and has more in common with .00011 mm per year, than it does with C, right, with respect to how your speed is perceived by others.

This shouldn’t be an avoided question to my thinking, right, relativity is a theory, not a person being examined on a table.

 

[DaveC426913]

Well, I must have misunderstood. I thought it has been said I will observe light pass me at a difference of C, “because all frames observe the speed of light as the same”.

Correct.

I took this to mean that if I’m stationary, it passes at C.

Correct.

If I’m one iota slower than light, at the same spot when it passes, that’s a reference frame, and therefore I see it pass at C.

Correct.

But then you said this:

if that special speed is reduced by the slightest amount, everything changes. Now it passes me according to classical physics motion analysis, meaning it barely passes me whatsoever.

Which is incorrect.

 

[DaveC426913]

... relativity is a theory, not a person being examined on a table.

The theory part is Einstein's curved space-time - a model of how light behaves the way we see it behave. But the observations are incontrovertible.

No matter what your frame of reference - whether you are moving or stationary, relative to some planet or star or galaxy, you will always observe the speed of light (in a vacuum) to be c.

Any theory that wishes to explain light must address that fact.

There are reasons for why it may not make sense at first, but it requires going down the rabbit hole of relativity. I'm not sure you are ready to make that journey yet. It requires throwing away your classical preconceptions and not drawing hasty conclusions from what you have learned about the classical world - which is kind of what you are doing now.

It's sort of beyond a single forum thread. The best way would be to pick up a basic primer book on relativity.

 

[hutchphd]

Without reaching the speed of light, by only the slimmest of margins, your speed of C minus one iota, is NOT a constant in all reference frames, and has more in common with .00011 mm per year, than it does with C, right, with respect to how your speed is perceived by others.

This shouldn’t be an avoided question to my thinking, right, relativity is a theory, not a person being examined on a table.

With respect. I have no desire to examine theory of relativity: I have made my peace with the intuitively difficult parts long ago, and recapitulating the tenets will likely not be useful to you. In fact what is subject to examination is your inability to understand the theory because of prejudices you do not seem to realize that you have. These are what is on the table. It requires you to ask specific guestions about the theory; not complain about our answers. Hopefully something will click and illumination will result. Or perhaps not. Your questions are not trivial or foolish but you need to be open to the very good answers being supplied to you. And please make your questions about theory and not epistomology

 

[Janus]

That’s new and different, then, from the thinking here, - that if I’m going C minus “1 iota”, a laser streams past me, at difference between no velocity, and C.

You are moving at say 0.9999c relative to laser emitter. To find the speed the light moves relative to you, As measured by you, Newtonian physics says you use 1c-0.9999c = 0.0001c.
With Relativity, you use (1c-0.9999c)/(1- 1c(0.9999c)/c^2. A bit of simplifying yields (1c-0.9999c)/(1-0.9999c^2/c^2) = (1c-0.9999c)/(1-0.9999) = c(1-0.9999)/(1-0.9999) = c. as long as the magnitude of one of the velocities you are working with is c, you get an answer of c.

 

[sophiecentaur]

The theory part is Einstein's curved space-time

That introduces GR. It is not part of the OP's question, which is just about Special Relativity. I think that one step at a time is appropriate here.

The OP's problem seems to be that he wants to introduce some particular speed which is a barrier between one set of observations and another. The fact is that Special Relativity operates, whatever speed you are going at; it's just that the effects are very small at speeds which are achievable by an observer.

I have made the point twice (and he doesn't seem to have picked up on it) that is there are other effects / consequences of the invariable speed of light. Apparent distances, apparent times and apparent frequencies change at (mostly) high relative speeds so the OP's problem of light suddenly behaving differently is irrelevant; Nothing is fixed apart from the speed of light; the rest of Physics fits itself around that. AND our personal views or mental models have to fit in with it too. There's no way out of that.

Relativity does not require suspension of any logical precepts, to my knowledge.

It certainly required suspension of the views of many very clever people in the early twentieth century. You, also, may need to re-examine what you mean by your 'logical precepts'. They are preconceptions and not necessarily logical; they do not fit with measurements, for a start.

 

[davidjoe]

New address for the thread SR. I still pose this sub question:

“If the speed of light is reduced by the slightest amount, (in the form of a fast object), is it observed to be constant, from all reference frames?”

My understanding is that this distinction applies to only light speed. It IS is a bright line difference.

 

[sophiecentaur]

New address for the thread SR. I still pose this sub question:

“If the speed of light is reduced by the slightest amount, (in the form of a fast object), is it observed to be Constant, from all reference frames?”

My understanding is that this distinction applies to only light speed. It IS is a bright line difference.

The speed of light IS the speed of light. You cannot change it. c is universal. Your post is meaningless.
A "fast object" cannot travel at c. I assume that you have gone to the trouble of finding out the formulae.

A "bright line" doesn't apply here - any more than a temperature of Zero K. You can get as close as you like - depending on how much money and energy you are prepared to expend but you are still a massive object and you will follow the SR formulae,

 

[davidjoe]

I’m not suggesting that light be slowed. And I think that is clear in the posts. Light travels at C. But objects can be accelerated to just an iota under C.

If it is inconvenient that a striking difference exists in how two speeds are observed in all reference frames, when the difference between them could be the smallest possible, that doesn’t make the question or the post meaningless, does it?

 

[Motore]

Maybe take a step back, choose a simpler scenario, clearly define everything: which reference frame measures which thing, which objects have which speed RELATIVE to each other, ...
Do not forget, that each object can regard itself at rest and each object sees the light going at c.
For example you have the earth and you have a laser that has 0.999c velocity relative to the earth. You are standing on the earth and you "see" (not literally) that the light emitted from the laser is going at velocity c and the laser is slowly lagging just behind that light. Now you go sit a on the laser and you are at rest, stationary and you "see" that the earth is going by with 0.999c velocity, but the light from the laser is still going away from you at c and you (the laser) are not lagging slowly just behind the emitted light.

 

[DaveC426913]

I’m not suggesting that light be slowed. And I think that is clear in the posts. Light travels at C. But objects can be accelerated to just an iota under C.

Objects can be accelerated to just under c, yes. And if you are on that object, and you measure a beam of light - any beam of light, in your object or outside it - you will measure it to be travelling at c.

If it is inconvenient that a striking difference exists in how two speeds are observed in all reference frames, when the difference between them could be the smallest possible, that doesn’t make the question or the post meaningless, does it?

What you are not factoring in is time dilation.

There is a simple scenario that involves a spaceship and a flashlight (or headlights) that shows what happens when two observers see different things, depending on where they are - I'm just not sure you're ready for it.

 

[A.T.]

If it is inconvenient that a striking difference exists in how two speeds are observed in all reference frames, when the difference between them could be the smallest possible, that doesn’t make the question or the post meaningless, does it?

Yes, Special Relativity is mathematically less convenient than Galilean Relativity, but it fits observation better. Convenience is also the reason why Galilean Relativity is still being widely used, where its errors are negligible.

 

[Vanadium 50]

f the speed of light is reduced by the slightest amount

I’m not suggesting that light be slowed.

You can see why we are puzzled.

 

[davidjoe]

Dave, I will definitely delve deeper. I have done a surface level look to see what NASA’s position is on this, just out of curiosity. Just begun so nothing to say about that.

I’m not for or against AE. I believe that is is possible to understand, be accepting of and critical of, a theory, all at the same time.

You can see why we are puzzled.

Fuller context than the quote box clears that up.

 

[sophiecentaur]

that doesn’t make the question or the post meaningless, does it?

It is meaningless to me. If you actually specify the precise situation and what exactly it is that the various observers are measuring then is all follows perfectly - as long as you realise that they will all see the same beam of light going past them at c. But its 'colour' may be different.

 

[davidjoe]

It is meaningless to me. If you actually specify the precise situation and what exactly it is that the various observers are measuring then is all follows perfectly - as long as you realise that they will all see the same beam of light going past them at c. But its 'colour' may be different.

If it’s not striking, then it’s possible that the point blended with everything else.

Envision, we’ve left port with sister cruise line ships that are so close to our speed we never lose sight of them, right?

If I’m being overtaken in the lead at such a slow rate from the Yucatán, that only by Galveston does the sister ship pass me, … BUT should that sister ship in the rear add .0000001 MPH to it speed, I will see it go by me at light speed, well, that is striking, no?

In my hypothetical, as this turned about halfway down, everything illustrated and discussed was either my ship 2 iotas under C, a missile pursuing it 1 iota under C, or light at C, both starting a second behind me. I look at the missile for what, 100 lifetimes, but add to the missile’s speed the slightest imaginable increase in speed, which is a laser’s speed, and I would see it pass me at light speed as it if I’m completely still.

 

[jbriggs444]

Nope. Light is going the fastest (straightest) between two points: anything sideways will be just slower.

In a flat space time, this is correct. In a curved space time, things are not quite so simple. There can be multiple trajectories between two events. Not all such trajectories need have equal transit times.

Consider, for instance, a laser beam that fires a few tens of meters away from your face on a starry night. It is aimed skyward at a black hole some hundreds of light years away, on a trajectory that nearly grazes the photon sphere so that the beam will loop around and come back to strike you some hundreds of years later.

In the mean time, light from dust that the beam has illuminated on its outward path might theoretically be seen hundreds of years before the final strike.

 

[A.T.]

I will see it go by me at light speed,

A ship cannot go at light speed. And what Lorentz Transformations give you is not what you actually see visually, but what is reconstructed to happen in a certain frame, after accounting for signal delay due to finite lightspeed.

 

[Vanadium 50]

Fuller context than the quote box clears that up.

I don't think it does.

Further, we are almost 80 messages in, and we are still reading confusing and impossible things from you - despite having been shown where these are wrong. For example, "I will see it go by me at light speed" is something that material objects do not do.

 

[davidjoe]

I don't think it does.

Further, we are almost 80 messages in, and we are still reading confusing and impossible things from you - despite having been shown where these are wrong. For example, "I will see it go by me at light speed" is something that material objects do not do.

The “it” going by me was light. And probably 4 people said that, before me. So…

If you’re referring to the cruise ships, man, is that not obviously an analogy?

 

[davidjoe]

Apart from the illustrations, - and perhaps this is too sensitive to the core of SR to even acknowledge - but only C (a very specific speed) is claimed to be observed as constant in all reference frames.

Take away one iota from light speed, the smallest conceivable change, and now that new speed, which objects can go, is NOT observed as a constant in ALL reference frames. That new speed is frame dependent. If you are matching it, beside it, looking at it, no relative motion is perceived.

Add the small delta increase to it, which only light can go, and now you see a beam of light pass you at light speed as if you are not moving at all.

 

[davidjoe]

A ship cannot go at light speed. And what Lorentz Transformations give you is not what you actually see visually, but what is reconstructed to happen in a certain frame, after accounting for signal delay due to finite lightspeed.

Right, a ship cannot. The three objects in question were my ship, a missile and the laser. Laser = C; missile = just under C; ship = just under laser.

 

[sophiecentaur]

If you’re referring to the cruise ships, man, is that not obviously an analogy?

You can only draw valid analogies when you understand your subject. You cleasrly do not so your anaolgy means nothing.
Have you actually read around this topic or is your only inout from your questions and PF's answers? Q and A is a hopeless way of learning on-line. It actually only works well for very receptive bright minds who are 'almost there'. Your random responses have ben taking you further and further from reality because you seem not to want to be wrong.

 

[A.T.]

Right, a ship cannot. The three objects in question were my ship, a missile and the laser. Laser = C; missile = just under C; ship = just under laser.

No, you were talking about a "sister ship" that you "will see go by me at light speed"

should that sister ship in the rear add .0000001 MPH to it speed, I will see it go by me at light speed

As already said, a ship cannot go at light speed.

 

[davidjoe]

No, you were talking about a "sister ship" that you "will see go by me at light speed"

As already said, a ship cannot go at light speed.

Somehow the original, ship, missile, laser hypothetical image blended.

The ships were recasting the same scenario in different terms. Some of my references to the “speed of light” or maybe light speed are not to light, but to that specific speed.

I agree, completely that the final step up in speed, is made by light. But the final step up need not be a big one. That is my point.

The final step up could be an inch a year, or infinitely smaller than that. But the final step up has enormous consequences to whether all reference frames are required to see light (the speed of light in a vacuum) as a constant, or not.

That smallest change in velocity is the difference between conceivably looking at a trailing missile for millennia, (if you and it are only slightly sub C) or seeing a laser pass by you, as if you are not moving at all.

 

[davidjoe]

If there are only three things in the universe, your ship, the missile and the laser, from slowest to fastest, whether you are going 1 mph and the missile 2 and the laser C, or you are two iotas under C, the missile is one, and the laser passes both at C, looking at the laser, you are going to see it pass you at the difference of C.

 

[Vanadium 50]

Increasingly complicated hypotheticals will not help.

1. There is no absolute speed. "One iota less than c" is observer dependent.
2. Massless objects (like light) always travel at c. All observers agree on this.
3. Massive objects always travel slower than c. Observers can disagree about their speed and even their direction.

 

[Nugatory]

If there are only three things in the universe, your ship, the missile and the laser, from slowest to fastest....

Some of your difficulty with relativity may come from not fully appreciating how this description is incorrect (or more precisely, includes a hidden assumption that is incorrect). It makes no sense to talk about the speed of the ship being more or less than the speed of the missile - all we can talk about is the speed of the ship relative to the missile when we consider the missile to be at rest and its negative, which will be the speed of the missile relative to the ship when we consider the ship to be at rest.

 

[DaveC426913]

The final step up could be an inch a year, or infinitely smaller than that. But the final step up has enormous consequences to whether all reference frames are required to see light (the speed of light in a vacuum) as a constant, or not.

This is not true. The enormous consequences you speak of are due to your misunderstanding of the scenario.

It looks like you are pretending that you could get your spaceship to a tiny bit below the speed of light and everything is classical, but if you could boost it juuuuuust that little bit extra, everything would change.

No.

All things with mass, such as spaceships and atoms, move at slower than c. Always.
All massless things, such as light, move at c. Always.

You cannot swap them back and forth. You cannot jump from your spaceship onto a beam of light - even hypothetically - and ask what you would see. It is non-sensical.

You in your spaceship, have a frame of reference in which you are stationary (that's the definition of a FoR). No matter how fast you are moving with respect to any other object in space, you are stationary in your own reference frame, and measure everything around you in that frame.

Light has no reference frame. It can't, because that would entail a reference frame in which it is stationary - which directly contradicts the axiom that light moves at c in all reference frames. It can't be stationbary and moving at c simultaneously.

So there is no "final step". There is massive object and massless objects, and ne'er the twain shall meet.

 

[Nugatory]

but only C (a very specific speed) is claimed to be observed as constant in all reference frames.

Well yes, that is a claim, but so is the claim that there is only one natural number $N$ such that $2+2=N$. That claim is backed up by a mathematical proof starting from the axioms of number theory, but of course you don't need a lot of advanced mathematics to satisfy yourself that $2+2=4$ and never anything else.

Likewise it has been mathematically proven that either there is no invariant speed, or if there is, then there is exactly one. And likewise, you don't need the math to satisfy yourself of this - just assume that there are two, play around with various objects moving at various speeds relative to one another and greater or less than one or both of your hypothetical invariant speeds, and you will quickly arrive at a contradiction.

So the question has always been whether there is no invariant speed, in which case we have Galilean relativity and velocities add as $w=u+v$; or there is one invariant speed, in which case we have Einstein's relativity and velocities add according to $w=(u+v)/(1+uv)$. (And note that the "very specific speed" doesn't show up in this formula at all because I've sensibly chosen units in which the invariant speed is unity).

It's actually something of an accident of history that we call the invariant speed "the speed of light" and represent it with the same symbol $c$ that we used for the measured speed of light. A more modern view would be that there is an invariant speed and the interesting question about light speed is whether light moves at that speed, or at some speed so close to it that our most sensitive measurements have been unable to find any difference.