General Relativity

[Wictorian]

6 minutes ago, Dash Lambda said:

  

Light travels at the speed of light relative to everything -so when we observe light, to us it travels exactly 299,792,458 m/s. If it's coming out of a moving flashlight, no matter how fast the flashlight is moving, the light is always going at the speed of light. That's why we get red/blue-shifting, because the wave gets stretched out or scrunched up.

 

Now, if you were to think about a photon's perspective, no time would pass. Photons don't change or evolve, they don't change direction (light bending around planets and stuff is technically it following a straight line, but that's GR being weird), they just appear and disappear. There's an effect that comes out of special relativity called length contraction, where the faster you're going the shorter distances appear to be, and due to length contraction, a photon would perceive the universe as totally flat, with no distance between where the photon begins and ends.

 

Physicists don't like referring to a photon's perspective, though. As I understand it, anything traveling at the speed of light doesn't actually have an inertial reference frame, so it can't be an 'observer' in the same way a slow-moving object can.

 

So back to the bus thing, if you have a bus going 99% the speed of light and you shoot a laser towards the front, the photons will still be going at the speed of light from every perspective. To someone outside the bus the frequency of the light is shifted super high, but to an observer on the bus the frequency is 'normal'. You can connect these two perspectives with length contraction -if you think about the light as a sine wave, then both observers see the same number of ups and downs, but the external observer literally sees a shorter bus, squeezing those waves into a shorter length.

 

If you extend that behavior to a bus traveling at the speed of light, then the bus would have no length. That means the external observer would see the photon and the bus travel together, at the same speed.

What makes light so special? (the pun) I can travel at different sppeds, why can't light?

 

What if you travel at the speed of the light for 1 hour (relative to us) and then come back to earth and slow down, will you think no time has passed and it will be like you fell asleep and woke up?

[Wictorian]

Just now, Wictorian said:

@tikker
What makes light so special? (the pun) I can travel at different sppeds, why can't light?

 

What if you travel at the speed of the light for 1 hour (relative to us) and then come back to earth and slow down, will you think no time has passed and it will be like you fell asleep and woke up?

 

[tikker]

30 minutes ago, Wictorian said:

Let's say you are travelling from A to B at the speed of light, and light is travelling from B to A. At the moment you meet, what do you see? Is it the same thing? You just see it at the speed of light and time slows down.

We can't travel at the speed of light, but say we can we still wouldn't see much. Ignoring reflections, the light to the side of us would just travel away from us at the speed of light. Light behind us would also travel away at the speed of light so it would just be dark as well. The light coming towards us would be hugely into super high energy radiation far outside the visible spectrum so we also wouldn't see anything because we would be too buys being vapourised.

 

With regards to the speed of light, we would "see" light coming towards us at the speed of light.

 

30 minutes ago, Wictorian said:

Also if an astronaut aged 5 years when we aged more, did he feel 5 years is past or more?

Time dilation is symmetric, because it's all relative. To us the astronaut is moving, but to the astronaut we are moving. That means

1. In our reference frame time is moving normally for us and the astronaut moves slower, because of time dilation.
2. In the astronaut's reference frame time moves normally for them and we move slower, because of time dilation.

(ignoring gravitational time dilation effects due to the astronaut being further from Earth)

 

17 minutes ago, Wictorian said:

What makes light so special? (the pun) I can travel at different sppeds, why can't light?

Being light. Maxwell's equations also show that the speed of light is a fixed value. Once derived, you end up with a so-called wave equation for electromagnetic radiation (i.e. light), in which the speed is defined by constants. From this it sort of intuitive to reason that there is no rest frame for photons. In that frame the photon would be stationary, except you have just shown that the speed of photons is a constant, so by proof of contradiction you can never "view" something form the perspective of a photon.

 

17 minutes ago, Wictorian said:

What if you travel at the speed of the light for 1 hour (relative to us) and then come back to earth and slow down, will you think no time has passed and it will be like you fell asleep and woke up?

You can't travel at the speed of light, because you have mass and only massless particles can travel at the speed of light. Once you attain light speed, as @Dash Lambda says, time and space break down. Distance doesn't exist anymore nor do time intervals.

[Dash Lambda]

  

3 minutes ago, Wictorian said:

What makes light so special? (the pun) I can travel at different sppeds, why can't light?

 

What if you travel at the speed of the light for 1 hour (relative to us) and then come back to earth and slow down, will you think no time has passed and it will be like you fell asleep and woke up?

Actually, there's nothing special about light aside from its abundance and interactivity. There are other particles and phenomena that travel at the speed of light, but photons are the easiest ones for us to play with, so we focus on light -hence the name 'speed of light'.

 

If you have mass you cannot travel at the speed of light, because accelerating to the speed of light would require unlimited energy. Things that travel at the speed of light are thus massless, but they have energy due to some weird phenomena that I'd have to read up on again, but the root of it is that they're traveling at the speed of light.

 

So, in order to 'exist' you need to represent some quantity of energy. If you have mass you have energy, and if you have no mass you must be traveling at the speed of light to have energy.

 

As for the second question, yes, if you traveled from A to B at the speed of light, from your perspective it would be instantaneous.

[tikker]

6 minutes ago, Dash Lambda said:

Actually, there's nothing special about light aside from its abundance and interactivity. There are other particles and phenomena that travel at the speed of light, but photons are the easiest ones for us to play with, so we focus on light -hence the name 'speed of light'

It was simply first I think. It'd be years or decades after Maxwell's equations before we would start inventing other massless particles that also travel at light speed.

11 minutes ago, Dash Lambda said:

but they have energy due to some weird phenomena that I'd have to read up on again, but the root of it is that they're traveling at the speed of light.

Thinking of the four-momentum perhaps? Energy = rest mass + momentum. Photons have no rest mass but have momentum, so they have energy.

[PCGuy_5960]

1 hour ago, Wictorian said:

What makes light so special? (the pun) I can travel at different sppeds, why can't light?

Light is an electromagnetic wave, so if you do the math and derive the wave equation you'll see that the speed of light depends on two constants (which only depend on the medium light is travelling in). More specifically, the speed of light is 1/sqrt(μ*ε). 

[PCGuy_5960]

1 minute ago, James Evens said:

But is it a wave or is it a particle? Is it maybe both? Maybe something entirely different?

True, but considering light as an electromagnetic wave is probably the simplest way of explaining why it has a constant velocity.

[Wictorian]

13 hours ago, tikker said:

We can't travel at the speed of light, but say we can we still wouldn't see much. Ignoring reflections, the light to the side of us would just travel away from us at the speed of light. Light behind us would also travel away at the speed of light so it would just be dark as well. The light coming towards us would be hugely into super high energy radiation far outside the visible spectrum so we also wouldn't see anything because we would be too buys being vapourised.

 

With regards to the speed of light, we would "see" light coming towards us at the speed of light.

 

Time dilation is symmetric, because it's all relative. To us the astronaut is moving, but to the astronaut we are moving. That means

1. In our reference frame time is moving normally for us and the astronaut moves slower, because of time dilation.
2. In the astronaut's reference frame time moves normally for them and we move slower, because of time dilation.

(ignoring gravitational time dilation effects due to the astronaut being further from Earth)

 

Being light. Maxwell's equations also show that the speed of light is a fixed value. Once derived, you end up with a so-called wave equation for electromagnetic radiation (i.e. light), in which the speed is defined by constants. From this it sort of intuitive to reason that there is no rest frame for photons. In that frame the photon would be stationary, except you have just shown that the speed of photons is a constant, so by proof of contradiction you can never "view" something form the perspective of a photon.

 

You can't travel at the speed of light, because you have mass and only massless particles can travel at the speed of light. Once you attain light speed, as @Dash Lambda says, time and space break down. Distance doesn't exist anymore nor do time intervals.

What if a bus is travelling at 2/3c and we are travelling at 2/3c too from the opposite direction? If c was smaller I think we would see the bus travelling faster than c. 

So if the astronaut aged 5 years he felt 5 years, right?

 

Idk this sounds like cheating. Because photons already do. 
 

Also I think scientist are over thinking about measuring the speed of light. First, why can't the clocks be synchronized? You can send a signal from a clock that is equally far to others. Or you can measure how much is the difference between them. You can simply send a light beam and measure the time it took to hit the censor instead of a mirror.

 

[Bombastinator]

56 minutes ago, Wictorian said:

What if a bus is travelling at 2/3c and we are travelling at 2/3c too from the opposite direction? If c was smaller I think we would see the bus travelling faster than c. 

So if the astronaut aged 5 years he felt 5 years, right?

 

Idk this sounds like cheating. Because photons already do. 
 

Also I think scientist are over thinking about measuring the speed of light. First, why can't the clocks be synchronized? You can send a signal from a clock that is equally far to others. Or you can measure how much is the difference between them. You can simply send a light beam and measure the time it took to hit the censor instead of a mirror.

 

Because the signal can’t travel faster than light.  The signal takes time to move too.  Light speed delay to Mars is apparently a real problem for the lander.  Something like 11 minutes?  If you look at Mars through a telescope on earth you’re not seeing Mars.  You’re seeing what Mars was doing 11 minutes ago.  As to the bus thing I’m not sure what would be seen but it wouldn’t look much like a bus. Something about red shift?  

[Wictorian]

1 hour ago, Bombastinator said:

Because the signal can’t travel faster than light.  The signal takes time to move too.  Light speed delay to Mars is apparently a real problem for the lander.  Something like 11 minutes?  If you look at Mars through a telescope on earth you’re not seeing Mars.  You’re seeing what Mars was doing 11 minutes ago.  As to the bus thing I’m not sure what would be seen but it wouldn’t look much like a bus. Something about red shift?  

I am asking about what speed would we see the bus travelling at. Actually if light speed isn't same in all directions then isn't it travelling faster than the speed of light?

[LordVetinari]

The speed of light given for 'c' (the universal constant) is the speed of light in a vacuum. 

 

You can slow light down in real life, but you can't get it to exceed 'c'. 

[tikker]

4 hours ago, Wictorian said:

What if a bus is travelling at 2/3c and we are travelling at 2/3c too from the opposite direction? If c was smaller I think we would see the bus travelling faster than c.

http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/einvel2.html#c1

Once things get relativistic the math changes. You can no longer just add velocities. In you example we would see the bus coming towards us at 0.92c. Still neatly below the speed of light.

2 hours ago, Wictorian said:

Actually if light speed isn't same in all directions then isn't it travelling faster than the speed of light?

No, it's just travelling at a different speed in different directions then. If you walk forwards quickly, but backwards slowly, are you travelling faster than yourself?

4 hours ago, Wictorian said:

Also I think scientist are over thinking about measuring the speed of light. First, why can't the clocks be synchronized?

This is one of those apparently simple, but actually deeply profound and complicated questions. Take a look at the Veritasium video again from 1:50 to 3:05.

4 hours ago, Wictorian said:

You can send a signal from a clock that is equally far to others.

The signal will arrive at those two clocks with a delay set by the speed of light, so you will not know when the clocks start ticking.

4 hours ago, Wictorian said:

Or you can measure how much is the difference between them.

Nicely explained by Veritasium. You cannot measure the difference between the clocks, because that difference is set by the speed of light, which you are trying to measure.

4 hours ago, Wictorian said:

You can simply send a light beam and measure the time it took to hit the censor instead of a mirror.

Also in the Veritasium video. With a single clock you'll never know when the light hit the detector. You need a second clock which you need to synchronize with the first and that is problematic.

[Rym]

On 2/18/2021 at 9:10 PM, Wictorian said:

I am no physicist but I find it hard to believe that time slows down when you approach the speed of light or with immense gravity. I know its about human psychology but is it 100% true or is there a chance it is wrong?

Well, it might not be time itself that slows down, but your perception of time. The entire universe is subject to the eyes of the beholder, the universe itself is meaningless and irrelevant in it's existence if there is no beholder (observer, someone to acknowledge it exists). It is like a flower that no matter how beautiful it is, it's irrelevant if there is no one to observe it's beauty.

 

Science still has a long way to go, and if you delve into quantum physics you will notice physically impossible things happening. Such as atoms re-arranging themselves into specific shapes that the observer expects them to be in, or at times atoms phasing out of existence itself and phasing back in at different locations. Literally ceasing to exist one moment and re-entering existence the next.

[Wictorian]

1 hour ago, tikker said:

No, it's just travelling at a different speed in different directions then. If you walk forwards quickly, but backwards slowly, are you travelling faster than yourself?

for example if light travels at c/2 to Mars, it wil travel instantly to earth. Instantly is faster than c.

[tikker]

30 minutes ago, Wictorian said:

for example if light travels at c/2 to Mars, it wil travel instantly to earth. Instantly is faster than c.

Yes the definition of the speed of light will then probably be adjusted if that is really the case. However, half the distance at c/2 plus half the distance instantly still gives a speed of c on average.

[Commodus]

This thread ultimately amounts to "recap decades of well-established physics research for me." 

 

Now, if you really want to break your mind, study what happened in the first seconds (or even fractions of a second) after the Big Bang. Many of the elements of the universe we take for granted, like photons, weren't there at the very start; it took a whole bunch of interactions in a very short period for things to settle down, so to speak. And even then, there were "Dark Ages" where there was no visible light until the first stars and galaxies formed.

[tikker]

16 minutes ago, Commodus said:

This thread ultimately amounts to "recap decades of well-established physics research for me." 

 

Now, if you really want to break your mind, study what happened in the first seconds (or even fractions of a second) after the Big Bang. Many of the elements of the universe we take for granted, like photons, weren't there at the very start; it took a whole bunch of interactions in a very short period for things to settle down, so to speak. And even then, there were "Dark Ages" where there was no visible light until the first stars and galaxies formed.

Doesn't make it less worth asking. Discussing with people is very different from just reading literature. The Big Bang isn't inherently a "better" or more interesting topic to study and its conception is also decades old already and a well established theory by now.

[Commodus]

38 minutes ago, tikker said:

Doesn't make it less worth asking. Discussing with people is very different from just reading literature. The Big Bang isn't inherently a "better" or more interesting topic to study and its conception is also decades old already and a well established theory by now.

Oh, it's certainly good to ask! The OP just started by suggesting that scientists might've got it wrong... well, if they did, it's something we can't really know. Not for a long time, at least!

 

I was suggesting the first moments after the Big Bang more as a subject of pure fascination than anything. It's wild that the very 'rules' of the universe were in flux for a very brief moment.

[Wictorian]

51 minutes ago, Commodus said:

The OP just started by suggesting that scientists might've got it wrong...

LOL. 

[Bombastinator]

10 minutes ago, Wictorian said:

LOL. 

Well “it must be wrong because I don’t understand” has been a pretty common thing lately.   It drove US politics for several years.  Society and science have become things that are too large for a single human mind to encompass for hundreds of years now.  One has to simply trust in the capacity of others to do as much as they can. I remember a quote once about magic being science we don’t understand.  We as people are fated to not understand most of what is going on around us.  Trust is occasionally abused as well which makes things even more complicated.  This question seems to be “is my trust in this particular thing being abused?” 

[tikker]

42 minutes ago, Bombastinator said:

Well “it must be wrong because I don’t understand” has been a pretty common thing lately.

True. Sometimes it feels like science has started to become just an opinion instead of an instrument to describe the Universe. I'm all for letting people believe what they want, but it also gets tricker and trickier to convince them the weirder an less intuitive things get, like relativity.

1 hour ago, Commodus said:

Oh, it's certainly good to ask! The OP just started by suggesting that scientists might've got it wrong... well, if they did, it's something we can't really know. Not for a long time, at least!

 

I was suggesting the first moments after the Big Bang more as a subject of pure fascination than anything. It's wild that the very 'rules' of the universe were in flux for a very brief moment.

Haha yeah all fair. If it was that simple we most likely would have figured it out by now (there are exceptions of course). The Big Bang is indeed fascinating, especially how matter somehow got the upper hand over antimatter.

[Bombastinator]

1 hour ago, tikker said:

True. Sometimes it feels like science has started to become just an opinion instead of an instrument to describe the Universe. I'm all for letting people believe what they want, but it also gets tricker and trickier to convince them the weirder an less intuitive things get, like relativity.

Haha yeah all fair. If it was that simple we most likely would have figured it out by now (there are exceptions of course). The Big Bang is indeed fascinating, especially how matter somehow got the upper hand over antimatter.

What do you mean “starting to”? That’s all it’s ever been for a thousand years.  Thats why things are theories.  The difference is it’s an informed unbiased opinion bs an uninformed biased opinion.  Marketers and politicians have latched onto this to be able to say well my opinion is just as good as yours so any stuff I make up that is convenient for me personally is just as likely to be how it works!” When it isn’t.   One of the really hard ones is it’s really really hard to keep bias out of such things.  Not only does it cause problems it gives the self serving ammunition.  Science is extremely limited.  It is specifically fundamentally unable to find truth directly.  What it IS capable of, and even this is extremely difficult to do is to find points of falsehood. Approximate Truth can be derived from a map of falsehood points if there are enough points of inquiry. Sort of like how a stencil can suggest an image with its lack of paint. The more points there are the fewer possibilities exist about what the truth is. That is all science can give.  It’s not nothing though.  A pray can throws dots of paint.  It throws billions though, hence its picture is fairly accurate.  One can make another kind of aerosol sprayer by taking a hollow drink stirrer(one needs a straw smaller than a drinking straw) and cutting it in half. When one half is used to blow over the top of the other half the bottom of which is submerged in paint one doesn’t get billions, but more like thousands.  The picture a stencil produces is much less clear.  Now imagine only one dot.  One dot by itself can mean almost nothing and it’s all humans can produce at a time.  It can take a lifetime to produce just one dot and it might not even happen. We have books though.  Dots build up.

Edited February 20, 2021 by Bombastinator

[dalekphalm]

On 2/19/2021 at 7:13 AM, Wictorian said:

Why is speed of the light constant? If velocity is distance/time and time isn't constant why can't velocity be relative? Why is it impossible for observers to see light travelling 2x the speed of light?

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.

[tikker]

4 minutes ago, Bombastinator said:

What do you mean “starting to”?

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".

8 minutes ago, Bombastinator said:

That’s all it’s ever been for a thousand years.  Thats why things are theories.

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.

1 hour ago, Bombastinator said:

Marketers and politicians have latched onto this to be able to say well my opinion is just as good as yours so any stuff I make up that is convenient for me personally is just as likely to be how it works!” When it isn’t.   One of the really hard ones is it’s really really hard to keep bias out of such things.  Not only does it cause problems it gives the self serving ammunition.

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.

[Wictorian]

2 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.

It's what politics is all about