braces_for_impact

I heard a statement today that I couldn't find fault with, but with which I still disagree. The statement seems flawed but I can't put my finger on it. On the other hand, I am admittedly lacking in anything but the most basic of physics education so I may very well be wrong in my 'gut instinct'. A little help please.

Confirm/disprove this statement?

"If there were no light in the universe, time would not exist."

Somehow this seems wrong to me, maybe it's just a non sequitur. Or, it could be accurate.

Tim Thompson

It's wrong, but it's a nice try. I think it's motivated by the role of the speed of light in relativity. For instance, in special relativity, time dilation depends on a factor that looks like sqrt(1-v^2/c^2). If there is no light, no c, what happens? If c = 0, is time undefinable? Of course, length contraction works the same way, and you could argue that without light, there would be no space either.

In general relativity, the speed of light becomes a factor of proportionality between space & time. What happens if there is no light? You might argue that there must be no time. But, then again, you could equally argue that there must be no space.

Some physicists have advocated having two "speeds of light",. or some such, one being the speed of propagation of electromagnetic waves, which we commonly think of as the "speed of light", and the other being the proportion between time & space, a physically independent quantity.

But I can imagine a universe in which time gets along just fine without c in either of its fundamental capacities. If you have space & time, who needs electromagnetic fields? And do space and time have to be proportional? I think not.

So, I would say that it might be clever, but it's wrong.

Jesse

The cosmic speed limit c should really be thought of as a property of the (Lorentzian) geometry of spacetime, it isn't solely associated with light (there are other massless particles besides photons, and all such particles travel at c) and it is even conceivable that photons could have some tiny rest mass, and that they only travel very close to c--this would not force us to throw out the theory of relativity, it would just force us to stop referring to c as "the speed of light", although I think such a finding would force us to get rid of (or at least modify) our current theory of electromagnetism, and probably areas of quantum field theory as well.

AdamWho

Maybe the person is referring to something more fundemental than relativity issues. If there is no light, then there are no photons, then there is no electro-magnetic force, then there is no ???

eh

See the DeSitter universe.

Answerer

Maybe that guy is refering to the beginning of the universe where all massless particles were identical to each other and photons could be said to mediate all kinds of force including gravitional. So, without photons, there will be no such thing as spacetime at the early stage of the universe.

nermal

Maybe more accurate would be:
If there were no light in the universe, time would be irrelevant.
It would still exist, as things move even in the absence of light, but without light, no information is transferred between objects not in contact, hence time would pertain to nothing.
Except for gravity, that kind of throws a wrench into things. But as I understand it, gravity is also limited to the speed of light, so the quote would in spirit include gravitational information as well.

Ed

Ernest Sparks

In short, 'NO light' means 'NO electromagnetism or photons'. Since EM seems to be intimately interlaced with the weak (decay) interaction and, I guess, with the strong (nuclear) interaction, I reckon that might leave out most of the stuff we know and love. But these interactions are believed to be lorenz-invariant in themselves, so maybe time-relations would be recoverable from some of them.

AdamWho

Another angle might be: If there were no photons then a universe might be able to exist but it would not be "our" universe.

braces_for_impact

Thank you folks, that was my impression, that it may be possible, but it certainly wouldn't be a universe anything like we know.