The Naked Scientists

The Naked Scientists Forum

Author Topic: How can someone measure things possibly not there?  (Read 2158 times)

Offline annie123

  • Sr. Member
  • ****
  • Posts: 252
    • View Profile
I hear often about measuring stars and their various planets, behaviours etc. but if these bodies are thousands of light years away then how does anyone know that they are measuring something that still exists? How can any statements be made about things so far away that what we see is probably not what's there? How do you know there's something out there at all beyond what fits into the parameter of time/light travel?


 

Offline flr

  • Sr. Member
  • ****
  • Posts: 302
  • Thanked: 2 times
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #1 on: 12/08/2013 00:39:16 »
We measure galaxies (or other bodies) by capturing the light these bodies emitted or reflected.
The light travel at finite rate (about 300,000 km/s)
So, when we look further away in space we also look back in time because light from far away objects needs time to get here.

We don't know if objects are still there right now at the time the measurement is done (according to our clock) , but we know they were there when the light we captured from them was emitted.

If those objects dissapeared meanwhile (i.e. 'unexpectedly' digested by an undetected black hole), our grand grand (grand)_n children may learn about it.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: How can someone measure things possibly not there?
« Reply #2 on: 12/08/2013 03:10:46 »
I hear often about measuring stars and their various planets, behaviours etc. but if these bodies are thousands of light years away then how does anyone know that they are measuring something that still exists? How can any statements be made about things so far away that what we see is probably not what's there? How do you know there's something out there at all beyond what fits into the parameter of time/light travel?
We don't know something in the sense that we "know" 1 + 1 = 2. We only "know" things that we use science to predict. We know things in the sense that we know the sun will rise tomorrow.

It takes a short amount of time for light to reach the earth from the sun. Therefore what we're seeing is not the sun as it is now but how it was a fraction of an hour ago. We assume that the sun is still there now even though we won't know for sure for many minutes. But nothing in our experience tells us that we should expect otherwise. And using what we know from observing nature we can make predictions of what to expect in the future. From this we can predict when a star will burn out to within a several million years. So if we're sure that the sun will rise tomorrow morning why should we doubt it will rise next year, or a hundred years, or a thousand years? Our knowledge allows us to make predictions and that's all we can do. So we really don't "know" things in the absolute sense that you may think we're saying we do. We say these things within certain levels of confidence,

This is a subject for the philosophy of science. A very good discussion of this is in a text by Fritz Rohrlich which is on my web site at http://home.comcast.net/~peter.m.brown/ref/philosophy_physics.pdf

It’s called “Philosophy and Logic of Physical Theory.”
 

Offline annie123

  • Sr. Member
  • ****
  • Posts: 252
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #3 on: 16/08/2013 23:09:27 »
Pmb- the text to which you refer is surely an abstract of a larger text? And one which would probably be something understood by physicists or mathematicians. I read it anyway, and extracted some of the overall ideas but this did not really throw much light on my original musings.I can understand a 'level' of confidence 'about some predictions within our solar system but in terms of things billions of AUs away I can't really see how we can be confident of anything since there are no reference points from the past when the instruments we have today reach distances quite impossible in the past.
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4126
  • Thanked: 247 times
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #4 on: 17/08/2013 02:44:46 »
Astronomers often examine stars, of which the closest one is our Sun, at about 8 light-minutes away.

Centuries ago, astronomers worked out that the Sun could not get all its energy from burning wood or coal, or from gravitational collapse. It was only in the 20th century that astronomers worked out that luminous stars get most of their energy from hydrogen fusion. Once this was understood, it was possible to make a mathematical model of the interior of a star, and understand how this would change over time - and predict how the Sun would look in the future, or in the past, even though we don't have any actual measurements of the current, future or far past behaviour. We are now able to confirm and tweak these calculations because we can now peer inside the Sun with helioseismology.
 
Astronomers also examine different stars of different brightness, chemical composition (using spectroscopy) and mass. These expand the mathematical models to new regions which are not currently seen in the Sun. These also allow prediction of past or future behaviour without actual measurements.

These models of stars can be extended to groups of stars (galaxies) and clusters of galaxies, which can be seen at far distances (ie far in the past), even when the individual stars can no longer be seen with our current telescopes.

These mathematical models make basic assumptions like:
  • "the laws of physics are the same everywhere in the universe" (an important hypothesis which astrophysicists test whenever they get an opportunity)
  • "this is what we understand with the information we have available today" (which could be completely superseded in 10 years time by a discovery made tomorrow)
  • "if nothing unexpected happens, then this will happen..." (which obviously could be completely wrong if a random black hole ran across your path tomorrow).
  • "if a tree falls in the forest when no-one is around, it really does make a sound" (ie the universe has an existence beyond my perception)
  • These assumptions are so fundamental to all of science that they are rarely repeated beyond introductory science courses (and sometimes, not even then).
But many astronomical observations don't rely on predicting the future - when astronomers announce "we have discovered a pulsar circling the (presumed) black hole at the center of our galaxy", they don't normally qualify that by saying "well, it was there 27,000 years ago", because anyone who understands astronomy knows that you are always looking into the past. A first measurement of the magnetic field near the center of any galaxy is important news for astronomers regardless of whether the measurement was made today or 27,000 years ago in our galaxy, or 2.5 million years ago in the Andromeda galaxy - it all helps astronomers refine their mathematical models of galaxies, and predict their behaviour in the future. [...and in the past - but should that be "postdict"?]

See: http://en.wikipedia.org/wiki/Stellar_evolution#Models
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11999
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #5 on: 18/08/2013 19:41:06 »
Annie, assuming that everything we can observe must obey 'c', everything you see exist for you at the moment you observe it. We are indeed looking into the past astronomically speaking, but the contributions of the mass you see looking out, will be the same as what your eye tells you about the mass/stars/planets etc you can 'see' at that moment (assuming you could see it all).

Just translate it into information instead, and define it such as all meaningful information obey 'c'. So what you see is what you get, although that 'information' might be a billion years old.
=

In fact, if it wasn't this way we, assuming a 'instantly 'propagating' gravity' not obeying 'c' for example, I think we should be able to measure it. Because we can extrapolate suns life lengths, depending on type, and then looking at distance and the time taken ('c') for the light to reach us, we should be able to test it, well, possibly so :) There's a lot of unknowns to consider I guess, as the 'real' size of a universe etc. We make some assumptions to simplify that, as the universe being isotropic and homogeneous (being the same no matter where you look out from). So, using those it should be test able, I think?
« Last Edit: 18/08/2013 19:56:58 by yor_on »
 

Offline annie123

  • Sr. Member
  • ****
  • Posts: 252
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #6 on: 18/08/2013 20:41:58 »
Thanks for your explanations.

“Maybe there's a whole other universe where a square moon rises in the sky, and the stars laugh in cold voices, and some of the triangles have four sides, and some have five, and some have five raised to the fifth power of sides. In this universe there might grow roses which sing. Everything leads to everything.”
― Stephen King
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4721
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: How can someone measure things possibly not there?
« Reply #7 on: 31/08/2013 18:00:29 »
If you take a classical view of simultaneity, everything you see no longer exists because in the time it too for the light to reach you, the object will have changed. The change may amount to no more than a molecular vibration or two, or it could be the total disintegration of a bomb or a supernova.

Fortunately, it seems that gravitational waves propagate at the speed of light, so the interactions we observe between distant objects probably represent what actually happened. No big deal. A lot of science depends on analysing recorded data rather than contemporaneous observation, and the availability of astronomical timeshifts from seconds to billions of years is an opportunity rather than a limitation on what we can discover. 
 

The Naked Scientists Forum

Re: How can someone measure things possibly not there?
« Reply #7 on: 31/08/2013 18:00:29 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums