Special Relativity - Could we see it ?

[Akiainavas]

So, this morning in my office there have been a big conundrum. My collegue ( PHD in Physics ) started a discussion on special relativity and an interesting question arised.

Quote

Hypothetically, if an object could move faster than the speed of light in a vacuum, would we be able to see it ?

It was really fun to listen to all theories but after the heated discussion died, none of us reached any conclusion.

Now, I understand that questions like that cannot be answered - since it's just wild theorizing - but I was wondering what you guys think about it ?

[cloudsweeper]

c is constant in all frames of reference, so Yes, you could see it.  

Apparently, some galaxies are receding at >c, and we can see them.

Just thoughts - strictly I'm out of my depth here!  I look forward to further contributions.

Doug.

[Akiainavas]

Here's an interesting point:

This picture represents the expansion of the universe in which light travels at 45 degrees:

A quick description:

Quote

The purple large tear-drop is the Hubble Sphere, which splits objects in the Universe into those moving away from us less than the speed of light from those moving faster than the speed of light due to the expansion of space. Those objects in the white region are the subluminal objects. Everything else is superluminal with respect to us.

This means that lots of those object actually move away from us faster than the speed of light - or have been when their light was emitted - and clearly we can still see them, as proven by Hubble's Deep Field for example. This, however, means that there are also objects that are moving faster than the speed of light right now that we will see in the future which makes things really wild if you think about it.

And now we've reacher a point where everyone was just scratching their heads and staring at diagrams... I'm sure this discussion is not over and the canteen will be buzzing today.

[SilverAstro]

3 hours ago, Akiainavas said:

So, this morning in my office

if an object could

It can not. End of discussion in any office   !!  Unless you are speaking of tachyons which can not travel slower than c, but they are hypothetical and thus not "objects".

and we all know that Hypothetically many hypothesises can spring into and out of existence even faster than light quantum foam.

 

[ollypenrice]

Wouldn't this involve time reversal?

Olly

[George Jones]

6 hours ago, Akiainavas said:

Here's an interesting point:

This picture represents the expansion of the universe in which light travels at 45 degrees:

A quick description:

This means that lots of those object actually move away from us faster than the speed of light - or have been when their light was emitted - and clearly we can still see them, as proven by Hubble's Deep Field for example. This, however, means that there are also objects that are moving faster than the speed of light right now that we will see in the future which makes things really wild if you think about it.

And now we've reacher a point where everyone was just scratching their heads and staring at diagrams... I'm sure this discussion is not over and the canteen will be buzzing today.

Since speed = distance/time, the definition of "speed" depends on the definitions of "distance" and "time". The definitions used in special relativity do not generalize easily to the curved spacetimes of cosmology, so the concept of speed in special relativity does not generalize to cosmology. The definitions used in cosmology are, however, easily applied in special relativity. When this is done, "cosmological speed" does not turn out to be "normal speed" in special relativity, it turns out to be something called the "velocity parameter". There does exist a definite mathematical relationship between these two concepts of speed.

[ollypenrice]

Ah, to have George's clarity of mind on these matters!

Olly

[Alien 13]

I a particle existed that had negative mass then it would get lighter the faster it traveled so could end up at infinite speed and we would never see or detect it.

Alan

[Macavity]

Well, maybe to add to the confusion... But I casually wonder if an object
moving closer and closer to the speed of light- In front of the observer,
would be increasingly Lorentz Contracted until it seemed of zero length!

https://en.wikipedia.org/wiki/Length_contraction (And seemingly so)

As as a result you may well not see (a whole lot of!) it anyway?  

P.S. I suppose one might also the worry about...
https://en.wikipedia.org/wiki/Length_contraction#Visual_effects
I tend to take the word of Roger Penrose in such things.
 

[cloudsweeper]

Good point @Macavity,  but the contraction would only be in the direction of travel, so the object wouldn't disappear. I think.......

Doug.

[vlaiv]

Here is an idea that is less farfetched than that of object moving above the speed of light.

If there is object moving close to speed of light at a trajectory close to observer (not straight at observer to avoid collision, but at a shallow angle and passing observer at a distance D), and observer is looking with their own eyes, what would be conditions to allow observer to see something (can be extended to CCD/lenses case)?

Given the quantum nature of light, there is limited number of photons being scattered of the surface of object. Distance to observer causes that photons to be inverse square dependent in terms of count of photons that reach the observer's eyes. Object far away will not be registered as there is no enough photons to cause eye stimulus (we can't see distant celestial bodies such as asteroids with naked eye). With object traveling close to speed of light there will be really small amount of time where object is in "visual" range of observer. Will enough photons be reflected of object as to cause detection? This is of course dependent both on reflectivity of object and intensity of illuminator.

I wonder what would equation that govern detection be like if we take all the variables to be known - size, shape and reflectivity of speeding object, detector threshold in photon count, pass by distance, angle of incidence and intensity of illumination source.

Something tells me that we might be surprised with solutions of such an equation - like having house sized object whizz by at 99.99% light speed, 50m away from observer in earths orbit illuminated by Sun - and observer will not have a clue it happened.

[Stub Mandrel]

Something moving at near light speed would build up a 'bow wave' of photos, just like a near-supersonic plane (although the photons would appear to speed away from the objects POV), so even if faint it would appear bright. Also doppler shift effects, so blue shifted so far it might be shining in x-rays! Similarly it would appear very dark when moving away and could appear as a radio source.

[vlaiv]

2 minutes ago, Stub Mandrel said:

Something moving at near light speed would build up a 'bow wave' of photos, just like a near-supersonic plane (although the photons would appear to speed away from the objects POV), so even if faint it would appear bright. Also doppler shift effects, so blue shifted so far it might be shining in x-rays! Similarly it would appear very dark when moving away and could appear as a radio source.

Would it? Photons don't act as static medium like air/water molecules that are present and relatively static to moving object - in object's frame of reference they still behave like everywhere else - whizzing around at C. In observers frame of reference they still move at C, so it will take less time between two photon's arrival than if object were standing still but will this compounding effect be enough to compensate for small amount of time photons are being reflected?

I do agree that there will be significant blue shift, but I guess that in case of Sun there will be IR part of spectrum that will be blue shifted into visible light - so there is reason to believe there will be visible light photons to detect.

[Tiki]

 

 

1 hour ago, Stub Mandrel said:

Something moving at near light speed would build up a 'bow wave' of photos, just like a near-supersonic plane (although the photons would appear to speed away from the objects POV), so even if faint it would appear bright.

I agree.

As an object passes through the 'speed of light barrier' you'd certainly expect an 'optical boom'. Some neutrino detectors look for an 'optical shockwave' of Cherenkov radiation which shows the acceleration of a particle faster than the speed of light (in that medium).

[Macavity]

Ironic(?) that Physics students are doubtless having to provide "correct"
answers to questions re. their course on Special Relativity... when it seems
that someone of the calibre of Roger Penrose can still write original papers
on the subject, without fear chance of contradiction.  Reassuring even? 

I suepect there might be some clues to the answer herein:
http://math.ucr.edu/home/baez/physics/Relativity/SR/penrose.html
Penrose and Teller (Penn & Teller?) seem to be the main protagonists.

There are one or two attempts to visualise this stuff on Youtube!
https://youtu.be/JQnHTKZBTI4  Perhaps based on... a simulation? 
The muddy(?) graphics and the "synth" voice are a bit weird though! 

[acey]

A good answer here (and some less good ones further down the page):

https://www.quora.com/Special-Relativity-Hypothetically-if-an-object-could-move-faster-than-the-speed-of-light-in-a-vacuum-would-you-be-able-to-see-it

[SilverAstro]

2 hours ago, Macavity said:

Ironic(?) that Physics students are doubtless having to provide "correct"
answers to questions

The muddy(?) graphics and the "synth" voice are a bit weird though! 

Weird indeed ! Sounds like the demented love-child of Stephen

Ironic, that you have caused this student to revisit the perceived rotation 50y after I was first asked to consider it (56y after Penrose et al.) and realize that I didnt fully understand it then , , , , and am not sure I do now either ! I had previously thought that the arrow on the side of a passing cube would get shorter but that the arrow on the rear face of the cube would become visible and progressively longer the faster a cube travelled (as the rear corner of the cube was shifted/contracted forward and out of the way of obscuring the rear side)  such that the visible aspect of the cube would remain a square and 'unshrunk' , I'm not convinced of that now although that confusing FAQ you quote would have us still think so with the carriage remaining on its tracks !

Interesting that Penrose writing in 1990 (Emperor's New Mind) makes no mention of this visibility/rotation problem and no mention of Terrell either, , , which may be why I had no need of revisiting it till now, I blame Macavity

[andrew s]

19 hours ago, Tiki said:

 

[andrew s]

Sorry about the blank post it seems you can't delete an accidental quote or at least I can't.

 

What I wanted to say was that the PhD physicist quoted in the OP should have known better if things don't exist it is idle to speculate as to what physics might say about them. Physics is about building predictive ( or retrodictive)  models about things we can observe not idly speculate about what we can't. It may be fun, and I have no problem with that, but it is not physics.

Secondly most pop science, even by eminent physicists, is normally wrong and introduces seductive images and analogies to entrap the lay person. They speak of virtual particles and processes none of which appear or would pass muster in peer reviewed journals. It is common to hear that photons are emitted from a source and then ask the path they take to a detector. This in QED is meaningless. All QED will say is what is the probability of a photon created by a source is absorbed by the detector. It is mute on everything in between. If you want to know about the propagation of electromagnetic wave then Maxwell's equations are your best bet. Hawkins, in his popular works, talks of virtual pairs being created at the event horizon of a black hole. One escapes and one is captured hence black bodies radiate. This description does not appear in his peer reviewed where there is never a mention of virtual particles!

If you really want to understand physics then you have to study it using valid text books and/or peer reviewed articles. 

I have no problem with pop science but maybe it should be in the lounge area...... only kidding.

Regards Andrew

[Stub Mandrel]

Not sure I agree with that. As a biologist I find 'Man After Man' by Dougal Dixon fascinating, and also the 'evolved Troodon' intelligent dinosaur. Such speculation  about what we can't know or test scan greatly help our understanding; not least making it easier to appreciate that parrots and ravens may be on the way to becoming 'intelligent dinosaurs'...

[andrew s]

37 minutes ago, Stub Mandrel said:

Not sure I agree with that. As a biologist I find 'Man After Man' by Dougal Dixon fascinating, and also the 'evolved Troodon' intelligent dinosaur. Such speculation  about what we can't know or test scan greatly help our understanding; not least making it easier to appreciate that parrots and ravens may be on the way to becoming 'intelligent dinosaurs'...

I was talking specifically about physics. I am not a biologist and have not read the books you mention so can't comment on them.

I would have thought observation of Parrot and Raven behavior would have led us to understand their behavior as intelligent without the need to speculate about dinosaurs but in the end I guess it does not matter what your inspirations are as long as they work be it Star Trek or Popeye.

Regards Andrew

[Macavity]

Where did it become "Pop Science"? "It's not Physics"? "Valid textbooks"? 
Virtual particles do become real - Pass muster, even in peer reviewed journals.
Who is this (Jim?) "Hawkins" guy? [teasing] Where is the controversy tho? 

Real Photons are detected by LHC (and previous generation!) experiments.
Their path through space can be accurately reconstructed to their origins.
They may be "invisible" meanwhile. But I don't see any conflict with QED
- Or anything else particularly? (No Pun!) Just getting a tad confused? 

[Stub Mandrel]

29 minutes ago, andrew s said:

I was talking specifically about physics. I am not a biologist and have not read the books you mention so can't comment on them.

I would have thought observation of Parrot and Raven behavior would have led us to understand their behavior as intelligent without the need to speculate about dinosaurs but in the end I guess it does not matter what your inspirations are as long as they work be it Star Trek or Popeye.

Regards Andrew

The examples I gave are 'what if extrapolations of evolution', as speculative as considering faster-than-light motion. The evolved Troodon is interesting as it was a speculation on dinosaurs evolving humanoid intelligence; the relevance of birds is that since Troodon was postulated it has been widely accepted that birds are theropod dinosaurs (like T. rex amongst many others) indicating that, in principle at least, the evolution of relatively capable intelligence among dinosaurs is fact rather than fiction.

Speculation is an essential part of scientific discovery - if we can't imagine things beyond what we already know how can we explore and expand  the frontiers of our knowledge?

[SilverAstro]

1 hour ago, andrew s said:

an accidental quote or at least I can't.

Yep, quotes hereabouts are slippery customers, almost as bad as the physics we are discussing

Eeekk ! I thort it already was in the lounge area, gulp, oh well lets soldier on regardless , , Only kidding
(/levity)

Well for what it is worth I agree with the assessment that the PhD should have known better, not sure I agree with the emphasis on "really understand" and "peer review". And "Pop science " should be in the same category as Politics !
It is silly of the PhD to speculate upon daft/impossible interpretations of 'known things', but singularities and other etceteras that are at the edge, or even beyond, current physics can be considered unknown knowns or is it known unknowns ( do I sound like someone we all know ! ) and are fair game. The speed of light ( or more correctly "c" ) is not one of those.

Peer reviewing is a holy grail of BrianC and serves us well for ordinary things that most peers can agree upon and is good to avoid blunders ( remember those faster than light neutrinos that resulted from a dry joint ? or was it whacky baccy ?! ) However in great conceptual matters eg. Steady State/ Big Bang (see me elsewhere)  and  the Earth in the middle or not,  then peer reviewing is no use whatsoever. We need peers like FredH who will stick to their guns till the bitter end, just to make sure. ( Yes it was a great shock when Arno Penzias and his chum shot that fox - I mean their pidgeons, and even worse because they were not looking for "IT" )

We dont all have the time to read up on these things to the extent of questioning Gamow et al, so we do need popularisers of science.

[andrew s]

2 hours ago, Macavity said:

Virtual particles do become real - Pass muster, even in peer reviewed journals.

"Virtual particles" come from Feynman diagrams are which is  a graphical representation of a perturbation expansion of the integrals in QED they have no physical significance any more than the terms in a series expansion of the SINE function do.

 

2 hours ago, Macavity said:

Real Photons are detected by LHC (and previous generation!) experiments.
Their path through space can be accurately reconstructed to their origins

I never said real photon don't exist I agree they are created and detected. Can you provide a reference to accurate reconstruction to their path through space?

 

2 hours ago, Macavity said:

Who is this (Jim?) "Hawkins" guy? 

Oops - I never could spell.

Regards Andrew