io9's "Ask the Physicist"


Last week, I posted an entry on io9.com calling for “Ask the Physicist” type questions. People responded, um…, with gusto. There were nearly 400 comments on the original post, and well over 200 unique questions.

I’ve started posting answers.

#1 – What is the universe expanding into

#2 – What happens if you’re traveling at the speed of light and turn on your headlights?

#3 – How smart do you have to be to collapse a wavefunction?

#4 – Why believe in Dark Matter?

That’s all for now!

-Dave

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2 Responses to io9's "Ask the Physicist"

  1. Naatz says:

    Ok, so I read all your current articles on io9, and well… I have one problem with something you said on the first one: What is the universe expanding into?

    You said: ‘1. You are not currently expanding. Neither is the earth. Nor the solar system. Nor the Milky Way Galaxy. The expanding universe is due to gravity, which means that in regions of high density, the dominant gravitational effects are entirely local. As a case in point, not all galaxies are moving away from the Milky Way. Our nearest neighbor, Andromeda, is actually hurtling toward us at around 80 miles per second, and will collide with us in a few billion years.’

    In my understanding, there is no possible way to prove that we, or every other matter in the universe, isn’t actually expanding, because all frames of reference would also expand just the same. On another note, why would the universe expand due to gravity, if it is an attractive force?

    best regards

  2. dave says:

    Your comment is correct – but only technically. Your atoms could be shrinking or growing, and so long as everything in the universe is shrinking or growing at the same rate (wavelengths of light, corresponding corrections to the speed of light, changes in the gravitational constant, etc), we wouldn’t know it. But that expansion is meaningless.

    When I say that the universe is expanding, I mean that (for example) twice as many water molecules could be placed end-to-end between two distant galaxies than could have fit 4 billion years ago. The same is not true of you or me or the sun.

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