# Time and motion Brian Greene

• November 1st, 2013, 09:29 PM
crafter82
Time and motion Brian Greene
So I was watching a Brian Greene doc on Youtube about Time and he says motion through space effects the passage of time. see video at 15 minutes til 17 minutes - can't link the video but its called The Illusion of Time.

I understand how distance between observers and gravity effects time but not motion. So If im sitting opposite a person and they walk 20 feet to their left but are still the same distance to me then why would that effect the time of my observation?
• November 1st, 2013, 10:02 PM
Janus
Quote:

Originally Posted by crafter82
So I was watching a Brian Greene doc on Youtube about Time and he says motion through space effects the passage of time. see video at 15 minutes til 17 minutes - can't link the video but its called The Illusion of Time.

I understand how distance between observers and gravity effects time but not motion. So If im sitting opposite a person and they walk 20 feet to their left but are still the same distance to me then why would that effect the time of my observation?

Distance does not effect time. If an event happens 1 light year from me, and I see it today at noon, that doesn't mean that I think it happened today at noon, I know that it took 1 yr to reach me, so I know that it happened at noon 1 yr ago according to my time keeping. Also, a person at the event sees that I see the event today at noon, he also knows that the light took one year to reach me and thus knows that the event happened 1 year ago by my and his time keeping.

With motion, it is different. The reason for this is that the speed of light remains the same for everyone. (this means as measured relative to themselves.) The simplest example is the following:

A man sits in the middle of a railway car. He flashes a light. For him, the light travels at the same speed in both directions and takes an equal amount of time to reach the two ends of the car.

I am moving relative to the railway car(or the car is moving relative to me, it makes no difference.

I also see the man flash the light. The light travels travels at the same speed (relative to me) in both directions. But the ends of the cars don't keep an equal distance from me( since the car is moving) and the light has to chase after one end of the car, while the other end of the car rushes towards the light. The light therefore does not hit both ends of the car at the same time.

The man in the car and I do not agree at what events happened when according to each others time keeping.

For a little more in depth discussion of this, check out this:

http://www.thescienceforum.com/physi...ty-primer.html
• November 1st, 2013, 10:02 PM
Daecon
It's called Time dilation.

Wikipedia has an article about it: Time dilation - Wikipedia, the free encyclopedia

It doesn't really apply unless you're moving close to the speed of light.