# B Why do we use infinity in Physics?

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1. Jul 20, 2016

### Phys12

When we talk about a particular problem in Physics. For instance, let's say that light is coming from somewhere to hit the earth. We often say that the light is coming from "infinity." Let's say that we're tackling a black hole and we have a person somewhere as an example and we say that let's say that the person is at "infinity."

My question: why "infinity"? Can't we say that it's at a distance x that we don't know, or some place far away? Why "infinity"?

2. Jul 20, 2016

### Staff: Mentor

It would be more accurate to say "in the limit approached as the distance becomes arbitrarily large", but that's a mouthful. Especially in informal discussions, it's easier to say "infinity" and move on.

3. Jul 20, 2016

### EnEE

you can say that it's at a distance x that we don't know. but for a lot of problems you want to omit parameters that make the solution complicated, for example if you want to find the electric field of a charged iron chair, it's may be very difficult... but if you say that you are in the "infinity" you can omit the changes in the surface of the chair and treat it like a single point charge.

"some place far away" - it's more accurate than "x that we don't know" but misleading... some times "some place far away" is only 1 micro-meter away. "after a very long time" can be just a second and so on

"infinity" it's just terminology for "you can omit the annoying stuffs from your equation" ;)

4. Jul 20, 2016

### Phys12

Perfect! :D

Thank you.

5. Jul 20, 2016

### Staff: Mentor

In optics, light coming from infinity is shorthand for the assumption that the light rays are perfectly parallel. For any x less than infinity, the rays radiate out at an angle relative to each other.

Infinity sounds far fetched, but parallel seems simple and ordinary.

6. Jul 20, 2016

### wud-wurks

In mathematics we talk about an exponential equation tending to infinity as it slides right on the X axis. getting closer to y=0 and North on the Y axis as it tend to X=0. As we do not offer a finite figure we use the term tending to infinity.