General Relativity

[Bombastinator]

3 hours ago, tikker said:

I've only noticed rising more quickly it the last few years in my environment. Could be another side effect of the interent of course, the world is much more connected now. most notably for me currently is that they're argueing with our government, for example, about their measures to keep the virus in check. Things like saying they are ungrounded because there is no emergency and the decisions have been based on predictions from "just some theories" that "don't necessarily reflect reality" and "have uncertainty associated with them".

I could agree that theories start with opinions, and that to a certain extent you could say a theory is "the opinion of many". However, they are much more than mere opinion. Opinions stop at "blue is the best colour". Theories are abstract frameworks made to help explain and think about something (supporting your opinion perhaps). In the context of this thread, it's the Theory of General Relativity, because it introduces the concepts of e.g. spacetime and its curvature to help us explain the law of gravity. You can say that the current scientific opinion is that GR is correct so far, as it has withstood numerous tests.

I feel this is the core comment really. Not so much argueing about science in general, but people ignoring inconvenient details. The "I can't see it so it doesn't exist" mentality.

There does seem to have been a gigantic jump in that sort of behavior in tha last few years.  It’s been building since the mid 80’s though.  It’s the combination of to almost but not actually quite right things that when combined together create complete BS.  It’s part of a general trend.  I take this point and this other point and combine them with this other thing that is generally crap and come up, unsurprisingly, with crap. But crap containing a thing wrapped in it. This is done over and over again.

[Dash Lambda]

On 2/20/2021 at 3:08 PM, dalekphalm said:

It should be noted that speed of light is constant in a vacuum. That's essentially the "top speed" of light. As light travels through different mediums (eg: Glass, air (earth atmosphere), water, etc) it has various different speeds, depending on the characteristics of those mediums.

 

Light traveling through water is slower than light traveling through the vacuum of space.

 

Why is light constant (medium dependent)? No one really knows why for sure. There are lots of theories and we can 100% verify this using observation and experimentation.

I feel like this phrasing is a little misleading. Light in transit, as far as we know, only and always travels at exactly c. The propagation of light through a medium will be slower because it is absorbed and re-emitted by anything in its path. So it's not that the speed of a photon is lower, it's that it turns into a chain of photons going from atom to atom with tiny pauses between being absorbed and emitted.

 

As for the question of why, there are many different ways to approach it, and we can answer some of them.

- What makes a photon different from, say, an electron? An electron has rest mass, whereas a photon is massless. So, while an electron can exist at rest, a photon can only exist when moving, and so it must be moving relative to all inertial reference frames, which means it's moving at the speed of light.

- What makes a photon travel at the same speed from every perspective? There's a fundamental connection between time and space that distorts concepts like duration and distance with respect to velocity and energy, where other features change to keep the speed of light constant.

- What is the purpose of a constant speed of light? Physics cannot answer that question. Not because physics as an entity is insufficient, but because the field of physics describes how things work, it doesn't address the meaning behind them. That's an engineering question, you'd have to ask the engineer who built the universe... whatever that means...

[Wictorian]

6 hours ago, Dash Lambda said:

I feel like this phrasing is a little misleading. Light in transit, as far as we know, only and always travels at exactly c. The propagation of light through a medium will be slower because it is absorbed and re-emitted by anything in its path. So it's not that the speed of a photon is lower, it's that it turns into a chain of photons going from atom to atom with tiny pauses between being absorbed and emitted.

 

As for the question of why, there are many different ways to approach it, and we can answer some of them.

- What makes a photon different from, say, an electron? An electron has rest mass, whereas a photon is massless. So, while an electron can exist at rest, a photon can only exist when moving, and so it must be moving relative to all inertial reference frames, which means it's moving at the speed of light.

- What makes a photon travel at the same speed from every perspective? There's a fundamental connection between time and space that distorts concepts like duration and distance with respect to velocity and energy, where other features change to keep the speed of light constant.

- What is the purpose of a constant speed of light? Physics cannot answer that question. Not because physics as an entity is insufficient, but because the field of physics describes how things work, it doesn't address the meaning behind them. That's an engineering question, you'd have to ask the engineer who built the universe... whatever that means...

As I said before, keeping light constant sounds like cheating. Is it impossible to surpass the speed of light? If so how do neutrinos do it?

[Jet_ski]

Time is measured with a frame of reference. On earth we measure time based on the rotation of earth around the sun, 365x24x60 seconds. If you think about it our measure of time actually depends on the velocity of the earth’s rotation.

 

In relativity, time is measure wrt the speed of light in vacuum which is its maximum. But light can also travel slower, in water for instance. So if you measure time in terms of light instead of earths rotation, then from our frame of reference, time can be slower somewhere in space because light isn’t traveling at its maximum speed!

[Dash Lambda]

54 minutes ago, Wictorian said:

As I said before, keeping light constant sounds like cheating. Is it impossible to surpass the speed of light?

It's not cheating, it's an observation and a foundational assumption that abruptly made our models categorically more powerful and accurate.

Have we dipped our hands into the great glowing coils of the universe and found the line in reality's specs that says the speed of light is constant? No.
Does everything work out eerily well when you consider the speed of light constant? Yes, to the point that it's integral to most of modern physics.

Could we be wrong? Sure, that's how science works.

Do we have any indication that we're wrong so far? No, it's just counterintuitive at first.

 

1 hour ago, Wictorian said:

If so how do neutrinos do it?

They don't, the ones we detect generally go something like 99.999999999999% the speed of light. In fact, neutrinos reaching the surface of the Earth is an excellent example of something only GR can explain. Their lifetimes are so short that in classical terms they should decay in the upper atmosphere, but because they travel so close to the speed of light time dilation extends their lifespan long enough for them to hit the ground.

[dalekphalm]

15 hours ago, Dash Lambda said:

I feel like this phrasing is a little misleading. Light in transit, as far as we know, only and always travels at exactly c. The propagation of light through a medium will be slower because it is absorbed and re-emitted by anything in its path. So it's not that the speed of a photon is lower, it's that it turns into a chain of photons going from atom to atom with tiny pauses between being absorbed and emitted.

 

As for the question of why, there are many different ways to approach it, and we can answer some of them.

- What makes a photon different from, say, an electron? An electron has rest mass, whereas a photon is massless. So, while an electron can exist at rest, a photon can only exist when moving, and so it must be moving relative to all inertial reference frames, which means it's moving at the speed of light.

- What makes a photon travel at the same speed from every perspective? There's a fundamental connection between time and space that distorts concepts like duration and distance with respect to velocity and energy, where other features change to keep the speed of light constant.

- What is the purpose of a constant speed of light? Physics cannot answer that question. Not because physics as an entity is insufficient, but because the field of physics describes how things work, it doesn't address the meaning behind them. That's an engineering question, you'd have to ask the engineer who built the universe... whatever that means...

You're right in that what I was saying was heavily dumbed down.

 

Light "travels" slower in other mediums not because the photon actually slows down, but because it's traveling through a medium with a higher rate of refraction (meaning, in short, the light reflects off of objects - often just atoms, bouncing the light around, making it take a longer path to the same destination).

 

Meaning that when light travels through water, for example, it would be comparable to traveling through the vacuum of space, but there are little mirrors placed all over the place bouncing the photon about as it goes along it's journey.

8 hours ago, Wictorian said:

As I said before, keeping light constant sounds like cheating. Is it impossible to surpass the speed of light?

Why does it sound like cheating? In every example we've tested, it's constant. As far as we know it's impossible to surpass the speed of light - though there are... well - areas of weirdness when it comes to Quantum Physics.

Particularly when looking at Quantum Entanglement (this is a phenomenon where two particles are paired on a quantum level, and interacting with one of those particles produces an effect on the other particle). This effect is seemingly instantaneous, regardless of distance, and has been shown in experimentation to seemingly react faster than light.

 

This SOUNDS amazing on paper, and the layman (myself included at times) might think Quantum Entanglement allows us to subvert the speed of light. Many Sci-Fi stories and universes use this as a means of FTL communication.

 

The problem is that the math tells us that while we can affect one of the pairs and measure that effect on the other, we can't actually transmit information using the pairs. Not yet anyway. Maybe one day.

 

But that's ultimately 

8 hours ago, Wictorian said:

If so how do neutrinos do it?

They don't.

 

There was an experiment in 2011 that measured FTL speeds for neutrinos. The test was faulty though - they had a loose fibre optic cable that was causing the bogus readings.

[LordVetinari]

45 minutes ago, dalekphalm said:

Particularly when looking at Quantum Entanglement (this is a phenomenon where two particles are paired on a quantum level, and interacting with one of those particles produces an effect on the other particle). This effect is seemingly instantaneous, regardless of distance, and has been shown in experimentation to seemingly react faster than light.

 

This SOUNDS amazing on paper, and the layman (myself included at times) might think Quantum Entanglement allows us to subvert the speed of light. Many Sci-Fi stories and universes use this as a means of FTL communication.

 

The problem is that the math tells us that while we can affect one of the pairs and measure that effect on the other, we can't actually transmit information using the pairs. Not yet anyway. Maybe one day.

 

 

Not to mention the slight bugger that it changes and collapses when you measure it. 

[Dash Lambda]

2 hours ago, dalekphalm said:

You're right in that what I was saying was heavily dumbed down.

 

Light "travels" slower in other mediums not because the photon actually slows down, but because it's traveling through a medium with a higher rate of refraction (meaning, in short, the light reflects off of objects - often just atoms, bouncing the light around, making it take a longer path to the same destination).

 

Meaning that when light travels through water, for example, it would be comparable to traveling through the vacuum of space, but there are little mirrors placed all over the place bouncing the photon about as it goes along it's journey.

The critical thing to clarify here is that 'bounce' means absorption and re-emission. A photon cannot simply hit something and change direction as it always travels in a straight line through spacetime, each time it bounces it's actually a different photon going in the new direction.

 

I agree that the path it takes is also significant in the propagation speed, I forgot about that.

[dalekphalm]

9 minutes ago, Dash Lambda said:

The critical thing to clarify here is that 'bounce' means absorption and re-emission. A photon cannot simply hit something and change direction as it always travels in a straight line through spacetime, each time it bounces it's actually a different photon going in the new direction.

Interesting distinction.

 

Now, this is probably getting outside the level of physics that most people have ever learned. I took High School physics in Grade 11 and 12 and we didn't touch on that at all.

 

But after doing some Googling, you're correct it seems:

https://phys.org/news/2007-01-mirror.html#:~:text=A typical household mirror works,surface of its metal backing.

Quote

A typical household mirror works like this: Photons (particles of light) bounce off an object or person, hit the mirror, and are absorbed by electrons on the surface of its metal backing. The electrons almost instantly emit “reflected” photons (not the same photons that came in, as those are absorbed and gone), which travel to our eyes, allowing us to see our image. Photons that strike the mirror head-on are reflected squarely back, and those hitting at an angle are reflected at the same angle in the other direction, forming a V-shaped path. This is the law of reflection.

The interesting thing here is that it only talks about the absorption by the mirror itself, not when it bounces off of other objects.

 

Also, reading about light, most sources that explain it simply refer to the action as "bouncing" or "reflecting" and don't really mention anything about absorption and re-emission.

9 minutes ago, Dash Lambda said:

I agree that the path it takes is also significant in the propagation speed, I forgot about that.

I mean that's really the only way to explain why light traveling through - say - water - is slower than when in a vacuum. If the individual photons are still traveling at the same speed, they must be taking a longer path to get to the end.

[Dash Lambda]

8 minutes ago, dalekphalm said:

Also, reading about light, most sources that explain it simply refer to the action as "bouncing" or "reflecting" and don't really mention anything about absorption and re-emission.

I suppose the reason is that it's usually correct enough, but when the discussion is about GR and the speed of light you need to go a little deeper.

 

10 minutes ago, dalekphalm said:

I mean that's really the only way to explain why light traveling through - say - water - is slower than when in a vacuum. If the individual photons are still traveling at the same speed, they must be taking a longer path to get to the end.

There's also a delay between absorption and re-emission. Usually it's very fast, but still not instantaneous.

For example, when people say that scientists brought light to a 'standstill' in a lab, what they're referring to is that the scientists were able extend the time between absorption and re-emission to be arbitrarily long.