Now you see it....... - 09/05/16 08:40 PM
I may well present three scenarios in this thread; it, sort of, depends on the responses. In each scenario there is a hypothetical space craft, travelling at c and an astronaut on the craft with a flashlight which he shines from the back of the craft towards the front. There will also be one or two remote observers, with hypothetical instruments that can be turned on and off very quickly.

(1) The astronaut turns on his flashlight. In his RF the light travels at c to the front of the craft.

Observer A makes her observation as the astronaut turns on his flashlight. She sees no light.
Re: Now you see it....... - 09/05/16 10:52 PM
Ok, There is a problem with your first idea, that the space craft is traveling at C. It just can't do that. And if it could I don't think that the math would work. The space ship would wind up with an infinite (relative) mass. There is a form of math that works with infinities, but it isn't the type you use with Special Relativity (SR). SR uses just plain algebra.

Bill Gill
Re: Now you see it....... - 09/05/16 10:53 PM
By the way, did you mean this to be in the physics forum?
Re: Now you see it....... - 09/06/16 01:56 AM
Originally Posted By: Bill
Ok, There is a problem with your first idea, that the space craft is traveling at C. It just can't do that.

Someone had to point that out!

Recall that Einstein had himself travelling at c, in order to make a point. I doubt that you would consider his conclusion invalid because of that.

In his thought experiment, Einstein didn’t end up with infinite (relative) mass, that would only have detracted from his line of reasoning.

Yes; I did mean to post in the physics forum. I don't know what happened there. I blame my age.
Re: Now you see it....... - 09/08/16 12:40 PM
there is no physical reason why someone could not travel at
c ... other than the capabilities of his propulsion system.

and there is no physical reason why his propulsion system could
not propel him to c.

claiming that there is a limit to the speed of physical objects
through the testing of the speed of light which has a speed limit
that is a result of its initial speed at the point of light emission
and the speed and direction at which the light source is moving
and the environment or medium that it then travels in is like
saying that a bullet that is fired from a riffle under water
from a fast moving submarine can not travel faster in the medium
of air above the water or even faster in the medium of space above
the air.

it is a untested pig headed claim.
and that is all it is.
Re: Now you see it....... - 09/09/16 12:38 PM
OK Paul, so where does that leave you on the statements in scenario 1 (OP)?
Re: Now you see it....... - 09/09/16 11:39 PM
you stated that the spacecraft and the astronaut and the flashlight
were traveling at c

so there must be a point somewhere that the speed measurement is
being taken from as the spacecraft moves away from it at c.

and were only talking about the light from the flashlight I suppose.

so an observer traveling outside the spacecraft at c would see the same
thing that the astronaut inside the spacecraft sees if there is a portal
that he can look through , they both would see the light shining onto the
front of the craft from the back of the craft.

and if light detection devices are placed along the path of the craft then
the light detectors should detect the same thing as the astronaunt and the
outside observer are able to see given that the detectors are properly designed
and calibrated to detect higher and lower frequency light because the craft
will be moving towards and away from the detectors at c as the craft passes
by them and the light from the flashlight will be traveling 1 x faster
than c as the craft is approaching the detectors and 1 x faster than c as
the craft is moving away from the detectors.

so the detectors would need to adjust for the changes in light frequency
that enters into the detection portions of the detectors.

like the red shift blue shift.

if we focus on what our eyes can see then we end up with false
math and a magic show to boot to sell books.

Re: Now you see it....... - 09/10/16 08:28 PM
A common response to the OP from those who are scientifically inclined is simply to dismiss it because "nothing massive can travel at c", so it's an impossible scenario. An interesting factor is that there may be many more reasion why it could not happen. You have touched on some of the snags, and my reason for raising the topic was to point to another.

The remaining two scenarios were as follows:

(2) Astronaut acts as in (1).

Observer A makes her observation when the light is half way from back to front. She would see it “frozen” in that position.

(3) Astronaut acts as in (1).

Observer A makes her observation as in (1). She leaves her instrument on, so sees no light.

Observer B starts her observation at the same point as A, but has her instrument programmed to turn off and on again such that it comes on again when the light is half way from back to front. She sees, first, no light, then the “frozen” beam.

Question: In the RF of an observer, does the light move only when it is not being observed?

To me, this seems like “fantasy physics”. The conclusion seems inconsistent with reality as we observe it. I know that does not necessarily mean it cannot happen, our intuition is often a poor guide. I wondered what others thought.
Re: Now you see it....... - 09/10/16 10:46 PM
Originally Posted By: Bill S.
2) Astronaut acts as in (1).

Observer A makes her observation when the light is half way from back to front. She would see it “frozen” in that position.

I see a several possible replies.

1 Observer A would not see anything. Since the space ship is traveling at light speed it has zero length, with respect to the observer. Therefore it would be invisible. This assumes that the observer is off to the side of the trajectory of the spaceship.

2 If Observer A was directly in line with the trajectory of the space ship then she would see a disc that represents the cross section of the spaceship all crushed together. If the space ship was transparent she would see the light moving along the spaceship trajectory.

3 Observer A would see what appears to be a supermassive black hole traveling along the path of the spaceship, since the relativistic mass of the ship would be infinite.

Bill Gill
Re: Now you see it....... - 09/12/16 01:30 AM
since the spacecraft is traveling at c already none of
the observers can "see" any light with their eyes except
any observers that are traveling with the spacecraft
at c and close to c

thats why I used detection equipment ... our eyes could not
see light that is outside of the wavelenghts of the
visible light spectrum.

and if you add c to any wavelenght in the visible light
spetrum that wavelenght drops out of the visible light
spectrum.

Re: Now you see it....... - 09/13/16 01:30 PM
Although the original scenario is impossible, a thread like this throws up some fascinating thoughts, for me, that is its value.

Originally Posted By: Bill
If Observer A was directly in line with the trajectory of the space ship then she would see a disc that represents the cross section of the spaceship all crushed together.

A few thoughts arise from this.

1. If the ship is travelling at c, observer A would not see the disc until it hit her.

2. If the ship is infinitely length contracted, it has no length. Can it be said to exist?

3. If the ship has a "headlight"; in the RF of the observer, the light and the ship would reach her at the same time. However, in the RF of the astronaut, the light would be leaving the ship at c. Does this mean that the astronaut would see the observer, but she would not see the ship?
Re: Now you see it....... - 09/13/16 01:36 PM
Originally Posted By: Paul[/quote
and if you add c to any wavelenght in the visible light spetrum that wavelenght drops out of the visible light spectrum

A valid point, Paul. The more you think about a scenario like this, the more reasons you find for labeling it "impossible".
Re: Now you see it....... - 09/13/16 04:35 PM
A few thoughts arise from this.

1. If the ship is travelling at c, observer A would not see the disc until it hit her.

if observer A is not moving in reference to the point that the
speed measurements are taken from to obtain the speed of the
spacecraft then observer A would never see the spacecraft.

unless his / her eyes can detect the light from the flashlight
moving at 2xc

and the only light that is available for any observer to "see" is the light from the flashlight that is moving at 2xc

2. If the ship is infinitely length contracted, it has no length. Can it be said to exist?

there is no physical reason for the spaceship to be contracted
in length.

so fake math and illusion or not the ship would exist and would
not contract in length or width or anything unless there is
a force that crushes the spaceship and no force of that magnitude
has been included.

3. If the ship has a "headlight"; in the RF of the observer, the light and the ship would reach her at the same time.

the light from a headlight traveling at 2xc would reach the observer first followed by the ship that is moving at 1xc.

she (her eyes ) would never "see / detect" the light or the ship.
and would never feel the impact.

unless her pain receptors send signals to her brain faster than c
and her brain processes that data faster than c.

However, in the RF of the astronaut, the light would be leaving the ship at c. Does this mean that the astronaut would see the observer, but she would not see the ship?

even though the astronaut can see the light "inside " it is only because he and the ship are traveling at c , he would not be able
to see any light reflections that would reflect off of observer A

the astronaut would never "see" the observer because he is
moving towards the observer at 1xc and any light reflecting off
of the observer would be traveling at 1xc which adds up to 2xc

his "eyes" cannot detect light below the visible light spectrum.

Re: Now you see it....... - 09/14/16 02:56 AM

Originally Posted By: Bill S.
1. If the ship is travelling at c, observer A would not see the disc until it hit her.

Observer A might be behind the ship.

Originally Posted By: Bill S.
2. If the ship is infinitely length contracted, it has no length. Can it be said to exist?

It must exist, since it has infinite relativistic mass. But just how it would exist is a problem. That's what happens when you try to think about things that can't really happen.

Originally Posted By: Bill S.
3. If the ship has a "headlight"; in the RF of the observer, the light and the ship would reach her at the same time. However, in the RF of the astronaut, the light would be leaving the ship at c. Does this mean that the astronaut would see the observer, but she would not see the ship?

In the RF of the ship Observer A would be traveling at the speed of light. So the problems would be reversed.

Bill
Re: Now you see it....... - 09/19/16 07:41 PM
Originally Posted By: Bill
Observer A might be behind the ship.

In which case she would never see the ship, or even know it was there. Here I use “see” with due deference to Paul’s comments.

Quote:
It must exist, since it has infinite relativistic mass. But just how it would exist is a problem. That's what happens when you try to think about things that can't really happen.

True, but I’m quite happy with that, as I was looking for/at reasons why massive objects might not be able to accelerate to c.

As you might suspect; the concept of infinite relative mass interests me. I’m trying to get my head round all that it implies.

Quote:
In the RF of the ship Observer A would be traveling at the speed of light. So the problems would be reversed.

So neither would see the other, and both would measure the speed of the headlight as c; except that they would not “see” it, so could not measure it.

As you say: that's the problem with impossible scenarios.
Re: Now you see it....... - 09/22/16 01:06 AM
Quote:
except that they would not “see” it, so could not measure it.

If they ever begin to use propulsion systems that could cause a
spaceship to travel at c and beyond they could use something
similar to the einstein bose condensate to "see" where theyre going
while traveling at and beyond c but I would imagine? that em wave sensors would be used in conjunction with navigation systems to avoid hitting large objects that would not be deflected by the bow shock or whatever means of deflection they decide to use.

lets just hope that they have tossed the junk science in the
trash can long before they achieve c speed flight.