What time is it?

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The concept of time feels so natural that we often forget just how peculiar it is. Time governs all our everyday lives. We keep track of it, waste it and wish we had more of it. It seems to go fast when we’re having fun, and drag when we’re not. But figuring out what exactly ‘time’ is remains one of the most difficult problems in physics. 

One of the biggest misconceptions is the notion of ‘now’. What we perceive as happening in the present is actually based on things that have happened in the past. This is because light does not travel instantaneously but takes time to reach us.

The Sun is about 93 million miles away from Earth (that’s about 150 thousand million metres) meaning that even though light travels at just under 300 million metres per second through space it still takes light from the Sun just over 8 minutes to reach us. That means we actually see the Sun how it was 8 minutes ago. Looking into the night sky we see the stars how they were millions or billions of years ago, as they are so far away the light has only just reached us. Even reading these words from your computer screen there is a delay of about 3 nanoseconds – so small you never notice it, but the fact is this intake of information is not instantaneous.


Everyday life is defined by hours, minutes and seconds. Scientific experiments regularly use much smaller intervals like nanoseconds, with some precise time measurements now on the attosecond (10-18) scale. This is still a relatively large division of time – the smallest unit of time in current theoretical physics is the Planck time of 10-43 seconds. It’s almost impossible to imagine how small the Planck length is (although try this). To put in in some kind of context, imagine a ‘grain’ of time being blown up to be the size of a grain of sand. If we also blew up real grain of sand in the same proportion they would  end up being 10000 trillion times the size of the Milky Way.

Einstein fundamentally altered the way we think about time. His theory was that time is a dimension just like the three spatial ones, and they are intertwined into a four-dimensional ‘spacetime’ that forms the fabric of the universe. Moving through  the time dimension is what gives us the feeling of time passing.

One of the well known laws of physics is that you can’t travel at, or faster than, the speed of light. More accurately, you can’t travel through space at the speed of light. This restriction doesn’t apply to that of time. Because space and time are together as 4D spacetime, the faster you travel through space, the slower you travel through time. This means that if you’re stationary in space, you’re travelling through time at the speed of light.

The nature of time is even stranger. It’s not just about how fast you’re moving, but also what you’re close to that influences it. This is because massive objects such as stars and planets – in fact anything with a gravitational field – warps the spacetime around it.

Because the Earth’s gravity warps spacetime, and this effect diminishes further from the planet’s centre of mass, each day our heads age around 10-11 seconds more than our feet. Live until 100 and that adds up to around 365 nanoseconds. This effect has been most notable in space missions – the Russian cosmonaut Sergei Krikalev has spent 803 days on space stations, meaning he’s around 21 milliseconds younger than he would have been had he stayed on Earth.


Black holes warp spacetime

In the 1960s Irwin Shapiro  conducted experiments that showed the Sun’s warping of spacetime causes a signal sent between Earth and Mercury to  travel at a different time to  that expected without this effect This validation of Einstein’s theory is called the Shapiro Time Delay.On earth the effect is miniscule, but out in the universe the vast mass of the stars and galaxies warp spacetime so much that time ticks all over the place.

There are even things that can warp spacetime so much they appear to not just slow down time, but stop it altogether. Black holes are phenomena that warp spacetime so much that they appear to stop time. Observing someone’s watch as they moved towards a black hole it would be seen to tick slower and slower, until when they reach its event horizon time would appear to stop completely.

Our intuition about time is that we live in the present. The past has happened and no longer exists, and the future is yet to happen. But if time is simply a dimension like those of space, as Einstein theorised, then we can come up with a completely counter-intuitive idea: that time and all events have always existed and always will. The past, the present, and the future all exist simultaneously.

Think of this as like your journey from home to work. As you travel away from your home, it still exists in space, and similarly past events still exist in time. Your work place already exists and you are simply travelling through space to it. Similarly future events already exist and you’re travelling through time to when they occur. Travelling through time is no different to travelling through space.

This idea that the future is as real as the past is poignant but doesn’t seem quite right. That’s because it’s a classical viewpoint and doesn’t take into account quantum mechanics. In the QM world nothing is certain and the future least of all. Teaming Einstein’s ideas with QM should give us a better grasp on the true nature of time.

An alternative theory is that time is not the smooth structure proposed by Einstein, but rather that it is granular. Intervals of time are like grains of sand, with the passing of time able to be thought of as like sand flowing through an hour glass.

If spacetime is grainy then it could grow grain by grain, event by event. This is not possible in Einstein’s vision, where spacetime is a continuum with all events already and always in existence. This theory is a bit strange, but does take into account the uncertainty of quantum mechanics which we know defines how the universe behaves at its smallest scale.

Time is a fundamental concept, governing not only our everyday lives but the entire universe. It’s remarkable that we still know so little about it. Will we ever understand the true nature of time? That’s uncertain. Or perhaps it isn’t.

IMAGES: Leo Reynolds, Andrew Coulter Enright and Alan Cleaver, via flickr.

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5 thoughts on “What time is it?

  1. Heya i’m for the primary time here. I came across this board and I in finding It truly helpful & it helped
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  2. Find copy of E-mil sent to another site.

    I have several news worthy items that relate to the same area of science.
    I really need to talk to someone who understands a little about physics.
    If you are prepared to listen then I have a big story for you. This would run & run.

    Example: There is NO precise, accurate, nailed down definition of what “TIME” actually, intrinsically is.
    Look in any dictionary and the supposed definitions become vague, ambiguous, thin, lacking in clarity, e.t.c.

    I have an absolute definition of what “TIME” is. I have written this down for all to read.
    It’s just getting people to read it. If you yourselves have any acumen at all (which I am sure you do) then you will be able read what I have written and be well able to understand it. It is simple, so simple that all to date have missed it, overlooked it, and just not recognised it.
    If you have any curiosity at all why not just read it for yourself.

    There is also ANOTHER explanation for the RED SHIFT observed when looking at distant astronomical objects. This has to be correct because it has been PROVEN to be so in laboratories.
    This RED SHIFT as you are no doubt aware is the basis that the big bang theory is based on, by those that BELIEVE in the big bang theory. Many intuitively, instinctively do not which includes Edwin Hubble.

    There IS an explanation for why galaxies spin as fast as they do without flying apart, which agrees with the above paragraph, so you have ONE new observation explaining TWO apparent anomalies.

    My name is Syd Wilcox, and I can be reach by phone on 0753 862 4439.

    If you are interseted in what I have then please get back to me. It will not cost you any thing but a little of your time.

  3. If you wish to find out what my definition of time is then all you have to do is e-mail me for it. I would like you to e-mail so that I have a record of receiving a request for the same. I did not put in my last reply as I thought that the E-mail address would go straight over but reading later I see that this is not so. My E-mail address is solarlibration@gmail.com

  4. Hi Syd. My apologies for the delayed reply to your comments. I have replied here, rather than the Extremes blog post (http://www.isciencemag.co.uk/blog/keen-on-science/extremes). I have an interest in science and merely offer my take on various things. I am no expert on cosmology or BBT, and I’m sure it will be of no surprise to you that Harlton Arp was never so much as mentioned in my undergraduate university level texts/lectures. I thank you for informing me about him now; his work sounds at least interesting if nothing more, though I have yet to have chance to read it. I hope to do so soon, and perhaps even offer a blog post here in due course as an alternative viewpoint that may interest other readers of this blog.

  5. This has an interesting post, It got me thinking about the hear and now and the different dimensions that we think exist. The Mayans must have been extremely advanced with their time keeping system and as with nealry all, is based on astrology and the movement of planets and stars.

    Thanks for the educational article.:)

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