Question related to speed of light. (2 Viewers)

[Galaxiom]

I eventually found this article:-
http://www.smithsonianmag.com/smart-news/speed-light-can-vary-180953949/

What a rubbish article.

 

[The_Doc_Man]

After a quick peek at the Smithsonian article, I have to agree with Galaxiom. While there might be something there, it has been so heavily glossed over that it has no apparent scientific value.

The critical experiment of sending two beams through two paths is where we run into the scientific weakness. If the fiber is a constant consistency throughout its length, then the index of refraction of the medium will have a highly predictable effect on the time it takes light to traverse the fiber. The other path mentioned therein has no firm description but says it does something to the light's structure and then undoes it. The question is, what it does and how it does it.

I have a further quibble with their use of "structure of light." A beam of light has a shape because it is the sum of many individual sub-beams (on down to the photon level OR to the energy-packet wave-front level, pick your poison). But when you break it down to the lowest-level interactions, OF COURSE they take time.

If you are going to change things in your light beam in some way, that change implies interactions, which in turn imply some sort of electron-cloud involvement. The interaction is usually that something gets absorbed and re-emitted. Without this kind of interaction, the light components wouldn't react at all. The trick is that there is a configuration change in the atoms that absorbed the light and a corresponding change when the light gets emitted again. This configuration change is not instantaneous. We have two words, fluorescence and phosphorescence, to describe what is essentially a continuum of speeds of re-emission. Phosphorescence is the "slow" interaction, fluorescence is faster. But NEITHER of them are actually instantaneous. So if you have an interaction to change the light, OF COURSE it takes longer to finish the path. But did that actually change the structure of the light, or of the light beam that is the aggregate of the photons / wave fronts?

For that reason, I consider the article a bit weak. Or maybe more than a bit.

 

[Frothingslosh]

This does not change the fundamental value of the speed of light constant (c). The slowing is the delay caused by absorption and reemission of particles in the medium.

I neither said nor implied that it did. I simply pointed out to Mike that light DOES slow down based on medium.

 

[Galaxiom]

I neither said nor implied that it did. I simply pointed out to Mike that light DOES slow down based on medium.

The discussion was about "The Speed of Light", meaning "c", the speed of light in a vacuum.

In fact it is the only speed that photons travel. Any other measured speed of light is corrupted by the time between absorption and reemission when the photon hits something.

To improve on Mike's car analogy, lower speeds are measured if the driver pulls over between stints of driving at 100 mph.

 

[Frothingslosh]

Discussions often involve tangents, and if you'll look at the post directly above mine, you'll note he asked if a denser medium actually changes the speed of light.

That is why c is specifically defined as 'speed of light in a vacuum', not 'speed of light'.

So again, nothing in my response either stated or implied that the value of c was changeable in the slightest; I simply confirmed that light does change speeds based on medium, and that if light really did encounter a region of space with appreciable density, that yes, the effective speed at which that light traveled really would decrease.

 

[Galaxiom]

if you'll look at the post directly above mine, you'll note he asked if a denser medium actually changes the speed of light.

I believe Mike's question was rhetorical.

 

[Mike375]

If I calculate the kinetic energy of a rifle bullet then is it correct that my answer will be 99.9999999 etc.% of the true figure as the mass of the bullet will have increased?

Likewise, if we had a ballistic pendulum then it would swing back by a greater amount than the transfer of momentum would illustrate?

 

[Galaxiom]

Mass, length, time and hence velocity and kinetic energy are all affected by Relativity.

The only property that measures the same from all frames of reference is momentum, the product of mass and velocity. Work backwards from that and all the answers can be derived.

Remember it is the relative motion of the object and the measurement frame of reference that matters.

 

[Galaxiom]

BTW Earlier it was pointed out that time stops for the speed of light space traveller. However this does not give them the impression that they travelled at infinite speed because the distance to the destination also becomes zero as observed by the traveller.

 

[Mike375]

Back to my original question as to time and mass if our traveller slows down.

Let's take the Apollo astronauts. Stage 3 reignites and takes them to escape velocity of about 26,000 mph and the stage is shut down and earhs gravity slows them to about 3000 mph when they are about 30,000 miles from the moon at which point the moon's gravity has more influence than Earth's.

Does time speed up for them during this "slow down" and does mass decrease?

Now to the next part. When the Lunar Module docks with the command and service module the service module fires up and takes them to about 5,000 mph to moon escape velocity and then they gradually accelerate to about earth's escape velocity of 26,000 mph and re entry. Does this cancel out of "even up" what has happened with time/mass on the trip to the moon?

As a side note, I was able to secure about a half an hour session with Eugene Cernan (Apollo 17) when he came to Australia....I have been a space freak since I was a young kid. The one thing that stands out from that meeting with him was his total inability to convey/explain what it was like to sit on top of the Saturn V as ignition sequence started and of course hitting the starter button on the Luna Module to leave the moon.

It is no wonder so many of those blokes finished up going a bit "funny" as not much else you could do......A Formula 1 is not quite like sitting 36 stories up while propellant is being consumed at 15 tons per second under you.

 

[The_Doc_Man]

Again, there is no slowdown. There is only a change of relative velocity. "Slow down" (as you are trying to use it in this discussion) implies that there might be an absolute frame of reference and thus there would be a strange effect of suddenly going at a "true velocity of 0" - but not only do we not think that such a thing CAN occur, but even if it DID, we don't think we would know it happened (since we still see things relatively). Your questions all seem to ignore the fact that everything you measure is relative to your own frame of reference, or to some other frame of reference that you can observe conveniently.

All you can ever do is ACCELERATE in a direction to cancel existing velocity with respect to your frame of reference, which is to say that if you have layered frames of reference, an outside observer in the outermost frame would see you and that space capsule synchronize speeds. I.e. the capsule would join your frame of reference. Thereafter, if there WERE any time dilation effects, you and the capsule would have the same level of that dilation.

 

[Mike375]

Your questions all seem to ignore the fact that everything you measure is relative to your own frame of reference, or to some other frame of reference that you can observe conveniently.

I don't that way at all. For example if I leave the earth and accelerate up towards the speed of light and return to earth several years have passed on earth but me 1 year or whatever has passed.

If we take two cars that leave each other at 50 mph then 1 hour later they are 100 miles apart. However, if one car remains stationery and the other car departs at 100 mph then again the cars a 100 miles apart after 1 hour but it will be the car that accelerated to 100 mph that experiences the time change and increase in mass. At least I think that is how it works.

I think I might go and play with Access for a while

 

[The_Doc_Man]

If we take two cars that leave each other at 50 mph then 1 hour later they are 100 miles apart. However, if one car remains stationery and the other car departs at 100 mph then again the cars a 100 miles apart after 1 hour but it will be the car that accelerated to 100 mph that experiences the time change and increase in mass. At least I think that is how it works

Nope, not how it works. Let's call that car M and car S. If car M moves and car S stands still (according to your setup),

- Car M moves at 100 MPH with respect to (w.r.t.) Car S, and thus experiences the mass and time effects based on an observer in car S.

- Car S moves at 100 MPH w.r.t. Car M, and thus appears to have the mass and time effects based on an observer in car M.

If you have an outside observer O (onlooker) who is standing still, this person is in the same frame of reference as car S, so isn't actually an outside observer.

If you have a traffic observer H (helicopter traffic report) whose pilot happens to be exactly following car M, this person is in the same frame of reference as car M, so isn't actually an outside observer.

If you have a passenger in a third car (Car T) moving at 50 MPH w.r.t. to car M, this person sees both Car M and Car S moving at 50 MPH away from him and sees corresponding changes in time and mass. (Miniscule for 50 MPH, but still there.)

Ain't relativity grand?

 

[Mike375]

If you have a passenger in a third car (Car T) moving at 50 MPH w.r.t. to car M, this person sees both Car M and Car S moving at 50 MPH away from him and sees corresponding changes in time and mass. (Miniscule for 50 MPH, but still there.)

Ain't relativity grand?

But it is Car M (its driver) that sees the greatest change in mass and time?

 

[Galaxiom]

But it is Car M (its driver) that sees the greatest change in mass and time?

No. A Doc said, it is all about relative motion.

None of the drivers see any change in the mass of their own car.

 

[Mike375]

No. A Doc said, it is all about relative motion.

None of the drivers see any change in the mass of their own car.

I don't get it.

If our intrepid traveller jumps on a light beam them from his perspective he reaches a planet at Alpha Centauri in zero time and there was zero distance. However, for observers on earth and the planet at Alpha Centauri he has travelled about 4 light years and he has travelled for about 4 years.

When he arrives at the planet they send a message back to earth. Our traveller spends 24 hours on the planet and then jumps on the light beam to head back to earth.

When he arrives back at earth 24 hours of his time will have passed and the message sent from the planet at Alpha Centauri will have arrived 24 hours before he arrived.

For the people on earth he will have been away for 8 years plus the 24 hours he spent at Alpha Centauri.

When he arrives back at earth they send a message to Alpha Centauri which arrives 8 years plus 24 hours after they sent their message to earth on his arrival at Alpha Centauri

Is this correct?

 

[The_Doc_Man]

When he arrives back at earth 24 hours of his time will have passed and the message sent from the planet at Alpha Centauri will have arrived 24 hours before he arrived.

Assuming instantaneous acceleration to speed c, yes. And your problem is?

I think that where you run into SERIOUS problems with this thought experiment is that by moving at an impossible speed you get an impossible result.

Shall I trigger yet another blown mind? If you could somehow convert to antimatter, you could travel as a tachyon, which can move faster than light ... but never slower. In fact, in theory a tachyon takes LESS anti-energy to move at speeds much faster than light. The closer to c it gets, the more massive (in anti-matter terms) the tachyon gets. Because the Lorenz-Fitzgerald equations work for tachyons, too. They just have a whole bunch of imaginary numbers floating around.

 

[Galaxiom]

Assuming instantaneous acceleration to speed c, yes. And your problem is?

I think that where you run into SERIOUS problems with this thought experiment is that by moving at an impossible speed you get an impossible result.

I don't see anything in the thought experiment inconsistent with Relativity.

Mike is actually struggling with the concept that it is the difference between the travellers' frame and the observers' frame that is the whole story with Relativity.

 

[Frothingslosh]

I don't see anything in the thought experiment inconsistent with Relativity.

Mike is actually struggling with the concept that it is the difference between the travellers' frame and the observers' frame that is the whole story with Relativity.

This.

The sticking point seems to be that he's welded to the idea of an absolute frame of reference, and seems to have some difficulty with the idea that there is literally no such thing as 'slowing down' or 'de-acceleration' when discussing physics, at least in regards to Relativity.

From what I've seen before, that's definitely one of the most difficult points for most people when they start wrestling with Relativity.

Asked in 1919 whether it was true that only three people in the world understood the theory of general relativity, [Arthur Stanley Eddington] allegedly replied: 'Who's the third?

 

[Mike375]

Let's try a real world one instead.

Astronaut leaves earth to dock and board the ISS.

He goes from zero to low earth orbit at about 17,500 mph in about 10 minutes. During this phase what happens with his time as compared to back on earth. My understanding has been his clock will be slightly behind earth time.

He stays on the ISS for 6 months. What happens to time in this "steady" velocity situation?

He then re enters and so goes from 17,500 mph to zero. What happens to time during this phase?

Lastly, it is simple to calculate the kinetic energy and momentum of a rifle bullet. However, is the "true" KE and momentum 1.0000000 etc.% greater than my calculation gives. If the bullet strikes a ballistic pendulum does the pendulum swing back a distance that is 1.0000000 etc.% greater than the calculation would show?