Need quickie layman explanation of relativity

I’m new here, just posting couple of questions. For example, this one: how would I explain to a child, looking outside an airplane’s window to the slow moving earth below, that we are in reality the ones that are moving, and moving at a speed almost 100 times faster than normally in the car? (ok the earth also moves) :confused:

Well, are you so sure you’re the ones moving? What makes you think that? (You asked about relativity, and these are the questions that lead to relativity).

Or you could use Google,GGLD:2004-08,GGLD:en&q=layman+relativity

When my daughter was just a toddler and was first getting to grips with the idea that the Earth is a round ball that rotates, she asked me (we were out driving at the time) “Are we actually moving - could we be staying still while the world turns underneath us?” Pretty insignful for a pre-schooler, I thought, but a good springboard for the whole relativity thing.

you could open the aircrafts window - that would convince most people

Funny, I’m discussing this with my physics students (mostly 9th graders) this week. You could say, “Do you remember when the plane took off and you got pressed back into your seat? That was the plane’s engine putting a force on the plane and speeding it up a great deal. When we land, you’ll feel a similar force in the opposite direction, meaning that the plane is slowing down a great deal.”

If you link the beginning of the trip to the end of the trip by the acceleration on each end, that might do it.


I am seeing two components to your question, neither of which is directly related to relativity. Relativity discusses the physics of moving objects and explains why from a physics standpoint it doesn’t matter which object moving at a constant velocity you consider to be moving with relation to another one, and how time and distance change with respect to speed. Your question, I think, can be addressed with classical physics, and with a minimum of it.

Anyway, the two parts are:

Why does it look like we are moving so slow if we are really moving fast? I get this question from my kids, and the same question applies to watching a plane from the ground. The answer is simple but hard to explain to young children. I draw a picture that shows how the subtended angle for a given travelled distance gets smaller and smaller as the object’s distance gets farther, but they need to be at least 6 before they get that.

Why does it feel like we are standing still but we see the earth moving? Because when you are going the same speed in a straight line, you won’t feel anything (unless you are hitting bumps). You can demonstrate the next time you are in the car. So you see something moving, especially out a small window like on a plane, and it looks like it’s what’s outside that’s moving. In a car, with windows on all sides, you see the outside world moving with a much greater sense of perspective, so it’s much easier to think of it as you moving rather than the car sitting still and the world moving past you.

I dunno. It’s one of those simple things that’s hard to explain. Maybe that’ll help a little.

Now, let us know when your child asks why it seems that time slows down when you’re on an airplane :wink:

Uh… relativity is a classical notion. Einsteinian relativity is (relatively) new. What you parsed as a second part goes directly to the heart of Galilean relativity.</nitpick>

That just demonstrates that the air is moving past the plane at a high velocity… and that there isn’t as much of it as there is (used to be) inside the pressurized cabin.

WHOOOOOSH!! :wink:

heh, yes. Despite what I said above, both of mine still have a problem with this (“Aaaaah! that clouds are following us!”)

But how many buttons exploded from your shirt when your toddling daughter asked, “Are we actually moving - could we be staying still while the world turns underneath us?”

Must have knocked your socks off as well. :slight_smile:

It was one of those moments when you feel your ears move (or am I the only person who gets that?)

The answer here would be to consider all the other cars out there travelling in other directions at the same time. How could the Earth turn so that everybody gets to where they’re going?

Obviously, it couldn’t (so they were all moving, while we stayed still and the Earth turned).

Only joking; I explained in the end that, yes, it was technically possible to be travelling fast enough that you seemed to stay still relative to the sun’s apparent position in the sky, but this would mean travelling much faster than my car would go, especially along country lanes that are neither straight, nor flat.

That is quite an intuitive leap for a child. You could use a globe to show how that might be possible if you lived within a few hundred miles of either pole but that you would need a very fast airplane to do it where you live.

The most basic concepts of Relativity can be had driving down the road.

Get on an expressway and go 60 mph. Point out to the person asking how signs at the side of the road appear to whiz past while other cars on the road appear to hardly be moving at all. Relative to the other cars you are motionless or moving rather slowly…good thing too else driving would be quite dangerous. The sign on the side of the road is not moving relative to you so your apparent speed is 60mph.

Taken a little further who is moving? The sign or you? From the sign’s viewpoint you just shot by at 60mph. From your view the sign passed you at 60mph. Technically both of the viewpoints above are correct. You can do thought experiments where it would be impossible to say who is moving (or maybe both are…no way to tell for certain). On earth, since you are in the car and know you accelerated, you know you are the one moving (acceleration is important in determining these things but if you are just coasting along and never experienced the acceleration you are back to not knowing who is moving).

Another way to bend the kids mind is to have them bounce a ball in the car. Hopefully this is done with little force so the ball doesn’t bounce all over. You want the ball to bounce straight up and down on the child’s lap (on a book or something). Point out to the child that the ball appears to be going straight up and down. Then point out how the trajectory of the ball would look to someone on the side of the road. As you drive by the ball is not only going up and down but has a horizontal vector as well. If they drew the path the ball took on a piece of paper the ball would make arches (as if you threw the ball forward).

It can get weirder than all of this of course but it is a good start.

Oh…I do not think the earth appearing to move slowly from an airplane can be laid at the feet of Relativity. I think that is more an issue of perception of distant versus near objects. Watch a freeway from a distant vantage point and the cars do not look like they are moving all that fast. Stand on the side of the road and the impression is much different.

Not sure why that is the impression we get (ever see a 747 about to land that looks nearly motionless despite it going ~200 mph?..that one messes with us more I think because the plane is far away but is so big seems closer than it is).

I once heard that this is the reason that railroad tracks are such dangerous places. Something about how it was beyond human visual perception to accurately gauge just how quickly that train off in the distance can close the gap and SPLAT!

In a typical airliner you’re actually traveling about 9 to 10 times as fast as in a car doing normal highway speeds.

Thank you all so much for your very insightful inputs. I understand that the question (or rather, yes, I think the question was two-folded) was simple but it’s bit difficult to find simple terms to explain.
Oh, and yeah, I’m sorry I meant the plane traveling at about 10 times faster than cars on highway speed, not 100 times. :smack:

Sure, sometimes when you’re surprised, your whole scalp moves around. I took that movement and gradually learned to do it conciously. Good comedians can do it at will. Then you learn to suppress the scalp movement part and concentrate on the ears, and voila, you’ve learned to wiggle your ears also

???How off-topic can I get???