speed of light?
Discussion
Dr Jekyll said:
We never said that about the speed of sound, it's just that there are certain engineering challenges in making an aircraft that can exceed Mach1 without disintegrating or going out of control.
Very true. Plenty of objects were travelling faster than sound before aeroplanes could. Artillery shells, rifle bullets and V2 rockets were all supersonic before aeroplanes.The challenges for supersonic flight was designing an aeroplane that could maintain its controllability at speeds up to and beyond Mach 1. It was an engineering and design issue, not something that required a fundamental change in our knowledge of physics or the universe.
aizvara said:
annodomini2 said:
Accurately measuring the speed of light is impossible, what we have this far is an average.
I'm probably being stupid/misunderstanding what you said, but I thought we defined all SI units around the speed of light and planck time/distance, so there is no measurement of the speed of light, it is a constant.https://youtu.be/pTn6Ewhb27k
The point about radiation travelling faster than light in a medium is interesting. Light is often said to travel slower through certain mediums. Although the average speed of light through the medium is slower than in a vacuum, this is only because light is absorbed by the atoms of the substance and re-emitted.
When this isn’t happening, light always travels at c.
When this isn’t happening, light always travels at c.
Fusion777 said:
The point about radiation travelling faster than light in a medium is interesting. Light is often said to travel slower through certain mediums. Although the average speed of light through the medium is slower than in a vacuum, this is only because light is absorbed by the atoms of the substance and re-emitted.
When this isn’t happening, light always travels at c.
I don't think this is correct.When this isn’t happening, light always travels at c.
As I understand it, the photons interract with electrons in the atoms of the medium causing the electrons to vibrate, which then generates a second EM wave. This combines with the light's EM wave to form a slower EM wave through the medium.
If the atoms actually absorbed and then re-emitted the light, the re-emission would generally be in a random direction, which isn't the case as a beam of light through a transparent medium (e.g. glass) continues in the same direction within the medium.
Video with explanation of what happens (and also explains why absorbtion and re-emission isn't the case).
67Dino said:
I find it’s easier to think of the speed of light differently to the speed of anything else.
I think it’s easier to think of it as a measure of the speed of “now”, the rate at which the current moment moves through the Universe. Nothing can cause anything else to happen faster than the speed of light travelling the distance (entanglement aside, another matter).
So whilst it takes light 8 minutes to get to us from the sun, the photon itself hasn’t spent any time at all. As far as it’s concerned, the moment it left the sun and the moment it arrived at Earth were the same moment.
It is often said that we are seeing the stars in the sky as they were thousands or millions of years ago, but personally I think this is confusing matters, since “now” only moves at the speed of light. It’s really meaningless to create a different sort of simultaneity where time is the same there as here, as that doesn’t exist.
Just aswell really, as if everything happened at the same time everywhere, we’d be blasted by every supernova going, regardless where it was.
HTH.
Yeah, best described as the 'speed of causality'. I think it’s easier to think of it as a measure of the speed of “now”, the rate at which the current moment moves through the Universe. Nothing can cause anything else to happen faster than the speed of light travelling the distance (entanglement aside, another matter).
So whilst it takes light 8 minutes to get to us from the sun, the photon itself hasn’t spent any time at all. As far as it’s concerned, the moment it left the sun and the moment it arrived at Earth were the same moment.
It is often said that we are seeing the stars in the sky as they were thousands or millions of years ago, but personally I think this is confusing matters, since “now” only moves at the speed of light. It’s really meaningless to create a different sort of simultaneity where time is the same there as here, as that doesn’t exist.
Just aswell really, as if everything happened at the same time everywhere, we’d be blasted by every supernova going, regardless where it was.
HTH.
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For the two things moving away from each other see 'velocity addition formulae' for relativistic speeds like 1/2c x2: https://en.wikipedia.org/wiki/Velocity-addition_fo... ; also see 'frames of reference' for what all that means. This doesn't mean that an outside observer wouldn't observe and calculate as if they were, but for the objects themselves their 'frame of reference' is changed.
...and for the laser on the moon the dot produced isn't a physical object.
This is an interesting video on why the speed of light has the value it does:
https://m.youtube.com/watch?v=FSEJ4YLXtt8
https://m.youtube.com/watch?v=FSEJ4YLXtt8
Dr Jekyll said:
We never said that about the speed of sound, it's just that there are certain engineering challenges in making an aircraft that can exceed Mach1 without disintegrating or going out of control.
I'd be surprised if it wasn't thought impossible, just as at other times it was thought that iron boats would sink, that flying machines were impossible and that at 60mh it would be impossible to breathe.This video explains it quite clearly.
https://youtu.be/A2JCoIGyGxc
We have observed that everything in the universe moves through spacetime at the speed of light. So if your velocity through space is the speed of light you don’t move through time.
To move through space faster than the speed of light you would have to go backwards through time (see hypothetical particle the tachyon).
Although this isn’t intuitive it means that you never actually accelerate ie you never get “faster”. You just shift the balance between moving through space and time. That only becomes apparent at velocities through space close to the speed of light, which is it something we can experience so it isn’t intuitive to us.
https://youtu.be/A2JCoIGyGxc
We have observed that everything in the universe moves through spacetime at the speed of light. So if your velocity through space is the speed of light you don’t move through time.
To move through space faster than the speed of light you would have to go backwards through time (see hypothetical particle the tachyon).
Although this isn’t intuitive it means that you never actually accelerate ie you never get “faster”. You just shift the balance between moving through space and time. That only becomes apparent at velocities through space close to the speed of light, which is it something we can experience so it isn’t intuitive to us.
Wacky Racer said:
Given that the speed of light is 186,000 miles per second, and the sun is 93 million miles away.
Am I right in assuming that when you look up to the sun you don't see it as it is now, but how it was eight minutes before?
Yep.Am I right in assuming that when you look up to the sun you don't see it as it is now, but how it was eight minutes before?
A measure of the speed of light that’s handier in everyday life is “a foot per nanosecond”, which is about 98% precise (a nanosecond is a billionth of a second).
When you turn on a ceiling light it takes a few nanoseconds for it to reach you (after the electricity has got there from the switch which takes time too).
So when you look around, you are not really looking at objects experiencing any consistent “now”. What you’re seeing is a wave of present times coming in from around you and arriving at the same time, creating your own sense of “now”.
Edited by 67Dino on Tuesday 28th June 05:58
67Dino said:
Yep.
A measure of the speed of light that’s handier in everyday life is “a foot per nanosecond”, which is about 98% precise (a nanosecond is a billionth of a second).
When you turn on a ceiling light it takes a few nanoseconds for it to reach you (after the electricity has got there from the switch which takes time too).
So when you look around, you are not really looking at objects experiencing any consistent “now”. What you’re seeing is a wave of present times coming in from around you and arriving at the same time, creating your own sense of “now”.
Unless I dreamed it, I am sure I read once that when an electric circuit is switched on, the electric current starts to flow instantaneously all around the circuit. That is, the information that the circuit is ‘on’ travels faster than the speed of light.A measure of the speed of light that’s handier in everyday life is “a foot per nanosecond”, which is about 98% precise (a nanosecond is a billionth of a second).
When you turn on a ceiling light it takes a few nanoseconds for it to reach you (after the electricity has got there from the switch which takes time too).
So when you look around, you are not really looking at objects experiencing any consistent “now”. What you’re seeing is a wave of present times coming in from around you and arriving at the same time, creating your own sense of “now”.
Edited by 67Dino on Tuesday 28th June 05:58
Roofless Toothless said:
Unless I dreamed it, I am sure I read once that when an electric circuit is switched on, the electric current starts to flow instantaneously all around the circuit. That is, the information that the circuit is ‘on’ travels faster than the speed of light.
You may or may not have dreamed it, but that's definitely wrong. Information travelling faster than than the speed of light would (a) violate causality and all kinds of other laws that we currently understand to be inviolable, and (b) absolutely revolutionise communication.In reality, electrical signals travel quickly but not faster than light https://en.wikipedia.org/wiki/Speed_of_electricity
Skeptisk said:
Although this isn’t intuitive it means that you never actually accelerate ie you never get “faster”. You just shift the balance between moving through space and time.
So space and time are interchangeable, just as matter and energy are...Yes. If I stop the hands on my clock, time elsewhere keeps going but I stay at the time set - so I'm going back in time relative to elsewhere. Genius!
deckster said:
You may or may not have dreamed it, but that's definitely wrong. Information travelling faster than than the speed of light would (a) violate causality and all kinds of other laws that we currently understand to be inviolable, and (b) absolutely revolutionise communication.
In reality, electrical signals travel quickly but not faster than light https://en.wikipedia.org/wiki/Speed_of_electricity
A good explanation of the speed of electricity is here:In reality, electrical signals travel quickly but not faster than light https://en.wikipedia.org/wiki/Speed_of_electricity
https://www.youtube.com/watch?v=oI_X2cMHNe0
Showing that it travels at the speed of light, but is not how you might think as it can take short cuts and so provide power faster than light could travel the length of the wires in a circuit.
deckster said:
Wacky Racer said:
Given that the speed of light is 186,000 miles per second, and the sun is 93 million miles away.
Am I right in assuming that when you look up to the sun you don't see it as it is now, but how it was eight minutes before?
Yes. Which is why it's so interesting for us to observe objects that are 13 billion light years away; it's not so much that they're a long way away, it's more than we genuinely are seeing things which happened just after the universe began. In a very literal sense, viewing very far away objects is actually looking at the past.Am I right in assuming that when you look up to the sun you don't see it as it is now, but how it was eight minutes before?
It's going to be a very dark sky in a trillion years.
ChocolateFrog said:
Following on from that as everything is expanding and therefore moving away from us. Stuff that was observable to us yesterday is now out of sight forever today and ultimately in the distant future everything bar very local cluster will beyond our observable sphere.
Only if they're moving away at more than the speed of light, shirley? Otherwise the movement will just delay the light's arrival.Simpo Two said:
ChocolateFrog said:
Following on from that as everything is expanding and therefore moving away from us. Stuff that was observable to us yesterday is now out of sight forever today and ultimately in the distant future everything bar very local cluster will beyond our observable sphere.
Only if they're moving away at more than the speed of light, shirley? Otherwise the movement will just delay the light's arrival.ChocolateFrog said:
Following on from that as everything is expanding and therefore moving away from us. Stuff that was observable to us yesterday is now out of sight forever today and ultimately in the distant future everything bar very local cluster will beyond our observable sphere.
It's going to be a very dark sky in a trillion years.
I lost an earlier reply, but It's going to be a very dark sky in a trillion years.
'Gravity also moves at lightspeedish, so if the sun were to suddenly disappear we'd continue to orbit nothing for another 8 minutes [light = causality]
In those 8mins about 15M stars will disappear forever behind the universal event horizon, and if we started travelling now at lightspeed we'd only ever be able to catch about 4% of the current observable universe'
[ETA deleted 'local cluster', 'cos I's da fuul.]
Edited by andy_s on Tuesday 28th June 20:46
I remember that scientists at Cern had discovered neutrinos travelling faster than the speed of light a while back.
https://www.bbc.co.uk/news/science-environment-150...
Unfortunately somebody hadn't plugged the cable in fully.
https://www.bbc.co.uk/news/science-environment-171...
https://www.bbc.co.uk/news/science-environment-150...
Unfortunately somebody hadn't plugged the cable in fully.
https://www.bbc.co.uk/news/science-environment-171...
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