Can light be still?

Can it? It seems kind of strange to think about conceptually, since we always think of light as moving so fast, it's essentially instantaneous when moving from point A to point B.

I think it can.

Perhaps I'm mistaken somewhere, but here's my argument: If something is moving at the speed of light in one direction and is giving off light in the opposite direction, the light coming off of it should be traveling at a speed of 0.


First of all, let's backtrack. Let's say I'm driving down the road in my truck going 60 mph. I have an orange in my hand. I throw the orange straight up. Naturally, the orange will continue moving in the same direction I'm heading, at the speed of 60 mph. (Of course, drag and all that stuff, but hold off on that.)

If I take that orange and I throw it backwards (the opposite way my truck is moving), at say a releasing speed of 20 mph, now that orange is actually going 40 mph in the same direction my truck is going. (Forget drag, gravity, etc.)

If I take the orange and throw it 80 mph behind me, it will really only be traveling 20 mph in that direction (since I'm going 60 mph forward). And if I throw it 60 mph behind me, then it won't move at all.


That being said, if an object (let's say a star) is traveling at the speed of light, the light given off of it ("behind" the star) should stand still.


Or does light have some weird property where it has to spread out (like air or water in a container)? Thus it can't be "still"? And if a star (at the speed of light) is heading in a direction directly away from you -- will you never see the light coming off of said star? [Assuming you're standing still.]

Ace beat me to it.

But it's one of the things I could never wrap my head around. Light always travels at 300,000kps. Always. Doesn't matter if a headlight is on a car coming at you at 200mph, or a brake light is on a car going away from you at 200mph. The speed of light always remains constant.

Go figure.

I can't.

Quote: RS

And if a star (at the speed of light) is heading in a direction directly away from you -- will you never see the light coming off of said star? [Assuming you're standing still.]

The light travels (in vacuum) with a constant speed in all inertial reference frames. That means it will travel at speed of c towards you regardless of the relative speed of the inertial frame its source is in.

My only guesses would be when time stopped, extreme gravity or absolute zero.

Quote: onenickelmiracle

My only guesses would be when time stopped, extreme gravity or absolute zero.

I thought absolute zero only applied to matter, not energy???

The frozen light article is a bit misleading. The photons don't actually stop, they are converted into something else (atomic spin) and then converted back.

The thing is that light doesn't behave like an orange would. And even an orange would not behave as expected if it travelled close to the speed of light. The speed of light is c in all frames of reference, no matter their relative speeds. One integral part for understanding this is time dilation.

Btw, the speed may not change, but the color does. See relativistic doppler effect.

Quote: aceofspades

frozen light

Interesting article. Had no idea about that.

Quote: Face

Ace beat me to it.

But it's one of the things I could never wrap my head around. Light always travels at 300,000kps. Always. Doesn't matter if a headlight is on a car coming at you at 200mph, or a brake light is on a car going away from you at 200mph. The speed of light always remains constant.

Go figure.

I can't.

Quote: rawtuff

The light travels (in vacuum) with a constant speed in all inertial reference frames. That means it will travel at speed of c towards you regardless of the relative speed of the inertial frame its source is in.

That seems strange, but at the same time not too strange. Light is some pretty weird shit.

But if light moves at a constant speed and can neither speed up nor slow down (maybe that's not worded properly?), then what about a black hole? If light cannot escape from it (?) and is being sucked in, would it get sucked to/toward the center of the black hole and move around in the ever dense black hole? Does it still move inside it?

Quote: Canyonero

The frozen light article is a bit misleading. The photons don't actually stop, they are converted into something else (atomic spin) and then converted back.

The thing is that light doesn't behave like an orange would. And even an orange would not behave as expected if it travelled close to the speed of light. The speed of light is c in all frames of reference, no matter their relative speeds. One integral part for understanding this is time dilation.

Btw, the speed may not change, but the color does. See relativistic doppler effect.

Cool links. I started reading on the doppler effect. Craziness.

Quote: rawtuff

The light travels (in vacuum) with a constant speed in all inertial reference frames. That means it will travel at speed of c towards you regardless of the relative speed of the inertial frame its source is in.

Yes, and one of the mindblowing byproducts of this is if you have two objects, both travelling at the speed of light, and both travelling directly towards one another, the speed at which they approach each other is the speed of light.

Quote: UP84

Quote: rawtuff

The light travels (in vacuum) with a constant speed in all inertial reference frames. That means it will travel at speed of c towards you regardless of the relative speed of the inertial frame its source is in.

Yes, and one of the mindblowing byproducts of this is if you have two objects, both travelling at the speed of light, and both travelling directly towards one another, the speed at which they approach each other is the speed of light.

Perhaps I'm confused on the wording, but I think I understand what you're saying.


If you have truck[A] going 80mph towards truck going 80mph, it can be said they are approaching eachother at a speed of 160mph, yes?

However, if both trucks are going speed of light (c), then they are approaching each other at the speed of light (c) and NOT approaching each other at the speed of light + speed of light (c + c). ??


If so, that is indeed mindblowing.



Reminds me of that one experiment (forgot what it's called). But basically they shoot electrons (or something like that) through a slit and it behaves one way. They do the same through 2 slits and behaves as expected (like water with ripples would). But when it's being "watched", the electrons behave differently. o_O

Quote: RS

That seems strange, but at the same time not too strange. Light is some pretty weird shit.

But if light moves at a constant speed and can neither speed up nor slow down (maybe that's not worded properly?), then what about a black hole? If light cannot escape from it (?) and is being sucked in, would it get sucked to/toward the center of the black hole and move around in the ever dense black hole? Does it still move inside it?

Once behind the event horizon, the only way it can move to is towards the center (insight) and not towards outside.

Because the timespace around a black hole is so distorted (curved) by its gravity that there is no future events able to happen beyond the event horizon, i.e no light wave can "happen" because it will need to have a speed faster than the speed of light to escape from the BH and through the EH.
Light needs space and time to spread in and it follows the curvature of the said timespace.

So the light captured in a black hole can only follow the curvature of this timespace which is towards the center of the BH.

Btw, light can be slowed down by putting it in a denser environment i.e water, glass etc.

But what happens if we sit at the event horizon and shoot a beam of light away from it? The gravity is still enormous, will it slow the light?

Explanations say no, we will still measure the speed of light as c, because it is in our local frame and the gravity itself does not slow the light as it's massless. But we would measure a delay when observing a beam of light travelling from point a to point b in different reference frames when it moves near super massive objects. Because it will need to travel through the curved spacetime.

Quote: aceofspades

I thought absolute zero only applied to matter, not energy???

It applies to heat energy and is impossible to reach. If the cosmos were to expand forever the total amount of heat would be equally distributed everywhere and the temperature would be the same everywhere but it would be slightly higher than 0 degrees Kelvin.

Quote: RS

Reminds me of that one experiment (forgot what it's called). But basically they shoot electrons (or something like that) through a slit and it behaves one way. They do the same through 2 slits and behaves as expected (like water with ripples would). But when it's being "watched", the electrons behave differently. o_O

Apparently so do cards, dice, and roulette balls. Especially BACCARAT CARDS.

Rawtuff, light can orbit a black hole, not just fall to the center. Look up "photon sphere."

Dr. Stephen Hawking has had some interesting things to say about black holes. You can look up "Hawking radiation" also.

UP84, Things can move towards eachother at faster than the speed of light. You can see that right here on earth in the large hadron collider.

Quote: pew

If the cosmos were to expand forever the total amount of heat would be equally distributed everywhere and the temperature would be the same everywhere but it would be slightly higher than 0 degrees Kelvin.

You say that as if you believed that the amount of thermal energy in the universe (or in any system) is a fixed quantity. Not so.

Quote: Face

But it's one of the things I could never wrap my head around. Light always travels at 300,000kps. Always. Doesn't matter ...

Go figure.

I can't.

We can't wrap our heads around it because we live in a world where we expect time and distance to be quite constant indeed. The ruler you used to measure for your blinds isn't going to change on you so that when you hang the blinds, all of a sudden they don't fit. The day is going to be 24 hours long, a minute is going to be a minute.

But for the speed of light to be constant "no matter what", wow, the universe made a strange rule for us there. Einstein figured it out, what we thought were the constants, can't be constant if light speed is to be what's constant!

Quote: Doc

You say that as if you believed that the amount of thermal energy in the universe (or in any system) is a fixed quantity. Not so.

Absolutely! The total amount of energy in the universe is absolutely fixed. It is a definite amount of heat. We can only guess at a figure though.

Where/when did you study thermodynamics? First of all, your terminology isn't standard or it isn't consistent. "Heat" is the term used to describe energy being transferred from one system to another due to a difference in temperature. Heat is not a term used to describe energy within a system. The closest terms I know of to match that concept are thermal energy or internal energy. Since energy exists in a number of forms and can be converted from one form to another, it is quite possible for the thermal energy (what some people mis-describe as "heat") of a system to change over time, even if that system is the entire universe. Consider either a chemical reaction or nuclear reaction in which chemical/atomic energy are converted to thermal energy. That can have a big impact on the temperature of the system.

Light in slow motion -- best Ted talk ever IMHO.

http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_frames_per_second?language=en

Quote: Doc

Where/when did you study thermodynamics? First of all, your terminology isn't standard or it isn't consistent. "Heat" is the term used to describe energy being transferred from one system to another due to a difference in temperature. Heat is not a term used to describe energy within a system. The closest terms I know of to match that concept are thermal energy or internal energy. Since energy exists in a number of forms and can be converted from one form to another, it is quite possible for the thermal energy (what some people mis-describe as "heat") of a system to change over time, even if that system is the entire universe. Consider either a chemical reaction or nuclear reaction in which chemical/atomic energy are converted to thermal energy. That can have a big impact on the temperature of the system.

The first law of thermodynamics states that matter and energy cannot be created or destroyed. Therefore the total amount of matter/ energy in the universe cannot under any circumstances be added to or changed in any way. Period.

Are you making a subtle attempt to change your position, or does that "Period." just completely overlook the very real possibility of converting mass to energy? You started off making a statement that sounded a lot as if it claimed the amount of "heat" in the cosmos is fixed and that continuing expansion would decree that everything tends to a uniform temperature barely above absolute zero. Your most recent statement that the total amount of mass and energy is a fixed amount is more accepted but practically irrelevant to the earlier claim.

Matter (or mass) to energy happens all the time. Eventually it will not happen because there will be no temperature difference anywhere so there will be no energy available to perform work and matter will be reduced to just some hydrogen atoms going real slow. I am saying that the total amount of mass/energy in the universe is a finite amount that cannot be changed except from the outside which would depend on there being something "outside".

When I used the term heat it is the same as saying matter, mass, energy, watts, ergs, btu's ect. they are interchangeable when discussing the heat death of our universe. On that scale entropy only increases.

Quote: pew

When I used the term heat it is the same as saying matter, mass, energy, watts, ergs, btu's ect. they are interchangeable

I'd be very interested in knowing what textbook or reference you saw that in. It has so many errors it's hard to know where to start. Here are a couple of things to consider:
(1) Heat is an entirely different concept than mass, not something that is "interchangeable."
(2) Do you usually measure heat, or any kind of energy, in watts? Do you also perhaps measure distances in miles per hour?
(3) Just how related do two entities need to be for you to consider them "interchangeable"? How about "friction" and "pressure", since they both involve forces?

Incredible.

Well it's snowing again so what the heck may as well exercise my tiny brain while I drink my coffee. Energy equivalent 1 ton of coal=27,778,245 BTU's= 7,000,000 Kilocalories =27,778.245 Cu.ft. natural gas =18292357188701500000000000000000000000000000 Electron volts= 29307600000000000000000 Ergs= 21616176446.16 Ft. lbs force= 2,931 Gigajoules=8141 Kilowatt Hrs= .7 tons of oil= 1-2 grams U235 ect......

Do you also perhaps measure distances in miles per hour? In normal human speech we don't, but if you asked me how far I traveled to get to the casino I could say 60 mph for 45 minutes and you would understand me I hope. of course you might think I was a little odd for expressing it that way.

Quote: Doc

Where/when did you study thermodynamics? First of all, your terminology isn't standard or it isn't consistent. "Heat" is the term used to describe energy being transferred from one system to another due to a difference in temperature. Heat is not a term used to describe energy within a system. The closest terms I know of to match that concept are thermal energy or internal energy. Since energy exists in a number of forms and can be converted from one form to another, it is quite possible for the thermal energy (what some people mis-describe as "heat") of a system to change over time, even if that system is the entire universe. Consider either a chemical reaction or nuclear reaction in which chemical/atomic energy are converted to thermal energy. That can have a big impact on the temperature of the system.

wow, I don't want in the middle of this, but in undergraduate chemistry courses heat/thermal energy were used interchangeably ... that's not to say it was all proper ... and in engineering it may indeed be a no-no ... signing out !!

In thermodynamics, whether it is the version taught in chemistry or physics or engineering, the basics are the same while different aspects may be emphasized. The set-up of the first and second laws makes it important to distinguish between energy within a system and energy being transferred between systems. Energy being transferred between two systems is categorized as either heat or work. There are several definitions that attempt to distinguish between those two, but the most common one is that heat is energy transferred due a difference in temperature, with work being any other energy transfer. An alternative definition is that work is an energy transfer that is equivalent to the lifting of a weight against a gravitational force. That definition often becomes more cumbersome.

Thermal energy is an ambiguous term that sometimes refers to heat being transferred and sometimes refers to a system's internal energy (or a change in that energy) that is indicated by its temperature. If you use the term "heat" to refer to internal energy, or any property/quantity contained within a system, you are quickly going to wind up confusing terms and drawing incorrect conclusions from statements from thermodynamics.

This thread started with some discussion of the speed of light and implications of relativity. I'm definitely more into the classic sciences, so I stayed out of it. Then, there was a comment or two that seemed to be based on classical thermodynamics but with some off the wall conclusions, so I spoke up. Perhaps I shouldn't have tried to make the discussion stick to standard terminology, but some of the statements seemed to be getting out of hand.

Now if someone wanted to discuss thermodynamics and debate whether the universe is an open or closed system, I'd probably let them fly off on that one. Some folks enjoy those issues, but I find them pointless for my life. I would put that topic in with discussion of such things as the shape of the boundary of the ever-expanding universe -- and what is beyond that boundary.

My original response addressed absolute zero which can't be reached and tried to point out that even with cosmic "heat death" the vast reaches of space would never attain it. I think current cosmology, that is big bang inflation states that the universe is a closed system with a definable beginning.

A cool video about how, in the vast expansive of the universe, light is still pretty slow


Riding Light

Quote: odiousgambit

wow, I don't want in the middle of this, but in undergraduate chemistry courses heat/thermal energy were used interchangeably ... that's not to say it was all proper ... and in engineering it may indeed be a no-no ... signing out !!

While so-called definitions are good in one sense - it takes different amounts of heat applied in different ways to a system to give it a specific thermal energy - wouldn't it be nice to begin with THAT ONE form of energy, and end up with THAT as something tangible or definitive?

Like realizing, eventually, the oneness of non-classical physics and classical physics. Good grief, the more determined we become, so too the more crazy the exceptions that show none of us in this time were ever really right either.

Quote: pew

Well it's snowing again so what the heck may as well exercise my tiny brain while I drink my coffee.

I'll start really believing in all of this stuff when people begin to actually benefit in meaningful ways relative to the human condition.

Faster cars with better brakes does not a safer car make. Nor one which relinquishes all control to computers in separate continents.

What is non-classical physics?

Quote: pew

What is non-classical physics?

Really?

Light can be slowed substantially, and even stopped. This is nothing new... been around for 15 years +.
Wiki "slow light".

Quote: Dalex64

Really?

I have no idea what the previous poster was saying so I was curious what he or she meant by the oneness of classical and non classical physics. I should have asked "what do YOU mean by..."

Quote: pew

I should have asked "what do YOU mean by..."

Indeed. And right on time. This stuff gives me a headache at night.

But at least nobody's trying to "peel an onion" over it. The beauty of saying what's on your mind.

You could google stuff, http://www.rc.umd.edu/praxis/complexity/plotnitsky/plotnitsky.html . However, that's not non-classical enough for me.

What happened to MrV? (His dull(?) determination the other way can be somewhat inspirational.)

Quote: teliot

Light in slow motion -- best Ted talk ever IMHO.

http://www.ted.com/talks/ramesh_raskar_a_camera_that_takes_one_trillion_frames_per_second?language=en

Thanks for sharing, that was amazing!