/ How does light travel in space?
Then perhaps sensing she had me on the ropes we talked about if we were in space with a torch and we turned it on for a couple of minutes what would happen to that light. I thought it might just keep travelling trough space as a light wave forever but I don't really know. I've promised her an answer by breakfast. She also wants to know if she were on the far side of space and was waiting for the light to arrive from the torch what would it be like.
You are correct, the light would travel as a wave and just keep going until it hit something that would absorb it (and likely re emit it - this is how we see the moon)
If you shone your torch for 1 minute, your daughter on the otherside of space would also see it for one minute, but would have to wait for the light to arrive.
To get her to understand the concept of light travel, tell her it takes about 8 mins for light to reach us from the sun, so when she sees the sun she's. Actually seeing back in time and what she sees is actually what the sun looked like 8 mins ago.
Next time its thundery point out the diff in speed of light and sound
Ps how old is she?
Tell her to ask her teacher if light is a wave or a particle.
If you were in space and you turned a torch on it would essentially be the same as turning a torch on on earth - it would illuminate the immediate space around you and you would consider that to be instantaneous because the speed of light (300000000 metres per second) is so much faster than the human eye can track.
If you were on the far side of space and someone a long way away turned a torch on, it would look exactly like someone turning a torch on from 100m away, except it would happen with a time delay corresponding to the time it took for the light to travel from the far side of space to you.
However, in practice this wouldn't happen, as a torch is a very weak light source and the light from it spreading out in a cone shape from the bulb would mean that the light would likely be far too diffuse to distinguish by the time it reached you. A laser might manage it, though - the light would need to be very directed and concentrated.
That might be pretty difficult for a young'un to take in!
tell her that its like throwin a ball here on earth.
The difference is, we have gravity and air, and the ball is going slowly.
The light is thrown out into space where there is no air, so nothing to absorb the light.
Sometimes though, light goes past a star which is very big, and the gravity makes the light change in direction.
Re the torch, if you did that, unforntunately, you'd never see it at the other side of the universe. the light is fanning out in a slight cone. But by the time that cone reaches the other side of space, it's SO wide, there is very little light where she is standing. To illustrate, just draw the rays coming off a torch on a bit of A4, fanning out. Tell her it's so far away, the amount of light hitting her eye would not be enough to trigger the rods and cones, then explain how the rods and cones each take a 'bit of light' to make an image.
> Then perhaps sensing she had me on the ropes we talked about if we were in space with a torch and we turned it on for a couple of minutes what would happen to that light. I thought it might just keep travelling trough space as a light wave forever but I don't really know. I've promised her an answer by breakfast. She also wants to know if she were on the far side of space and was waiting for the light to arrive from the torch what would it be like.
Yes, the light from the stars just keeps going in waves through the vacuum, and since very little of it hits anything it just carries on until it gets to something solid - namely, your eyes.
The answer with the torch is "sort of". First thing you can do is shine a torch at a wall from two metres away. Result - small bright patch of light on the wall. Shine it at the wall from ten metres away. Result - bigger, less bright patch of light on the wall. If it's a clear night, it's not getting absorbed by anything in the gap, it's just that it doesn't all go in the same direction, so it doesn't all end up in the same place, so the light is spread more thinly. So in practice if you were millions of miles away from the torch then even in the vacuum of space it'd be so thinly spread over such a big area that you wouldn't be able to see it.
The same thing happens with light from the stars - they're enormous balls of continuous nuclear explosion kicking out huge amounts of light, but because that light is spread out over a ball 1000,000,000,000,000 km in radius by the time it reaches us, we only see them as tiny pinpricks of light.
Give me a couple of minutes, I'll go look for some Carl Sagan stuff on it.
Answers hopefully suitable for the young...
They are very, very bright, many of them are even brighter than the sun. And there is nothing (vacuum is, by definition, nothing) to stop the light so we see it here. Light moves very fast, but it's not instant, and the light has spent years travelling to get to us.
That light would go out into space as a long, thin beam, travelling very fast. It would spread out as it travelled, and it wouldn't take long until it was spread so thinly that it was invisible even with the most powerful of telescopes. A torch is a hundred million million million million times dimmer than our sun, and our sun is small enough to be nearly invisible from the other side of the galaxy, never mind the other side of space. Space is huge.
If instead of using a torch just blew up a bunch of big stars, then the explosion would be bright enough to see from a long way away. You could see it in the next galaxy over (which is called Andromeda by the way), but it would take 2.5 million years to travel there.
We don't know how far away the far edge of the universe is, or even if it has an edge. It's possible it wraps round on itself somehow, but that's getting complicated. If it has a far side, then the light would probably not get there before the end of the universe.
If it did get there, it would arrive as radio waves due to the cosmological redshift. That's due to general relativity, which is notoriously hard to understand, so I'm not even going to try to explain it.
In reply to cuppatea:
Naughty! Don't bring quantum mechanics into this it's complicated enough already.
For the curious it is neither or both (depending on definitions); it is somewhere in between.
Don't even go there!! I still have a hard time explaining this and I sort of understand it.
Again though, there's a Carl Sagan youtube vid which does a great job.
You could point out that light behaves both as waves and as particles (just don't try to explain this too much;-)). Now if you consider it as particles, you can say that the tourch puts out a large number of photons (billions), which when they strike our cells in our eyes cause a tiny voltage, and hence we 'see' the light from the torch.
Now point out that the beem from the torch is a cone and the further you are away the more area the same number of photons have to cover (you can demonstate this by holding the torch at different distances from a wall, and showing the increasing size of the spot it makes). This means that at even reasonable distances, like across the road or from the end of the garden, the torch looks much less bright. Tomorrow night get her to shine a torch at you as you are stood against a wall and walk backwards, point out that you stay the same size but the spot of light around gets bigger, meaning there are less photons hitting your eyes, as more and more are hitting the wall as she gets further away from you.
Now taking this to the massivly large distances of space, the photon's are so spread out, that even say from the moon you'd be luck if even 1 photo of light hit your eye. This means that it would be impossible to distingish it from the background 'noise' of other photons.
Now you can use the analogy that the stars are like huge candles, and space is a dark field. You can see the candle in the field at night because although only a few thousand photons from it hiit your eyes, the background is dark. The Stars are billions and billions of times brighter than your torch so still a large number of photons hit your eyes and you see them, even in the huge distance of space. In the daylight the number of photons from our own sun is so large that you can't see the stars above the background 'noise'.
The other point is that space is not a perfect vacuum. Even in deep space there are still a few particles, and if one of the photons strikes one of these the the light will be absobed, so not all the light will make it in the direction of observation.
So yes the light from her torch goes for huge distances in space, but you can't see it as it is so spread out.
Hope this helps.
Basically, yes. Tell her the stars she can see are a million times bigger than the whole world. But a torch is too tiny.
In truth, IF you could get the light to leave the torch in exactly parallel lines (you can't) you WOULD see it the other end.
Problem is, it fans out and becomes too weak to see.
The light from stars does the same. That's why they don't look as bright as our sun, because we are only seeing a teeny bit of the light.
Re flying alongside at speed of light
We are getting in to the theory of relativity here. Imagine a train carriage with it's lights on, to the observer in the field a packet of light is wizzing by, but to someone on the train it's just light constantly.
So if travel along side the light beam at the speed of light in space in a kids world, yes you would be able to see it next to you, the same as if you were walking next to a rolling ball. In a more "real" situation you couldn't see it unless there was some substance to absorb and re emit it (or you flew along with your head in the beam looking back to where the light was emitted)
Still intrigued know how old she is, this is the sort of questions that could develop into a great physics brain if nurtured
About travelling at the speed of light: you can't do it. Light is not like anything else your daughter (or we) are used to. It goes at a constant speed, whatever you do.
This is an amazing, remarkable fact. A train might be going at 100mph through a station, but if you drive alongside in a car at 60mph, it looks like it's only going 40mph. You can't do this with light: no matter how fast you chase it, it will always be going at the same speed. It seems pretty counter-intuitive, and it should do: Einstein had to completely remodel our understanding of space and time to make sense of it.
Delighted to see this thread and realise that the internet is not just for porn.
I guess she's old enough to learn that science can be quite complicated, but not yet old enough for the thought that it is merely the best-fit model for what can be observed...or hypothethised
Explain refraction then.
Diffuse light is light coming in at you from different angles - eg if you've got a single light source (like the sun) spreading out and then some of the spread out stuff getting scattered back in (eg by going through clouds) - so on a cloudy day you get diffuse light, but not much of it. If it doesn't get scattered back in, then you only get the fraction of the light from the star that is coming right at you, and don't get the stuff that was going out a fraction of a degree to the side.
Well, you sort of do because there's a little bit of diffusion from the atmosphere, but not enough to make a star into a smeary diffuse light...
The constant speed refers to light travelling through a vacuum, traveling through other substances it differs with density amoung other things
Do waves need a medium in which to travel, or is the wave form self-perpetuating? I always thought a wave was a movement of energy through another medium, not a physical thing in itself. Perhaps this is where light as a particle comes in?
I realise this would mean that radio waves couldn't travel through a vacuum, and clearly they do. I guess that means I'm wrong, or perhaps radio waves are also strange.
> Do waves need a medium in which to travel, or is the wave form self-perpetuating? I always thought a wave was a movement of energy through another medium, not a physical thing in itself. Perhaps this is where light as a particle comes in?
First three paragraphs. :-)
I may be teaching you to suck eggs, but its the most simplistic way to explain it so everyone undersands it :D
If I may interject, as a Physicist...
Wow. Good question. Another guy once asked this exact same question. His name was Albert Einstein. This exact question was the one which kick-started his discovery (or invention, interesting discussion there) of special relativity. I.e. - you can never go faster than the speed of light, if you try the universe will stop you.
I will be happy to write more when I don't have to be up in the morning and I am sober. However, the take-away message is that this is a hugely insightful (and complicated) question.
No you wouldn't. You can not travel at the speed of light, even in a 'kid's world'. It is disingenuous to claim to a child that this is possible, and the mere statement that 'in a kid's world you would see this...' is incorrect because it is a physical impossibility.
> No you wouldn't. You can not travel at the speed of light, even in a 'kid's world'. It is disingenuous to claim to a child that this is possible, and the mere statement that 'in a kid's world you would see this...' is incorrect because it is a physical impossibility.
We all know this, but it's explaining it to a very young child. So the best way is to say you can't go as fast as light, but if you could........ then treat it as a thought experiment.
Feed the enquiring mind. don't crush it.
> First three paragraphs. :-)
Then I assume a vacuum is not actually a vacuum? If waves really can travel through complete nothingness then they wouldn't need to retain any dimensional form. Complete nothingness is non-dimensional in itself (I know it looks pretty gap-filling from the point of view of an observer).
> We all know this, but it's explaining it to a very young child. So the best way is to say you can't go as fast as light, but if you could........ then treat it as a thought experiment.
I completely agree with your intent, but don't start with a false premise. You'll only end up with reductio ad absurdum in this case. Start with the idea, and explore it correctly.
You're preaching to the converted!
This gets rather deep. Our current model of reality is such that there are lots of fields permeating space. Think of these as drum skins - they are just sitting there not making a sound, but they have the potential to do so, and in different ways.
Each of these different drum skins has different properties - thickness, tension, for example.
If you strike the drum skin, a wave will propagate along it in a way determined by those properties. Where the drum skins meet, they can transfer energy from one to another - a wave in one skin becomes a wave in one (or more - always two or more, actually) other drum skin.
We call a vibration of a field (Drum skin) a particle, like an electron, for example, and a coupling between fields an interaction.
Mathematically we call these constructs 'fields'. In our current model of the universe, everything physical is a result of a vibration of a field, and every interaction between physical objects is a result of couplings between fields (the transfer of energy between the drum skins).
So. If the universe is of infinite size (and mass?) But it is also expanding...then what is it expanding *into* ?
What you need is to tell her about the powerful LASER that they can point at a reflector left on the moon during the landing, that 'torch' light is parallel and powerful enough to travel to the moon and back, so she could witness her own 'torch' light returning.
No, you would not be safe in saying that. The statement as given is largely meaningless, and it would be misleading to try to make it mean something. Sorry.
(I'll let myself out...)
Fiver apiece on Amazon.
Get her to work out how long (in miles) her 60 sec beam of light actually is - helps to get a (small) handle on the scales involved.
> So. If the universe is of infinite size (and mass?) But it is also expanding...then what is it expanding *into* ?
It's a rather tricky one to answer that, as in the mathematical framework of spacetime, it doesn't exist 'in' anything - it's the spacetime itself that is expanding. There is no concept of the universe being 'in' anything else - what would that thing be in, and that thing, and that thing...
> So. If the universe is of infinite size (and mass?) But it is also expanding...then what is it expanding *into* ?
Nothing. Boundaries, starts and ends are a very human abstraction.
Surely that depends on where she is and how fast she's travelling relative to you.
Thanks :) I was more interested in the
theory than the answer.
The old "humans can't understand it so there" gets tiresome.
> Surely that depends on where she is and how fast she's travelling relative to you.
Brilliant question about moving aloungside the beam of light at the same speed. This is the exact question Einstein asked in his thought experiment that lead to the theory of relativity!
Have a look at Maxwells equations of electromagnetic radiation, which model the propagation of light through a vacuum. (via that wiki page you linked to)
Like metres meters and seconds?
Is theoretical physics any less tangible because it cannot be seen, or measured?
I'm enjoying this thread, there a few Brian Cox's in the UKC midst!
> Like metres meters and seconds?
> Is theoretical physics any less tangible because it cannot be seen, or measured?
The whole point of theoretical physics is that it's usually NOT tangible, it's postulated as extensions of what we do know, or pissibilities which fit the available facts.
These theories, if they hold enough merit, are often then tested where possible by doing practical experiments. CERN etc.
The temperature of space is a tricky question. Remember that it is a hard vacuum, and vacuums insulate very well. The battery will eventually cool down to the point of uselessness, but it won't happen for a while.
A powerful LED light intended for use on earth but used in space will likely cook itself in short order, as it will have been designed to lose heat by conduction and convection, neither of which are available to it in a vacuum.
In reply to balmybaldwin:
You could stick your head into the pulse of light and see it, but you can only see light if it hits your retinas! A pulse of light passing beside you is quite invisible.
> Nothing. Boundaries, starts and ends are a very human abstraction.
This is not quite true. There's a very good article in New Scientist covering this ground. You need to register to read it, but it is free.
That was sort of my point. Metres kilos seconds etc are a human abstraction. All science is theoretical.
This forum needs an irony button, or some smilies :)
> That was sort of my point. Metres kilos seconds etc are a human abstraction. All science is theoretical.
> This forum needs an irony button, or some smilies :)
I do not understand why SI units being abstract has any meaning???
Yes, they're abstract. But it doesn't matter what units you use, so long as they are compatible units, the maths works.
Science is ONLY abstract in the sense that to a small extent it relies on allowing that we are actually 'here'
apart from that........ no, it's usually all provable by observation and/or experiment.
> I'm enjoying this thread, there a few Brian Cox's in the UKC midst!
I think there's loads of Cox on UKC
> Delighted to see this thread and realise that the internet is not just for porn.
What porn were you sreaching for when you came across this thread?
Ah. I think chaos theory I'd playing a part. I was answering a point Milesy had made,but without quoting him. My ironical answer was perhaps taken as as if i was making a point as if I knew what I was on about. My degree was Mech Eng, so I know little of the more theoretical side of science.
Anyway. What's a nuclear scientists favourite food?
Not really related, but discussing whether something is abstract or not always reminds me of this, which must be the best abstract to a scientific paper ever written:
although this one comes close:
How could you see the light next to you? You can only see light when it either comes straight into your eye, or is reflected/refraceted/absorbed-retransmitted into your eye... Just passing by doesn't count. Thus even if you were travelling (perhpas just slghtly slower than it) you wouldn't see it unless it hit something. Assuming that by this point it is not incident on your eye.
Is this correct?
More interesting, if you were travelling backwards at say half the speed of light looking at the light source would you see it for two minutes. I think you would, but half asbright as if you were standing still.
Am I right? Can a real scientist help out?
Imagine you are standing at a train station and you watch a train go by. On this train someone throws a ball. You, standing on the platform, see the ball travel at the speed it was thrown (v) PLUS the speed of the train (u), however if someone standing on the train was to measure the speed of the ball, they would only see it traveling at the speed it was thrown at.
So the point of that^^ was that speed USUALLY depends on your frame of reference, ie what you are comparing the speed of the ball with.
Light, however, is different. Light (along with the rest of the electromagnetic spectrum AFAIK) appears to travel at the same speed no matter what your frame of reference is.
Now your question is vaguely related to that, although it is important in this thread so I thought I would add it in. what is relevant to your question however, is an interesting theory called redshift. It is based on the Doppler effect ( http://en.wikipedia.org/wiki/Doppler_effect ) occurs when a waveform form has an initial velocity (u in the example above) or the observer is moving relative to the source. it is what makes a truck horn sound higher as it approaches you and sound lower as it drives away.
As light travels at the same speed no matter what your frame of reference (see above) the Doppler effect shouldnt apply in the same way, but it does. Confusing, I know and I dont even pretend to understand it.
Now for my answer: if you were to look at the torch while moving away, the light would appear to move towards the red end of the spectrum. In the case of white light this means that the blue end would start to disappear and the colour would look more orangey-red. I dont know, however, if you would only see the light for one minute due to lights constant speed, or if you would see the light for the 2 minutes, which would be suggested by the Doppler effect. My gut reaction would be that you would only see the light for 1 minute as light has a constant velocity no matter what yor frame of reference, but as I say, I dont know, so lets wait for someone with a little more knowledge than me.
Not that it really matters in this context but vacuum isn't by definition "nothing" vacuum is defined as the absence of traditional matter. There's all sorts of "stuff" in a vacuum, photons of light just being one of the many things.
Also, space isn't a total vacuum, it's got a handful of atoms of helium/hydrogen knocking about in every cubic meter :o)
If you pass light through a Bose-Einstein condensate it'll slow down. I think they've recorded it going at 30ish MPH, a fair bit slower than it's more traditional 670600000 ish MPH
It travels alpine style (fast and light) !
Both those papers have excellent abstracts!
(And both papers have strong Bristol connections - some positive press to counter the attention reflected on us by our somewhat unenlightened Christian Union this week.)
Hence why the atmosphere buggers up land based astronomic observations; non-homogeneous atmospheric delay.
> This gets rather deep. Our current model of reality is such that there are lots of fields permeating space. Think of these as drum skins - they are just sitting there not making a sound, but they have the potential to do so, and in different ways.
> Each of these different drum skins has different properties - thickness, tension, for example.
> If you strike the drum skin, a wave will propagate along it in a way determined by those properties. Where the drum skins meet, they can transfer energy from one to another - a wave in one skin becomes a wave in one (or more - always two or more, actually) other drum skin.
Think that might be a bit beyond me.
When I was young and taking lots of trips, I came to the conclusion that the only logical explanation of the universe was that it didn't actually exist. All that existed was the idea that it existed,and the maths extrapolated from that. Or in other words, the universe is just a set of equations where all the variables are zero.
Not wishing to sound like a dick, but you were off your face and perhaps not as informed as cutting edge science is.
"Or in other words, the universe is just a set of equations where all the variables are zero. " is just meaningless words.
I think that was my point, actually...
As in "I am the master of the universe, the wind of time is blowing through me; and its all moving relative to me, its all a figment of my mind", perhaps?
Back to the OP. If your daughter was on the other side of space, and your torch (let's assume its bright enough) was shining at her, is the reality that the light would be bent around so much en route by gravity that you wouldn't be able to aim it in the right direction?
One minute. You lost faith in yourself there after a pretty good answer!
As to why the stars look so small it's not a good explanation to say that the light is spreading out a lot and getting faint. It has to do with the angle the star subtends at the eye. It's a very small angle and there are a limited number of receptors in the eye which is not very wide in the first place. Only a few receptors will fire and the star therefore looks very small, or equivalently not very bright (ignoring the fact that some stars are brighter than others). A torch at that distance will be such a vanishingly small angle that it will not fire any receptors, and it will be sending out less photons from its glowing element than the equivalent area on a star I think. Telescopes are built very large to counteract this and collect more photons.
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